EFFECT OF HLA-DM TRANSFECTION ON HEN EGG LYSOZYME PRESENTATION BY T2.A(K) CELLS

T2 cells have a large homozygous deletion in the MHC II region, Transf ection of MHC class II genes into T2 cells allows presentation of pept ide but not native protein Ags, This defect in protein presentation ha s been attributed to the lack of HLA-DM, an MHC class Ii-related prote in that facilitates the release of an invariant chain peptide (CLIP) i ntermediate from nascent MHC class II proteins within the endocytic co mpartment of APC, Here, we show that A(k) molecules within isolated la te endosome fractions of T1.A(k) (wild-type) vs T2.A(k) (HLA-DM-defici ent) bind biotin-HEL46-61 at comparable levels, consistent with previo us observations that A(k) molecules on T2 cells are not predominantly occupied with CLIP, However, A(k) molecules in the late endosomes of T 2.A(k) fail to present peptide to a T hybrid, whereas the late endosom es from T1.A(k) have no such defect, Transfection of HLA-DM A and B in to T2.A(k) partially restores protein Ag presentation by T2.A(k) cells , These data suggest that HLA-DM can play a role in Ag presentation in addition to its role in CLIP release, However, even after DM transfec tion there remains a 10-fold difference in the dose-response curve for hen egg lysozyme presentation by T1.A(k) vs T2.A(k)/DM cells, In addi tion, HLA-DM transfection fails to restore presentation by late endoso me fractions, The failure to fully restore Ag presentation in T2.A(k) cells by DM transfection suggests that another gene product, required for efficient Ag presentation, may be absent from the late endosomes o f T2.