DIFFERENT ENDOCYTIC COMPARTMENTS ARE INVOLVED IN THE TIGHT ASSOCIATION OF CLASS-II MOLECULES WITH PROCESSED HEN EGG LYSOZYME AND RIBONUCLEASE-A IN B-CELLS

The processing of exogenous antigens and the association of peptides w ith class II molecules both occur within the endocytic pathway. 2A4 B lymphoma cells of the H-2(k) haplotype were grown in the presence or t he absence of two different exogenous antigens (hen egg lysozyme and r ibonuclease A) internalized by fluid-phase endocytosis. Using subcellu lar fractionation techniques, we demonstrate that, in the presence of hen egg lysozyme, newly synthesized SDS-stable class II molecules are detected in a dense endocytic compartment which does not have the char acteristics of neither early and late endosomes nor lysosomes. In cont rast, no SDS-stable class II molecules are observed between ribonuclea se A and newly synthesized class II molecules. Interestingly, when cla ss II molecules are analyzed at steady state, SDS-stable class II mole cules induced by ribonuclease A are found in a compartment cosedimenti ng with late endosomes. These results suggest that the tight associati ons between ribonuclease A or hen egg lysozyme with class II molecules occur in distinct endocytic compartments and that these associations may depend on the sensitivity of antigens to proteolysis.