Porcine Reproductive and Respiratory Syndrome Virus promotes SLA-DR-mediated antigen presentation of non-structure proteins to evoke a non-neutralizing antibody response in vivo

The humoral immune response against PRRSV infection is characterized by a rapid induction of non-neutralizing antibodies (NAbs) against non-structure proteins (NSPs). Here, we systematically investigated the potential mechanism for the induction of PRRSV NSP-specific non-NAbs. Our data suggested that PRRSV-NSP-specific NAbs appeared within 10 days after PRRSV infection in vivo In the in vitro model, functional upregulation of the SLA-DR was observed in bone marrow-derived dendritic cells (BM-DCs) and porcine alveolar macrophages (PAMs), whereas remarkable inhibition at the mRNA level was observed after infection by both PRRSV-1 and PRRSV-2 isolates. Notably, the inconsistency in SLA-DR expression between the mRNA and protein levels was resulted from the de-ubiquitination of SLA-DR via the OTU domain of PRRSV-NSP2 to inhibit ubiquitin-mediated degradation. Moreover, mass spectrometry-based immunopeptidome analysis identified immunopeptides originating from multiple PRRSV-NSPs within SLA-DR of PRRSV-infected BM-DCs. Meanwhile, these PRRSV-NSP-derived immunopeptides could be specifically recognized by serum from PRRSV-infected piglets. Notably, certain NSP-derived immunopeptides characterized in vitro could be identified from PAMs or hilar lymph nodes from PRRSV-infected piglets. More importantly, in vitro neutralizing assay indicated that serum antibodies against NSP-immunopeptides were unbale to neutralize PRRSV in vitro Conversely, certain Structure Proteins (SPs)-derived immunopeptides were identified and could be recognize by pig hyperimmune serum against PRRSV, further indicates that NSP-derived antibodies response is non-protective in vivo In conclusion, our data suggested that PRRSV infection interferes with the MHC-II molecule-mediated antigen presentation in APCs via promoting SLA-DR expression to present immunopeptides from PRRSV-NSPs, which contributes to the induction of non-NAbs antibodies in vivoIMPORTANCE PRRSV has haunted the swine industry for over 30 years since its emergence. Besides the limited efficacy of PRRSV Modified live vaccines (MLV) against heterogeneous PRRSV isolate, rapid induction of non-neutralizing antibodies (non-NAbs) against PRRSV-NSPs after MLV immunization or wild strains infection was one of the reasons hampered development of an effective vaccine. By using in vitro generated BM-DCs as models to understand the antigen presentation process of PRRSV and, our data indicated that PRRSV infection of BM-DCs promotes functional SLA-DR upregulation to present PRRRSV-NSPs- derived immunopeptides for evoking non-NAbs response in vivo Our work not only uncovered novel mechanism regarding interference of host antigen presentation by PRRSV but also revealed novel insight for understanding the rapid production of non-neutralizing antibodies against PRRSV-NSPs, which may benefit for developing an effective PRRSV vaccine against PRRSV in future.