Murine lymphoma-induced immunosuppression: requirement for direct tumour cell contact

Subpopulations of suppressor cells in chickens infected with cells of a transplantable lymphoblastic leukemia

Three distinct subpopulations of cells with suppressor activity were separated by Ficoll density gradient centrifugation methods from the spleens of 3- to 4- week-old chickens infected with cultured lymphoblastoid cells (JM-VLC), derived from JM-V leukemia: (i) a subpopulation of nonadherent cells, which separated in the T-cell-rich gradient fraction of leukemic chicken spleen, inhibited proliferative responses to concanavalin A in mixed cultures with normal chicken spleen cells; (ii) phagocytic cells (macrophages), which were the most effective suppressor cells of all subpopulations in the mixed culture assay, were recovered among the cells of greatest density in the spleens of both normal and leukemic chickens; (iii) JM-VLC cells in the buoyant gradient fractions of leukemic chicken spleens also effected suppression. In the later stages of lymphoproliferative disease, the number of spleen cells of buoyant density was increased, apparently as a result of infiltration of the spleens with JM-VLC cells.

Suppression of T cell-mediated immunity by tumor cells: immunogenicity versus immunosuppression and preliminary characterization of the suppressive factors

In studying the immunogenicity of spleen cells and tumor cells in the generation, of cytotoxic T lymphocyte (CTL) in the allogeneic mixed lymphocyte culture (MLC) or mixed lymphocyte tumor cell culture (MLTC) reactions, we have found that the tumor cells not only appear to be poorly immunogenic, but are also immunosuppressive. This was shown by the ability of the tumor cells or their cell-free extracts to suppress standard MLC reactions. This suppression was acting mainly at the induction phase of the cytotoxic response. It could not interfere with the killing activity of the fully generated CTLs. In a Friend virus-induced leukemia FBL-3 system, at least two major components could be attributed to the cause of immunosuppression; one was of viral origin and the other was of non-viral origin. The viral component was sensitive to UV-irradiation and could be pelleted after ultracentrifugation at 100,000 g. The non-viral component was UV-resistant and was retained in the supernatant fraction after ultracentrifugation. Friend virus and 12 commonly found murine viruses have been excluded as the possible candidates causing the immunosuppression. The immunosuppressive viruses are very likely of endogenous origin and are defective in replication as shown by electromicroscopy, and by the virus focus-inducing and reverse transcriptase assays. These findings indicate that probably all tumor cells possess the immunosuppressive factor(s) which may account for their apparent lack of immunogenicity and the lack of proper immune responses in the tumor-bearing hosts.