Deletion of a C-terminal sequence of the class II-associated invariant chain abrogates invariant chains oligomer formation and class II antigen presentation.

The MHC class II-associated invariant chain (Ii) is involved in Ag processing and presentation. Physical association of MHC class II molecules with Ii and an effect of Ii on peptide loading to class II have been demonstrated, but to date these functions have not been related to a particular region of Ii. We investigated luminal deletion mutants of Ii and their role in Ag processing and presentation. IAk-expressing L cells were transfected with deletion mutants of the Ii gene and assayed for their ability to present hen egg lysozyme to three different T cell hybridomas. It is shown that the sequence aa 131-191 of Ii is important for the presentation of native hen egg lysozyme. In addition, this C terminal region is shown to be responsible for Ii oligomer formation. It is therefore conceivable that oligomer formation of Ii is a prerequisite for class II-restricted Ag processing and presentation.

The segment of invariant chain that is critical for association with major histocompatibility complex class II molecules contains the sequence of a peptide eluted from class II polypeptides.

Major histocompatibility complex class II molecules present peptides from an extracellular source of antigens to CD4+ T lymphocytes. The class II-associated invariant chain affects this role of alpha and beta polypeptides by restriction of peptide loading to endocytic vesicles. Up to now no specific portion of the invariant chain has been defined as the class II binding site. We constructed recombinant invariant chain genes and inspected association of the mutant invariant chains with class II polypeptides. Here we demonstrate that an extracytoplasmic sequence of the invariant chain (aa 81-109) that is only 23 residues away from the transmembrane region is essential for contact with class II polypeptides, whereas the remaining C-terminal part is dispensable for binding. The sequence of invariant-chain-derived peptides that were eluted from class II molecules is contained in this segment and may define the class II binding site of the invariant chain. The membrane-proximal position of this region suggests that the invariant chain and invariant-chain-derived peptides isolated from class II molecules bind to a domain distinct from the class II pocket.