MORE EFFICIENT PEPTIDE BINDING TO MHC CLASS-II MOLECULES DURING CATHEPSIN-B DIGESTION OF I(I) THAN AFTER I(I) RELEASE

The binding of a T cell-presented peptide to MHC class II alpha,beta chains occurs as a concurrent process with the release of the associated invariant chain (I(i)) by cathepsin B. I(i) was digested by cathepsin B from solubilized, MHC class II alpha,beta I(i) complexes in the presence of N-hydroxysuccinimidyl-4-azidobenzoate-conjugated, I-125-labeled, influenza virus matrix (18-29) peptide. The peptide was crosslinked where it became bound. This HLA-DRI-restricted peptide bound about three times more efficiently to class II alpha,beta chains of DRI-positive B cells when present during cathepsin B digestion of I(i) than when added afterward, also at pH 5.0. Binding was competed by similarly DR-restricted peptides. Cathepsin D cleaved I(i) but did not enhance peptide binding. However, a trace level of cathepsin D, added to the assay for peptide binding in the presence of cathepsin B, further enhanced peptide binding about three times. These experiments support an hypothesis for the staged release of I(i) fragments by cathepsin D and cathepsin B, catalyzing at one point the insertion of a peptide into the antigen binding site formed by class II alpha and beta chains.