Endocytosis and dissociation of class I MHC molecules labeled with fluorescent β-2 microglobulin

Membrane class I MHC molecules of Con-A activated and lymphoma murine cells have been labeled by exchange of the cell's β₂m with soluble fl-β₂m. It has previously been shown that this method of labeling is specific and does not affect the biologic properties of class I MHC Ag. With this labeling it has been possible to demonstrate the constitutive endocytosis of class I MHC by fluorescence microscopy and by measuring the resistance to quenching by crystal violet of the internalized fl-β₂m molecules. We could also follow the kinetics of β₂m dissociation from the class I molecules at different pH. At pH 5.5, that is the average pH of endosomes, there is considerable dissociation within 15 to 20 min, that is the average recycling half time of class I MHC containing endosomes in activated T cells. Inasmuch as the process is reversible it is likely that, in the recycling endosomes of T cells, class I MHC molecules undergo conformational changes with β₂m going off and on and with consequent changes of the peptide binding site. This process might be involved in Ag presentation, but, because it is apparently limited to T cells, it would play a role in the presentation of the cell's own TCR in idiotypic interactions between T cells.