Chemical reactivity of an HLA-B27 thiol group

Techniques have been developed to measure the reactivity of free thiols in the HLA class I antigen-binding cleft. HLA-B27, which sequencing predicts has a free cysteine at position 67, reacts rapidly with the positively charged thiol reagent monobromotrimethyl-ammoniobimane bromide (qBBr) to give products which are identifiable by isoelectric focusing. HLA-B38, B39, B64 and B65, all of which have a similar Cys 67, react less strongly. Several other class I molecules, notably HLA-C antigens, are reactive in this system, and it may be capable of recognizing subtypes such as A*0207 which also carry free cysteine. The accessibility of thiol to qBBr depends both on the chemistry of the class I molecule and other factors in the cell. Two human cell lines which are known to carry identical B27 genes but do not present the same peptides, differ considerably in the accessibility of their B27 thiol. Evidence from mouse cells transfected with mutant B27 genes suggests that a unique lysine at position 70 in the wild-type molecule increases reactivity to thiol-reactive metabolites. The failure of B27 to give a complete reaction with qBBr in our model systems suggests that it can exist in more than one chemical form. This may leave the molecule susceptible to oxidation, causing errors in T cell recognition and an exaggerated inflammatory response.