Involvement Of The ITIM In PoFcyRllb Cytoplasmic Domain To Antibody-dependent Enhancement Of Porcine Reproduction And Respiratory Syndrome Virus Infection

Porcine reproductive and respiratory syndrome (PRRS) is an economicallysignificant viral disease of swine world wide. The syndrome was initially observed inthe United States in1987and subsequently in Europe in1990. PRRS is characterizedby respiratory disorders, abortion in pregnant sows, and high mortality in piglets. Thecausative agent of PRRS is PRRSV, which is an enveloped positive-strand RNA virusin the family Arteriviridae.Pigs infected with PRRSV develop a strong and rapid humoral response, butthese initial antibodies do not confer protection and can even be harmful by mediatingan antibody-dependent enhancement (ADE), since these antibodies can facilitate theentry of the virus into targets cells in vitro. Antibody-dependent enhancement is oneof the major problem in prevention and control PRRS. In vitro infection of alveolarmacrophages by PRRSV is significantly enhanced in the presence of sub-neutralizinganti-PRRSV antisera, and the duration of viremia is greater in pigs injected withsub-neutralizing antibodies prior to virus challenge than in pigs injected with normalIgG. These observations strongly suggest that ADE of PRRSV infection has thepotential to enhance the severity of disease and possibly the susceptibility to PRRSVinfection in pigs with declining levels of PRRSV-specific antibodies of maternalorigin, or with low-titre antibodies induced by exposure to wild-type or vaccinestrains of PRRSV. The development of protective antibodies is the basis of vaccinedevelopment against PRRSV. However, the discovery of the phenomena of ADE inPRRSV infection is a significant obstacle to the development of effective vaccines forcontrolling PRRS. Because of these features of PRRSV infection, PRRS has been oneof the most challenging subjects of research in veterinary viral immunology. The mechanism of ADE for PRRSV is still obscure. Previous studies haveindicated that Fcγ receptor (FcγR)-mediated entry of infectious PRRSV immunecomplexes into macrophages is a key event in the pathogenesis of the disease.Previously, we have showed that porcine FcγRIIb is a key molecule mediating ADEof PRRSV infection. FcγRIIb is the only inhibitory receptor that after binding immunecomplexes can prevent the activation of immune cells by recruitment of SHIP-1to itscytoplasmic ITIM (immunoreceptor tyrosine-based inhibition motif). The intracellularmechanisms and implications of enhanced pathogenesis may be the result of theinhibitory signaling after interaction of infectious immune complexes with FcγRIIb.This study demonstrates that PRRSV was able to suppress the transcription ofkey antiviral genes TNF-α and IFN-β, when infection was via the ADE pathway.Investigation of this infection pathway found that PRRSV suppresses the antiviralgenes by disrupting the transcription of the genes coding for the associatedtranscription factors IRF-1, IRF-3, NF-κB and nuclear translocation of NF-κB.However, the signal transduction of poFcγRIIb in mediating ADE of PRRSV remainsobscure.The present study was undertaken to determine whether modification of theconserved motifs of the cytoplasmic region of poFcγRIIb affects the ability of thereceptor to mediate ADE of PRRSV. We evaluated the possible roles of thecytoplasmic and ITIM regions of poFcγRIIb in mediating ADE of PRRSV. Consistentwith previous findings, we introduced a series of mutations in the cytoplasmicdomains of wild-type (WT) poFcγRIIb and examined the capacity for ADE. Theresults indicated that disruption of the ITIM motifs and removal cytoplasmic domainof the poFcγRIIb eliminated the ability of poFcγRIIb to mediate ADE of PRRSV.These findings suggest that the specific structure of the poFcγRIIb cytoplasmicdomain is essential for the ability of poFcγRIIb to mediate ADE. Consistent withprevious findings, the siRNA of SHIP-1was synthesized and the effective of SHIP-1in ADE was examined. The results indicated that the IFN-β transcription was elevatedand the virus replication was reduced, when the expression of SHIP-1was interfered.These findings suggest that the signal transduction of the ITIM is important for theability of poFcγRIIb to mediate ADE. The results provide a profound implication forour understanding of the mechanism of PRRSV entry into cells in the presence of antibody. The ADE mechanism described in this study furthers our understanding ofpathogenesis following PRRSV infection and is of general relevance to virallyinduced disease and in relation to antiviral vaccination strategies.