Cattle infected with bovine leukaemia virus may not only develop persistent B-cell lymphocytosis but also persistent B-cell lymphopenia

We investigated the distribution of B and T cells in the peripheral blood of haematologically inconspicuous (non-persistent lymphocytotic, PL- ) cattle infected with the bovine leukaemia virus (BLV). Flow cytometric data were obtained from six PL- cattle and compared with six age-matched animals with persistent lymphocytosis (PL+ ) and five non-infected healthy controls (BLV- ). In the PL- group, the percentage and number of surface immunoglobulin-positive (sIg(+) ) B cells were significantly reduced. Whereas in BLV- cattle, about 40% of the peripheral blood lymphocytes (PBL) were sIg(+) and 24% were sIgM(+) B cells. In the PL- group, less than 20% of the PBL were sIg(+) and sIgM(+) B cells. Only 5% of the PBL co-expressed sIgM(+) and CD5(+) versus 16% in BLV- . This decrease was persistent over 3 years and predominantly affected: (i) B cells that did not express sIgM; (ii) sIgM(+) B cells co-expressing CD5 and CD11b; and (iii) equally both lambda- and kappa-type light chain B-cell subpopulations. In contrast, the number of all circulating lymphocytes, CD5(-) and CD11b(-) sIgM(+) B cells and CD2(+) T cells did not differ. In PL+ animals, about 75% of the PBL were sIgM(+) CD5(+) B cells. These cells were of polyclonal origin, as light chains of the lambda- and kappa-type were expressed in a ratio of 4:1 (57.7% of PBL lambda(+) , 14% kappa(+) ) as in BLV- animals (33.6% of PBL lambda(+) , 8.7% kappa(+) ). In PL+ cattle the absolute number of B-cells and, therefore, their relative percentage is significantly increased. For this reason, even in case of absolutely increased T-cell numbers, the relative percentage of T-cells could be lower than in normal controls. The cause for the observed B cell decrease in PL- cattle is unknown, but it can be assumed that cytotoxic T cells are involved in this B-cell lymphopenia