Alveolar macrophages. III. Studies on the mechanisms of inhibition of T-cell proliferation.

Unlike macrophages from the peritoneal cavity or the peripheral blood, rat alveolar macrophages actively inhibit mitogen-induced T-cell proliferation. Studies on the characteristics of this inhibitory activity revealed the following: the macrophages must be live, but mitomycin C does not block their activity; they must be added to lymphocyte cultures soon after the initiation of the cultures, and prolonged pre-incubation of macrophages in vitro diminishes their cytostatic activity; suppressive activity is most obvious in cultures of rapidly proliferating lymphocytes, and the lymphocytes themselves may be syngeneic or allogeneic; the suppressive activity may be duplicated by a low molecular-weight dialysable component of macrophage culture supernatants, and suppression of RNA synthesis is as readily demonstrable as suppression of DNA synthesis in target cells; the cytostatic effects of alveolar macrophages in lymphocyte cultures do not appear to result from target cell destruction. Studies involving repeated endobronchial lavage of rats revealed the presence of two alveolar macrophage subpopulations, one (weakly adherent in vivo) supportive to T-lymphocyte proliferation, and another (strongly adherent) strongly inhibitory; the latter population comprises the majority of alveolar macrophages in normal rats.