ABSTRACT
An African swine fever virus (ASFV) gene with similarity to the T-lymphocyte surface antigen CD2 has been found in the pathogenic African isolate Malawi Lil-20/1 (open reading frame [ORF] 8-DR) and a cell culture-adapted European virus, BA71V (ORF EP402R) and has been shown to be responsible for the hemadsorption phenomenon observed for ASFV-infected cells. The structural and functional similarities of the ASFV gene product to CD2, a cellular protein involved in cell-cell adhesion and T-cell-mediated immune responses, suggested a possible role for this gene in tissue tropism and/or immune evasion in the swine host. In this study, we constructed an ASFV 8-DR gene deletion mutant (Δ8-DR) and its revertant (8-DR.R) from the Malawi Lil-20/1 isolate to examine gene function in vivo. In vitro, Δ8-DR, 8-DR.R, and the parental virus exhibited indistinguishable growth characteristics on primary porcine macrophage cell cultures. In vivo,8-DR had no obvious effect on viral virulence in domestic pigs; disease onset, disease course, and mortality were similar for the mutant Δ8-DR, its revertant 8-DR.R, and the parental virus. Altered viral infection was, however, observed for pigs infected with Δ8-DR. A delay in spread to and/or replication of Δ8-DR in the draining lymph node, a delay in generalization of infection, and a 100- to 1,000-fold reduction in virus titers in lymphoid tissue and bone marrow were observed. Onset of viremia for Δ8-DR-infected animals was significantly delayed (by 2 to 5 days), and mean viremia titers were reduced approximately 10,000-fold at 5 days postinfection and 30- to 100-fold at later times; moreover, unlike in 8-DR.R-infected animals, the viremia was no longer predominantly erythrocyte associated but rather was equally distributed among erythrocyte, leukocyte, and plasma fractions. Mitogen-dependent lymphocyte proliferation of swine peripheral blood mononuclear cells in vitro was reduced by 90 to 95% following infection with 8-DR.R but remained unaltered following infection with Δ8-DR, suggesting that 8-DR has immunosuppressive activity in vitro. Together, these results suggest an immunosuppressive role for 8-DR in the swine host which facilitates early events in viral infection. This may be of most significance for ASFV infection of its highly adapted natural host, the warthog.
An immortalized porcine macrophage cell line competent for the isolation of African swine fever virus
