NKG2D-deficient mice are defective in tumor surveillance in models of spontaneous malignancy.

Ligands for the NKG2D stimulatory receptor are frequently upregulated on tumor lines, rendering them sensitive to natural killer (NK) cells, but the role of NKG2D in tumor surveillance has not been addressed in spontaneous cancer models. Here, we provided the first characterization of NKG2D-deficient mice, including evidence that NKG2D was not necessary for NK cell development but was critical for immunosurveillance of epithelial and lymphoid malignancies in two transgenic models of de novo tumorigenesis. In both models, we detected NKG2D ligands on the tumor cell surface ex vivo, providing needed evidence for ligand expression by primary tumors. In a prostate cancer model, aggressive tumors arising in NKG2D-deficient mice expressed higher amounts of NKG2D ligands than did similar tumors in wild-type mice, suggesting an NKG2D-dependent immunoediting of tumors in this model. These findings provide important genetic evidence for surveillance of primary tumors by an NK receptor.

Critical Roles of Chemoresistant Effector and Regulatory T Cells in Antitumor Immunity after Lymphodepleting Chemotherapy.

Antitumor immunity is augmented by cytotoxic lymphodepletion therapies. Adoptively transferred naive and effector T cells proliferate extensively and show enhanced antitumor effects in lymphopenic recipients. Although the impact of lymphodepletion on transferred donor T cells has been well evaluated, its influence on recipient T cells is largely unknown. The current study demonstrates that both regulatory T cells (Tregs) and effector CD8(+) T cells from lymphopenic recipients play critical roles in the development of antitumor immunity after lymphodepletion. Cyclophosphamide (CPA) treatment depleted lymphocytes more efficiently than other cytotoxic agents; however, the percentage of CD4(+)CD25(+) Foxp3(+) Tregs was significantly increased in CPA-treated lymphopenic mice. Depletion of these chemoresistant Tregs following CPA treatment and transfer of naive CD4(+) T cells augmented the antitumor immunity and significantly suppressed tumor progression. Further analyses revealed that recipient CD8(+) T cells were responsible for this augmentation. Using Rag2(-/-) mice or depletion of recipient CD8(+) T cells after CPA treatment abrogated the augmentation of antitumor effects in CPA-treated reconstituted mice. The transfer of donor CD4(+) T cells enhanced the proliferation of CD8(+) T cells and the priming of tumor-specific CD8(+) T cells originating from the lymphopenic recipients. These results highlight the importance of the recipient cells surviving cytotoxic regimens in cancer immunotherapies.

Cohesin-SA1 deficiency drives aneuploidy and tumourigenesis in mice due to impaired replication of telomeres.

Cohesin is a protein complex originally identified for its role in sister chromatid cohesion, although increasing evidence portrays it also as a major organizer of interphase chromatin. Vertebrate cohesin consists of Smc1, Smc3, Rad21/Scc1 and either stromal antigen 1 (SA1) or SA2. To explore the functional specificity of these two versions of cohesin and their relevance for embryonic development and cancer, we generated a mouse model deficient for SA1. Complete ablation of SA1 results in embryonic lethality, while heterozygous animals have shorter lifespan and earlier onset of tumourigenesis. SA1-null mouse embryonic fibroblasts show decreased proliferation and increased aneuploidy as a result of chromosome segregation defects. These defects are not caused by impaired centromeric cohesion, which depends on cohesin-SA2. Instead, they arise from defective telomere replication, which requires cohesion mediated specifically by cohesin-SA1. We propose a novel mechanism for aneuploidy generation that involves impaired telomere replication upon loss of cohesin-SA1, with clear implications in tumourigenesis.

A distinct chemokine axis does not account for enrichment of Foxp3(+) CD4(+) T cells in carcinogen-induced fibrosarcomas.

The frequency of CD4(+)  Foxp3(+) regulatory T (Treg) cells is often significantly increased in the blood of tumour-bearing mice and people with cancer. Moreover, Treg cell frequencies are often higher in tumours compared with blood and lymphoid organs. We wished to determine whether certain chemokines expressed within the tumour mass selectively recruit Treg cells, thereby contributing to their enrichment within the tumour-infiltrating lymphocyte pool. To achieve this goal, the chemokine profile of carcinogen-induced fibrosarcomas was determined, and the chemokine receptor expression profiles of both CD4(+)  Foxp3(-) and CD4(+)  Foxp3(+) T cells were compared. These analyses revealed that the tumours are characterized by expression of inflammatory chemokines (CCL2, CCL5, CCL7, CCL8, CCL12, CXCL9, CXCL10 and CX3CL1), reflected by an enrichment of activated Foxp3(-) and Foxp3(+) T cells expressing T helper type 1-associated chemokine receptors. Notably, we found that CXCR3(+) T cells were significantly enriched in the tumours although curiously we found no evidence that CXCR3 was required for their recruitment. Instead, CXCR3 marks a population of activated Foxp3(-) and Foxp3(+) T cells, which use multiple and overlapping ligand receptor pairs to guide their migration to tumours. Collectively, these data indicate that enrichment of Foxp3(+) cells in tumours characterized by expression of inflammatory chemokines, does not occur via a distinct chemokine axis, thus selective chemokine blockade is unlikely to represent a meaningful therapeutic strategy for preventing Treg cell accumulation in tumours.

Host genetic background impacts modulation of the TLR4 pathway by RON in tissue-associated macrophages.

Toll-like receptors (TLRs) enable metazoans to mount effective innate immune responses to microbial and viral pathogens, as well as to endogenous host-derived ligands. It is understood that genetic background of the host can influence TLR responsiveness, altering susceptibility to pathogen infection, autoimmunity and cancer. Macrophage stimulatory protein (MSP), which activates the receptor tyrosine kinase recepteur d'origine nantais (RON), promotes key macrophage functions such as motility and phagocytic activity. MSP also acts via RON to modulate signaling by TLR4, which recognizes a range of pathogen or endogenous host-derived molecules. Here, we show that RON exerts divergent control over TLR4 activity in macrophages from different mouse genetic backgrounds. RON potently modulated the TLR4 response in macrophages from M2-prone FVB mice, as compared with M1-skewed C57Bl6 mice. Moreover, global expression analysis revealed that RON suppresses the TLR4-dependent type-I interferon gene signature only in FVB macrophages. This leads to attenuated production of the potent inflammatory mediator, tumor necrosis factor-α. Eliminating RON kinase activity markedly decreased carcinogen-mediated tumorigenesis in M2/Th2-biased FVB mice. We propose that host genetic background influences RON function, thereby contributing to the variability in TLR4 responsiveness in rodents and, potentially, in humans. These findings provide novel insight into the complex interplay between genetic context and immune function.

xCT deficiency accelerates chemically induced tumorigenesis.

During the course of inflammation and its resolution, macrophages are exposed to various cytotoxic materials, including reactive oxygen species. Thus, macrophages require a protective machinery against oxidative stress to survive at the inflammatory site. Here, we showed that xCT, a component of transport system x(c)(-), was significantly up-regulated in activated infiltrating cells, including macrophages and neutrophils at the inflammatory site. System x(c)(-) mediates the uptake of extracellular L-cystine and is consequently responsible for maintenance of intracellular glutathione levels. We established a loss-of-function mouse mutant line of xCT by N-ethyl-N-nitrosourea mutagenesis. Macrophages from xCT(mu/mu) mice showed cell death in association with the excessive release of high mobility group box chromosomal protein 1 upon stimulation with LPS, suggesting that xCT deficiency causes unremitting inflammation because of the impaired survival of activated macrophages at the inflammatory site. Subcutaneous injection of 3-methylcholanthrene (3-MCA) induced the generation of fibrosarcoma in association with inflammation. When 3-MCA was injected s.c. into mice, xCT mRNA was up-regulated in situ. In xCT(mu/mu) mice, inflammatory cytokines (such as IL-1beta and TNFalpha) were overexpressed, and the generation of 3-MCA-induced fibrosarcoma was accelerated. These results clearly indicate that the defect of the protective system against oxidative stress impaired survival of activated macrophages and subsequently enhanced tumorigenecity.

Helicobacter pylori, a carcinogen, induces the expression of melanoma antigen-encoding gene (Mage)-A3, a cancer/testis antigen.

Cancer/testis antigens (CTAs) are known to be expressed in various cancer types but are minimally or not expressed in normal tissues except for germline tissues. CTAs are attractive targets for cancer immunotherapy and diagnosis because of their restricted expression. The mechanisms of CTAs expression are unclear because the inducers of CTAs expression remain to be elucidated. We hypothesized that carcinogens may induce the cellular expression of CTAs. To prove this, we attempted to inoculate Helicobacter pylori, a known carcinogen, in Meth-A cells, normal gastric cells, and normal splenocytes and induce the expression of a CTA. Melanoma antigen-encoding gene (Mage)-A3, one of the CTAs, was not expressed in both normal cells but in Meth-A cells inoculated with H. pylori. Furthermore, we performed limiting dilution using Meth-A cells inoculated with H. pylori and established derivative clone from Meth-A designated as Meth-A/pylori/3C3 which permanently express Mage-A3 after excluding H. pylori. We herein report the first successful induction of a CTA in a cell line via exposure to a carcinogenic agent. Furthermore, the establishment of Meth-A/pylori/3C3, which is Meth-A expressing Mage-A3, is considered to contribute to the resolution of the mechanism of CTAs expression.

Sialylation of 3-methylcholanthrene-induced fibrosarcoma determines antitumor immune responses during immunoediting.

Sialylation of tumor cells is involved in various aspects of their malignancy (proliferation, motility, invasion, and metastasis); however, its effect on the process of immunoediting that affects tumor cell immunogenicity has not been studied. We have shown that in mice with impaired immunoediting, such as in IL-1α(-/-) and IFNγ(-/-) mice, 3-methylcholanthrene-induced fibrosarcoma cells are immunogenic and concomitantly bear low levels of surface sialylation, whereas tumor cells derived from wild type mice are nonimmunogenic and bear higher levels of surface sialylation. To study immune mechanisms whose interaction with tumor cells involves surface sialic acid residues, we used highly sialylated 3-methylcholanthrene-induced nonimmunogenic fibrosarcoma cell lines from wild type mice, which were treated with sialidase to mimic immunogenic tumor cell variants. In vivo and in vitro experiments revealed that desialylation of tumor cells reduced their growth and induced cytotoxicity by NK cells. Moreover, sialidase-treated tumor cells better activated NK cells for IFN-γ secretion. The NKG2D-activating receptor on NK cells was shown to be involved in interactions with desialylated ligands on tumor cells, the nature of which is still not known. Thus, the degree of sialylation on tumor cells, which is selected during the process of immunoediting, has possibly evolved as an important mechanism of tumor cells with low intrinsic immunogenicity or select for tumor cells that can evade the immune system or subvert its function. When immunoediting is impaired, such as in IFN-γ(-/-) and IL-1α(-/-) mice, the overt tumor consists of desialylayed tumor cells that interact better with immunosurveillance cells.

Tumor immunoediting by NKp46.

NK cells interact with a wide variety of hazardous cells including pathogen-infected and tumor cells. NKp46 is a specific NK killer receptor that recognizes various influenza hemagglutinins and unknown tumor ligands. It was recently shown that NKp46 plays a significant role in the in vivo eradication of tumor cells; however, the role played by NKp46 in vivo with regard to tumor development is still unclear. In this study, we used the 3-methylcholanthrene (MCA)-induced fibrosarcoma model in NKp46-deficient mice to test the NKp46 recognition of carcinogen-induced tumors. We show that although the rate of MCA-induced tumor formation was similar in the presence and in the absence of NKp46, the expression of its unknown ligands was NKp46 dependent. The unknown NKp46 ligands were nearly absent in tumors that originated in wild-type mice, whereas they were detected in tumors that originated in the NKp46-deficient mice. We demonstrate that the interactions between NKp46 and its MCA tumor-derived ligands lead to the secretion of IFN-gamma but not to the elimination of the MCA-derived tumor cells. In addition, we show that the in vivo growth of MCA-derived tumor cells expressing high levels of the NKp46 ligands is NKp46 and IFN-gamma dependent. Thus, we present in this study a novel NKp46-mediated mechanism of tumor editing.

NKG2D function protects the host from tumor initiation.

The activation NKG2D receptor has been shown to play an important role in the control of experimental tumor growth and metastases expressing ligands for NKG2D; however, a function for this recognition pathway in host protection from de novo tumorigenesis has never been demonstrated. We show that neutralization of NKG2D enhances the sensitivity of wild-type (WT) C57BL/6 and BALB/c mice to methylcholanthrene (MCA)-induced fibrosarcoma. The importance of the NKG2D pathway was additionally illustrated in mice deficient for either IFN-gamma or tumor necrosis factor-related apoptosis-inducing ligand, whereas mice depleted of natural killer cells, T cells, or deficient for perforin did not display any detectable NKG2D phenotype. Furthermore, IL-12 therapy preventing MCA-induced sarcoma formation was also largely dependent on the NKG2D pathway. Although NKG2D ligand expression was variable or absent on sarcomas emerging in WT mice, sarcomas derived from perforin-deficient mice were Rae-1(+) and immunogenic when transferred into WT syngeneic mice. These findings suggest an important early role for the NKG2D in controlling and shaping tumor formation.

Development of a quantitative bioassay to assess preventive compounds against inflammation-based carcinogenesis.

Reducing cancer incidence and mortality by use of cancer-chemopreventive agents is an important goal. We have established an in vitro bioassay that is able to screen large numbers of candidate chemicals that are positive for prevention of inflammation-related carcinogenesis. To accomplish this we have added candidate chemicals or vehicles and freshly isolated, fluorescent dye-labeled inflammatory cells that were overlaid on TNF-alpha-stimulated mouse endothelial cells in a 96-well plate. Inhibition of inflammatory cell attachment to the endothelial cells by the chemicals was quantified by the intensity of fluorescence from the adherent inflammatory cells after removing unattached cells. Using this assay, we selected two chemicals, auraptene and turmerones, for further study. As an in vivo test, diets containing these test chemicals were administered to mice with a piece of foreign body, gelatin sponge, that had been implanted to cause inflammation, and we found that the number of inflammatory cells that infiltrated into the subcutaneously implanted gelatin sponge was reduced compared to that found in the mice fed with a control diet. Moreover, diets containing either of the two chemicals prevented inflammation-based carcinogenesis in a mouse model. We found that the compounds reduced not only the number of infiltrating cells but also the expression of inducible nitric oxide synthase (iNOS) or formation of 8-hydroxy-2'-deoxyguanine (8-OHdG) in the infiltrated cells. Moreover, both compounds but not controls sustained the reducing activity in the inflammatory lesion, and this finding was confirmed by using non-invasive in vivo electron spin resonance. The newly established in vitro screening assay will be useful for finding biologically effective chemopreventive agents against inflammation-related carcinogenesis.

Proliferative and molecular effects of the dual PPARalpha/gamma agonist tesaglitazar in rat adipose tissues: relevance for induction of fibrosarcoma.

The dual peroxisome-proliferator-activated receptor (PPAR) α/γ agonist tesaglitazar has been shown to produce fibrosarcomas in rats. Here, the authors studied morphology, proliferation, differentiation, and inflammation markers in adipose tissue from rats exposed to 1, 3, or 10 µmol/kg tesaglitazar for 2 or 12 weeks, including recovery groups (12 weeks treatment followed by 12 weeks recovery), and 3 or 10 µmol/kg tesaglitazar for 24 weeks. Subcutaneous white and brown fat revealed reversible dose-related histopathological alterations and after 12 and 24 weeks developed areas of thickened skin (fatty lumps). There was a dose-dependent increase in proliferation of interstitial cells in white and brown fat as shown by increased mitotic index in all dose groups after 2 weeks. This was limited to the high dose after 12 and 24 weeks in white fat. Gene expression analyses showed that while tesaglitazar induced differentiation of adipose tissue characterized with a switch in cyclin D1 and D3 mRNA by 12 weeks, longer exposure at high doses reversed this differentiation concurrent with a reappearance of early adipocyte and inflammatory markers. These data suggest that sustained increased turnover of mesenchymal cells in adipose tissues, concomitant with onset of inflammation and fibrosis, drives development of fibrosarcomas in rats treated with tesaglitazar.

Flt3 ligand induces tumor regression and antitumor immune responses in vivo.

Daily treatment of mice with recombinant human Flt3 ligand (huFlt3L) results in a dramatic numerical increase in the number of dendritic cells (DCs) in vivo. Since DCs are pivotal in the induction of immune responses, we tested whether Flt3L treatment of mice challenged with a syngeneic methylcholanthrene (MCA)-induced fibrosarcoma would augment the generation of effective antitumor immune responses in vivo. Flt3L treatment not only induced complete tumor regression in a significant proportion of mice, but also decreased tumor growth rate in the remaining mice. A preliminary characterization of the cellular mechanisms involved suggests that Flt3L may be important in the treatment of cancer in situ through the generation of specific antitumor immune responses.

Decoupling tumor cell metastasis from growth by cellular pilot protein TNFAIP8.

Cancer metastasis accounts for nearly 90% of all cancer deaths. Metastatic cancer progression requires both cancer cell migration to the site of the metastasis and subsequent proliferation after colonization. However, it has long been recognized that cancer cell migration and proliferation can be uncoupled; but the mechanism underlying this paradox is not well understood. Here we report that TNFAIP8 (tumor necrosis factor-α-induced protein 8), a "professional" transfer protein of phosphoinositide second messengers, promotes cancer cell migration or metastasis but inhibits its proliferation or cancer growth. TNFAIP8-deficient mice developed larger tumors, but TNFAIP8-deficient tumor cells completely lost their ability to migrate toward chemoattractants and were defective in colonizing lung tissues as compared to wild-type counterparts. Mechanistically, TNFAIP8 served as a cellular "pilot" of tumor cell migration by locally amplifying PI3K-AKT and Rac signals on the cell membrane facing chemoattractant; at the same time, TNFAIP8 also acted as a global inhibitor of tumor cell growth and proliferation by regulating Hippo signaling pathway. These findings help explain the migration-proliferation paradox of cancer cells that characterizes many cancers.

Multicellular origin of fibrosarcomas in mice induced by the chemical carcinogen 3-methylcholanthrene.

The cellular origin of tumors induced by the chemical carcinogen 3-methylcholanthrene (MCA) was studied in mice with X-chromosome inactivation mosaicism. Because only one of the two X-chromosomes is active in XX somatic cells, a female heterozygous at the X-linked phosphoglycerate kinase (PGK-1) locus for the usual Pgk-1b gene and the variant Pgk-1a has two populations of cells, in the cells of one population, Pgk-1b is active and B-type enzyme is synthesized, whereas in cells of the other population, A-type enzyme is produced. Both enzyme types are found in normal tissues from these mosaic mice. A tumor developing from a single cell exhibits only one of the two PGK enzyme types, whereas a tumor with a multicellular origin expresses both enzymes (i.e., it has a double-enzyme phenotype). Five fibrosarcomas developing at the site of injection of 0.2 or 2.0 mg of MCA were analyzed. 36 of 38 fragments from the five tumors had double-enzyme PGK phenotypes. One piece from each of two tumors showed a single-enzyme phenotype. Histological, cell culture, and cloning studies indicate that the double-enzyme phenotypes reflect the presence of both types of malignant cells and not admixture of normal with neoplastic elements in the specimens tested for PGK. The results suggest strongly that these fibrosarcomas have a multicellular origin.

Cellular immune responses to methylcholanthrene-induced fibrosarcoma in BALB/c mice.

Several in vitro parameters of cellular immunity were examined in BALB/c mice with an experimentally induced fibrosarcoma tumor. The results of capillary migration of spleen cells in high tumor cell dose inoculated mice show appearance of cellular immune response in the early stages of the tumor growth. As the tumor progresses, the cellular response declines and rapidly disappears, culminating in stimulation values near the time of the death of these mice. The blastogenic studies also show early cellular recognition of tumor antigen by mouse spleen cells and whole blood (Z24 h). After the 2nd day following tumor injection, no blast transformation is noted. However, the results obtained with a lower inoculating tumor cell dose demonstrate an initial cellular recognition on the 7th day. This response gradually disappears by the 19th day and remains negative up to the time of the death of these mice. This cellular immunity was confirmed by the cytotoxic experiments showing that the primary cells responsible for this cellular reactivity were the immune cells. An interesting finding was the presence of a factor(s) capable of blocking the cytotoxic effect. The nature and mechanism of this blocking factor(s) is now under investigation.

Rejection of tumor allografts by mouse spleen cells sensitized in vitro.

This paper reports a model system of cellular immunity in which allosensitization of mouse spleen cells is induced in vitro. Allosensitization was achieved by culturing spleen cells upon monolayers of allogeneic fibroblasts. The ability of the spleen cells to inhibit the growth of tumor allografts in vivo served as a functional assay of sensitization. We found that unsensitized spleen cells or spleen cells sensitized against unrelated fibroblast antigens had no inhibitory effect on the growth of allogeneic fibrosarcoma cells when they were injected together into irradiated recipients. In contrast, spleen cells which were specifically allosensitized in vitro were found to be highly effective in inhibiting the growth of an equal number of allogeneic tumor cells. Several times more spleen cells from mice sensitized in vivo were required to produce a similar immune effect. This confirms the findings of previous studies which indicate that sensitization in cell culture can promote the selection of specifically sensitized lymphocytes. Preincubating sensitizing fibroblasts with allo-antisera blocked the allosensitization of spleen cells. This suggests that antibodies binding to fibroblasts may inhibit the induction of sensitization by competing with lymphocytes for antigenic sites. Mouse spleen cells which were able to recognize and reject tumor allografts in vivo were unable to cause lysis of target fibroblasts in vitro. Such fibroblasts, however, were susceptible to lysis by rat lymphoid cells sensitized by a similar in vitro method. These findings indicate that the conditions required for lymphocyte-mediated lysis of target cells may not be directly related to the processes of antigen recognition and allograft rejection in vivo.

Tumor-secific immunity to chemically induced tumors. Evidence for immunologic specificity and shared antigenicity in lymphocyte responses to soluble tumor antigens.

Experiments were designed to explore the apparent paradox that methylcholanthrene-induced tumors of mice evoke tumor-unique transplantation immunity but reveal almost complete cross-reacting antigenicity in tests of lymphocyte behavior in vitro. The approach involved use of tumor membranes solubilized in 3 M KCl, employed both as the stimulating antigen source in a new in vitro proliferation assay of lymphocyte recognition, and as immunogens in vivo. The kinetics of the assay resembled those of in vitro tests of mitogen or specific antigen stimulation in other systems. Lymphoid cell proliferation was assessed in peripheral blood leukocytes, lymph nodes (LN), and spleen over the course of tumor bearing, and in animals immunized by tumor amputation or with the solubilized antigens. The pattern of spread of reactivity was from regional LN to spleen, peripheral blood, and nonregional nodes in each circumstance. An unexplained low antigen dose inhibitory phenomenon was encountered in spontaneously proliferating cell subpopulations taken from some tumor-bearing animals. In vitro responses to some but not all solubilized antigens made from multiple syngeneic tumors were detected in each circumstance. The soluble antigens also induced shared resistance to some tumors. The patterns of spread of responsiveness to syngeneic tumor antigens, the time-course, and relative intensity were most compatible with independent clonal responses to multiple tumor-borne antigens, some but not all of which are shared in any family of syngeneic tumors.

Cyclophosphamide-facilitated adoptive immunotherapy of an established tumor depends on elimination of tumor-induced suppressor T cells.

On the basis of preceding studies showing that tumor-induced, T cell-mediated immunosuppression serves as an obstacle to adoptive immunotherapy of the Meth A fibrosarcoma, it was predicted that cyclophosphamide treatment of tumor bearers would remove this obstacle and allow passively transferred immune T cells to cause tumor regression. It was found that infusion of immune spleen cells alone had no effect on tumor growth, and cyclophosphamide alone caused a temporary halt in tumor progression. In contrast, combination therapy consisting of intravenous injection of 100 mg/kg of cyclophosphamide followed 1 h later by intravenous infusion of tumor-immune spleen cells caused small, as well as large tumors, to completely and permanently regress. Tumor regression caused by combination therapy was completely inhibited by intravenous infusion of splenic T cells from donors with established tumors, but not by spleen cells from normal donors. These suppressor T cells were eliminated from the spleen by treating the tumor-bearing donors with 100 mg/kg of cyclophosphamide. Immune T cells, in contrast, were resistant to this dose of cyclophosphamide. These results show that failure of intravenously-infused, tumor-sensitized T cells to cause regression of the Meth A fibrosarcoma growing in its syngeneic or semi-syngeneic host is caused by the presence of a tumor-induced population of cyclophosphamide-sensitive suppressor T cells.

Inhibition of methylcholanthrene-induced carcinogenesis by an interferon gamma receptor-dependent foreign body reaction.

The foreign body reaction is one of the oldest host defense mechanisms against tissue damage which involves inflammation, scarring, and encapsulation. The chemical carcinogen methylcholanthrene (MCA) induces fibrosarcoma and tissue damage in parallel at the injection site. Tumor development induced by MCA but not due to p53-deficiency is increased in interferon-gamma receptor (IFN-gammaR)-deficient mice. In the absence of IFN-gammaR, MCA diffusion and DNA damage of surrounding cells is increased. Locally produced IFN-gamma induces the formation of a fibrotic capsule. Encapsulated MCA can persist virtually life-long in mice without inducing tumors. Together, the foreign body reaction against MCA prevents malignant transformation, probably by reducing DNA damage. This mechanism is more efficient in the presence of IFN-gammaR. Our results indicates that inflammation and scarring, both suspected to contribute to malignancy, prevent cancer in certain situations.