Direct binding of a soluble natural killer cell inhibitory receptor to a soluble human leukocyte antigen-Cw4 class I major histocompatibility complex molecule.

Natural killer (NK) cells expressing specific p58 NK receptors are inhibited from lysing target cells that express human leukocyte antigen (HLA)-C class I major histocompatibility complex molecules. To investigate the interaction between p58 NK receptors and HLA-Cw4, the extracellular domain of the p58 NK receptor specific for HLA-Cw4 was overexpressed in Escherichia coli and refolded from purified inclusion bodies. The refolded NK receptor is a monomer in solution. It interacts specifically with HLA-Cw4, blocking the binding of a p58-Ig fusion protein to HLA-Cw4-expressing cells, but does not block the binding of a p58-Ig fusion protein specific for HLA-Cw3 to HLA-Cw3-expressing cells. The bacterially expressed extracellular domain of HLA-Cw4 heavy chain and beta2-microglobulin were refolded in the presence of a HLA-Cw4-specific peptide. Direct binding between the soluble p58 NK receptor and the soluble HLA-Cw4-peptide complex was observed by native gel electrophoresis. Titration binding assays show that soluble monomeric receptor forms a 1:1 complex with HLA-Cw4, independent of the presence of Zn2+. The formation of complexes between soluble, recombinant molecules indicates that HLA-Cw4 is sufficient for specific ligation by the NK receptor and that neither glycoprotein requires carbohydrate for the interaction.

Proteolysis of major histocompatibility complex class II-associated invariant chain is regulated by the alternatively spliced gene product, p41.

Invariant chain (Ii) is an intracellular type II transmembrane glycoprotein that is associated with major histocompatibility complex class II molecules during biosynthesis. Ii exists in two alternatively spliced forms, p31 and p41. Both p31 and p41 facilitate folding of class II molecules, promote egress from the endoplasmic reticulum, prevent premature peptide binding, and enhance localization to proteolytic endosomal compartments that are thought to be the sites for Ii degradation, antigen processing, and class II-peptide association. In spite of the dramatic and apparently equivalent effects that p31 and p41 have on class II biosynthesis, the ability of invariant chain to enhance antigen presentation to T cells is mostly restricted to p41. Here we show that degradation of Ii leads to the generation of a 12-kDa amino-terminal fragment that in p41-positive, but not in p31-positive, cells remains associated with class II molecules for an extended time. Interestingly, we find that coexpression of the two isoforms results in a change in the pattern of p31 degradation such that endosomal processing of p31 also leads to extended association of a similar 12-kDa fragment with class II molecules. These data raise the possibility that p41 may have the ability to impart its pattern of proteolytic processing on p31 molecules expressed in the same cells. This would enable a small number of p41 molecules to modify the post-translational transport and/or processing of an entire cohort of class II-Ii complexes in a manner that could account for the unique ability of p41 to enhance antigen presentation.