The expression of immune response–associated (la) antigens on the surface of mouse strain GR (H-2 dx) ascites leukemia (GRSL) cell lines was studied by cytotoxic tests, immunofluo–rescence, and immunoprecipitation assays. la expression varied among the three GRSL cells lines (GRSL 2, GRSL 14, and GRSL 15) studied by cytotoxic assay. GRSL 14 cells showed the strongest expression of la antigens among these three cell lines. A time-course study of tumor growth in mice revealed that Ia antigens on the tumor cells demonstrated the strongest expression 10 days after injection of GRSL cells into GR mice, and that subsequently it decreased until the death of the animal. Cells treated with neuraminidase exhibited more readily detectable la antigens, especially in the late stages of leukemia, which suggested that la antigens had been masked by sialic acid. Immunoprecipitation studies revealed that Ia molecules on the leukemia cell had the same molecular weight as those on the normal lymphocytes. Immunofluorescence studies disclosed that la antigens were distributed diffusely on the surface of the tumor cells. The I (immune response) region of the mouse major histo-compatibility complex codes for gene(s) controlling a series of lymphocyte membrane alloantigens (la) and many immunologically related functions, including genetically controlled immune response (2, 22), graft-versus-host reactivity (16), and mixed lymphocyte reactivity (1). Ia determinants are expressed principally on lymphocytes but also are found on macrophages, sperm, and epidermal cells (12). Although during the period when Ia antigens initially were being described there was some controversy about the expression of Ia determinants on T cells (9, 11, 20), currently, there is unanimous agreement that la antigens are expressed on at least a subset of T cells as well as on most B cells (6, 10). I region determinants expressed exclusively on T cell subpopulations also have been described (17, 23). Previous studies on tumor cells by direct cytotoxic assays suggested some expression of la antigens, but results of absorption testing were negative (David, unpublished data). Tumor cell line S1 (BALB/c, H-2d), a lymphocytic lymphoma, was shown to be positive for Ia.7 by both direct tests and absorption analyses (5). Further characterization of Ia antigens on these tumor cells, however, became very difficult because of interference by murine leukemia virus (MuLV) antigens that are expressed on these cells and detected by the alloantisera, which contain anti-MuLV antibodies on almost all cases. Screening of several AKR (H-2k) thymomas also revealed reactivity caused by MuLV antigens (25). In these studies, we used lymphatic leukemia cells maintained as ascites cells in the strain GR (H-2dx), which has a high incidence of mammary cancer and an incidence of about 20% thymic lymphomas in male mice. GR ascites leukemia (GRSL) cells have the advantage that they lack detectable MuLV antigen (13), although the Gix antigen is present (14). Furthermore, these cells have been characterized extensively for other cell surface markers (15), which would be of value in ascertaining their relationship to la antigen. We tested to see whether all of the individual Ia specificities expressed on normal GR strain lymphocytes are expressed also on GRSL tumor cells. For further confirmation of the expression of Ia antigens on the surface of tumor, immunoprecipitation assays and immunofluorescence techniques were also used.
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