Murine B-lymphocyte colony formation: the effects of cyclophosphamide and azathioprine.

The present studies assessed the relationship between murine splenic B-lymphocyte colony-forming cells and the number of mature B lymphocytes in the spleen following administration of either cyclophosphamide or azathioprine. Cyclophosphamide (200 mg/kg) reduced the number of splenic B lymphocytes by a mean of 77%. However, a greater reduction in colony-forming cells was noted. Five days after cyclophosphamide administration, the number of colonies per spleen was reduced by more than 90%. Furthermore, the recovery of colony-forming activity was delayed in comparison with restoration of the B-cell system. The number of B lymphocytes returned to normal by day 10; colony formation was still reduced on day 28. Azathioprine, administered for 5 consecutive days, caused only a minimal reduction in B-cell clonal growth. These findings suggest that colony-forming cells may not be progenitor cells which are required for recovery from drug-induced lymphocyte destruction. Rather, this activity may be a property of mature B cells. The majority of potential colony-forming cells appear to be non-proliferating, as the phase-specific drug azathioprine caused only minimal reduction in colony-forming activity.