Evaluation of in vivo and in vitro effectivity of immune defense against a spontaneously arising, nonlymphoid rat tumor. I. Analysis of natural immune defense

Interleukin-1 produced by tumorigenic fibroblasts influences tumor rejection

Oncogene-transformed BALB/c-3T3 fibroblasts which spontaneously or upon immune-activation with cytokines and lipopolysaccharide (LPS) generate IL-1 alpha, were tested for their tumorigenicity as well as their interaction with natural immune defense by NK cells and macrophages. Oncogene-transformed fibroblasts were weakly tumorigenic, since not all mice developed tumors despite application of high doses of tumor cells. This was independent of the immune status of the host. However, in the immunocompetent host those transformed fibroblast lines which spontaneously produced IL-1 alpha grew only transiently and then regressed. After induction of IL-1 alpha production, a decrease in the rate of tumor take was noted and the rate of regression of developing tumors was increased. Regression of IL-1-producing transformed fibroblasts was strongly reduced but not completely abolished in sublethally irradiated mice. This indicated that IL-1 production may predominantly influence T-cell-mediated defense, but some influence on non-adaptive immunity could not be excluded a priori. IL-1 production did not influence susceptibility of transformed fibroblasts towards NK cells and macrophages. However, IL-1-producing transformed fibroblasts were most potent stimulators of NK cells and macrophages, the stimulatory effect being locally restricted. In conclusion, IL-1 producing, oncogene-transformed fibroblasts which generated the cytokine constitutively or upon immune-activation, were rejected from the tumor-bearing host following initial growth. Fibroblast-induced local activation of NK cells and macrophages was shown to play some role in tumor graft rejection. The influence of IL-1 production of transformed fibroblasts on T-cell-mediated defense is addressed in the accompanying report.

IFN-treatment of B16-F1 versus B16-F10: relative impact on non-adaptive and T-cell-mediated immune defense in metastatic spread

Treatment of tumor cells with interferon-gamma (IFN) frequently reduces their susceptibility towards NK cells and results in augmented expression of MHC antigens, which may increase immunogenicity of tumor cells. Depending on the relative strength of these opposing effects, i.e. escape from non-adaptive immune defense versus facilitated activation of T-cell-mediated defense, IFN-treatment may be beneficial or disadvantageous for the tumor-bearing host. This is demonstrated for the variants F1 and F10 of the B16 melanoma, which differ in metastasizing capacity. IFN-treatment of B16-F1 melanoma cells significantly reduced susceptibility towards non-adaptive immune defense, and increased metastasizing potential. On the other hand, H2K antigen expression was augmented by a factor of 50; concomitantly, lysability by CTL was increased, together with the number and expansion rate of cytotoxic T-cell precursors (CTLp) recruited after immunization with IFN-treated B16-F1. The benefit of increased antigenicity and immunogenicity outweighed the disadvantage or reduced susceptibility towards non-adaptive immune defense. B16-F10 cells were less susceptible to NK cells, expression of MHC antigens was found to be stronger and they were more immunogenic than B16-F1 cells. After IFN-treatment, susceptibility to non-adaptive immune defense was further reduced. Expression of MHC antigens as well as antigenicity and immunogenicity were only moderately augmented. As a consequence, the decreased susceptibility to non-adaptive immune defense was dominating in the tumor bearing host and could not be counterbalanced by immunization with IFN-treated B16-F10 cells. We interpret these data to show that a precise knowledge of the relative decrease in susceptibility to non-adaptive immune defense, the relative increase in MHC antigen expression, antigenicity and immunogenicity may allow a more precise prognosis of the influence of IFN on metastatic capacity in the B16 system, and eventually also in a clinical therapeutic regimen.

Interferon-gamma treatment of B16 melanoma cells: opposing effects for non-adaptive and adaptive immune defense and its reflection by metastatic spread

The impact of interferon-gamma (IFN) treatment of tumor cells on non-adaptive and adaptive immune defense and its reflection by metastatic spread were evaluated using a weakly metastasizing variant of B16 melanoma (B16-FI). Treatment of B16-FI with IFN resulted in a decrease in binding structures for NK cells and concomitantly in augmented metastasizing capacity. In line with this, activation of NK cells and Mo, which led to reduction of metastatic nodes, was less efficient with IFN-treated B16-FI, while after elimination of non-adaptive immune defense, the number of metastases increased significantly, but irrespective of IFN treatment. On the other hand, IFN-treated B16-FI cells become more prone to killing by cytotoxic T-cells (CTL). This was due to increased lysability by CTL and to increased immunogenicity; i.e., a higher frequency of B16-specific CTL was observed after immunization with IFN-treated than with untreated B16-FI. The reverse phenomenon was observed with anomalous and/or lymphokine-activated killer cells (AK/LAK). The common cause of increased antigenicity and immunogenicity may reside in increased expression of class-I and de novo expression of class-II MHC antigens after IFN treatment. Increased antigenicity and immunogenicity of IFN-treated B16-FI was reflected by significant reduction of metastatic nodes, prolonged survival and increased TD100 in animals immunized with IFN-treated vs. untreated melanoma cells. Comparison of the divergent effects of IFN treatment on B16-FI melanoma cells showed that the benefit of increased antigenicity/immunogenicity clearly outweighed the disadvantage of reduced susceptibility to non-adaptive immune defense.

Acquired resistance towards immune defense during metastatic progression represents a secondary phenomenon

The role of natural immune defense in the control of metastatic spread of tumor cells was evaluated by adapting an s.c.-grown, antigenic, but non-immunogenic rat fibrosarcoma (Bsp6S) to ascitic growth (BSp6A) in the strain of origin (BDX), where peritoneal cells display a high level of NK and macrophage activity. Parallel tests were performed to determine whether tumor cells can selectively adapt to non-specific immune defense, i.e. whether antigenicity of the non-immunogenic tumor remains unaltered, and whether this is accompanied by metastatic progression. During adaptation to ascitic growth the tumor line gradually lost susceptibility to NK cells and macrophages. This was due to the appearance of an increasing number of resistant clones (BSp6S, none; BSp6A, 50% of clones), which had lost binding structures for NK cells and macrophages. No alteration could be observed in antigenicity of the BSp6A variant as revealed by clonal analysis using LD of CTL. Neither BSp6S nor BSp6A cells metastasized. When the ascitic variant was retransplanted s.c. (BSp6AS), susceptibility to NK cells and macrophages was further decreased, in the sense that all clones of the BSp6AS variant became resistant. Furthermore, the BSp6AS variant had lost some of the tumor-associated antigens (TAAs) found on BSp6S and A variants. More important, upon s.c. transplantation BSp6AS regularly metastasized to the draining LN, contrary to BSp6S and BSp6A. When locally growing tumors were excised, rats implanted with BSp6AS frequently died with metastatic tumor burdens in LNs and lung, while other animals survived after excision of BSp6S or BSp6A. The data indicate a correlation between resistance to lysis, morphology and metastatic capacity. But the initial loss of susceptibility to NK cells and macrophages during adaptation to ascitic growth, which can be considered as an escape mechanism, was not accompanied by increased metastatic capacity. Hence, we suppose that with respect to metastatic progression of the BSp6AS variant, resistance to lysis by loss of binding structures represents only a secondary, but not a causative, element.