Characterization of the blood lymphocyte population in cattle infected with the bovine leukemia virus

Blood leukocytes of cattle characterized in terms of bovine leukemia virus (BLV) infection and persistent lymphocytosis (PL) were examined for the presence of lymphocyte subpopulation markers and viral antigens. The percentages of cells with surface and intracytoplasmic immunoglobulin M (IgM) and erythrocyte-antibody-rosetting cells agreed closely in all infected cattle. This correlation and the results of double labeling experiments indicate that virtually all the surface IgM-positive B-lymphocytes in the blood of these animals carry Fc receptors. In PL cattle, the percentages of surface IgM-positive cells were more than twice those of normal cells and accounted for all the increase in peripheral blood lymphocytes. B-cells accounted for most of the increase in peripheral blood lymphocytes seen in cattle with PL. In contrast, most BLV-infected, nonlymphocytotic cattle had normal percentages of B-cells. Thus, the expansion of the B-cell population in blood, while being a conspicuous characteristic of PL, is not necessarily a consequence of BLV infection per se. Comparisons of the percentages of IgM-positive and erythrocyte-antibody complement-rosetting cells, together with the results of double labeling experiments, indicate that about one-half the B-cells in the blood of cattle with PL lacked C-3 receptors. The proportion of these cells (most likely immature B-lymphocytes) was smaller in the blood of BLV-infected nonlymphocytotic cattle. Direct comparison showed that, in BLV-infected cattle with or without PL, and in BLV-free cattle, virtually all erythrocyte-rosetting blood cells had peanut agglutinin receptors. With only one exception, the numbers of erythrocyte-positive cells in the blood of BLV-infected cattle with or without PL were within normal values. The "null" blood cell population, estimated as the difference between the IgM-positive and erythrocyte-positive populations, was essentially unaffected in BLV-infected cattle without PL, but it was absent in PL cattle. The large majority of the B-lymphocytes present in the blood of cattle with PL were infected with BLV. The proportion of infected B-lymphocytes in the blood of BLV-positive nonlymphocytotic cattle was much lower. Even in cattle with low or moderate levels of BLV-infected blood lymphocytes, the percentages of these cells were remarkably constant during the 12-month period of the study. The data indicate that most of the BLV-infected B-lymphocytes of cattle with PL lack C-3 receptors.