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 c hains occurs as a concurrent process with the release of the associate d invariant chain (I(i)) by cathepsin B. I(i) was digested by cathepsi n B from solubilized, MHC class II alpha,beta I(i) complexes in the pr esence of N-hydroxysuccinimidyl-4-azidobenzoate-conjugated, I-125-labe led, influenza virus matrix (18-29) peptide. The peptide was crosslink ed where it became bound. This HLA-DRI-restricted peptide bound about three times more efficiently to class II alpha,beta chains of DRI-posi tive B cells when present during cathepsin B digestion of I(i) than wh en added afterward, also at pH 5.0. Binding was competed by similarly DR-restricted peptides. Cathepsin D cleaved I(i) but did not enhance p eptide binding. However, a trace level of cathepsin D, added to the as say for peptide binding in the presence of cathepsin B, further enhanc ed peptide binding about three times. These experiments support an hyp othesis for the staged release of I(i) fragments by cathepsin D and ca thepsin B, catalyzing at one point the insertion of a peptide into the antigen binding site formed by class II alpha and beta chains.