IMMUNOBIOLOGICAL CHARACTERIZATION OF THE MURINE LB LEUKEMIA AND THE LBC CELLULAR-LINE

LB leukemia is a nonimmunogenic T cell tumor which spontaneously arose in a BALB/c mouse; efforts to induce immunological rejection of the l eukemic cells have always failed. The leukemic cells grow rapidly and progressively in the syngeneic host invading spleen, lymph nodes and l iver. A cell line (LBC) was developed from the original tumor. Both th e original tumor and the cell line have been characterized as expressi ng the Thy 1+, CD3-, CD25+, MHC class I+, class II-, CD4- (original tu mor), CD4+ (cell line), CD8+, gp70-, J11d.2+ phenotypes. Immunization of syngeneic mice with irradiated LBC cells induced cytotoxic T lympho cytes as well as anti-LBC antibodies which reacted with components of 14, 16 and 27 kDa present on LB tumor cells, LBC cell line and normal thymocytes but not on normal lymph node cells. Immunization of syngene ic mice with LBC cells partially protected them against subsequent cha llenge with the original tumor cells. The effect of sera from tumor-be aring mice and the super-natants from short term cultures were studied on cell proliferation. An inhibitory activity was demonstrated in the se fluids, which was abrogated by addition of exogenous IL-2. ELISA sh owed the presence of soluble IL-2R alpha chain both in the conditioned medium as well as in the serum, which was demonstrated to be responsi ble for the inhibitory activity. The soluble IL-2R was produced by LB leukemic cells and exerted the inhibitory activity blocking cell proli feration and modulating immune response by binding to free IL-2. Using reverse-transcription PCR, mRNA for IL-2 was found to be present in t umor cells. Our findings indicate that LB cell proliferation is mediat ed by an autocrine pathway involving endogenous IL-2 generation, despi te the fact that these cells are not dependent on exogenous IL-2 to gr ow in culture. The relationship between tumorigenicity and expression of MHC class II was also investigated. In vitro treatment with IFN gam ma failed to induce the expression of class II antigens in LBC cell li ne. Therefore these cells were tri-transfected by a liposome-mediated protocol with I-A alpha(d), I-A beta(d) genes and pSV2neo. Cells were selected to grow in medium containing Genetecin (G418) and surviving t ransfectants were cloned. Three I-A+ clones were obtained (LBCT) and w ere used to induce a specific CTL response against tumor cells. Syngen eic mice inoculated with 10(3) LBCT cells failed to develop a tumor wh ile the DT50 of mice injected with 10(6) LBCT cells was three times th e value for mice injected with LBC cells (I-A-). It is suggested that neoexpression of MHC class II molecules enhances anti-tumor response b y transforming tumor cells into professional antigen-presenting cells, which may be used to improve tumor-specific immunity in the autologou s host.