CHEMICAL-REACTIVITY OF AN HLA-B27 THIOL-GROUP

Techniques have been developed to measure the reactivity of free thiol s in the HLA class I antigen-binding cleft. HLA-B27, which sequencing predicts has a free cysteine at position 67, reacts rapidly with the p ositively charged thiol reagent monobromotrimethyl-ammoniobimane bromi de (qBBr) to give products which are identifiable by isoelectric focus ing. HLA-B38, B39, B64 and B65, all of which have a similar Cys 67, re act less strongly. Several other class I molecules, notably HLA-C anti gens, are reactive in this system, and it may be capable of recognizin g subtypes such as A0207 which also carry free cysteine. The accessib ility of thiol to qBBr depends both on the chemistry of the class I mo lecule and other factors in the cell. Two human cell lines which are k nown to carry identical B27 genes but do not present the same peptides , differ considerably in the accessibility of their B27 thiol. Evidenc e from mouse cells transfected with mutant B27 genes suggests that a u nique lysine at position 70 in the wild-type molecule increases reacti vity to thiol-reactive metabolites. The failure of B27 to give a compl ete reaction with qBBr in our model systems suggests that it can exist in more than one chemical form. This may leave the molecule susceptib le to oxidation, causing errors in T cell recognition and an exaggerat ed inflammatory response.