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Characterization of a suppressor cell-activating factor (SCAF) released by adherent cells treated with M. tuberculosis.

Peripheral blood adherent cells ingesting killed Mycobacterium tuberculosis release a suppressor cell-activating factor (SCAF) into their culture supernatants. When adherent cells ingested 125I-labeled M. tuberculosis, radioactivity could be detected in the supernatant within 2 hr. When this supernatant was fractionated on a Sepharose 2B column, the fraction with suppressor cell-activating activity was also found to contain the majority of the radiolabel, which suggests that the macrophage processed bacteria (or bacterial product) constituted the major portion of the SCAF. This fraction also contained a high proportion of lipid, and the fraction with suppressor activity resided purely within the phospholipid fraction. By employing thin layer chromatography, the phospholipids responsible were identified as phosphatidylethanolamine and phosphatidylinositol. These results indicate that when macrophages ingest mycobacteria, they release phosphatidylethanolamine and phosphatidylinositol of bacterial origin into their culture supernatants, which are responsible for activating suppressor T cells.