Release of DR molecules from complexes with invariant chain through the formation of a C-terminal 25 kDa invariant chain fragment.

To investigate how class II major histocompatibility complex (MHC) molecules are released from complexes with invariant chain (Ii), we studied a 25 kDa Ii fragment (p25) detected by Western blotting in affinity chromatographed DR preparations. The p25 species corresponds to the non-transmembrane, C-terminal Ii fragment 107-232. It was determined by gel filtration chromatography that the p25 fragment has a relative molecular mass (M(r)) of 46 kDa, indicating that this Ii fragment is present as dimers in B cell lysate. Two independent approaches were followed to demonstrate that generation of the p25 fragment takes place shortly before, or concomitantly to, loading of class II MHC molecules with antigen fragments. First, it was shown that a fraction of the p25 molecules is resistant to endoglycosidase H digestion, indicating that the p25 polypeptide can exit the endoplasmic reticulum (ER) and is transported at least to the cis-Golgi compartment. Second, treatment of class II MHC-positive B cells with leupeptin blocks the formation of p25, further indicating that this Ii fragment is generated in the endosomal compartment. The role of the p25 Ii species in the assembly of complexes between peptides and DR molecules was then investigated. While the p25 fragment was totally unable to prevent binding of a synthetic tetanus toxin peptide to DR molecules, the full-length Ii species (p33/35) effectively inhibited peptide binding, indicating that, by contrast with the p33/35 species, the p25 fragment does not occlude the peptide binding site of DR molecules. We concluded that the p25 fragment, which is produced by proteolytic cleavage at the N-terminal side of Methionine 107, has a decreased affinity for DR molecules as compared with the p33/35 species. Dissociation of the p25 fragment from DR molecules exposes the peptide binding site, which is thus made accessible for antigen fragments. This model of the complexes between DR and antigen fragments proposes that a stretch of Ii prevents peptide binding by occluding the peptide binding site without directly occupying it.