CELLULAR MECHANISMS OF EXOGENOUS PEPTIDE BINDING TO HLA CLASS-II MOLECULES IN B-CELLS

We have investigated the ability of APC Class II molecules to bind and release exogenous peptides, two phenomena that are still poorly under stood. In order to investigate the half-life of the complex of an exog enous peptide with DR molecules we have evaluated the uptake and relea se of the radiolabeled peptide 17-29-Tyr of influenza virus matrix pro tein (MA 17-29-Y) by a B-EBV cell line at different times and under di fferent conditions. We have found that the kinetics of both binding an d release of the peptide are very fast in living cells; using glutaral dehyde-fixed cells, the kinetics of the two phenomena are slow, closel y resembling those observed with the same peptide and purified, immobi lized DR molecules. As confirmed by the study of a specific T-cell clo ne activation, the Class II-MA 17-29-Y complexes are short-living ones , with an average half-life of 55 min, and the DR molecules that bind exogenous peptides continuously undergo peptidic exchange. These data, taken together, suggest that the APC are endowed with cellular mechan isms that increase the efficiency of both the loading and the unloadin g of Class II HLA with exogenous peptides. These mechanisms do not app ear to require ATP or to involve newly synthesized Class II molecules, intracellular acidic compartments, or the microtubule-microfilament s ystem. On the other hand, an undamaged cell membrane appears to be cru cial for an efficient binding. (C) 1994 Academic Press, Inc.