Involvement Of Porcine CD16 In Antibody-dependent Enhancement Of PRRSV Infection

Porcine reproductive and respiratory syndrome virus(PRRSV) is the causative agent of porcine reproductive and respiratory syndrome(PRRS), which is an important acute infectious disease, caused substantial economic losses to the swine industry worldwide. To control this disease effectively, researchers have developed a few PRRSV vaccines. However, the immunological effect and the progression of PRRS areinfluenced by a variety of factors, vaccines are always less efficient, in whichantibody-dependent enhancement(ADE) of infection is one crucial factor. ADE means that subneutralizing serum can enhance virus entry into target cells and improve the efficiency of infection.At present, ADE effect has been found in more than 40 kinds of virus infection in various genera, such as dengue virus, human immunodeficiency virus, West Nile Virus, and so on. Common features of these viruses are that they replicate in immune cellsand cause persistent infection, which are similar to PRRSV. Conventional vaccinesoften failedto control these ADE viruses;on the contrary, the vaccine-produced antibodies enhanced the induction of viruses. So it is important to understand the mechanismsof ADE by virus model.In this study, wetried to illustrate the pathogenic mechanism of PRRSV from the perspective of ADE. We found that subneutralizing anti-PRRSV serum enhanced PRRSV infection in porcine alveolar macrophages(PAMs)in vitro, it means that subneutralizing serum can enhance PRRSV entry and replication. It has been reported thatthree classes of Fcγ receptors(FcγRs): FcγRI(CD64), FcγRII(CD32), and FcγRIII(CD16) can mediate ADE of different viruses infection. Quantitative RT-PCR revealed that CD16 transcripts were the most abundant among three types of FcγRs in PAMs. Western blotting and flow cytometrydata showed that a high level of CD16 protein was detected in PAMs. Thus,we mainly studied the role of porcine CD16 in the ADE of PRRSV infection.First, functional blocking antibody was used to determine whether CD16 is responsible for mediating ADE of PRRSV. The results showed that treatment with anti-CD16 m Absignificantly inhibited the ADE of virus infection in PAMs, suggesting that CD16 is involved in the enhanced virus production in PRRSV-antibody immune complexinfected PAMs. Because PAMs express three different FcγR isoforms, we evaluated the effects of CD16 alone in FcγR-nonbearing cells(HEK293-T and COS-7 cells) by transfection. When these two cell lines transfected with CD16 were incubated with PRRSV-immune complex, the immune complex can attach to these cells andsubsequently entry into cells. Furthermore, the internalized virus canreplicate and produceprogeny virus. Wealso showed that the efficient expression of porcine CD16 requires the association of the Fc R γ-chain.This research first reported that CD16 is involved in ADE of PRRSV infection, which provide a new theory to explore the pathogenic mechanism of PRRSV. This CD16-mediated ADE may induce a shift in PRRSV tropism towards CD16-expressing cells, distributing virus to more organs during virus infection, thusour findings provide significant new insights for developing PRRSV vaccines.