EXTENDED REPERTOIRE OF PERMISSIBLE PEPTIDE LIGANDS FOR HLA-B-ASTERISK-2702

Recognition of self peptides bound to the class I major histocompatibi lity complex molecule HLA-B27 is thought to trigger proliferation of a utoreactive T cells and result in autoimmune arthritic diseases. Previ ous work from other laboratories established that a predominant featur e of endogenous peptides eluted from purified B27 is an arginine at po sition 2. We studied the binding of peptides containing both natural a nd unnatural amino acids by the subtype HLA-B2702, with the goal of g aining insight into peptide binding by this B27 subtype that is associ ated with susceptibility to arthritic disease. A soluble form of B270 2 was depleted of endogeneous peptides. We tested the binding of pepti des substituted with cysteine, homocysteine, or an alpha-amino-epsilon -mercapto hexanoic acid side chain (Amh) instead of the naturally occu rring arginine at position 2, to determine whether the peptide sulfhyd ryl residue could be covalently linked to cysteine 67 in the B2702 bi nding cleft. Although none of the altered peptide sequences bound cova lently to B2702, the affinities of the homocysteine- and Amh-substitu ted peptides were close to that of the native peptide sequence. Substi tutions at position 2 with other side chains, such as glutamine and me thionine, also resulted in peptides that bound with only slightly redu ced affinity. These results demonstrate that peptide side chains other than arginine at position 2 can be accomodated within the B2702 pept ide binding site with only minor reductions in affinity. This extended repertoire of permissible B27-binding peptides should be taken into a ccount for a consideration of disease-associated peptide sequences.