Objective:To study immunosuppression mediated by the porcine FcγRⅢ in porcine reproductive and respiratory syndrome virus ( PRRSV ) infection to pulmonary alveolar macrophages ( PAMS ).Methods: In this study pulmonary alveolar macrophages cells were treated with containing 200 TCID50 PRRSV,lipopolysaccharide (LPS) (100 ng/ml) and purified mouse anti-pig FcγRⅢIgG (550 μg/ml) separately,simultaneously,PAM cells treated with purified mouse anti-pig FcγRⅢIgG (550 μg/ml) was infected by 200 TCID50 PRRSV ,untreated PAM cells as the control group.Each group were post-cultured 12,24,36,48,60,72 h, the cells and the supernatant were collected.The dynamic variation of PRRSV RNA copies in inoculation group were detected by using real-time fluorescence quantitative PCR method.mRNA level of IFN-αand TNF-αin each group were detected by using relative fluorescence quantitative PCR.Results:The result showed that mRNA level of IFN-αwas improved during PRRSV infection to PAMS 12-24 h,and mRNA level of IFN-αwas inhibited during 36-72 h,then mRNA level of IFN-αrecovered normally; mRNA level of TNF-αwas increased slightly post-infection 12-72 h.IFN-αand TNF-αmRNA levels of PAM cells treated with LPS were both up-regu-lated,using the purified mouse anti-pig FcγRⅢ IgG to treat the PAM cells,selective activation of porcine FcγRⅢ in the PAM cells down-regulated significantly mRNA levels of IFN-αand TNF-α.PRRSV infection assay mediated by selective activation FcγRⅢof the PAM cells inhibited antiviral cytokine ( IFN-αand TNF-α) mRNA levels.Conclusion:The results show selective activation of FcγRⅢinhibited significantly mRNA levels of the antiviral cytokine IFN-αand TNF-αof host cells,and innate antiviral immune response to PRRSV infection.