Association of GP51 expression and persistent CD5+ B-lymphocyte expansion with lymphomagenesis in bovine leukemia virus infected sheep.

Alterations in the circulating CD5+ B-lymphocyte population, in vitro GP51 expression, and in vivo tax/rex expression that may precede lymphomagenesis were characterized prospectively in ten experimentally BLV-infected sheep. Infection with pathogenetic BLV resulted in a significant expansion of the circulating CD5+ B-lymphocyte population in six infected sheep. Of the remaining four infected sheep that did not have persistently elevated CD5+ B-lymphocyte counts, three developed lymphoid neoplasia within 14 months post-inoculation. Neoplastic cells from two of these three sheep were CD5- B-lymphocytes, while cells from the third were CD5+ B-lymphocytes. In vitro GP51 expression was a consistent feature of circulating lymphocytes from all three sheep developing tumors, but high level tax/rex gene transcription was not detected in circulating lymphocytes prior to lymphomagenesis. Neither in vitro GP51 expression nor high level tax/rex gene transcription was associated with expansion of the CD5+ B-lymphocyte population in sheep with significantly elevated CD5+ B-lymphocyte counts. These observations indicate that BLV infection in sheep results in expansion of the circulating CD5+ B-lymphocyte population, and that this expansion is not required for the subsequent development of BLV-associated lymphoid neoplasia.