Relative ability of different bovine leukocyte populations to support active replication of rinderpest virus

Bovine peripheral blood mononuclear cells (PBMC) were infected with the pathogenic Saudi isolate of rinderpest virus (RPV) in order to identify the cell subpopulation(s) susceptible to active replication of this virus. Flow cytometry analysis, using a monoclonal antibody recognizing the H glycoprotein of RPV, showed that monocytes were the main subpopulation in which the virus replicated, whereas <2% of lymphocytes expressed viral antigen. The activation of PBMC with concanavalin A before infection resulted in an increase in the capacity of lymphocytes to support RPV replication; >90% of CD4+ and CD8+ T lymphocytes expressed viral antigen at 3 days postinfection, although < or = 40% of gamma/delta T cells were productively infected. B-lymphocyte activation with pokeweed mitogen also resulted in increased replication of this virus in these cells, involving up to 40% of B lymphocytes. An enhancement of lymphocyte susceptibility to infection and active replication by RPV was observed upon coculture of RPV-infected PBMC on bovine endothelial cells. Such enhancement was most marked with the B-cell and CD4+ T-cell subpopulations. Contact between lymphocytes and extracellular matrix components did not alter the capacity of RPV to replicate in lymphocytes. This intercellular contact with endothelial cells increased the viability of certain lymphocyte subpopulations, but it alone could not explain the increased sensitivity to RPV. Intercellular signalling, which resulted in interleukin-2 receptor upregulation, probably played a role. In summary, monocytes are the main target for active, productive infection by RPV. Similar replication in lymphocytes depends on their activation state and on contact with accessory cells such as endothelial cells. These characteristics have important implications for virus traffic in vivo and the pathogenesis of this disease.