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SUMMARYWhile a sorting signal in the cytoplasmic tail of the major histocompatibility complex (MHC) class II molecules is known to in¯uence their endocytic transport, potential effects of the transmembrane (TM) domain of the MHC class II molecules on endocytic transport remain unclear. We have examined the role of the TM domain by comparing antigen-presenting functions of the wildtype (WT) I-A b and mutant (MT) I-A b molecule substituted in the b-chain TM with a chain TM. A20 cells transfected with WT I-A b were able to present antigen (hen egg lysozyme) better to some hybridomas, while those transfected with MT I-A b consistently outperformed WT for other hybridomas recognizing different epitopes. This difference in antigen processing and presentation is not caused by the differences in H-2M (DM) requirement or association with Ii. The time required for processing of speci®c epitopes appears to be different, suggesting sequential involvement of various endocytic compartments in the antigen processing. Although both WT and MT molecules were found in the early endocytic (transferrin receptor-rich) compartments, MT molecules accumulated in these compartments in higher quantities for longer time periods. Similarly, the MT molecule is retained for a longer time period than WT in late endocytic (LAMP-1 associated) compartments. Together, our data indicate an important role of the TM domain of the MHC class II molecules in the intracellular traf®cking and, consequently, antigen processing and presentation.

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