Conformational alterations during biosynthesis of HLA-DR3 molecules controlled by invariant chain and HLA-DM

HLA-DM is known to catalyze the exchange of class Ii-associated invariant c hain (li) peptide (CLIP) for cognate peptide during biosynthesis. In DM-neg ative cells HLA-DR3 molecules have been shown to predominantly present CLIP and to lack the DR3-specific mAb epitope 16.23, which has led to the assum ption that CLIP prevents binding of mAb 16.23. In the present study we show that CLIP does not prohibit 16.23 epitope expression, but that the formati on of this epitope is directly influenced by interactions of the DR molecul e with ii and DM. Detergent solubilized DR3 from wild-type as well as DM- c ells bound CLIP in a 16.23(+) mode. On cells, however, neither CLIP nor ant igenic peptide bound to DR3 in a 16.23(+) conformation, unless HLA-DM was e xpressed. Thus, HLA-DM appears to alter the conformation of DR3 in a peptid e-independent fashion. Since in DM-deficient cells that also lack ii, DR3 m olecules assembled in a 16.23(+) conformation, we conclude that during bios ynthesis ii and DM exert opposing conformational constraints, characterized by suppressing or releasing 16.23 epitope expression. These results imply that DR3/peptide complexes, including DR3/CLIP, can exist in two conformati ons depending on previous interaction with DM, but independent of the natur e of the peptide bound. We show that these naturally occurring class II con formers can be selectively recognized by T cells.