Porcine FcγRIIb Mediates Antibody-dependent Enhancement Of PRRSV Infection

Porcine reproductive and respiratory syndrome (PRRS), caused by the porcine reproductive and respiratory syndrome virus (PRRSV), is one of the most important infectious diseases that affecting pig farms, and it has caused huge losses to swine industry all over the world. Because of the antibody-dependent enhancement (ADE), the pattern of persistent, the high heterogeneity of the virus, as well as the subclinical infection with occasional epidemic outbreaks, measures to the clinical prevention and control have been complicated. Antibody-dependent enhancement (ADE) of virus infection, means that sub-neutralizing antibodies can enhance the replication and infection of virus. Antibody-dependent enhancement is a significant obstacle to the development of vaccines to control many viral diseases.ADE is mainly mediated by the Fc receptors (FcRs) on the surface of immune cells. Fc receptors (FcRs), which expressed on the surface of immune cells, are a group of important molecules. They can bind the Fc region of immunoglobulins and have lots of biological functions. Porcine FcγRs are consisted of three types of molecules, FcγRⅠ (CD64), FcγRⅡ (CD32) and FcγRⅢ(CD16). Of the three types of FcyRs, porcine FcyRI and FcγⅢ, because of the ITAM motif in y chain, are activatory Fc receptors. Whereas, poFcyRⅡ, in which an ITIM motif was found, belongs to inhibitory Fc receptors. FcyRⅠ and FcγⅢ, the activatory Fc receptors, have been shown to facilitate antibody-mediated enhancement of dengue virus in human macrophage previously. However, the inhibitory Fc receptor whether mediated ADE has not been reported. The purpose of this study is to investigate that whether poFcyRIIb, as an inhibitory receptor, can mediate antibody-mediated enhancement of PRRSV infection.Stable Marc-145cell lines expressing poFcγRⅡb (Marc-poFcyRII) were established in the early. On this basis the expression of porcine FcγRⅡ and the characteristics of PRRSV replication were studied. The expression of porcine FcγRⅡ in the Marc-145cell lines expressing poFcγRⅡ was verified by RT-PCR and flow cytometry, and then virion production and virus infection in the absence of PRRSV antibody were quantified using both parent Marc-145cells and Marc-poFcγRⅡ, the results showed that the infection and the efficiency of PRRSV replication in the two cells were similar. In the stable Marc-145cell lines expressing poFcγRⅡb (Marc-poFcγRⅡ), the relative viral yield was significantly increased in the presence of sub-neutralizing levels of anti-PRRSV antibody. In Marc-145cells not bearing FcγRs, the enhancement was not evident. The Fab fragment and the normal porcine sera had no effect neither. When cells were infected in the presence of anti-PRRSV antibody, the enhancement of infection was inhibited by anti-poFcγRⅡ antibody These results indicate that the enhancement of PRRSV infection in the presence of anti-PRRSV virus antibody is FcγRⅡ-mediated.To further study the characteristics of the inhibitory IgG Fc receptor-mediated ADE of PRRSV, the infection rate of PRRSV ADE and non-ADE infection of Marc-poFcγRⅡ cell lines were compared. In Marc-poFcγRⅡ cell lines, the initial PRRSV infectivity had no difference between ADE and non-ADE infection, while after24h, a obvious increase in infectivity was found, with higher numbers of infected cells under ADE infection conditions, indicating that the post-entry intracellular activities are very important. In order to better understand the pathogenesis of PRRSV infections and the PRRSV-porcine cell interactions, it is helpful to elucidate the role of porcine FcγRⅡb in PRRSV ADE infections.