Effective tumor immunotherapy may require not only activation of anti-tumor effector cells, but also abrogation of tumor-mediated immunosuppression. The cytokine, TGF-β, is frequently elevated in the tumor microenvironment and is a potent immunosuppressive agent and promoter of tumor metastasis. OX40 (CD134) is a member of the TNF-α receptor superfamily and ligation by agonistic antibody (anti-OX40) enhances effector function, expansion and survival of activated T cells. In this study, we examined the therapeutic efficacy and anti-tumor immune response induced by the combination of a small molecule TGF-β signaling inhibitor, SM16, plus anti-OX40 in the poorly immunogenic, highly metastatic, TGF-β-secreting 4T1 mammary tumor model. Our data show that SM16 synergizes with anti-OX40 to elicit a potent anti-tumor effect against established primary tumors, with a 79% reduction in tumor size, a 95% reduction in the number of metastatic lung nodules and a cure rate of 38%. This positive treatment outcome was associated with a 3.2-fold increase of tumor-infiltrating, activated CD8+ T cells, an overall accumulation of CD4+ and CD8+ T cells, and an increased tumor-specific effector T cell response. Complete abrogation of the therapeutic effect in vivo following depletion of CD4+ and CD8+ T cells, suggests that the anti-tumor efficacy of SM16 + anti-OX40 therapy is T cell dependent. Mice that were cured of their tumors were able to reject tumor re-challenge and manifested a significant tumor-specific peripheral memory IFN-γ response. Taken together, these data suggest that combining a TGF-β signaling inhibitor with anti-OX40 is a viable approach for treating metastatic breast cancer.

Summary

TGF-β blockade significantly slows tumor growth through many mechanisms, including activation of CD8+ T-cells and macrophages. Here, we show that TGF-β blockade also increases neutrophil-attracting chemokines resulting in an influx of CD11b+/Ly6G+ tumor-associated neutrophils (TAN) that are hypersegmented, more cytotoxic to tumor cells, and express higher levels of pro-inflammatory cytokines. Accordingly, following TGF-β blockade, depletion of these neutrophils significantly blunts anti-tumor effects of treatment and reduces CD8+ T-cell activation. In contrast, in control tumors, neutrophil depletion decreases tumor growth and results in more activated CD8+ T-cells intra-tumorally. Together, these data suggest that TGF-β within the tumor microenvironment induces a population of TAN with a pro-tumor phenotype. TGF-β blockade results in the recruitment and activation of TAN with an anti-tumor phenotype.

Background

Transforming growth factor β (TGF-β) plays a complex role in breast carcinogenesis. Initially functioning as a tumor suppressor, this cytokine later contributes to the progression of malignant cells by enhancing their invasive and metastatic potential as well as suppressing anti-tumor immunity. The purpose of this study was to investigate the efficacy of SM16, a novel small molecule ALK5 kinase inhibitor, to treat a highly metastatic, TGF-β-producing murine mammary carcinoma (4T1).

Materials and Methods

Mice bearing established 4T1 tumors were treated with SM16 intraperitoneally (i.p.) or orally and primary and metastatic tumor growth was assessed.

Results

SM16 inhibited Smad2 phosphorylation in cultured 4T1 tumor cells as well as primary and metastatic 4T1 tumor tissue. Blockade of TGF-β signal transduction in 4T1 tumor cells by SM16 prevented TGF-β-induced morphological changes and inhibited TGF-β-induced invasion in vitro. When delivered via daily i.p. injection or orally through mouse chow, SM16 inhibited the growth of primary and metastatic 4T1 tumors. Splenocytes isolated from mice on the SM16 diet displayed enhanced IFN-γ production and anti-tumor CTL activity. Furthermore, SM16 failed to inhibit the growth and metastasis of established 4T1 tumors in immunodeficient SCID mice.

Conclusion

Taken together, the data indicate that the anti-tumor efficacy of SM16 is dependent on an immune mediated mechanism and that SM16 may represent a safe and effective treatment for metastatic breast cancer.

Locally-produced TGF-β promotes tumor-induced immunosuppression and contributes to resistance to immunotherapy. This paper explores the potential for increased efficacy when combining immunotherapies with TGF-β suppression using the TGF-β type I receptor kinase inhibitor, SM16. Adenovirus expressing IFNβ (Ad.IFNβ) was injected intratumorally once in established subcutaneous AB12 (mesothelioma) and LKR (lung cancer) tumors or intratracheally in a K-ras orthotopic lung tumor model. Mice bearing TC1 (lung cancer) tumors were vaccinated with two injections of adenovirus expressing HPV-E7 (Ad.E7). SM16 was administered orally in formulated chow. Tumor growth was assessed and cytokine-expression and cell populations were measured in tumors and spleens by real time-PCR and flow cytometry. SM16 potentiated the efficacy of both immunotherapies in each of the models and caused changes in the tumor microenvironment. The combination of SM16 and Ad.INFβ increased the number of intratumoral leukocytes (including macrophages, NK cells, and CD8+ cells) and increased the percentage of T-cells expressing the activation marker CD25. SM16 also augmented the anti-tumor effects of Ad.E7 in the TC1 flank tumor model. The combination did not increase HPV-E7 tetramer-positive CD8+ T cells in the spleens, but did induce a marked increase in the tumors. Tumors from SM16-treated mice showed increased mRNA and protein for immunostimulatory cytokines and chemokines, as well as endothelial adhesion molecules, suggesting a mechanism for the increased intratumoral leukocyte trafficking. Blockade of the TGF-β signaling pathway augments the anti-tumor effects of Ad.INFβ immune-activating or Ad.E7 vaccination therapy. The addition of TGF-β blocking agents in clinical trials of immunotherapies may increase efficacy.

Purpose

Adoptive cellular immunotherapy has promise as an approach to eradicate established tumors. However, a significant hurdle in the success of cellular immunotherapy involves recently identified mechanisms of immune suppression on cytotoxic T-cells at the effector phase.

Transforming growth factor-β (TGF-β) is one of the most important of these immunosuppressive factors because it affects both T-cell and macrophage functions. We thus hypothesized that systemic blockade of TGF-β signaling combined with adoptive T-cell transfer would enhance the effectiveness of the therapy.

Experimental Design

Flank tumors were generated in mice using the OVA-albumin (OA) expressing thymoma cell line, EG7. Splenocytes from transgenic OT-1 mice (whose CD8 T-cells recognize an immunodominant peptide in OA) were activated in vitro and adoptively transferred into mice bearing large tumors in the presence or absence of an orally available TGF-β receptor-I kinase blocker (SM16).

Results

We observed markedly smaller tumors in the group receiving the combination of SM16 chow and adoptive transfer. Additional investigation revealed that TGF-β receptor blockade increased the persistence of adoptively transferred T-cells in the spleen and lymph nodes, increased numbers of adoptively transferred T-cells within tumors, increased activation of these infiltrating T-cells, and altered the tumor microenvironment with a significant increase in TNF-α and decrease in arginase mRNA expression

Conclusions

We found that systemic blockade of TGF-β receptor activity augmented the anti-tumor activity of adoptively transferred T-cells and may thus be a useful adjunct in future clinical trials.