03_Evidence for immunosuppressive properties_Wasin.pmd

18 Charerntantanakul W.PRRSV was reported to possess immunosuppressiveproperties. Such properties werecharacterized by weak innate and delayed andweak adaptive immune responses. Pigsinfected with PRRSV developed low levels ofinnate antiviral cytokines, humoral and cellmediatedimmune (CMI) responses after 2-4weeks of infection, while the animals producedrobust innate antiviral cytokines, humoral andCMI responses within one week of pseudorabiesvirus and swine influenza virus infections. (15)During acute PRRSV infection, pigs developedsignificantly reduced humoral immune responseto other swine pathogens eg. classical swinefever virus (CSFV), indicative of PRRSV abilityto cause generalized humoral immunosuppression.PRRSV-infected pigs reportedly demonstratedincreased susceptibility to co- andsecondary infections and developed moresevere disease than pigs infected with coinfectedpathogens alone. (16-18) Commonly foundco-infected pathogens are Streptococcus suis,Hemophilus parasuis, Mycoplasma hyopneumoniae,Actinobacillus pleuropneumoniae, Salmonellasp., and swine influenza virus. (16-18)The present review summarizes evidence forPRRSV-induced immunosuppression (Table 1).The first part of this review emphasizes onevidence for PRRSV-induced innate immunesuppression. The second part discussesPRRSV and suppression of humoral immunity.The last part describes PRRSV and CMIsuppression.

Evidence for immunosuppressive properties of PRRSV 19PRRSV-induced innate immune suppressionPigs demonstrate significantly reducedpopulations of peripheral blood monocytes andpulmonary alveolar macrophages (PAM) afterPRRSV infection. (6-8,17) The virus replicates inmyeloid cells and induces them to undergoapoptosis. (2-5) PAM and pulmonary intravascularmacrophages infected with PRRSV havereduced cell activities in phagocytosis, productionof reactive oxygen species eg. superoxideanion (O 2-) and hydrogen peroxide (H 2O 2) andmicrobial killing. (3,19) PRRSV-infected PAM produceminute level of interferon alpha (IFNα) andtumor necrosis factor alpha (TNFα); both areessential for inhibition of PRRSV replication. (19-22)The levels of IFNα and TNFα in bronchoalveolarlavage (BAL) fluids of PRRSV-infected pigsare low, which allow PRRSV to replicate withouteffective innate immune defense. (20,22) Themechanisms of PRRSV in suppressing innateimmune responses of pigs were not known.ORF5 product of the virus was reported toassociate with apoptosis of myeloid cells. (23)Reduced innate immune response is a characteristicof viruses in the family Arteriviridae.The members of this family of virus, in additionto PRRSV, comprise lactate dehydrogenaseelevatingvirus, equine viral arteritis virus, andsimian hemorrhagic fever virus. (24) These virusesare myelotrophic and capable of suppressingIFNα and TNFα production, and inducing apoptosisof myeloid cells. (24,25) The mechanisms ofthese viruses in suppressing innate immuneresponse of their hosts are not known.PRRSV-induced humoral immune suppressionHumoral immune response to PRRSVappear approximately 2-4 weeks after infection.(26-29) This is relatively late, compared tohumoral immune response to other swine pathogenseg. pseudorabies virus and swine influenzavirus which appear within three days toone week of infection. (15) The humoral immuneresponse to PRRSV is predominantly againstnucleocapsid proteins of the virus which are nonneutralizingepitopes. (30,31) Neutralizing antibodyto PRRSV (against glycoprotein 4 and glycoprotein5) appear later than non-neutralizingantibody and has low titers (2 3 -2 5 ) throughoutthe course of infection. (26,28,29) The neutralizingantibody to PRRSV at very low titer (≤ 2 2 ) maynot be protective and may enhance PRRSVinfection of macrophages via antibody-dependentenhancement of infection. (32) Pigs thathave low or very low titers of neutralizing antibodiesto PRRSV may demonstrate prolongedviremia, in which the virus could be isolated fromserum and peripheral blood mononuclear cells(PBMC) for at least five weeks after infection.(28,33-37) The virus could also be isolated fromtissues eg. palatine tonsils, heart, small intestine,and brain for months after infection. (6,7,28,36,38,39)PRRSV-seropositive pigs could shed infectiousvirus via semen, urine, saliva, nasal secretion,colostrum, milk, feces, and transplacentaat late gestation. (6,12,13,28,39-44)The causes of delayed and weak neutralizingantibody response to PRRSV are notentirely known. Molecular analysis of PRRSVglycoprotein 4 and 5 reveal that there are heavyglycosylations of neutralizing epitopes whichmight reduce antigen recognition by B cells. (45)

20 Charerntantanakul W.In addition, there are decoy neutralizingepitopes in glycoprotein 5 which induce B cellsto produce non-protective antibodies. (46)Antibody response to PRRSV has differentkinetics with respect to clinical virulence of thevirus. The response is faster and more robustfollowing high virulent PRRSV infection than afterinfection with low virulent virus. (9) This isbecause high virulent PRRSV replicates fasterand thus produces more antigens for B cellsthan low virulent one. (9,33,38,47) This samephenomenon of different kinetics of antibodyresponse due to varying virus virulence wasreported in infections with other viruses eg.bovine viral diarrhea virus, (48) foot-and-mouthdisease virus (49) , and influenza virus. (50)Pigs infected with PRRSV developedsignificantly reduced humoral immune responseto other swine pathogens during acute PRRSVinfection. (51) This was shown in pigs infectedwith PRRSV one week prior to immunizationwith CSFV modified-live virus (MLV) vaccine inwhich pigs demonstrated significantly reducedCSFV-specific neutralizing antibody production,as compared to naïve pigs immunized withCSFV MLV vaccine alone. (51) The animals,however, did not develop significantly reducedantibody response to other swine pathogensafter they were recovered from acute PRRSVinfection. This was shown in pigs inoculatedwith PRRSV two weeks prior to immunizationwith killed pseudorabies virus vaccine in whichpigs showed no reduction in pseudorabiesvirus-specific immunoglobulin production whencompared to naïve pigs immunized with pseudorabiesvirus vaccine alone. (37) Pigs vaccinatedwith PRRS MLV vaccine showed no significantreduction in antibody response to porcinecircovirus type 1 after five weeks of MLV vaccination.(36)PRRSV-induced CMI suppressionCMI responses to PRRSV were determinedby alteration of T cell subpopulations in vivo,cytokine gene expression and protein productionin vivo, and antigen-specific lymphocyteproliferation and cytokine production in vitro.Alteration of T cell subpopulations invivoPigs were reported to have transient lymphocytopeniaof CD4 + CD8 - and CD4 - CD8 + Tcells from three days to approximately fourweeks after PRRSV infection. (6,52-55) The causeof lymphocytopenia was not known but wasbelieved to be due to virus-induced apoptosisof lymphocytes. (56-59) PRRSV induces apoptosisof lymphocytes via mechanisms mediatedby PRRSV-infected myeloid APC; precisemechanisms are not yet known. (59) After fourweeks of PRRSV infection, the populations ofCD4 - CD8 + T cells were recovered to normal leveland then increased dramatically. The increaseof CD4 - CD8 + T cell population suggests its significancein PRRSV infection. (53-55) Like humansand mice, CD4 - CD8 + T cells are cytotoxic T cellsof the pigs. (60-62) This T cell subset is importantin controlling infection of intracellular pathogenssuch as virus, rickettsia, and chlamydia of pigs,mice, and humans. (63) The population of this Tcell subset is usually increased during infectionwith intracellular pathogens. (63)Cytokine gene expression and proteinproduction in vivoPigs demonstrated significantly

Evidence for immunosuppressive properties of PRRSV 21increased interleukin-6 (IL-6), IL-10, and IFNγgene expression and protein production in theirserum, PBMC and BAL cells after PRRSVinfection. (64-69) The increased cytokine responsewas relatively delayed, primarily detected twoweeks after PRRSV infection, as compared tothree days of IL-6 and IFNγ response after pseudorabiesvirus and swine influenza virus infection.(15) The cause of delayed cytokine responsewas not known but was proposed toassociate with increased production of IL-10.IL-10 is an anti-inflammatory cytokine, reportedto inhibit pro-inflammatory cytokine productioneg. IL-1, TNFα, and IFNγ. (70,71) TNFα and IFNγare crucial for controlling PRRSV replication andenhancing antigen processing and presentationof myeloid APC. (21,72) Increased IL-10 productionmay facilitate PRRSV replication andreduce antigen presentation by myeloid APCto T cells. No significant change in othercytokine gene expression ie. IL-1, IL-2, IL-4, IL-8, IL-12, IL-15, IL-18, and TNFα was observedafter PRRSV infection. (65,66,69,73)Antigen-specific lymphocyte proliferationPRRSV-specific lymphocyte proliferation invitro was detected approximately four weeksand persisted until approximately three monthsafter infection. (73-75) The appearance of PRRSVspecificlymphoproliferative response was verydelayed, compared to the response to otherswine pathogens such as pseudorabies virusand swine influenza virus which arose approximatelyone week after infection. (15) The causeof delayed lymphoproliferative response was notknown but was believed to associate with increasedproduction of IL-10. (51)A challenge infection with PRRSV inducedanamnestic lymphoproliferative response inwhich lymphocytes proliferated after two weeksof challenge. (74) T cell subsets responsible forlymphoproliferation were CD4 + CD8 + andCD8 + γδ + . (61,76-78) In pigs, CD4 + CD8 + T cells arememory T-helper cells, and CD8 + γδ + T cells arenatural killer-like cells. (63)Long-term exposure to PRRSV or PRRSVantigen may induce reduced T-cell proliferativeresponse. This was reported in pigs repeatedlyvaccinated with PRRS MLV (Ingelvac ®PRRS MLV, Boehringer Ingelheim Vetmedica,Inc, St. Joseph, MO) or killed virus(PRRomiSe TM , Intervet Inc., Gainesville, GA)vaccines. (75) The cause of reduced T cellresponse after repeated exposure to the virusor virus antigens was not known. Reduced Tcell response to vaccine virus and vaccine antigensafter repeated immunizations is uncommon.Animals usually develop better immuneresponse following subsequent immunizations.Animals may develop reducing immuneresponse in the second vaccination if theyreceive overwhelming loads of vaccine antigensin their primary immunization; the phenomenonis known as high-zone tolerance. (79) Whetherthis high-zone tolerance phenomenon appliesto PRRSV immunizations requires further investigation.Antigen-specific cytokine productionIFNγ response of PRRSV-specific T cellswas detected approximately 3-10 weeks afterinfection and PRRS MLV vaccination. (27,36,80) Theresponse was very delayed compared to theIFNγ response of T cells specific for pseudorabiesvirus and swine influenza virus which

22 Charerntantanakul W.appeared within one week of infection. (15) Thecause of delayed IFNγ response was notentirely known but was shown to be mediatedby IL-10. (5) PRRSV-specific T cell subsetsresponsible for IFNγ production reportedly comprisedCD4 - CD8 + , CD4 + CD8 + , and γδ Tcells. (27,80,81)SummaryPRRSV causes diseases in pigs of all ages.The virus induces poor innate, humoral immune,and CMI responses. Innate immune responseto PRRSV is reduced by various determinations.Humoral immune and CMI responses toPRRSV are weak and delayed, compared tohumoral immune and CMI responses to otherswine pathogens. The mechanisms of PRRSVaffecting immune responses are not clearlyunderstood.Poor immune response to PRRSV interfereswith the detection and elimination program ofPRRSV-infected pigs. Detection of PRRSVseroconversionrequires at least two weeksafter infection; the duration is long enough forvirus to spread from pigs to pigs. The virusalso increases susceptibility of pigs to otherpathogens, making the disease more severeand increasing economic loss. Intensive farmmanagement should be considered. Forveterinary practitioners in Thailand, the Thaiveterinary medical association under royalpatronage has published the clinical practiceguideline for PRRS in pig farms in Thailand, towhich proper farm management procedures arereferred.Future study on PRRSV immunology shouldfocus on determination of PRRSV strategies toreduce host immune response. The determinationshould include characterization ofPRRSV components contributing to reducedhost innate and adaptive immune responses,and mechanisms of virus induction of IL-10expression. Inhibition of PRRSV expression ofsuch components and virus induction of IL-10expression will be the next step of future study.Technology of RNA interference may be usedto control expression of PRRSV, IL-10, andother yet undiscovered immunosuppressivecomponents.AcknowledgementsThe author thanks Dr.James A.Roth andDr.Ratree Platt of Iowa State University for guidanceduring the preparation of the manuscript.References1. Meulenberg JJ, Hulst MM, de Meijer EJ,Moonen PL, den Besten A, de Kluyver EP, etal. Lelystad virus, the causative agent ofporcine epidemic abortion and respiratorysyndrome (PEARS), is related to LDV and EAV.Virology 1993;192:62-72.2. Voicu IL, Silim A, Morin M, Elazhary MA. Interactionof porcine reproductive and respiratorysyndrome virus with swine monocytes. VetRec 1994;134:422-3.3. Thanawongnuwech R, Thacker EL, Halbur PG.Effect of porcine reproductive and respiratorysyndrome virus (PRRSV) (isolate ATCC VR-2385) infection on bactericidal activity of porcinepulmonary intravascular macrophages(PIMs): in vitro comparisons with pulmonaryalveolar macrophages (PAMs). Vet ImmunolImmunopathol 1997a;59:323-35.4. Thacker EL, Halbur PG, Paul PS, Thacker BJ.Detection of intracellular porcine reproductiveand respiratory syndrome virus nucleocapsidprotein in porcine macrophages by flowcytometry. J Vet Diagn Invest 1998;10:308-11.

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Evidence for immunosuppressive properties of PRRSV 2774. Lopez Fuertes L, Domenech N, Alvarez B,Ezquerra A, Dominguez J, Castro JM, et al.Analysis of cellular immune response in pigsrecovered from porcine respiratory and reproductivesyndrome infection. Virus Res 1999;64:33-42.75. Bassaganya-Riera, J, Thacker BJ, Yu S, StraitE, Wannemuehler MJ, Thacker EL. Impact ofimmunizations with porcine reproductive andrespiratory syndrome virus on lymphoproliferativerecall responses of CD8+ T cells. ViralImmunol 2004;17:25-37.76. Summerfield A, Rziha HJ, Saalmuller A. Functionalcharacterization of porcine CD4+CD8+extrathymic T lymphocytes. Cell Immunol1996;168:291-6.77. Zuckermann FA, Husmann RJ. Functional andphenotypic analysis of porcine peripheralblood CD4/CD8 double-positive T cells. Immunology1996;87:500-12.78. Saalmuller A, Werner T, Fachinger V. T-helpercells from naive to committed. Vet ImmunolImmunopathol 2002;87:137-45.79. Swinton J, Schweitzer AN, Anderson RM. Twosignal activation as an explanation of highzone tolerance: a mathematical explorationof the nature of the second signal. J TheorBiol 1994;169:23-30.80. Meier WA, Galeota J, Osorio FA, HusmannRJ, Schnitzlein WM, Zuckermann FA. Gradualdevelopment of the interferon-gamma responseof swine to porcine reproductive andrespiratory syndrome virus infection or vaccination.Virology 2003;309:18-31.81. Olin MR, Batista L, Xiao Z, Dee SA, MurtaughMP, Pijoan CC, et al. Gammadelta lymphocyteresponse to porcine reproductive andrespiratory syndrome virus. Viral Immunol2005;18:490-9.

28 Charerntantanakul W.กกกก ก, , . . กกกกกกก กกกก กกกก, กก, ก ก pulmonary alveolar macrophage pulmonaryintravascular macrophage กกกกก กกก กกกก กกกกกก 2550;5(1):17-28.:กกก ก