Characterization of the Blood Lymphocyte Population in Cattle Infected with the Bovine Leukemia Virus1

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 erythrocyteantibody-rosetting cells agreed closely in all infected cattle. This correlation and the results of double labeling experiments indi cate 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 re sults 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-lympho cytes) 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 erythrocyterosetting 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 nonlympho cytotic cattle was much lower. Even in cattle with low or mod erate 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 BLVinfected B-lymphocytes of cattle with PL lack C-3 receptors. 1This work was supported by a grant from the Kleberg Foundation. 2Present address: Cátedra de Virologia, Facultad de Ciencias Veterinarias, U. N. C. P. B. A., Pinto 399, Tandil, Buenos Aires, Argentina. 3Present address: Cambridge BioScience Corporation, 35 South Street, Hopkinton, MA 01748. 4To whom requests for reprints should be addressed. Received 10/15/84; revised 3/25/85; accepted 3/26/85. INTRODUCTION BLV5 infection in cattle is, as a rule, persistent even though virus-neutralizing antibodies are continuously present, usually in high titers, in all infected animals. Only a small fraction (probably less than 10%) of the BLV-infected cattle develop leukemia (lymphosarcoma). The rest of these animals either remain as asymptomatic carriers or develop a condition known as PL (for review, see Ref. 1). It has been assumed that PL represents a preleukemic stage. However, most cattle with PL are otherwise clinically normal and do not develop leukemia even when kept until advanced ages. Moreover, there is no evidence that these animals harbor neoplastic cells (2). Thus, PL in cattle should be regarded as an essentially benign response to BLV infection. The available evidence strongly suggests that the development of lymphosarcoma and PL in cattle depends to a large extent upon the host genetic constitution (3, 4). Several workers (5-11) have found increased percentages of PBL bearing slg in cattle with PL, but they did not distinguish whether these cells had synthesized or adsorbed the immuno globulin. DeLima and Mitsherlich (12) showed that PL-positive cattle have elevated percentages of I SL bearing C-3 receptors, as determined by EAC rosetting. Kjmar ef al. (6) reported increases in the percentages of slg-positive cells and cells bear ing C-3 receptors (as determined by binding of aggregates of human IgG) in the blood of 5 cows with PL. These 2 studies, however, do not clarify the relation of BLV infection to the observed increases in PBL bearing B-cell markers, because neither the control nor the PL-positive animals were examined for BLV. Takashima ef al. (11) showed that the concentration of slgpositive or EAC-positive PBL was higher in BLV-infected cattle with PL than in BLV-free cattle. Since BLV-infected cattle without PL were not included, this study also fails to clarify whether or not BLV infection per se was responsible for the changes in the concentration of PBL bearing B-cell markers. More conclusive information on this question was reported by Kenyon and Piper (5) who studied animals that had been examined for both BLV infection and PL. This study showed that, as compared with BLV-free cattle, about 65% of the BLV-infected cattle without PL had normal concentrations of slg-positive or EAC-positive PBL. In contrast, the percentages of these cells were greatly increased in all cattle with PL. The available information on the changes in the concentration of T-cells in the blood of BLV-infected cattle is scant. In 3 PL cattle examined by Paul ef al. (8), the percentages of PBL forming erythrocyte rosettes were about one-half of that of nonlympho5The abbreviations used are: BLV, bovine leukemia virus; AIP, amplified immunoperoxidase assay; EA, erythrocyte-antibody rosetting; EAC, erythrocyte-antibody complement rosetting; PBL, peripheral blood leukocytes; PBS, phosphatebuffered solution; PL. persistent lymphocytosis; PNA, peanut agglutinin; slg, sur face immunoglobulin; slgM, surface immunoglobulin M. CANCER RESEARCH VOL. 45 JULY 1985