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

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-gamma (IFN-gamma) -activated macrophages infected with Salmonella typhimurum secreted peptides able to load empty MHC Kb molecules on co-cultured TAP-2-deficient RMA-S cells, added as targets for peptide loading. The increase in class I Kb on the RMA-S cells, resulting from the macrophage-derived peptides, exhibited a comparable stability as the direct addition of an exogenous Kb-binding peptide (OVA257-264) to the RMA-S cells. In both cases, the Kb 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 Kb 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-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 presentation may be involved in priming cytotoxic T lymphocytes against intracellular pathogens in vivo.