The murine class I molecules are a family of cell-surface glycoproteins that includes the transplantation antigens H-2K, H-2D, and H-2L, as well as the lymphoid differentiation antigens Qa-1, Qa-2, Qa-3, and thymus leukemia

The murine class I molecules are a family of cell-surface glycoproteins that includes the transplantation antigens H-2K, H-2D, and H-2L, as well as the lymphoid differentiation antigens Qa-1, Qa-2, Qa-3, and thymus leukemia (TL) 1 antigen. These antigens are encoded in the major histocompatibility complex (MHC) on chromosome 17 (Fig. 1). Class I molecules have molecular masses of 40-45 kilodaltons (kD), and associate noncovalently with/32-microglobulin. The transplantation antigens are important cell-cell recognition molecules for cytotoxic T lymphocytes (1). The function of the Qa/TL antigens is unknown, but their presence on lymphoid cells, coupled with their homology to transplantation antigens, suggests that they too may be involved in cell-cell interactions in the immune system. TL antigen is a class I molecule of special interest because of its varied patterns of expression in both normal and leukemic cells. Transplantation antigens are expressed on virtually all somatic cells, while TL antigens are expressed only on thymocytes, some thymic leukemias, and activated T lymphocytes (2-4). In prothymocytes, TL antigen expression is induced in response to thymic hormones during the maturation of these cells into thymocytes (5, 6). Mature T cells that migrate to the peripheral lymphoid system no longer express detectable TL antigen, except when stimulated to proliferate (4). Six seroiogically defined alleles of Tla exist, and mice having the three most commonly studied alleles, Tla a, T la b, and Tla ~, differ in the quantity of TL antigen expressed on their thymocytes, as well as in serological determinants (7, 8). Thymocytes from Tla ~ mice express about 20 times as much TL antigen as those from Tla c mice, while thymocytes from Tla b mice are generally considered to be TL-; however, one report (8) suggests that Tla b thymocytes may express very low amounts of TL antigen. In contrast, leukemias of all three strains may express TL antigen at levels comparable with Tla" thymocytes. In the past four years, our understanding of class I genes has increased dramatically due to the isolation of complementary DNA (cDNA) clones from both human and mouse class I genes (9-11). Steinmetz et al. (12, 13) used a class