MHC CLASS-II COMPARTMENTS AND THE KINETICS OF ANTIGEN PRESENTATION INACTIVATED MOUSE SPLEEN DENDRITIC CELLS

MHC class II (MHC-II) molecules bind fragments of exogenous Ags in an intracellular endocytotic compartment. In view of divergent data on th e MHC-II distribution in different cell lines, it was of interest to l ocalize MHC-II molecules in a natural and the most potent APC type, th e dendritic cell (DC). By using immunogold labeling of ultrathin cryos ections of cultured mouse spleen DC, we found that MHC-II molecules we re present abundantly at the plasma membrane and in intracellular comp artments containing internal membrane vesicles and/or membrane sheets. The majority of these compartments was situated late in the endocytot ic route, as demonstrated by the late appearance (after a lag of 30 mi n) of internalized exogenous tracer. These compartments contained the lysosomal enzymes cathepsin D and beta-hexosaminidase, but lacked the late endosomal marker cation-dependent mannose-6-phosphate receptor. W e conclude that most of the intracellular MHC-II molecules in cultured spleen DC reside in a compartment with (pre)lysosomal characteristics , resembling the so-called MHC-II-enriched compartments (MIIC), origin ally described in B cells. We also investigated whether the presence o f MHC-II molecules in endocytotic compartments was related to the kine tics of Ag processing and presentation by these cells. Pulse-chase end ocytosis experiments with hen egg lysozyme (HEL) as a model Ag showed that activated spleen DC were able to efficiently process and present this Ag to an HEL-specific T hybridoma cell line. However, presentatio n started only after a lag of 2 h and was maximal after 6 h. The diffe rence in time between the arrival of Ag in proteolytic endocytotic com partments, in particular MIIC, and effective Ag presentation is discus sed in the context of DC maturation.