Lymphocyte subpopulations in sheep during the early stage of experimental infection with bovine leukaemia virus

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Abstract
Sheep were experimentally infected with bovine leukaemia virus (BLV) by the inoculation of PBL from leukaemic sheep. Antibodies to viral structural proteins were detected at from 2 to 6 weeks after inoculation. At seroconversion, all sheep had a marked increase in the number of circulating lymphocytes, due essentially to an increase in the number of B cells. The number of circulating B cells then decreased but remained higher than pre-infection levels. A second increase in this population preceded the development of a B cell lymphoblastic leukaemia. Generalized lymphosarcoma was diagnosed at necropsy of all sheep. Variation between individual sheep in the time from infection to the development of tumours allowed two clearly delineated groups of nine sheep to be compared. A study of changes in the B cell and T cell populations during the first 16 weeks of infection suggested that the initial response to infection influences the subsequent rate of leukaemogenesis. At seroconversion the number of circulating B cells was significantly higher in group 1 (10.16 1.51 109/1) than in group 2 (6.47 2.76 109/1). Group 1 sheep became leukaemic at 20–50 weeks after infection, whereas group 2 sheep did not do so until 70–95 weeks after infection.