Biochemical evidence for a separate, MHC-linked locus encoding H-2.28 antigens

In comparing the tryptic peptide maps of the H-2L and H-2D glycoprotein antigens isolated from NP-40 lysates of RADA1 (H-2 alpha) leukemic cells, no more than 37% of the observed arginine-containing tryptic peptides are found to be homologous. Thus, the primary amino-acid sequences of these two antigens are probably less than 90% homologous. This constitutes the strongest evidence to date that the MHC-linked H-2L region encodes H-2L antigens separately from the H-2D region, even though H-2L antigens bear D-end-associated antigenic determinants of the H-2.28 family. The anti-H-2.28 alloantiserum (k X r anti h2) used to precipitate H-2L antigens in this investigation was the NIH contract antiserum D28b. As the tryptic peptide maps also suprisingly revealed, D28b precipitates H-2D antigens as well and, thus, anti-H-2.4 immunoadsorbants were employed to isolate H-2L free of H-2D antigens. In light of the dual specificity of D28b, its reactivity with BALB/c-H-2dm2 mutant cells was re-examined. Even though mutant lymphocytes, which lack H-2L but not H-2D antigens, are not cytotoxically lysed by D28b (as are parental H-2d cells), D28b appears to precipitate H-2D antigens from NP-40 extracts of mutant splenocytes.