A novel macrophage receptor enhances MHC II-peptide loading and surface expression of a hapten-protein conjugate

A receptor possessing specificity for fluorescein was previously identified on murine macrophage. The goal of the present study was to determine if this receptor influenced MHC II-peptide loading and surface expression of a hapten-protein conjugate within murine macrophage. Although inhibition of fluid-phase pinocytosis had no detectable effect, lower levels of intracellular MHC II-peptide complexes were observed upon inhibition of receptor-mediated endocytosis. Moreover, lower levels of surface expressed MHC II-fluoresceinated peptide complexes were also detected. Following subcellular fractionation experiments, it was revealed that the receptor altered the endocytic trafficking of the antigen within the cell. Namely, degraded antigen and MHC II-peptide complexes were not observed in dense transferrin receptor positive, cathepsin D positive, LAMP-1 positive organelles upon inhibition of the receptor. Previous studies also suggested that this receptor enhanced MHC II-peptide loading by concentrating high levels of antigen to endocytic organelles. The implications of these findings on subsequent development of the immune response were also discussed.