1082 STRUCTURAL HOMOLOGY BETWEEN Qa - 2 AND H - 2

The murine 17th chromosome contains two regions of particular interest to biologists , the Tit complex and the major histocompatibility complex (MHC). 1 The T/t complex controls a number of loci that have important functions in embryonic development (1); the MHC encodes gene products that are involved in cellular interactions critical for immune function (2). A third region, Tla, located telomeric to H-2D (3), has recently been described. The Tla region controls the expression of several serologically defined lymphoid cell surface alloantigens termed TL and Qa antigens (3). At present, five Qa antigens (Qa-l-5) have been described and the expression of these antigens appears to be confined primarily to hematopoietic cells (4-7). Earlier studies (3-12) indicated that Qa antigens were found predominantly on T cells but that Qa-1, Qa-2, Qa-3, and Qa-4 are expressed on mitogen-stimulated B cells as well. More recent studies have demonstrated that Qa-2 is expressed on a variety of hematopoietic cells, including subsets of B cells, myeloid progenitor cells, and multipotential stem cells (13). In addition, Qa-4 and Qa-5 antigens are expressed on natural killer cells (14). Antisera recognizing Qa-1 or Qa-2 antigens define cell surface glycoproteins of 44,000 mol wt that are noncovalently associated with 8-2 microglobulin (lfi, 16). Thus, Qa-1 and Qa-2 may be related to H-2 alloantigens at the primary structural level. In the present report, we describe studies of the Qa-2 antigen at the primary structural level. Isotopically labeled Q a-2, H-2K, and H-2D antigens from lysates of mitogen-stimulated C57BL/6 spleen cells were immunoprecipitated with congenic alloantisera. Comparative mapping of the tryptic peptides obtained from Qa-2 and H-2 molecules indicate that these two molecules have different primary structures, but that there is considerable structural homology between them. The data suggest, therefore, that the genes encoding the Qa-2 and H-2 molecules have arisen from a common primordial gene.