Interferon-gamma differentially regulates antigen-processing functions in distinct endocytic compartments of macrophages with constitutive expression of class II major histocompatibility complex molecules.

RAW264.7 cells were transfected to express constitutively the murine class II major histocompatibility complex (MHC-II) molecule, I-Ak. The resulting RAW.Ak cells presented HEL(46-61) peptide to 3A9 T hybridoma cells, but they were unable to process and present HEL protein in their resting state. However, IFN-gamma stimulation induced the ability of RAW.Ak to process and present HEL protein, with little effect on their ability to present HEL(46-61) peptide. Antigen catabolism showed little change with IFN-gamma stimulation, suggesting that the production of peptides was not the regulated step in the processing pathway. Furthermore, HEL(46-61) peptide delivered directly into lysosomes by acid-resistant liposomes was also presented only upon IFN-gamma stimulation, while the presentation of peptides delivered into endosomes by acid-sensitive liposomes showed a lesser dependence on IFN-gamma stimulation. Thus, IFN-gamma regulated the ability of peptides delivered into certain lysosomal compartments to meet with MHC-II molecules and form peptide-MHC complexes, or to transport subsequently to the plasma membrane. Two other antigens, ribonuclease A and haemoglobin, were processed by RAW.Ak cells without IFN-gamma stimulation, suggesting that these antigens could be processed by different mechanisms, perhaps in earlier endocytic compartments. Thus, different antigens may be processed in distinct endocytic compartments, and an IFN-gamma-regulated mechanism controls the rescue of peptides from lysosomal compartments for presentation at the plasma membrane.