Antibody 12‐15 cross‐reacts with mouse Feγ receptors and CD2: study of thymus expression, genetic polymorphism and biosynthesis of the CD2 protein

Abstract

Previously we described a monoclonal antibody (mAb 12–15) that reacted with murine Fc receptor proteins (betal, beta2 and alpha) and an undefined molecule of 37 kDa (beta3) on certain types of cells. Here we present serological and biochemical evidence that the beta3 chain is expressed on mouse thymocytes and that it is identical to the CD2 antigen. By immunofluorescence staining and cytofluorographic analysis >95% of thymocytes were positive. Brightly staining cells coincided with cortison-resistant thymocytes suggesting that mature thymocytes expressed higher levels of the antigen. Biosynthetic labeling of DBA/2 thymocytes with [³⁵S]methionine showed that the size of the CD2 precursor molecule was 43 kDa which was processed to approximately 55–65 kDa in the mature molecule. mAb 12–15 was also reactive with the tunicamycin-treated form of the CD2 antigen suggesting that the cross-reactive epitope was of protein nature. Comparison of different mouse strains indicated that two molecular forms of CD2 exist. On BALB/c thymocytes, the relative mass of the native molecule was approximately 60–70 kDa (CD2.1) and slightly larger than in DBA/2 (CD2.2). Following endoglycosidase F treatment both proteins still showed a slight difference in electrophoretic mobility. Several inbred mouse strains were analyzed for expression of CD2 forms. When mAb 12–15 was used in cyotoxic T lymphocyte inhibition experiments using specific CTL and tumor target cells it was found that the antibody could specifically inhibit CTL-mediated lysis presumably by interfering with CD2 function.