Roles of reactive nitrogen intermediates and transforming growth factor-β produced by immunosuppressive macrophages in the expression of suppressor activity against T cell proliferation induced by TCR stimulation

The suppressor activity of splenic macrophages induced by Mycobacterium intracellulare infection (MI-Mϕs) against T cell concanavalin A (Con A) mitogenesis is mediated by MI-Mϕ's mediators, such as reactive nitrogen intermediates (RNIs), phosphatidylserine, free fatty acids, prostaglandin E2 and to a minor extent TGF-β. Here, we have compared the roles of RNIs and TGF-β in the expression of MI-Mϕ's suppressor activity against Con A mitogenesis and anti-CD3 monoclonal antibody (mAb)- and anti-CD28 mAb-induced mitogenesis (TCR signal-induced mitogenesis) of the target T cells, and have found the following. First, NG-monomethyl-l-arginine (NMMA) inhibited MI-Mϕ's suppressor activity against TCR signal-induced mitogenesis as well as Con A mitogenesis. Second, anti-TGF-β mAb weakly restored the MI-Mϕ-mediated suppression only in the case of Con A mitogenesis, under limited conditions, such as very low cell densities of MI-Mϕs. Third, the blocking effects of NMMA plus anti-TGF-β mAb were somewhat more prominent in the case of Con A mitogenesis than in the case of TCR signal-induced mitogenesis. Fourth, Con A- or TCR signal-stimulated MI-Mϕs secreted significant amounts of the latent TGF-β but not the active one. These findings indicate that RNIs, but not TGF-β, play important roles in the MI-Mϕ-mediated suppression of TCR signal-induced mitogenesis, as well as Con A mitogenesis, of the target T cells.