Interleukin-1 inhibition of B lymphopoiesis is reversible

Interleukin-1 (IL-1) has multiple effects on the hematopoietic system. The present data demonstrate that IL-1 and/or products induced by it reversibly suppress B-cell differentiation. Upon the addition of 50 U/mL (2.4 ng/mL) of recombinant IL-1 alpha (rIL-1 alpha) to lymphoid long-term bone marrow cultures at their initiation, very few B lymphocytes could be detected, and the majority of cells present were myeloid. This inhibition of B lymphopoiesis did not appear to be due to effects on proliferation of mature B cells because IL-1 did not affect the proliferative response of B cells to form B-cell colonies (CFU-B). The actions of the monokine were further examined by using myeloid and lymphoid long-term bone marrow culture systems. The transfer of myeloid long-term bone marrow cultures to lymphoid conditions usually results in the cessation of myelopoiesis and initiation of B lymphopoiesis. Exposure of early B-cell precursors present under the myeloid conditions to 50 U/mL of RIL-1 did not affect their subsequent differentiation into B cells upon transfer of the cultures to lymphoid conditions. However, myelopoiesis was sustained, and B lymphopoiesis did not initiate if 50 U/mL of rIL-1 was added to myeloid bone marrow cultures at the time of their transfer to the lymphoid conditions and during biweekly feedings thereafter. Upon removal of IL-1, myelopoiesis ceased, and B lymphopoiesis initiated. Thus, the effects of IL-1 on inhibition of B lymphopoiesis are reversible.