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Hybrid Membrane IgM with the Transmembrane Region of I-A Facilitates Enhanced Presentation of Distinct Epitopes to T Cells¹

Ko-Jiunn Liu^2,*, Michael Schwen^*, Philip W. Tucker and Byung S. Kim^3,*

^* Department of Microbiology-Immunology, Northwestern University Medical School, Chicago, IL 60611; and Institute for Cellular and Molecular Biology, University of Texas, Austin, TX 78712

The role of B cell Ag receptors (membrane Ig or mIg) in the efficient Ag presentation to T cells, including the requirement of mIgM-associated Ig/Igß, remains unclear. We report here that mIgM, substituted with greater than two-thirds of the NH₂-terminal A transmembrane (TM) regions of the MHC class II molecule, are capable of mediating the efficient presentation of specific Ag to some (Group 1) but not all (Group 2) T cell hybridomas. In contrast, the generation of epitopes recognized by the Group 2 hybridomas can be mediated only by the wild-type mIgM. Tyrosine phosphorylation appears to be necessary for the enhanced Ag presentation to Group 2 hybridomas, while it does not for Group 1 hybridomas. In addition, differential sensitivity of Ag processing to leupeptin, different duration required for epitope generation/presentation, as well as the involvement of distinct epitopes for stimulation of these groups of T cell hybridomas were observed. These results suggest that transport of the mIgM/Ag complexes to an endocytic compartment(s) for generation of certain T cell epitopes may be mediated by the N-terminal TM sequence of mIgM, independent of Ig/Igß association. This function can be replaced by two-thirds of the NH₂-terminal TM region of A chain of class II molecules.