Suppression of lymphoproliferation by alveolar macrophages in the guinea pig

The relationship between alveolar macrophages (AM) and lymphocytes may be important in the early establishment of infection with Mycobacterium tuberculosis. AM in several species have been shown to suppress lymphoproliferation by producing inhibitors that include nitric oxide (NO).

To study this phenomenon in the guinea pig, the mitogen-induced proliferation of splenic lymphocytes was quantified under various conditions of co-culture with resident AM.

Guinea pig AM consistently and profoundly suppressed proliferation in the co-cultures at AM:lymphocyte ratios of 1:4 or greater. The inclusion of a NO synthesis inhibitor, N-monomethyl-L arginine (NMMA), in the co-culture medium did not influence the suppression of Con A-induced lymphoproliferation by resident guinea pig AM. No nitrite could be detected in supernatant fluids of co-cultured AM and splenocytes. Attempts to stimulate guinea pig AM with LPS in combination with recombinant murine and human IFN-gamma, infection with live Listeria monocytogenes, or incubation with the supernatants from ConA-activated guinea pig lymphocytes failed to generate NO metabolites. The addition of catalase or indomethacin to the Con A-induced AM-splenocyte co-cultures, to inhibit hydrogen peroxide (H2O2) or prostaglandin E2 (PGE2), respectively, did not counteract the suppression mediated by AM. Cell contact was necessary for the co-cultures to generate their inhibitory effects on lymphoproliferation, however, the suppression was actually mediated, at least in part, by soluble factors produced in the co-cultures.

These results suggest that resident alveolar macrophages suppress lymphocyte proliferation in the guinea pig, but that the effect is not mediated by NO, PGE2 or H2O2. The failure to demonstrate NO synthesis under a variety of stimulatory conditions, which resulted in macrophage activation, suggests that the guinea pig is similar to the human in that regard.