Bone marrow suppressor B cells in vitro.

Marrow cells from intact CBA and (CBA x C57B1)F1 mice added to a culture of syngeneic splenocytes at its initiation suppressed the generation of anti-SRBC antibody-forming cells. Removal of cells bearing surface immunoglobulins and/or MBLA led to a substantial reduction in the marrow suppressive activity. Marrow cells from mice pretreated with five doses of hydroxyurea did not suppress the immune response of syngeneic splenocytes to SRBC. While the total number of cells in the marrow of such animals declined 5-fold, the relative content of Ig-positive cells was somewhat increased. The proportion of blast cells dropped from 10.3 to 2.4%. No blast cells bearing surface immunoglobulins were observed. The proportion of cells incorporating [3H]-thymidine fell 40-fold. We conclude that suppression of the immune response to SRBC in vitro is mediated by immature precursors of the B lymphocyte series which are present in the bone marrow of intact mice.