Macrophages present exogenous antigens by class I major histocompatibility complex molecules via a secretory pathway as a consequence of interferon‐γ activation

Summary Macrophages can process and present exogenous antigens on major histocompatibility complex (MHC) class I molecules through an alternative mechanism involving the internalization of antigens and the secretion of peptides loading MHC class I molecules at the cell surface. In this paper, we found that interferon‐γ (IFN‐γ) ‐activated macrophages infected with Salmonella typhimurum secreted peptides able to load empty MHC K b molecules on co‐cultured TAP‐2‐deficient RMA‐S cells, added as targets for peptide loading. The increase in class I K b on the RMA‐S cells, resulting from the macrophage‐derived peptides, exhibited a comparable stability as the direct addition of an exogenous K b ‐binding peptide (OVA 257–264 ) to the RMA‐S cells. In both cases, the K b complexes were stable for at least 3 hr after separating the RMA‐S cells from the macrophages. The endosomal inhibitors, leupeptin and ammonium chloride, did not inhibit the release of peptides and the increase in K b staining on the RMA‐S cells in the co‐culture systems. Brefeldin A also had no effect. P815 cells previously co‐cultured with Salmonella ‐infected macrophages became targets for cytotoxic T lymphocytes isolated from Salmonella ‐infected BALB/c mice. Taken together, our data suggest that IFN‐γ‐activated macrophages process exogenous antigens in an intracellular compartment where serine proteases generate peptides released to the external environment for loading empty MHC class I molecules at the cell surface. This TAP‐independent mechanism for the MHC class I presentation may be involved in priming cytotoxic T lymphocytes against intracellular pathogens in vivo .