A new B-cell alloantigen, TB21, coded for in the HLA-D/DR region

A new B-cell alloantigen, designated as TB21, was serologically defined by 2 monospecific sera, T383 and T2200. Both T383 and T2200 had cytotoxic titers of 1:8 against TB21 positive B cells and were highly correlated to each other (r = 0.876). The phenotype frequency of TB21 was approximately 55% in the Japanese population. TB21 was strongly associated with DRw9 and weakly with DR5, but was definitely distinct from the known supertypic specificities such as MT or MB antigens. Family studies showed that TB21 segregated with different DR antigens on different haplotypes, indicating that TB21 is coded for by a gene at a closely linked but distinct locus from HLA-DR. Furthermore, TB21 was preferentially detected on B cells but not on monocytes by complement-dependent cytotoxicity. Failure of detection of TB21 on monocytes further distinguished the TB21 antigen from DR and MT antigens. Results of this study suggest that there exist 2 kinds of B-cell alloantigens coded in the HLA region: DR and MT antigens, and other B-cell alloantigens represented by TB21.