Characterization of a peptide-loading compartment by monoclonal antibodies

Whether or not peptide-loading compartments are classical or specialized compartments of the endocytic pathway of antigen presenting cells is still a matter of debate. One way to solve this discrepancy would be to characterize specific markers for the peptide-loading compartment. We chose to generate monoclonal antibodies against the peptide-loading compartment that we previously characterized as lysozyme loading compartment (LLC) [Escola, J.M., Grivel, J.C., Chavrier, P., Gorvel, J.P., 1995. Different endocytic compartments are involved in the tight association of class II molecules with processed hen egg lysozyme and ribonuclease A in B cells. J. Cell Sci. 108, 2337; Escola, J.M., Deleuil, F., Stang, E., Boretto, J., Chavrier, P., Gorvel, J.P., 1996. Characterization of a lysozyme-major histocompatibility complex class II molecule-loading compartment as a specialized recycling endosome in murine B lymphocytes. J. Biol Chem. 271, 27360]. A preliminary screening by dot blot enabled us to identify several monoclonal antibodies recognizing the LLC and not early and late endosomes. One of these antibodies, the 20C4, was then characterized. It is directed against mature class II molecules of all murine haplotypes. By electron microscopy, 20C4 labeling was restricted to both the plasma membrane and the LLC. These reagents may be useful in the further characterization of the specialized function of these intracellular organelles.