Regulation of macrophage populations. IV. Modulation of Ia expression in bone marrow-derived macrophages.

The expression of I region-associated (Ia) antigens was studied in macrophages derived from bone marrow cell precursors cultured in L cell-conditioned medium (LCM) as a selective growth stimulator. Very few macrophages expressed Ia either while growing actively in LCM or when quiescent, after the removal of LCM from the culture. The addition of T cell lymphokines, however, stimulated the biosynthesis and membrane expression of Ia. Uptake of bacteria by the macrophage during exposure to the lymphokines produced a synergistic increase in Ia expression. The lymphokine-induced stimulation of Ia was reduced by the simultaneous presence of LCM, suggesting that growth and differentiative stimuli have opposing effects on Ia induction. All macrophage colonies derived from single precursors contained cells that could be induced to express Ia.