Surface appearance and instability of empty H-2 class I molecules under physiological conditions.

Recent evidence suggests that endogenously produced antigenic peptides are required for assembly of major histocompatibility complex class I chains with beta 2-microglobulin and transport to the cell surface. The RMA-S mutant cells are thought to be defective in intracellular peptide loading to class I molecules and, therefore, devoid of class I surface expression. Here we report that at physiological temperature (37 degrees C) "empty" class I molecules appear at the cell surface of RMA-S cells where they can be trapped with H-2 antibodies. In the absence of the stabilizing ligand, the class I molecules rapidly alter their conformation but remain at the cell surface as demonstrated with a rabbit antiserum. Such denatured H-2 molecules can also be found on normal wild-type RMA cells. However, their amount is strongly reduced after culture of RMA cells with a class I binding peptide. These findings indicate that empty class I molecules appear at the surface not only on mutant but also on normal cells, suggesting that in normal cells the supply with peptides is limited.