Cell surface immunoglobulins of murine plasma cell tumors

Prostaglandins, but not tumor-derived IL-10, shut down concomitant tumor-specific CTL responses during murine plasmacytoma progression

IL-10 is assumed to be a major immunosuppressive factor produced by most B-cell tumors. The immunosuppressive role of tumor-derived IL-10 was analyzed using the MHC class II-negative BALB/c plasmacytoma ADJ-PC-5 as a model tumor. Immune monitoring of tumor-bearing mice was based on the measurement of tumor burden, tumor-specific CTL cytotoxicity and intracellular cytokine staining using FACS. ADJ-PC-5 tumor progression in syngeneic recipients is associated with strong, concomitant, tumor-specific CTL responses during early stages of tumor progression which are sufficient to cause rejection of small s.c. autologous test tumors. These initial CTL responses gradually decline during later tumor stages. Blocking of IL-10 in vivo did not abolish CTL suppression or retard tumor growth. More strikingly, application of anti-IL-10 antibodies during early tumor stages abrogated CTL induction and markedly accelerated tumor growth. In contrast to anti-IL-10 treatment, application of cyclo-oxygenase inhibitors to ADJ-PC-5 tumor-bearing mice led to enhanced tumor-specific CTL responses throughout all stages of tumor progression, paralleled by retarded tumor growth and a significantly delayed onset of suppression. Both findings contradict a dominant immunosuppressive role of IL-10 during B-cell tumor progression. Tumor-derived IL-10 must therefore be considered an immunostimulating factor, which accounts for the high immunogenicity of B-cell tumors, whereas prostaglandins, which are not produced by the tumor cells themselves, are the dominant immunosuppressors in this system.

CD8+ tumor-specific Tc cells primed in vivo or in vitro against the BALB/c plasmacytoma ADJ-PC-5 use the same TcR V beta families but display distinct TC1 or TC2 characteristics

The involvement of counteractive CD8+ T cell subsets in tumor-specific unresponsiveness was analyzed in a syngeneic murine tumor model. CD8+ cytotoxic T cells against the IL-10 producing BALB/c plasmacytoma ADJ-PC-5 can be easily induced in vitro, in a primary syngeneic mixed lymphocyte tumor cell culture (MLTC), or in vivo, by repeated immunization of syngeneic BALB/c mice with high doses of X-irradiated ADJ-PC-5 tumor cells. Long term cultivated CD8+ ADJ-PC-5-specific Tc lines use either TcR of the V beta 6 or V beta 8.1/8.2 type, irrespective if the lines were derived from a primary MLTC or from immunized mice. While most of the Tc lines produce type-1 cytokines (IFN-gamma, no IL-4) upon stimulation, at least two of them, which were derived from a primary MLTC, display a type-2 cytokine spectrum (IL-4, no IFN-gamma). The primary in vitro Tc response against ADJ-PC-5 cells shows characteristics of a TC2 response: CD8+ Tc cells which are induced in a primary MLTC do not produce IFN-gamma, and the tumor-specific Tc response is enhanced by IL-4 but suppressed by IFN-gamma or IL-12. In contrast, ADJ-PC-5-specific CD8+ Tc cells from immunized mice are IFN-gamma producing TC1 cells. Since the primary in vitro Tc response against the tumor is suppressed even by lowest numbers of irradiated ADJ-PC-5-specific TC1 cells via IFN-gamma, these TC1 cells behave similar to a previously described regulatory subset of IFN-gamma producing CD8+ T cells, which are induced during early stages of ADJ-PC-5 tumorigenesis and inhibit the induction of a tumor-specific Tc response from naive BALB/c spleen cells in vitro.

Regulation of immune responses against the syngeneic ADJ-PC-5 plasmacytoma in BALB/c mice. IV. Tumor-specific T suppressor cells, induced at early stages of tumorigenesis, act on the induction phase of the tumor-specific cytotoxic T cell response

The BALB/c plasmacytoma ADJ-PC-5 induces specific non-cytolytic T suppressor (Ts) cells under experimental conditions which closely resemble early stages of tumorigenesis. The phenotype of these Ts cells is Thy-1.2+, Lyt-1.2-, Lyt-2.2+, I-Ad-. They inhibit the induction of the cytotoxic T cells in a primary mixed lymphocyte tumor culture (MLTC) against the ADJ-PC-5 plasmacytoma. Specificity of suppression can be demonstrated by testing them in MLTC against a panel of syngeneic control tumor. Only the induction phase of a primary MLTC can be suppressed, and within 24 h of MLTC, cytotoxic T (Tc) cells become resistant towards suppression. ADJ-PC-5-specific Tc cell clones are also resistant to suppression. The fact that Ts cell-mediated specific suppression can be overcome by addition of semipurified IL2 to the MLTC suggests that Th cells, and not Tc cell precursors, are the target. The findings are discussed with respect to the potential use of Tc effector cells in cancer therapy.