Macrophages present exogenous antigens by class I major histocompatibilitycomplex molecules via a secretory pathway as a consequence of interferon-gamma activation

Macrophages can process and present exogenous antigens on major histocompat ibility complex (MHC) class I molecules through an alternative mechanism in volving the internalization of antigens antigens and the secretion of pepti des loading MHC class I molecules at the cell surface. In this paper, we fo und that interferon-gamma (IFN-gamma) -activated macrophages infected with Salmonella typhimurum secreted peptides able to load empty MHC K-b molecule s 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 direc t 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 a nd the increase in K-b staining on the RMA-S cells in the co-culture system s. Brefeldin A also had no effect. P815 cells previously co-cultured with S almonella-infected macrophages became targets for cytotoxic T lymphocytes i solated from Salmonella-infected BALB/c mice. Taken together, our data sugg est that IFN-gamma -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 presentat ion may hp involved in priming cytotoxic T lymphocytes against intracellula r pathogens in vivo.