Arginine 45 is a major part of the antigenic determinant of human beta 2-microglobulin recognized by mouse monoclonal antibody BBM.1.

Monoclonal antibody BBM.1 (Brodsky, F.M., Bodmer, W. F., and Parham, P. (1979) Eur. J. Immunol. 9, 536-545) identifies an antigenic determinant of human beta 2-microglobulin (beta 2-M). The antibody binds free and HLA-A,B-associated beta 2-M with similar affinity, showing that the BBM.1 antigenic determinant does not involve residues of beta 2-M that interact with HLA-A,B heavy chains. Peptides (SWH.1-5) synthesized from residues 35-50 of the beta 2-M sequence specifically inhibit the binding of BBM.1 to cell surfaces. Their inhibitory activity is destroyed by trypsin treatment. The observations (i) that BBM.1 does not bind to beta 2-M of species other than man, gorilla, and chimpanzee and (ii) that arginine 45 is the only human-specific residue between positions 35 and 50 suggested that this residue might be part of the BBM.1 antigenic determinant. This hypothesis was confirmed by reversible modification of arginine residues with cyclohexanedione. Modification of arginines in native beta 2-M and of the single arginine, corresponding to position 45, in the peptide SWH.5 resulted in up to 95% loss of BBM.1 inhibitory activity. Reversal of the modification by treatment with hydroxylamine resulted in complete recovery of activity. Rabbit antibodies elicited by immunization of SWH.5 conjugated to bovine serum albumin showed no detectable reaction with native beta 2-M but did specifically react with human beta 2-M after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electrophoresis onto nitrocellulose. These results thus identify a region around residue 45 of the beta 2-M polypeptide which is exposed to the environment and not involved in binding HLA-A,B heavy chain. Analysis of the beta 2-M sequence by calculating local hydrophilicity indices (Hopp, T. P., and Woods, K. R. (1981) Proc. Natl. Acad. Sci. U. S. A. 78, 3824-3828) agree with this region being a major antigenic determinant. Models of beta 2-M structure as an immunoglobulin domain show this region of polypeptide to be part of a loop between the two layers of beta-pleated sheet, also consistent with it being a major antigenic determinant. The position of the loop favors a model in which beta 2-M interacts with HLA across the four-stranded beta-pleated sheet like an immunoglobulin constant region domain.