Characterization and measurement of CD5+ B cells in normal and Trypanosoma congolense-infected cattle.

CD5+ B cells in cattle are present in peripheral blood and spleen, but not in lymph nodes, tonsils or Peyer's patches. Compared to classical B cells, they express similar levels of B cell surface markers, but have higher levels of surface IgM. We failed to find evidence for IgD on bovine B lymphocytes. The CD5+ B cells expressed CD11b (Mac-1). Another small subpopulation of B cells carried CD11b but not CD5. In cattle infected with Trypanosoma congolense, a dramatic increase in the percentage of CD5+ B cells in blood and spleen was observed. This increase occurred 7-10 days after parasites were first detected in the blood and correlated with the increase in serum IgM and the increase in the absolute number of B cells that is typical to trypanosome-infected animals. The increase in B cells was found to be due mainly to the expansion of the CD5+ B cell subpopulation. The cause of the amplification of the CD5+ B cells and their possible involvement in the production of autoantibodies and non-parasite-specific antibodies which have been described in trypanosome-infected animals are discussed.