Increased MHC Class II and CD25 expression on lymphocytes in the absence of persistent lymphocytosis in cattle experimentally infected with bovine leukemia virus

We recently observed that a group of cattle experimentally infected with bovine leukemia virus (BLV) had enhanced antibody responses to recall antigens. None of the cattle in this group were classified as persistently lymphocytotic, but they did have significantly increased numbers of circulating T and B cells. In order to investigate the potential mechanisms of BLV-induced immune activation, dual-color flow cytometry was used to compare the expression of MHC class II (MHC II) molecules and the inducible IL-2 receptor alpha chain, CD25, on lymphocyte subsets in freshly isolated and cultured PBMC from these same BLV-infected cattle (n=5) with that of age-matched, uninfected controls (n=3). Freshly isolated peripheral blood mononuclear cells (PBMC) from BLV-infected cattle were found to contain a significantly higher percentage of B cells that expressed MHC II molecules (p<0.01). In addition, an increased proportion of CD4+ T cells from BLV-infected cattle expressed MHC II molecules after 20 h of Concanavalin A (Con A) stimulation (p<0.05), and MHC II expression was increased on both CD4+ (p<0.01) and CD8+ (p<0.05) T cells from BLV-infected cattle after 68 h in vitro, even in the absence of exogenous mitogen. Although CD25 expression was not increased on freshly isolated lymphocytes from BLV-infected cattle, an increased percentage of B cells from BLV-infected cattle expressed CD25 after 20 h of culture, either in the presence (p<0.05) or absence (p<0.01) of Con A. Thus, in addition to causing alterations in absolute numbers of circulating lymphocytes, BLV infection appears to cause a functional activation of both B and T cells, even in cattle that are non-lymphocytotic. It is likely that these BLV-induced alterations in lymphocyte activation status contributed to the previously observed enhancement of antibody responses in vivo.