Bacterial infection and immunity  （Long Min)

The term infection describes theprocess that pathogenic microorganisms multiply, release toxin   within the body and produce achange in the normal physiology of the body.  The microorganisms capable of causingdisease are called pathogen. Some bacteria, such as Lactobacillusacidophilus, are considered to be nonpathogens,because they rarely or never cause human disease. The normal florae of the bodyare usually not pathogenic but under certain condition can cause disease. Theyare called opportunistic pathogens. Because of the adaptability ofbacteria and the detrimental effect of modern radiation therapy, chemotherapy,and immunotherapy on resistance mechanisms, some bacteria previously consideredto be nonpathogens are now known to cause disease. Serratia marcescens, for example,is a common soil bacterium that causes pneumonia, urinary tract infections, andbacteremia in compromised hosts.

Normal Flora and Opportunistic pathogens

Normal Flora

The microbial populations that inhabit the surfacetissues of the skin, oral cavity, respiratory tract, gastrointestinal tract,and genitourinary tract are referred to as normal Flora (Fig. 6-1).  The humanbody, which contains about 10¹³ cells, routinely harbors about 10¹⁴bacteria. The normal microbial flora is relatively stable, with specific generapopulating various body regions during particular periods in an individual'slife after birth. 

Some membersof the normal flora may aid the host. For example, normal flora in theintestinal tract synthesizes vitamin K and aid in the absorption of nutrients.On mucous membranes and skin, normal flora may prevent colonization ofpathogens by competing receptor or binding sites on host cell, competingnutrients and producing antibiotic materials. On the other hand, microorganismsof the normal flora may harm the host by causing dental caries, abscesses, orother infectious diseases. Some of them may exist as commensalsthat inhabit the host for long periods without causing detectable harm orbenefit. Even though most elements of the normal microbial flora are harmlessin healthy individuals, these organisms frequently cause disease in compromisedhosts.

Germ-free animals are available to study the effect of normal microbialflora. Two interesting observations were made. First, the germ-free animalslived almost twice as long as conventional animals, and second, infection oftencaused death in conventional animals, but intestinal atoniafrequently killed germ-free animals. Although the result indicates thatbacterial flora may be not essential to life, studies with antibiotic treatedanimals suggest that the flora protects individuals from pathogens.Investigators have used streptomycin to reduce the normal flora and have theninfected animals with streptomycin-resistant Salmonella. Normally, about 10⁶organisms are needed to establish a gastrointestinal infection, but instreptomycin-treated animals whose flora is altered, fewer than 10 organismswere needed to cause infectious disease.  In germ-free animals, the alimentarylamina propria is underdeveloped, little or noimmunoglobulin is present in sera or secretions. It means that normal floraenhance the development of immune organs in host body.

Opportunisticpathogens

Somemembers of the normal flora may produce disease under certain circumstances.  Opportunistic infections occur when thehost defense mechanisms are impaired, microbes arepresent in large numbers, or when microbes reach vulnerable body sites. In compromised patients, whose defenses are weakened, these bacteriaoften cause opportunistic infectious diseases when entering the bloodstream andtissues. If forcefully removed fromthe restrictions of that environment and introduced into the bloodstream ortissues (after surgery, catheterization or other treatmentmodalities), normalflora may become pathogenic. Forexample, streptococci of viridans group arethe most common resident organisms of the upper respiratory tract. If largenumbers of them are introduced into the bloodstream, they may settle ondeformed or prosthetic heart valves and produce infective endocarditis.Bacteroides species and Enterobacteriaceae are the commonest residentbacteria of the large intestine and are quite harmless in that location. Ifintroduced into the free peritoneal cavity or into pelvic tissues along withother bacteria as a result of trauma, they cause suppuration and bacteremia. When the normal flora areadversely affected by antibiotics that are used to treat a disease, an imbalancemay occur that leads to the development of disease. For example, the use ofantibiotics sometimes disrupts the balance of the microbial community of thegastrointestinal tract, permitting the growth of Clostridium difficile; this causes a severe and sometimes fatalgastrointestinal tract infection.

Nosocomial Infections

Infectious diseases acquired as a result ofa hospital stay are known as Nosocomial Infections. Nosocomial Infections are hospital-acquired infections

So many factors unique to the hospital environment are led to nosocomial infections. Some patients become infected whensurgical procedures and lower defenses permit resident flora to invade theirbodies. Indwelling devices such as catheters, prosthetic heart valves, grafts,drainage tubes, and tracheostomy tubes form a readyportal of entry and habitat for infectious agents. Other patients acquireinfections directly or indirectly from fomites,medical equipment, other patients, medical personnel, visitors, air, and water.The health care process itself increases the likelihood that infectious agentswill be transferred from one patient to another. Treatments using instrumentssuch as respirators and thermometers constitute a possible source of infectiousagents. 

Bacterial Pathogenesis 

Pathogenicity refers to the qualitative ability ofbacterial to cause disease. Various pathogenic bacterial can cause differentdiseases. For example, Mycobacterial tuberculosiscauses tuberculosis and Salmonella typhi led toTyphoid.

Virulence quantitatively describes theextent of a microorganism’s ability to cause disease. The term virulence isderived from the Latin virulentia, meaningpoison.

Pathogenicity is amulti-factorial process which depends on the virulence of the particularbacterial, the dosage(numbers) of that microorganisms,and the immune status of the host.  Forethemore,specific bacterial species (or strains within aspecies) initiate infection after being transmitted by different routes tospecific sites in the human body. For example, bacteria are transmitted inairborne droplets to the respiratory tract, by ingestion of food or water or bysexual contact.

Virulence of Pathogenic Bacterial

Invasiveness and toxins contribute to thevirulence of bacterial pathogens. 

Invasiveness

Invasiveness refers to the ability ofmicrobes to invade human cells and tissues and to multiply on or within them.

Capsules

Capsules interfere with the ability of phagocytic blood cells to engulf and destroy bacteria andprotect bacteria against some antimicrobial substances. So bacteria canmultiply rapidly and cause diseases in human body. Capsules surrounding thecells of strains of streptococcus pneumoniae, forexample, permitting these bacteria to evade the normal defencemechanisms of the host, allowing them to reproduce and causing the symptomatology of pneumoniae.

 Capsules of some bacteria mimic host molecules in structure: Escherichiacoli K1 and Neisseria meningitides B(neuraminic acid)capsules mimic the neural cell adhesion molecule.This molecular mimicry of host molecules helpsthese pathogens evade recognition by the immune system. It may also trigger anautoimmune response.

Adhesin

Bacterial infections are usually initiatedby adherence of the microbe to a specific epithelial surface of the host.Otherwise the organism is removed e.g. by peristalsis and defecation (from thegut) ciliary action, coughing and sneezing (from therespiratory tract) or urination (from the urogenitaltract). Adhesion is not non-specific "stickiness". Specific interactionsbetween external constituents on the bacterial cell (adhesins)and on the host cell (receptors) occur i.e. an adhesin-receptorinteraction. The presence or absence of special receptors on mammalian cellscontributes significantly to tissue specificity of infection. Specific factorsthat enhance the ability of a microorganism to attach to the surfaces ofmammalian cells are termed adhesins. There are twokinds of adhesins. One is Fimbrialadhesin. The other is Non-fimbrialadhesin.

Fimbrial adhesins

Fimbrial are involved in mediatong attachment ofsome bacteria to mammalian cell surfaces. The antigenic composition of fimbriae can be complex. For instance, two fimbrial antigens called colonization factor antigens (CFA)ⅠandⅡhave been detected in enteropathogenic E. coli.Strains of E. coliwith different surface characteristics cause distinct diseases. Among the mostthoroughly studied pili are those of uropathogenic E. coli. Certain adhesinspresent on the tips of fimbriae of E. colifacilitate binding to epithelial cells. Type 1 fimbriaebind to mannose containing receptors. Whilst P fimbriaeallow binding to galactose containing glycolipids (e.g. cerebrosides)and glycoproteins present on epithelial cells. Theyare referred to as "P" fimbriae since theywere originally shown to bind to P blood group antigens on human erythrocytes.

Non-fimbrialadhesin

Non-fimbrial adhesin include the filamentous haemagglutininof Bord.pertussis, a mannose-resistant haemagglutinin from Salmonella serotype Typhimuriumand a fibrillar haemagglutininfrom Helicobacter pylori. Outer membrane proteins are involved in the adherenceof N.gonorrhoeae and enteropathogenicEsch.coli to cell surfaces.GroupA streptococcus pyogeneshave hair-like appendages. Lipoteichoic acid and M protein are found on the appendages. The lipoteichoicacid causes adherence of the streptococcus to buccalepithelial cells. M protein acts as an antiphagocyticmolecule.

In addition to preventing loss of the pathogen from the host,adhesion induces structural and functional changes in mucosal cells, and thesemay contribute to disease.

Other extracellular aggressins

Many species of bacteria produce enzymesthat may be involved in the pathogenic processes．Some of these enzymes are discussed below．

S aureus producescoagulase， which works in conjunction with serum factors to coagulate plasma． Coagulasecontributes to the formation of fibrin walls around staphylococcal lesions，which helps them persist in tissues．Coagulase also causes deposition of fibrin on the surface of individualstaphylococci，which mayhelp protect them from phagocytosis or fromdestruction within phagocytic cells．

Many hemolytic streptococci producestreptokinase（fibrinolysin)， a substance that activates a proteolyticenzyme of plasma．This enzymeis then able to dissolve coagulated plasma and probably aids in the rapidspread through tissues.

Many other degradativeenzymes, including mucinases, phospholipasesand hyaluronidases,areproduced by pathogenic bacteria.

Toxins

Two major types of toxin have beendescribed: Exotoxins, which are produced extracellularly by both Gram-negative and Gram-positivebacteria, and endotoxin, which is a component of theouter membrane.

Exotoxins

Exotoxins are proteins secretedmainly by Gram-positive bacteria. For example, Clostridium botulinum, Clostridium tetani, Corynebacterium diphtheriae,Staphylococcus and Streptococcus pyogenes canproduce Exotoxins. Some Gram-negative bacterial i.e E. coli, VibrioCholerae, Shigella dysenteriae also produce them. Examplesof exotoxin are botulism, anthrax, cholera anddiphtheria. Typically, protein exotoxins are excretedinto the surrounding medium. The effects of exotoxinare specific to the microorganism of producing toxin, and these toxins causedistinctive clinical symptoms.

Exotoxins are generally more potent than endotoxin,and far smaller amounts are needed to produce serious disease symptoms than arerequired for disease symptoms due to LPS. Protein Exotoxinsare extremely potent: about 30 g of diphtheria toxin could kill 10million people, and 1g of botulinum toxin cankill everyone in the United States (over 225million people). Those Exotoxin have a partially characterized site of action e.g.botulinum toxin, tetanus toxin and anthrax lethaltoxin. Botulinum toxin acts by causing inhibition ofrelease of acetylcholine at the neuromuscular junction. Tetanus toxin is takenup at neuromuscular junctions and transported in axons to synapses. It thenacts by inactivating inhibitory neurons.

Protein exotoxinsare readily inactivated by heat. A protein exotoxincan normally be inactivated by exposure to boiling water for 30 minutes. Some exotoxins can be partially denatured (e.g.byformaldehyde treatment) to generate toxoidswhich lack toxic activity but still induce protective immunity. It can beused as vaccine.

Exotoxin may be categorized according to the symptoms. For examples,neurotoxins affect the nervous system, enterotoxins cause an inflammation of the tissues of the gastrointestinaltract, and cytotoxin interfere with cellularfunction.

Table 1. Some Major Exotoxins

Most bacterial exotoxins are composed of areceptor protein component (B subunit) that attaches to a target cell and atoxin component (A subunit) that enters the cell and disrupts normal cellactivity. They are called A - B Toxins. The classicaltoxins demonstrated to act in this fashion are those of cholera and diphtheria.Examples of some pathogenic mechanisms associated with exotoxinare given below.

Endotoxins

Endotoxins are toxiccomponents of Gram-negative bacterial cell wall. The classical and most potent endotoxin is lipopolysaccharide(LPS). The substances are heat-stable，havemolecular weight more than 10 million. They have three main regions: O-specialpolysaccharide (outmost in cell wall), core polysaccharide and lipid A withKDO. However, peptidoglycan(PG) displays many endotoxin-likeproperties. Certain peptidoglycans are poorlybiodegradable and can cause chronic as well as acute tissue injury. Althoughall Gram–negative bacterial have LPS in their cell walls, LPS is not toxinunless it is released from the outer layer of the cell. When the Gram –negativebacterial die, the cell walls disintegrate, releasing the LPS toxin. Somegrowing Gram –negative bacterial also release LPS toxin due to sloughing or “blebbing” of outer membrane; in this cases, the LPS canhave a toxin effect on a host organism. The term endotoxinwas originally introduced to describe the component of Gram –negative bacteriaresponsible for the pathophysiology of endotoxin shock, a syndrom withhigh mortality, particularly in immnocompromised orotherwise debilitated individuals.

 The effects of LPS toxins are generally the same for all species ofGram-negative bacteria because of the common nature of lipid A. Toxicity isassociated with the lipid portion of the LPS molecule, termed lipid A. LPS cantrigger complement cascade by the alternative pathway. Both LPS and Lipid A arepotent activators of macrophages, resulting in the induction of a range of cytokines(IL-1,TNF and other cytokines) which are involvedin the regulation of immune and inflammatory responses. In addition, endotoxin interacts with the blood clotting system viaHageman factor (factor Ⅻ). This may result in the inappropriate clotting of blood in theperipheral vasculature, called disseminated intravascular coagulation (DIC).The pathophysiological effects of LPStoxins include fever,circulatorychanges, and other general sympotoms, such asweakness and nonlocalized aches. The pathophysiologic effects of LPS toxins are generally thesame for all species of Gram-negative bacteria because of the common nature oflipid A. 

 Injection of LPS produces fever after 60-90 minutes, the time need forthe body to release IL-1. Injection of IL-1produces feverwithin 30 minutes. Repeated injection of IL-l produces the same feverresponse each time but repeated injection of  LPS causes a steadily diminishing feverresponse because of tolerance due in part to reticuloendothelialblockade and in part to IgM antibodies to LPS．

 Injection of LPS produces early leukopenia，as does bacteremiawith gram-negative organisms．Secondary leukocytosis occurs later． The early leukopeniacoincides with the onset of fever due to liberation of IL-1. LPS enhances glycolysis In many cell types and can lead to hypoglycemia．

 Endotoxin shock occurs in gram-negative bacteremia．Endotoxin affect avariety of cell as Kupffer cells, granulocytes,macrophages, platelets, and lymphocytes. The effects of endotoxinon host cells are known to stimulate prostaglandin,cytokines synthesis and to activate the kallikrein system, the kininsystem, the complement cascade via the alternative pathway, the clottingsystem, the fibrinolytic pathways. There may bewidespread after arteriolar and venular constrictionfollowing by peripheral vascular dilatation, increased vascular permeability，decrease in venous return， lowered cardiac output， stagnation in the microcirculation，peripheral vasoconstriction，hypotention, shock，and impaired organ perfusion and its consequences．Dissemnated intravascular coagulation also contributes to these vascularchanges．

 LPS are among the many different agents that can activate thealternative pathway of the complement cascade，precipitating a variety of complement-mediated reactions（anaphylatoxins， chemotacticrespnses， membrane damage， etc) and a dropin serum levels of complement components（C3，C5-9)．

 Dissemnated intravascularcoagulation is a frequent complication of gram-negative bacteremiaand can also occur in other infections． LPS initials the blood clotting system via Hageman factor (factor Ⅻ). Thus fibrinogen isconverted into fibrin. At the same time， plasminogen can be activated by LPS to plasmin（a proteolytic enzyme) which can attack fibrin  with the information of fibrin splitproducts．Reduction inplatelet and fibrinogen levels and detection of fibrin split products  are evidence of  DIC． 

 LPS causes platelets to adhere to vascular endothelium and occlusion ofsmall blood vessels,causing ischemic or hemorrhagic necrosis In various organs．

 Endotoxin levels can be assayed by the limulustest： a lysate of amebocytesfrom the horseshoe crab (limulus) gels   or  coagulates   in   the   presence   of  0.00001ug/ml of endotoxin.

The Portal of Entry

To initiate an infection, a microbe entersthe tissues of the body by a characteristic route, the portal of entry. Themajority of pathogens have adapted to a specific portal of entry, one thatprovided a habitat for further growth and spread.Ifcertain pathogena enter the “wrong” portal, they willnot be infectious. For instance, inoculation of the nasal mucosa with theinfluenza virus invariably gives rise to the flu, but if this virus contactsonly the skin, no infection will result. Occasionally, an infective agent canen會(huì)計(jì)資格ter by more than one portal. For instance, Mycobacterium tuberculosis enters throughseveral portals of entry such the skin, respiratory and gastrointestinaltracts.

The Size of The Inoculum

Another factor crucial to the course of aninfection is the quantity of microbes in the inoculating dose. For most agents,infection will proceed only if a minimum number (the infectious dose) ispresent. The infectious dose depends on the virulence of pathogenic bacterialand immune defense in human body. In general, microorganisms with smallerinfectious doses have greater virulence. A large inoculumis needed for the weakly virulence species. The infectious dose varies fromabout 10 cells in tuberculosis, giardiasis, and coccodioidomycosis, to 1,000 cells for gonorrhea and 10,000cells for typhoid fever in contrast to 1,000,000,000 cells in cholera.

Host Immunology

The principal physiologic function of theimmune system is to protect the host against pathogenic microbes. Innateor non-specific immune system, which include the skin, phagocytic cells, and various antimicrobial chemicals, formthe first line of defense against foreign organisms. A second line of defenseis the specific or adaptive immune system. It involves theproduction of antibodies and cell-mediated responses in whichspecific T and Bcells are activated and destroyinvaders. The response to a second round of infection is often more rapid thanto the primary infection because of the activation of memory B and T cells.The cells of the immune system interact with one another by a variety of signalmolecules so that a coordinated response may be mounted. These signals may beproteins such as lymphokines which areproduced by cells of the lymphoid system, cytokines and chemokines that are produced by other cells in animmune response, and which stimulate cells of the immune system. 

Non-Specific Immunity  

Innate immunity consists of thepre-existing defenses of an animal such as barrier layers and secretions. Itpresents from birth and does not depend upon prior exposure to any particularmicroorganisms. The innate defense mechanisms are non-specific. It means thatthey are effective against a wide range of potentially infectious agents. Innateimmunity does not change with repeat exposure to the same pathogen.

The elements of the non-specific (innate) immune system include:Anatomical barriers, secretory molecules and phagocytic cells.

Table 2.  Physico-chemicalbarriers to infections

Anatomical barriers

Skin and mucous membranes

The intact skin and mucous membranes of thebody afford a high degree of protection against pathogens. The outer hornylayer of skin is consist mainly of keratin that isindigestible by most microorganisms. So it can stop the entry of organism into our bodies.If the skin is damaged by traumatic injury or surgery, infection will beinitiated. Intestinal movement,oscillation of broncho-pulmonary cilia swallowof salive and continuous trickle of urine, whicheject noxious substances or microorganisms from the body. 

Secretory molecules

These include organic acids in skinsecretions, lactic acid of sweat, thiocyanate insaliva, low molecular weight fatty acids in the lower bowel, complement, acutephase proteins, etc. in serum, interferon and tumor necrosis factor (TNF) atthe site of inflammation. An additional defense in tears and saliva is Lysozyme, an enzyme that breaks down the bacterial cellwall (peptidoglycan). The intestine’s digestivejuices and bile acids are potentially destructive to microbes. Interferonand TNF-alpha inhibits viral replication and activates other cells whichkill pathogens.

NormalFlora

The composition and protective effectexerted by normal flora were discussed before. Its presence can block theaccess by pathogens to epithelial surfaces and can create an unfavorableenvironment for pathogens.

Phagocytic cells

Neutrophils (PMN) and Mononuclear phagocytesare the most important cellular components of the non-specific immune system.Neutrophils (polymorphonuclear,PMN) only stay in bloodstream for about 10 hours and then enter the tissues.The lifespan of these cells is 1-3days. Mononuclear phagocytes are theother population of phagocytic cells and include monocytes in circulation, histiocytesin tissues, microglilal cells in the brain, Kupffer cells in the liver and macrophages in serouscavities and lymphoid organs. Monocytes stay in bloodstreamfor about 2-3 days. After that the cells enter various tissues and develop tomacrophages.

When pathogenic bacterial penetrate the skin or mucous membranes andinvade the body tissues, neutrophils are the firstcells that are attracted to the site of infection, where they ingest thepathogens and also utilize their microbicidalmechanisms to kill and/or restrict the spread of the microbes. In general, thepathogens are killed by neutrophils. If they are notkilled, the bacterial enter the lymphatics, lung,bone marrow, liver or bloodstream to be engulfed and killed by macrophages.

Phagocytosis Response to infection

Binding

During infection, phagocyticcells respond to chemotactic factors and migrateacross the capillary wall to the site of infection/inflammation.   

 Once at the site of infection, phagocytes can bind to bacteriavia receptors for bacterial polysaccharides (scavenger receptor) or hostproteins that act as opsonins (proteins which aid phagocytosis: fibronectin,complement and IgG antibody). Receptor interactionswith these ligands promote phagocytosisand activation for efficient killing of pathogens.

Ingestion

Whether mediated by specific receptors ornot, the foreign bacteria is engulfed by the pseudopodsof phagocytes and produces an endosome or phagosome within the cells. For small particles such asvirus, the cell membrane of phagocytes invaginatesand form pinosome.

Digestion

The phagosomemigrates to and fuses with lysosomes to form aphago-lysosome. The anti-microbial substancesin lysosomes such as lysozyme,myeloperoxidase, lactoferrin,defensins, reactive oxygenintermediate (ROI) and reactive nitrogen intermediate (RNI) can killmicroorganisms. Degradative enzymes such as proteases,phospholipases, RNases, andDNase degrade engulfed particles. The remainingmaterial is transported to the cytoplasmic membraneof the human cells within a vacuole; it is removed by exocytosisor is consumed within the phagocytic cell.

Phagocytes kill or degrade engulfed microorganisms in at least threedifferent ways: (1) oxygen-dependent mechanisms, (2) oxygen-independentmechanisms, and (3) nitrogen-dependent mechanisms. Phagocytosisincreases oxygen consumption and causes the activation of the respiratoryburst which results in the production of superoxideanion, singlet oxygen, hydroxyl ion and hydrogen peroxide.These molecules are microbicidal and cause killing oforganisms in the phagosome. Oxygen-independentmechanisms include lysozyme, lactoferrin, defensins,proteases (elastase, cathepsinG, etc.), phospholipases,RNases, and DNase, whichcontribute to the destruction of the ingested microorganisms. Another mechanismof killing involves reactive nitrogen intermediates (RNIs).These include nitric oxide (NO), nitrite (NO₂^-), andnitrate (NO₃^-). Nitric oxide is likely to be the mosteffective killing agent. It is produced from arginineby macrophages.when they are stimulated by interferons or tumor necrosis factor. All of these forms ofnitrogen are toxic to microorganisms.

Outcome of phagocytosis

The phagocyticcells engulf and destroy most bacteria that attempt to invade the body. Somebacterial have developed mechanisms to evade killing and degradation. Thesebacterial such as Mycobacterial tuberculosis cansurvive as intracellular pathogens by preventing phagosome-lysosomefusion. Other bacterial evade phagocytosis byproducing leukocidins, which are toxic proteins thatdestroy the phagocytic cells. In Chédiak-Higashi,most microorganisms are phagocytosed normally, butintracellular killing is impaired, because a defect in a cytoplasmicprotein causes abnormal granule membrane fusion, leading to lysosomaldysfunction and recurrent infection by pyogenicbacteria.

Humoral defencemechanisms

 A number of microbicidalsubstances are present in the tissue and body fluids.

 Lysozyme is found in neutrophils and in serum, saliva, tear. It splits sugarsoff the structural peptidoglycan of the cell wall ofmany Gram-positive bacterial and thus causes their lysis.In Gram-negative bacterial the peptidoglycan appearsto be protected by other wall components, e.g.lipopolysaccharide.The action of complement may be needed to remove this protection and led Gram-negativebacterial to be destructed by lysozyme.

Complement is a family of more than 11glycoprotein molecules. The complement cascade can be initiated or triggered byspecial antigen-antibody complexes. This is called classical pathway. Thenonspecific initiation of the complement is referred to as the alternatepathway. It can be activated by Gram-negative endotoxin.The activation of complement by either pathway give rise toC3b and the generation of a number of factors that can aid variousnonspecific defense responses in the host. These factors can act as opsonin, chemotactic agents and anaphylatoxin. They also can enhance phagocytosisand trigger inflammation.

Defensins are thesmall molecular weight proteins synthesized by neutrophils.They are called human neutrophil proteins (HPNs) and are labeled HPN-1, HPN-2, HPN-3, and HPN-4. Defensins are stored in cytoplasmicgranules and delivered to phagocytic vacuoles. Theycan increase the permeability of bacterial (especially extracellularbacteria) and fungal cell membranes and affect enveloped viruses. Alternations inthe cytoplasmic memberanesof these microbes contribute to their death or inactivation.

Acquired immunity

This is a response to a specific immunestimulus (antigen) that involves cells of the immune system and frequentlyleads to a state of immune memory. In adaptive immunity, which occurs after alag period during which immune B and T cells become activated, invadingorganisms are destroyed.

This is a defense response acquired by human body after cells ofimmune system contact with antigens of the invading microbes. Two features ofit are specificity and memory. This is a response to a specific immune stimulus.The body acquires immunity only after exposure to an antigen. And it permits arapid and specific secondary response on reexposureto that same substance. In acquired immunity, there is a lag period (10-14days), during which immune B and T cells become activated, invading organismsare destroyed.

Acquired immunity includes antibody-mediated immunity andcell-mediated immunity.

In the antibody-mediated immunity, helper (CD4) T lymphocytesrecognize the pathogen’s antigens complexedwith classⅡMHC proteins on surface of an antigen-presenting cell( macrophage orT cell) and produce cytokines that activate B cells expressing antibodys that specifically match the antigen. The B cellsundergo clonal proliferation and differentiate toplasma cells that can produce specific immunoglubulins(IgG, IgM, IgA, IgD, IgE). Major defense function ofantibodies include neutralization of exotoxinand viruses and opsonization of the pathogens.

In the cell-mediated immunity, the antigen-MHC Ⅱ class complex is recogonized by helper (CD4) T lymphocytes, while theantigen-MHCⅠclass complexis recogonized by cytotoxic(CD8) T lymphocytes. Each class of T cells produces cytokines, becomeactivated, and expands by clonal proliferation. Themost important cells are helper (CD4) T lymphocytes and cytotoxicT cell. Foreign antigen-activated helper (CD4) T lymphocytes can secretevarious cytokines, leading to activation of neutrophils,macrophages, and cytotoxic T cell, causing “hypersensitivity”.

Immunity to extracellularbacteria

Extracellular bacteria are capable of replicating outside host cells, e.g., inthe circulation, in extracellular connective tissues,and in various tissue spaces such as the airways and intestinal lumens. Thesebacteria include Staphylococcus, Streptococcus pneumoniae,Neisseria meningitidis, Neisseria gonorrhoeae, Clostridiumtetani, Corynebacterium diphtheriae, and Bordetella pertussis, Shigella dysenteriae, Vibrio cholerae.

Extracellularbacteria cause disease by two principle mechanisms. First, they induceinflammation. Second, many of these bacteria produce toxin.

A principal mechanism of natural immunity to these microbes is phagocytosis and killed by neutrophil,monocytes and tissue macrophages.

A principal mechanism of protective specific immunity to thesemicrobes is humoral immunity.

Cell walls and capsules of extracellularbacteria are T cell non-dependent antigens. Such antigens directly stimulate Bcell, giving rise to strong specific IgM responses. Proteinsof extracellular bacteria are defined as T celldependent antigens. Extracellular microbes andsoluble antigen are phagocytosed and processed byantigen presenting cells (APCs). CD4⁺ Tcells are activated to stimulate antibody production and to activate the phagocytic and microbicidalfunctions of macrophages.

The effects of special antibody are as below. 1.IgG antibodies opsonize bacteria and enhance phagocytosisby binding to Fcγreceptors on monocytes, macrophages and neutrophils. Both IgM and IgG antibodies activate complement, generating C3b and iC3b,which bind to specific type 1and type 3 complementreceptors. Respectively, and further promote phagocytosis.2. Both IgM and IgGantibodies neutralize bacterial toxins, preventing their binding to targetcells, and promote their clearance by phagocytosis.3. Both IgM and IgG antibodiesactivate the complement system, leading to theproduction of the microbicidal MAC (Membrane attackcomplex) and the liberation of by-products that are mediators of acuteinflammation. The lytic function of the MAC isimportant for the elimination of only some microbes, especially Neisseria. 4.IgA in mucosalsecretions block the colonization of pathogens.

Some bacterial toxins such as Staphylococcal enterotoxinand toxic shock syndrome toxin stimulate large numbers of CD4+ T cells. Theyare active at very low concentration (10^-9mol/L). Any one of thesetoxins can stimulate all the T cells in an individual that express a particularset or family of V_βT cell receptor genes. These toxins have been called super-antigens.They can activate many T cells, resulting in large amounts of cytokinesproduction, including IL1and TNF. Super-antigens cause clinicopathologicabnormalities that may be similar in some respects to endotoxinshock. The polyclonal lymphocyte activation induced by bacterial endotoxins and super-antigens may also contribute to thedevelopment of autoimmunity. A late complication of the humoralimmune response to bacterial infections may be the generation ofdisease-producing antibodies. The best-defined examples are rheumatic fever andglomerulonephritis.

Immunity to intracellularbacteria

The most important intracellular bacteriainclude Mycobacterium tuberculosis, Mycobacterium leprae,Legionella pneumophila, L.monocytogenes, Salmonella typhi,Brucella, Rickettsia,Chlamydia.

The host immune response is the principle cause of tissue injury anddisease in infections by some intracellular bacterial. Intracellular microbescan resist phagocytes and often persist for long periods in cells, even inpersons with effective cell-mediated immunity. Persistent organisms providechronic antigenic stimulation. This may lead to local collection of activatedmacrophages, called granulomas. It not only preventsthe spread of microbes, but also causes tissue necrosis and extensive fibrosis.

The major protective immune response against intracellular bacteriais T cell-mediated immunity. Both CD4⁺ and CD8⁺ T cellscontribute to protective immunity against intracellular bacteria. The principalfunction of CD4⁺ T cell is the production of cytokines, particularlyIFN-γ. The consequence of IFN-γ production is the activation of macrophages,including ones that are infected. IFN-γ stimulates phagocyticand degradative functions of macrophages, leading toenhanced bacterial killing and elimination. The cytokines produced by CD4+ Tcells can activate CTL cells (cytotoxic cells) and mediateDTH reactions. For instance, in mycobacterialinfections, antigens such as the purified protein derivative (PPD) stimulateCD4⁺ T cells and these cells mediate DTH reactions to skin challengewith PPD in previously infected persons. CTL cells contain perforin,which is a monomeric protein related to thecomplement component C9. In the presence of Ca²⁺ ions the monomersbind to the target cell membrane and polymerize to form a transmembranepore and lead to cell death. The intracellular bacteria are escaped from hostcells,   opsonizedby antibody and killed by phagocytes. CTL cells also produce TNF-α, lymphokines,IFN-γto destroytarget cells.

Intracellular bacteria have many kinds evasion of immune mechanisms.Mycobacteria do this by inhibiting phagolysosome fusion, perhaps by interfering with lysosome movement. The phenolic glycolipid of M. leprae functionsas a scavenger of reactive oxygen species. Virulent strains of L. monocytogenes produce a protein called hemolysin,which promotes intracellular bacterial survival, probably by forming pores inthe phagosome membrane ,thereby releasing bacteria into the cytoplasm and preventing their degradationin phagolysosomes.

The originate and progress of infection

The source of the infection

The source of the infectious agent can be exogenous,originating from a source outside the body (the environment or another personor animal), or endogenous, already existing on or in the body (normalflora or latent infection).

In order for an infectious agent to continue to exist and be spread,it must have a permanent place to reside. The reservoir is the primary habitatin the natural world from which a pathogen originates. Often it is a human oranimal carrier, although soil, water, and plants are also reservoirs.  Living Reservoirs

 Persons or animals withfrank symptomatic infection are obvious sources of infection.

 A carrier is anindividual who inconspicuously shelters a pathogen and spreads it to otherswithout any notice. Asymptomatic (apparently healthy) carriers are indeedinfected, but they show no symptoms. Other asymptomatic carriers, calledincubation carriers, spread the infectious agent during the incubation period.Recuperating patients without symptoms are considered convalescent carrierswhen they continue to shed viable microbes and convey the infection to others.   

Many vectors and animal reservoirs spread their own infections tohumans. An infection indigenous to animals but naturally transmissible tohumans is a zoonosis. Spread of thesediseases are promoted by close associations of humans with animals, andpeople in animal-oriented or outdoor professions are at greatest risk. At least150 zoonoses exist worldwide. The biological vectorsuch as fleas, mosquitos, flies, and ticks,communicates the infectious agent to the human host by biting, aerosolformation, or touch.

Nonliving Reservoirs

Soil harbors the vegetative forms of bacteria, protozoa, helminthes,and fungi as well as their resistant or developmental stages such as spores,cysts, ova, and larvae. Regular bacterial pathogens include the anthraxbacillus and species of Clostridium that are responsible for gas gangrene, botulism,and tetanus. Pathogenic fungi in the genera Coccidioidesand Blastomyces spread by spores in the soil anddust. Natural bodies of water carry fewer nutrients than soil, but stillsupport a number of pathogenic species such as Legionella,Cryptosporidium, and Giardia.

Routes of pathogen transmission

Respiratory infections

Mucus, sputum, nasal drainage, and other moist secretions are themedia of escape for the pathogens that infect the lower or upper respiratorytract. The most effective means of releasing these secretions is coughing andsneezing, although they can also be released during talking and laughing. Tinyparticles of liquid released into the air form aerosols or droplets that canspread the infectious agent to other people. The agents of tuberculosis,influenza, measles and chickenpox most often leave the host through airbornedroplets.  

Wound infections

Most wound infections in the skin and mucosa are caused by thepatient’s own microorganisms such as staphylococci and streptococci. Spores ofClostridium that are responsible for gas gangrene, botulism, and tetanus existin soil and feces of human and animal. When the spores enter the wound, gasgangrene, botulism, and tetanus will be occur.

Intestinal infections

Feces containing pathogens are a public health problem when allowedto contaminate drinking water and food or when used to fertilize crops. Typhoidfever, dysentery and Cholera are caused by ingesting contaminate drinking waterand food. The most important vectors are human hand, water and flies.

Contact infections

 The infections are caused by directlycontact between the skin or mucous membranes of the infected person or animaland that of healthy person. Most sexually transmitted diseases are spreaddirectly. Indirectly contact by intermediate conveyor also causes someinfections. The intermediate conveyors are some public objects such asdoorknobs, telephones, push buttons contaminated by touching. Shared bedlinens, handkerchiefs, toilet seats, toys, clothing, personal articles also causethe infections.

Animal bites infections

The majority of animal vectors are arthropods such as fleas, mosquitos, flies, and ticks. They transmit the infectiousagents to the human host by biting, aerosol formation, or touch. Blood-feedinginsects such as mosquitos and fleas are commontransmitters of pathogens. In the case of biting vectors, the animal can (1) injectinfected saliva into the blood (the mosquito), (2) defecate around the bitewound (the flea), or (3) regurgitate blood into the wound (the tsetse fly).

Patterns of Infection

Aided by virulence factors，microbes eventually settle in aparticular target organ and continue to cause damage. The type and scope ofinjuries varies greatly, depending on the properties of the specific microbialstrain and the physiologic state of the host.

Apparent Infection

When an infection causes pathological changes leading to disease, itis often accompanied by a variety of signs and symptoms. This infection iscalled apparent infection.

Infections that come on rapidly, with severe but short-livedeffects, are called acute infections. The pathogenic bacteria includesNeisseria meningitides, Vibriocholerae, enterotoxigenic E.coliet al. Infections that progress and persist over a long period oftime are called chronic infections. Intracellular bacteria such as Mycobacteriumtuberculosis, Mycobacterium leprae often causeschronic infection. This infection persists several months to several years. Sub-acuteinfections do not come on as rapidly as acute infections or persist as longas chronic ones. A focal infection is said to exist when the infectiousagent breaks loose from a local infection and is carried into other tissues.This pattern is exhibited by tuberculosis or by streptococcal pharyngitis that gives rise to scarlet fever.

Localized infection, the microbe entersthe body and remains confined to a specific tissue. Examples of localizedinfections are boils, fungal skin infections, and warts. When an infectionspreads to several sites and tissue fluids, usually in the bloodstream, it iscalled a generalized, or systemic, infection. Many infectious agents donot remain localized but spread from the initial site of entry to othertissues.

Toxemia, the pathogen itself remainslocalized at the portal of entry but the toxin produced by these pathogens isspread by the blood to the actual target tissue. For example,tetanus and diphtheria. Bacteremia, which means that these microbes are present in the blood but arenot necessarily multiplying. Bacteremia can betransient or persistent. It allows bacteria to spread widely in the body andpermits them to reach tissues particularly suitable for their multiplication. Endotoxaemia, which means that Gram-negativebacteria enter blood to produce a lot of endotoxin. Septicemiarefers to a general state in which microorganisms are multiplying in the bloodand are present in large numbers.

Infections That Go Unnoticed

It is rather common for an infection to produce no noticeablesymptoms, even though the microbe is active in the host tissue. In other words,even though the microbe is active in the host tissue. In other words, althoughinfected, the host does not manifest the disease. Infectionsof this nature are known as asymptomatic, subclinicalor inapparent because the patient experiences nosymptoms or disease and does not seek medical attention. However, it isimportant to note that most infections are attended by some sort of sign. Inthe section on epidemiology, we further address the significance of subclinical infections in the transmission of infectiousagents.

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Viral Infections And Immunity

Viral Pathogenesis

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The consequences of a viralinfection depend on a number of viral and host factors that affectpathogenesis. Viral infection was long thought to produce only acute clinicaldisease but other host responses are being increasingly recognized. Theseinclude asymptomatic infections, induction of various cancers, chronicprogressive neurological disorders and possible endocrine diseases.

Effects of viral infection on bodies

1. The forms of viral infection

Viruses enter intohosts via skin or mucosal surfaces (eye, genital tract, respiratory tract,alimentary tract, in some cases by direct inoculation into the blood stream andinfection of fetus in utero.

Viral infections are usuallyself-limiting. Inapparent or subclinicalinfection refers to the many infections that give no overt sign of theirpresence at the host-parasite level. Most viruses that infecthumans, such as those that cause routine respiratory infections (e.g., coldviruses, influenza viruses) and gastrointestinal infections (e.g., Rotaviruses,Norwalk virus), cause acute infections. Acute viral infections are of relatively short duration with rapidrecovery（fig.1).

fig.1 Acute viral infection, followed by viralclearance

Sometimes,however, the virus persists for long periods of time in the host. Envelopedviruses such as paramyxoviruses, some herpesviruses eg. EBV, retroviruses andarenaviruses appear particularly suited to initiatepersistent infections. Infections appear to persist because the virus does notdisrupt the essential housekeeping functions of the cells (DNA, RNA and proteinsynthesis). Some persistently infected cells, such as in measles (SSPE) may beassisted by the capacity of humoral Abs to cap viral Ags on the cell surface. This promotes the shredding ofviral Ags from the cell surface, leaving the cellsurface free of viral glycoproteins and thus theinfected cell is protected from CTLs and K cells. Long-term virus-hostinteraction may take three forms.

Chronic Persistent Infections

Chronic infections are those in whichvirus can be continuously detected; mild or no clinical symptoms may be evident(fig.2). They occur with a number of animal viruses, and the persistence incertain instances depends upon the age of the host when infected. In humans,for example, rubella virus and cytomegalovirus infections acquired in utero characteristically result in viral persistence thatis of limited duration, probably because of development of the immunologiccapacity to react to the infection as the infant matures. Infants infected withhepatitis B virus frequently become persistently infected (chronic carriers);most carriers are asymptomatic. Animal studies have shown that in chronicinfections the viral population often undergoes many genetic and antigenicchanges.

Fig.2  Acuteviral infection, followed by chronic infection

Latent Virus Infections

Latent infectionsare those in which the virus persists in an occult, or cryptic, form most ofthe time. There will be intermittent flare-ups of clinical disease; infectiousvirus can be recovered during flare-ups. In latent infections, overtdisease is not produced, but the virus is not eradicated. This equilibriumbetween host and parasite is achieved in various ways by different parasitesand hosts. The virus may exist in a truly latent noninfectious occult form,possibly as an integrated genome or an episomalagent, or as an infectious and continuously replicating agent, termed apersistent viral infection.

Infectious agents causing chronic persistent infections have found a wayof escaping a cell-mediated immune response. The mechanisms include generationof cells that escape a cell-mediated immune response, down regulation of MHCproduction in infected cells so that they are not recognized and destroyed by Tcells and infection of cells in immunoprivilegedsites such as the brain.

Fig.3 Acute viral infection, followed by latent infection and periodicreactivation

Slow Virus Infections

Slow virus infections have a prolongedincubation period, lasting months or years, during which virus continues tomultiply. Clinical symptoms are usually not evident during the long incubationperiod. Unlike latent and chronic infections, slow infection may not begin withan acute period of viral multiplication. The best example of this type of “slowvirus” infection is scrapie in sheep (fig.4). Kuru and Creutzfeldt-Jakob disease occur in humans. Themechanism by which this burning group of agents induce disease is unknown．

Fig.4 Slow chronic infection

Some viruses can maintain more than onetype of persistent infection at the same time, but in different cells. The typeof persistent infection may not be dependent on the cell type and thephysiologic state of the cell. For example, Epstein-Barr virus (EBV) latentlyinfects B cells, but in the same individual it is released for long periodsfrom productively infected pharyngeal epithelial cells (chronic infection).Therefore, in one individual persistent infection with a single virus mayinvolve multiple types of persistence, each of which may become more or lessimportant as the individual responds to the disease.

 2. Course and Outcome of virus infection

Virus infectionmay remain localized to the organ through which virus gained entry (respiratorytract, intestine, skin, genital tract, or eye) or it may spread to other partsof the body.  Cell and tissue tropism is determined by specific virusreceptors. Additional cell functions may be required for productive virus infection. For examples, cells must be capable of cleaving influenza virus hemagglutinin to be infected by this virus; T cellactivation (i.e., induction of cellular DNA synthesis) is necessary forintegration and expression of HIV-1 provirus DNA.

Before theirspread to the target organ, viruses usually multiply at or near the site oftheir entry into the host.  For example, primary replication of poliovirusoccurs in the oropharyngeal and intestinalmucosa.  From there the virus spreads to the Payer's patches of the gutand in mesenteric lymph nodes, followed by spread to other organs and,ultimately, to the CNS via the hematogenic route.

Cell and tissueinjury is direct consequence of the "cytopathiceffect" of infecting virus and due to cellular products induced by viruses(e.g., cytokines) or the immune responses of the host. There are four outcomesfor the cells infected by viruses. (a)Death resulted from acute virus infectionor from complications following acute infection. (b) Viruses wereeliminated from the body with complete recovery of function. (c) Viruses wereeliminated of with sequelae (e.g., paralyticpoliomyelitis). (d) Failure to eliminate virus leaded persistentinfection.

For disease torecur in a latent infection, the virus must be reactivated and beginreplicating. Some factors associated with reactivation are infection with otherviruses (as in HIV), nerve trauma (e.g., herpes facialisfollowing surgery of the trigeminal ganglion), physiologic and physical changes(e.g., fever, menstruation, and sunlight), and immunosuppression(as in cytomegalovirus disease). Papovavirusencephalitis in immunosuppressed patients mayrepresent exacerbation and spread of a chronic infection.

Effects of viral infection on cells

In most cases, thedisturbances of bodily function that are manifested as the signs and symptomsof viral disease result from the direct effects of viruses on cells. Knowledgeof the morphologic,  biologic, biochemical,and physiologic effects of viruses on cells is essential in understanding the pathophysiolog of viral disease and developing accurate diagnosticprocedures and effective treatment．

Virus-host cell interactionsmay produce either cytocidal infections, in whichproduction of new infectious virus kills the cell, persistent infections, inwhich the virus or its genome resides in some or all of the cells withoutkilling most of them, or transformation, in which the virus does not kill thecell but produces morphologic,biochemical, physiologic, and biologic Change that may result in the acquisitionof malignant properties by the cell (malignant transformation). The type of virusinfection and the virus-induced effects on cells are dependent on the virus,the cell type and species, and often the physiologic state of the cell.

Cytocidal infecetion

Most productive infectionsare called cytocidal because they klllthe host cell. Infection of permissive cells with virus leads to cytocidal infection.

The first effectsof the replication of cytocidal viruses to bedescribed were the morphologic changes known as cytopathiceffect. Many types of cytopathic effect occur. Oftenthe first sign of viral infections is a rounding of the cells. A particularlystriking cytopathic effect of some viral infectionsis the formation of syncytia, or polykaryocytes,which are large cytoplasmic masses that contain manynuclei and are usually produced by fusion of infected cells. The mechanism ofcell fusion during viral infection probably results from the interactionbetween viral gene products and host cell membranes.

The interaction ofvirus and cell may result in rapid changes in the cell membranes, including movementof ions and the metabolism of secondary messengers such as cyclic nucleotides.Viruses often inhibit the synthesis of host cell macromolecules, including DNA,RNA and protein. Expression of viral genes may alter the biologic functioningof Infected cells. For example, virus-specific dprotelns inserted in the membrane may alter the cell’santigenic or immune properties, and changes in cytoskeletalelements may alter the cell’s shape and behavior.

Alterations ofcellular genetic material occur following some viral infections including chromosomedamage.

Persistentinfection

In persistent infections,viral nucleic acid remains in specific host cells indefinitely; progeny virus mayor may not be produced.

Autoimmune injury and other forms ofcell damage may occur during persistent infections. Budding virionsand viral peptides associate with the cell membrane change the antigeniccharacteristics of the cell so that the immune system may recognize it asforeign. The cell then may be attacked by the humoraland cellular immune system of the host and will die even though it was infectedby a noncytocidal virus.

The immuneresponse also may cause formation of circulating antigen-antibody complexesinvolving viral antigens. These complexes may deposit in the kidneys and leadto renal disease. The long-term association of the virus with specific targetcells may lead to altered function or responses; this type of mechanism isresponsible for the progressive neurologic diseaseassociated with slow virus-type persistent infections such as kuru, Creutzfeldt-Jakob disease, or subacutesclerosing panencephalltis.

Integratedvirus infection

There is another type of persistent virus-host cell interaction,integrated virus infection, in which all or part of the viral nucleic acidbecomes integrated into the genome at specificor any site in host cell. Progeny virions may never be assembled or released from the hostcell. New virus-specific antigens, however, can be detected within the cell oron the cell surface. Infection with retroviruses is a classic example of thismechanism.

Apoptosis

One of the methods bywhich cytotoxic Tlymphocytes (CTLs) killvirus-infected cells is by inducing apoptosis. Some cancer-causing viruses usetricks to prevent apoptosis of the cells they have transformed.Several human papillomaviruses (HPV)have been implicated in causing cervical cancer. One of them produces a proteinthat binds and inactivates the apoptosis promoter p53.Epstein-Barr Virus(EBV) produces a protein similarto Bcl-2 and another protein that causes the cell to increase its ownproduction of Bcl-2. Both these actions make the cell more resistant toapoptosis.

Transforrning of cells

Some persistentInfections in which part or all of the viral genome is retained in the hostcell (either as episomes or integrated into the hostDNA) can cause the host cell to undergo oncogenictransformation. Transformation generally takes place in more than one step. Forexample, a first (initiating) event that immortalizes a cell (makes it capableof dividing indefinitely) will not result In a tumorunless a second (promoting) event releases the cell from contact inhibition.Viruses may either initiate or promote transformation. Because DNA viruses are often cytocidal,transformation by DNA viruses takes place usually in abortive infections; fewor no viral genes are expressed. Transforming RNA viruses, in contrast, mayproduce infectious progeny. Viruses transform cells either via the expressionof viral oncogenes or by altering the expression of cellularoncogenes. In the former case, the relevant segmentsof the viral genome must be retained intact in the host cell.

Effects of viral infection on immune system

Viruses haveevolved a multitude of mechanisms for exploiting weaknesses in the host immunesystem and avoiding, and sometimes actually subverting, immune mechanisms. Someviruses are so successful in avoiding host defences thatthey persist in the host indefinitely, sometimes in a latent form without producingdisease.

One of the mostimportant strategies developed by viruses is to infect cell of the immunesystem itself. The effect of this is often to disable the normal functioning ofthe cell type that has been infected. Many common human viruses including rubella,mumps, measles and herpesviruses infect cells of theimmune system, as does the human immunodeficiency virus (HIV). The consequencesviral infected of cells of the immune system have been categorized in two waysinfections that cause temporary immune deficiency to unrelated antigens andsometimes to the antigens of the infecting virus. It is known that infectionwith influenza, rubella, measles and cytomegalovirus predisposes to bacterialand other infections. This is sometimes associated with depressed immunoglobulinsynthesis and interference with the antimicrobial functions of phagocytes.

Viruses thatcannot enter and replicate within phagocytic cells willbe destroyed they are engulfed by a neutrophil ormacrophage. Since neutrophils are short-lived cellsthey do not usually give rise to progeny virus. On the other hand, monocytes and macrophages are long-lived cells and can be responsiblefor disseminating a virus throughout the body. Viruses that do replicate withinmacrophages must escape from the phagosome veryrapidly before it fuses with the lysosome. Reovirus infection of macrophages is actually helped by thelysosomal enzymes which initiate ‘uncoating’of the virus and therefore enhance viral replication.

Antiviral Immunity

Viruses are small, obligate intracellularparasites which cause infection by invading cells of the body and multiplyingwithin them. Within their life cycle they have a relatively short extracellular period, prior to infecting the cells, and alonger intracellular period during which they undergo replication. The immunesystem has mechanisms which can attack the virus in both these phases of itslife cycle, and which involve both non-specific and specific effector mechanisms.

Non-Specific Mechanisms

Most viralinfections are limited by nonspecific defenses, which include various hormones, temperature, NK cells, interferons and phagocytes.. Viralinfection of cells directly stimulates the production of interferonsNK cell activity is increased druing early stages of infection with many viruses andvirus-infected cells in vitroare susceptible to lysis by NK cells. These findings, together with othercircumstantial evidence, suggest that interferons andNK cells play a role in the recovery from virus infections.

Interferons:

At the time of their discovery in 1957 the term interferon identifieda factor produced by cells in response to viral infection that protected othercells of the same species from attack by a wide range of viruses. It is nowclear that this activity is mediated by members of regulatory proteins (Fig.5)．

Fig.5 Virally infected cells release interferon that protects neighboring uninfected cells from viral infections. Themechanisms of interferon action are very complex and result in degradation ofviral RNA and blockage of viral protein synthesis.

In humans, as in anumber of other species, there are three types of interferon; α-interferon（IFN-α)and β-interferon (IFN-β), Produced by peripheral blood mononuclear cells andfibroblasts, respectively, and γ-interferon (IFN-γ), a lymphokine produced in responseto a specific antigenic signal. The production of interferonsis under strict inductional control. IFN-αand IFN-βareproduced in response to the presence of viruses and certain intracellularbacteria. Double-stranded RNA may be the important inducer. IFN-γ whichhas an extensive role in the control of immune responses, is produced byantigen-activated T lymphocytes and natural killer  (NK) cells. IFNsnow can be produced relatively easily by recombinant DNA techniques in E. colior animal cells and are being used therapeutically in some virus infections andmalignancies including Kaposi's sarcoma accompanying AIDS, chronic activehepatitis B and hepatitis C. 

Like all cytokines, IFNsare hormone-like agents.  Their action on cells is initiated by binding tospecific receptors on the cell surface.  Two types of cellular receptorsfor IFN have been identified:  one type for the IFN-alpha/beta species,and another one for IFN-gamma.  Many cellular genes are activated as aresult of IFN action, leading to the synthesis of several new proteins. One protein specifically induced by IFNs is a proteinkinase that phosphorylatesthe small subunit of peptide chain initiation factor eIF-2, therebyinactivating eIF-2 and thus leading to inhibition of protein synthesis. Another IFN-induced protein is an oligonucleotide synthetase (2'5' A synthetase)that generates 2'5' pppA(pA)n.  The latter oligonucleotide then causes activation of a latent endonuclease, which in turn leads to degradation of mRNA(both viral and cellular).  Activation of both the protein kinase and 2'5' A synthetase also requires double-stranded RNA and ATP.Double-stranded RNA is generated during many virus infections.  Thus, theaction of these enzymes is not fully expressed until a cell is infected by avirus.  This feature possibly explains why IFN does not have a stronginhibitory effect on protein synthesis and many other cellular functions inuninfected cells while it efficiently inhibits virus replication.

Induction of theprotein kinase and 2'5' A synthetase accounts for some of the antiviral activitiesand some other actions of IFNs.  However, notall antiviral activities are based on the activation of these twopathways.  The exact mechanisms by which IFNsinhibit growth and by which their many effects on cells of the immune systemare mediated are also not yet fully understood in molecular terms.  These immunomodulatory actions (e.g., increase in class I MHC,activation of NK cells and CTLs) are also importantin host defense to viruses.

Natural Killer Cells:

Natural killer(NK) cells are a subset of lymphocytes found in the blood and tissues, whichlack antigen specific surface receptors (TcR orimmunoglobulin receptors). Phenotypically, NK cellsdo not express the characteristic cell surface markers that define T cells andB cells, and so NK cells represent a distinct lineage of lymphocytes. In man,the principal NK cell is the large granular lymphocyte (LGL)which comprise 2-5% of peripheral blood lymphocytes. However, not all lytic cells are LGLs and not all LGLs are NK cells.

NK cells possessthe ability to recognise and lysevirally infected cells and certain tumour cells.Whilst not showing antigen specificity, they clearly exhibit some degree ofselectivity in targeting "abnormal" cells for lysis.The nature of the receptor (or receptors) that confer this selectivity oftarget recognition has not been clearly defined, but it has been shown recentlythat the expression of "self" MHC molecules inhibits NK lysis of target cells. The main advantage that NK cellshave over antigen-specific lymphocytes in antiviral immunity is that there isno "lag" phase of clonal expansion for NKcells to be active as effectors, as there is with antigen-specific T and Blymphocytes. Thus NK cells may be effective early in the course of viralinfection, and may limit the spread of infection during this early stage, whileantigen-specific lymphocytes are being recruited and clonally expanded.

There is overlap of the NK population with K cells. The Fc receptor of the NK cell is however, not involved in the lytic process. There are also mechanistic differences and Kcell activity is less consistently augmented by interferon and other immunemodulators. NK activity is subject to both positive and negative regulation invivo and in vitro. Interferon gamma and IL-2 are potent inducers. Besidesproducing lysis, NK cells can producealpha-interferon.

The targetmolecules recognized but NK cells have not been defined but it appears thatsome determinants are ubiquitous whilst others have a more restricteddistribution. An alternative suggestion is that NK cell susceptibility dependson the absence of normal cell surface antigens such as MHC molecules. Theimportance of NK cells in viral infection is partially understood. It had beenshown that mice depleted of NK cells by treatment with Abagainst asialo GM1 show an increased susceptibilityto CMV.

Specific Mechanisms

Specific immuneprocesses are especially important in the resistance to reinfectionby the same or related virus after recovery from primary infection. However, components of the immune system are also clearly important indetermining the course of primary virus infections.

Viruses are strongly immunogenic and induces 2 types of immune responses; humoral and cellular. Both humoral and cell mediated arms of the immune response playa role as specific effector mechanisms in antiviralimmunity. The characteristics of the immune reaction to the same virusmay differ in different individuals depending on their genetic constitutions.

Presenation of viral antigen

The molecular basis ofantigen recognition by T cells is well understood. The TcRrecognizes short antigen-derived peptide sequences presented in associationwith self MHC class I or MHC class II molecules at the surface of an antigenpresenting cell (APC). T cell recognition, therefore, involves direct cell-cellcontact between the antigen-specific TcR on the Tlymphocyte and an MHC compatible cell which presents the processed antigen inassociation with surface MHC molecules. The finding that self MHC molecules areinvolved in the recognition of antigen by T lymphocytes led to the concept of"MHC restriction" of T cell responses, and pointed to the importantrole that products of the major histocompatibilitycomplex play in the cell mediated immune response. The major histocompatibility complex consists of a cluster of genes,most of which encode products with immunologicallyrelated functions.

The viral epitopes that the immune system responds to have beenstudied to give an insight into the mechanisms involved in the host response to these pathogens andalso to aid in the development of better vaccines．The recognition of viral antigens is similar to thatfor all foreign material．Bcells and immunoglobulin are able to combine with exposed epitopeswhile processed viral fragments presented in the context of MHC molecules arerecognized by T cells．TheB cell will recognize a conformationally determined epitope while the T cell epitopeis a sequential epitope.

Antigen-specific Bcells can act as antigen-presenting cells and therefore support the generationof an immune response by presenting viral antigens in association with MHCclass II molecules to helper T(TH) cells．B cells will present antigen to T cells and in returnwill be stimulated by growth and differentiation molecules. Intramolecularhelp may explain hapten-carrier effects. For virusparticles the uptake of an intact virion will meanthat the B cell will be able to present peptides derived from internal proteinsto T cells. Thus, a B cell specific for a surface antigen can receive help froma T cell specific for another molecule as long as it is present within the sameparticle, i.e. intrastructural help.

In most cases， exogenous virus proteins, i.e.those derived from an extracellular virus and takeninto a cell, will be presented in the context of MHC class II molecules andstimulate TH cells. Cells that are supporting viral replication express virus－derived peptides in association with MHCclass I molecules, i.e. the endogenous pathway. Thefact that some endogenous viral proteins are presented in the context of MHCclass II molecules suggests that the cellular compartment in which they arefound is not transected by the MHC class I pathway. In addition to the interactionbetween MHC/antigen and the TcR, MHC class I andclass II molecules also bind to the CD8 and CD4 T cell surface molecules,respectively. Thus, MHC class I molecules present antigen to CD8+ T cells; MHCclass II molecules present antigen to CD4+ T cells.

Humoral effector mechanisms
Specific antibodies are important in and may protect against viralinfections. Antibody production per se has been dealt with in detail already inthe course, and so is only described here in relation to its role in antiviralimmunity.

Neutralizing Antibody:

The most effectivetype of antiviral antibody is "neutralizing" antibody - this isantibody which binds to the virus, usually to the viral envelope or capsid proteins, and which blocks the virus from bindingand gaining entry to the host cell. IgM, IgG and IgAantibodies can "neutralize" extracellularvirus particles; such inactivation involves multiple mechanisms. Those of the IgM and IgG class are especiallyrelevant for defense against viral infections accompanied by viraemia, whereas those of the IgAclass are important in infections acquired through a mucosa. (the nose, the intestine) which maybe protective against infection at these surfaces. (This is the basis of immunisation with the current oral polio vaccine).Virusspecific antibodies may also act as opsonins inenhancing phagocytosis of virus particles - thiseffect may be further enhanced by complement activation by antibody-coatedvirus particles.

During the course ofa viral infection, antibody is most effective at an early stage, before thevirus has gained entry to its target cell. In this respect, antibody isrelatively ineffective in primary viral infections, due mainly to the lag phasein antibody production. Preformed antibody, particularly neutralisingantibody, however, is an effective form of protective immunity against viralinfections, as witnessed by the success of many viral vaccines, which work bystimulating virus-neutralising antibody responses.

Antibody-dependent cell-mediated cytotoxicity

In addition, in thecase of some viral infections, viral proteins are expressed on the surface ofthe infected cell. These may act as targets for virus-specific antibodies, andmay lead to complement-mediated lysis of the infectedcell, or may direct a subset of natural killer cells to lysethe infected cell through a process known as antibody-directed cellular cytotoxicity (ADCC). This can contribute to the elimination of virus fromthe body, but it also increases damage to virus-infected organs.

Antibody-mediated enhancement

Not all antibodies toviruses are protective, however, and in certain cases antibody to the virus mayfacilitate its entry into a cell through Fcreceptor-mediated uptake of the antibody coated particle. Such antibodies arecalled enhancing antibodies. Dengue haemorrhagic fever (thisis associated of infection in an individual who had prior infection with adifferent serotype) and RSV (Children immunized with inactivated RSV developunusually severe disease if subsequently infected with the same virus) could bedue to binding of un-neutralized virus-Ab complexesto cell surface Fc receptors, and thus increasing thenumber of cells infected

Cell-mediated immunity(CMI)

There is a greatdeal of clinical and experimental evidence for the role of CMI in thesusceptibility to primary infections by measles, varicella-zoster,herpes simplex, cytomegalo-, respiratory syncytial and vaccinia viruses.Patients who lack immunoglobulinsbut develop CMI ordinarily recover from viral diseases without difficulty. Whereas patients with defective CMI but normal humoralimmunity recover poorly from certain viral infections, such as vaccination withvaccinia. CMI also appears to play a criticalrole in maintaining latent virus infections. Such infections are frequentlyreactivated in patients undergoing organ transplants whose CMI is suppressed bytherapy.

The majormechanism of CMI appears to be the lysis ofvirus-infected cells by CTL.  Another important mechanism is thelysis of virus-infected cells by antibody dependentcellular cytotoxicity (ADCC), mediated by various Fc receptor expressing cells.  Production ofinterferon and other cytokines by sensitized T cells upon stimulation by viralantigen is also important.

Cytotoxic T Cells

The principal effector cells which are involved in clearing establishedviral infections are the virus specific CD8+ cytotoxicT lymphocytes (CTL). These cells recognise (viral)antigens which have been synthesised within cell'snucleus or cytosol, and which have been degraded.They are presented at the cell's surface as short peptides associated with selfclass I MHC molecules. The recognition of antigen by CD8+ T cells is,therefore, distinct from that of CD4+ T cells in several respects. It requiressynthesis of the target antigen within the cell (and is therefore restrictedlargely to virally infected or tumour cells); it is"restricted" by class I MHC molecules (as opposed to MHC class IIrestriction for CD4+ T cells); MHC class I molecules are expressed on almostall somatic cells, so virtually any cell, on infection with virus, can act as a"target" cell for antigen specific CTL (contrasts with the limitedtissue distribution of class II MHC); recognition of an antigen presenting cell(APC) by an antigen-specific CTL usually results in the destruction of the APC.The importance of CTL in the clearance of virus infection has been demonstratedin a wide variety of viral infections in both laboratory animals and in man.

Cytokines

Cytokines areimportant in both the specific and nonspecific arms of host resistance to virusinfections.  The contact of sensitized T cells with antigen (includingviral antigen) leads to the generation of several T cell-derived cytokines; ofthese interferon (IFN)-* and tumor necrosis factor (TNF) are probably the mostsignificant in virus infections.  Synthesis of some cytoki8nes istriggered directly by virus infection; this type of induction does not involvespecific immune recognition, but depends on the direct interaction of the viruswith the producing cell.  The latter type of induction is characteristicfor IFN-alpha/beta, TNF and interleukin-6 (IL-6).  Monocytesand macrophages are the major source of these cytokines during virus infection.

Immunopathogeny

The immune response itself often contribute to the production ofdisease The examples of enhanced viral injury could be due toone or a mixture mechanisms. Antibodies produced from immune complexes with thevirus, and are deposited in the kidney resulting in glomerulonephritis.In addition, necrotic lesions appear in the liver, brain, spleen and otherorgans, apparently resulting from the reaction between virus-sensitized killerT-lymphocytes and viral Ags present on the surface ofmany cells. As with virus-specific antibodyresponses, however, not all CTL responses to virus are beneficial to the host,and in some cases the tissue destruction caused by the virus-specific CTL isgreater than the damage done by the virus itself; and example of this would bethe fulminant hepatitis associated in a smallproportion of cases with infection with hepatitis B virus, in which the liverdamage is caused by virus-specific CTL rather than directly by the virus.

(Zhao Wei)