African Swine Fever Virus CD2v protein promotes β-Interferon expression and apoptosis in swine cells

Abstract

African swine fever (ASF) is a disease of swine characterized by massive lymphocyte depletion in lymphatic tissues due to apoptosis of B and T cells, most likely triggered by proteins or factors secreted by infected adjacent macrophages. Here we describe a role for the ASF virus (ASFV) protein CD2v in apoptosis induction in lymphocytes. CD2v is a viral homolog of host CD2 that has been implicated in viral virulence and immunomodulation in vitro; however, its actual function remains unknown. We show that CD2v is secreted into culture medium of CD2v-expressing swine cells; and expression of-or treatment with CD2v led to significant induction of IFN-{beta}/ISGs transcription and antiviral state. CD2v expression led to enhanced NF-{kappa}B-p65 nuclear translocation in these cultures and incubation with a NF-{kappa}B inhibitor reduced CD2v-induced NF-{kappa}B-p65 nuclear translocation and IFN-{beta} transcription. We show that CD2v binds CD58, the natural CD2 ligand, and that CD58 siRNA knock-down results in significant reduction in NF-{kappa}B-p65 nuclear translocation and IFN-{beta} transcription. Treatment of swine PBMC with purified CD2v led to enhanced NF-{kappa}B-p65 nuclear translocation and induction of IFN-{beta} transcription. Further, induction of caspase-3 and PARP1 cleavage was observed in these swine PBMC at later times, providing a mechanism for CD2v-induced apoptosis of lymphocytes. Finally, IFN-{beta} induction and NF-{kappa}B activation was inhibited in swine PBMC treated with purified CD2v pre-incubated with antibodies against CD2v. Overall, our results indicate that ASFV CD2v is an immunomodulatory protein that, by promoting lymphocyte apoptosis, may contribute to bystander lymphocyte depletion observed during ASFV infection in pigs. IMPORTANCEASF, a severe hemorrhagic disease of domestic swine, represents a significant economic threat to swine industry worldwide. One critical pathological event observed in pigs infected with virulent isolates is an extensive destruction of lymphoid tissue and massive lymphocyte depletion. However, viral factor/s involved in this event are yet to be identified. Here we show that, by inducing NF-{kappa}B-dependent IFN signaling, ASFV CD2v is able to promote apoptosis in swine PBMC. We propose that CD2v released by ASFV-infected macrophages contributes to the massive depletion of lymphocytes observed in lymphoid tissues of infected pigs. Results here improve our understanding of ASFV pathogenesis and will encourage novel intervention approaches.