Enhancement of macrophage Fc-dependent phagocytosis by resident thymocytes: effect of a unique heat-stable lymphokine

Lymphocytes, activated by lectins or specific antigens, have been shown to enhance macrophage phagocytosis through the elaboration of a heat-labile soluble factor(s). Recent evidence from our laboratory revealed that resident (nonactivated) murine thymocytes and splenic lymphocytes increase peritoneal macrophage glucose metabolism through the elaboration of a heat-stable soluble factor(s). Therefore, we investigated the effect of resident lymphocyte subpopulations on macrophage Fc-dependent phagocytosis. Thioglycollate-elicited and resident peritoneal macrophages from BALB/c mice were cultured in serum-free media with syngeneic resident thymocytes or splenic T lymphocytes. Macrophage Fc-dependent phagocytosis was assayed by measuring the ingestion of 51CrSHEA. After 4 days in vitro, resident thymocytes produced a mean 160 (+/- 31) and 136% (+/- 22) increase in Fc-dependent phagocytosis by thioglycollate-elicited (thio-macrophages) and resident peritoneal macrophages, respectively. Splenic T lymphocytes increased thio-macrophage phagocytosis by 112% (+/- 41) under similar conditions. Macrophage Fc-dependent phagocytosis was increased after 24 hr of co-culture by supernatant derived from resident thymocytes and could be further enhanced by supernatant from Con A-activated thymocytes. Supernatant from guinea pig embryo fibroblasts did not increase macrophage phagocytosis. The soluble factor(s) was produced by resident thymocytes after 24 hr of preculture. This factor was active despite heating at 100 degrees C for 30 min whereas the effect of Con A-activated thymocyte supernatant was heat-labile. The stimulatory effect of resident thymocyte supernatant was not observed when the macrophages and supernatant were cultured in 2% FCS. In contrast to the factor(s) produced by resident thymocytes, the factor(s) in FCS that increased phagocytosis was heat-labile. These data suggest thymocytes and splenic T lymphocytes promote macrophage Fc-dependent phagocytosis in the absence of antigenic or lectin stimulation. This previously unrecognized effect of resident thymocytes is due to a unique heat-stable soluble factor(s) that is concealed in the presence of serum.