Medical Microbiology

Continued:AvoidanceMechanismsofPathogens(withexamples)

Influenceonlymphocytesandimmunosuppression.

&Directdestructionoflymphocytes,ornegativeregulationoftheirfunction

(HIV?).

&InductionofimmunopathologicalT-cellresponses(insomecasesthesecanbe

immunosuppressive,e.g.HIV).

&Inductionofimmunosuppressiveautoantibodies.

Influenceonselection,induction,anddeletionofTcells.

&NegativeselectionofTcells;ifviralantigensarepresentinthethymusresponsive

Tcellswillbedeleted.

&Exhaustiveactivation,andsubsequentdeletion,ofperipheralTcells;insome

overwhelmingperipheralvirusinfectionsalloftherespondingTcellsaredeleted

(HBV,HCV).

Interferencewithcytokines,cytokineandchemotaxinreceptors(R),etc.Many

virusesproducesubstancesthatblockorinhibitreceptorsforthehumoralcompo-

nentsoftheimmunedefensesystem,forinstance:

&IL-1bR,TNFaR,IFNcR;herpesvirus,smallpoxvirus.

&Chemotaxinreceptor;cytomegalovirus.

&IL-10R;theEpstein-BarrvirusproducesB-cellreceptorfactorI,whichbindstothe

IL-10RthuspreventingactivationofTH2cells.

&Viral-inducedinhibitionofinterleukinproduction.

ImpairmentofMHCantigenexpression.Down-regulationofMHCclassIand/or

classIIexpression:

&Adenovirus;E1 9 proteinreducesexpressionofMHCclassIoninfectedcells.

&Murinecytomegalovirus;preventstransportofMHCclassItotheGolgiappa-

ratus.

ImmuneProtectionandImmunopathology..............

Whethertheconsequencesofanimmuneresponseareprotectiveorharmful
dependsonthebalancebetweeninfectiousspreadandthestrengthofthe
ensuingimmuneresponse.Asformostbiologicalsystems,theimmunede-
fensesystemisoptimizedtosucceedin 50 – 90 %ofcases,notfor 100 %of
cases.Forexample,immunedestructionofvirus-infestedhostcellsduring
theeclipsephaseofavirusinfectionrepresentsapotentmeansofpreventing
virusreplication(Fig.2. 15 ,p.79).Fromthispointofview,lyticCD8+T-cell
responsesmakegoodsenseasthehostwilldieifproliferationofacytopathic
virusisnothaltedearlyon.Ifanoncytopathicvirusisnotbroughtunderim-
mediatecontrol,theprimaryillnessisnotsevere—however,thedelayedcy-
totoxicresponsemaythenleadtothedestructionofverylargenumbersof
infectedhostcellsandthusexacerbatedisease(Tables2. 9 and2. 10 ).Sincean
infectionwithnoncytopathicvirusesisnotinitselflife-threateningtothe

ImmuneDefensesagainstInfectionandTumorImmunity 103

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Kayser, Medical Microbiology © 2005 Thieme

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