Sharon A. Oldford

A Critical Role for Mast Cells and Mast Cell-Derived IL-6 in TLR2-Mediated Inhibition of Tumor Growth

Several TLR agonists are effective in tumor immunotherapy, but their early innate mechanisms of action, particularly those of TLR2 agonists, are unclear. Mast cells are abundant surrounding solid tumors where they are often protumorigenic and enhance tumor angiogenesis. However, antitumor roles for mast cells have also been documented. The impact of mast cells may be dependent on their activation status and mediator release in different tumors. Using an orthotopic melanoma model in wild-type C57BL/6 and mast cell-deficient KitW-sh/W-sh mice and a complementary Matrigel–tumor model in C57BL/6 mice, mast cells were shown to be crucial for TLR2 agonist (Pam3CSK4)-induced tumor inhibition. Activation of TLR2 on mast cells reversed their well-documented protumorigenic role. Tumor growth inhibition after peritumoral administration of Pam3CSK4 was restored in KitW-sh/W-sh mice by local reconstitution with wild-type, but not TLR2-deficient, mast cells. Mast cells secrete multiple mediators after Pam3CSK4 activation, and in vivo mast cell reconstitution studies also revealed that tumor growth inhibition required mast cell-derived IL-6, but not TNF. Mast cell-mediated anticancer properties were multifaceted. Direct antitumor effects in vitro and decreased angiogenesis and recruitment of NK and T cells in vivo were observed. TLR2-activated mast cells also inhibited the growth of lung cancer cells in vivo. Unlike other immune cells, mast cells are relatively radioresistant making them attractive candidates for combined treatment modalities. This study has important implications for the design of immunotherapeutic strategies and reveals, to our knowledge, a novel mechanism of action for TLR2 agonists in vivo.

Mast cells as targets for immunotherapy of solid tumors

Mast cells have historically been studied mainly in the context of allergic disease. In recent years, we have come to understand the critical importance of mast cells in tissue remodeling events and their role as sentinel cells in the induction and development of effective immune responses to infection. Studies of the role of mast cells in tumor immunity are more limited. The pro-tumorigenic role of mast cells has been widely reported. However, mast cell infiltration predicts improved prognosis in some cancers, suggesting that their prognostic value may be dependent on other variables. Such factors may include the nature of local mast cell subsets and the various activation stimuli present within the tumor microenvironment. Experimental models have highlighted the importance of mast cells in orchestrating the anti-tumor events that follow immunotherapies that target innate immunity. Mast cells are long-lived tissue resident cells that are abundant around many solid tumors and are radiation resistant making them unique candidates for combined treatment modalities. This review will examine some of the key roles of mast cells in tumor immunity, with a focus on potential immunotherapeutic interventions that harness the sentinel role of mast cells.

Ranitidine modifies myeloid cell populations and inhibits breast tumor development and spread in mice.

ABSTRACT Histamine receptor 2 (H2) antagonists are widely used clinically for the control of gastrointestinal symptoms, but also impact immune function. They have been reported to reduce tumor growth in established colon and lung cancer models. Histamine has also been reported to modify populations of myeloid-derived suppressor cells (MDSCs). We have examined the impact of the widely used H2 antagonist ranitidine, on both myeloid cell populations and tumor development and spread, in three distinct models of breast cancer that highlight different stages of cancer progression. Oral ranitidine treatment significantly decreased the monocytic MDSC population in the spleen and bone marrow both alone and in the context of an orthotopic breast tumor model. H2 antagonists ranitidine and famotidine, but not H1 or H4 antagonists, significantly inhibited lung metastasis in the 4T1 model. In the E0771 model, ranitidine decreased primary tumor growth while omeprazole treatment had no impact on tumor development. Gemcitabine treatment prevented the tumor growth inhibition associated with ranitidine treatment. In keeping with ranitidine-induced changes in myeloid cell populations in non-tumor-bearing mice, ranitidine also delayed the onset of spontaneous tumor development, and decreased the number of tumors that developed in LKB1−/−/NIC mice. These results indicate that ranitidine alters monocyte populations associated with MDSC activity, and subsequently impacts breast tumor development and outcome. Ranitidine has potential as an adjuvant therapy or preventative agent in breast cancer and provides a novel and safe approach to the long-term reduction of tumor-associated immune suppression.

Mast Cell Modulation of the Tumor Microenvironment

Mast cells are myeloid derived immune effector cells that have been most widely studied in the context of allergic disease. However, over recent years it has become apparent that they also play critical roles in the regulation of tissue remodelling and host defence. In solid tumors, mast cells are abundant at the tumor periphery in close proximity to blood vessels and are frequently considered to function in a tumor promoting capacity. They release potent angiogenic cytokines that augment tumor blood vessel formation, tumor enhancing growth factors and tissue-degrading enzymes that enable tumor metastasis. Mast cells can also release mediators in the tumor microenvironment that enhance aspects of immune suppression, such as interleukin 10. These observations have led to the consideration of inhibiting mast cells as an approach to cancer therapy. In marked contrast to this negative picture of mast cells in tumors, some studies of human disease have suggested that increased mast cell numbers can be associated with an improved prognosis. It has also been demonstrated, in mouse models, that mast cells might be important targets for immune activation during immunotherapy. Since mast cells are resistant to radiation and normally serve a sentinel cell role recruiting effector cells such as natural killer cells and T cells to sites of infection, they might have substantial potential as an additional target for therapy in the context of more traditional treatments. Our understanding of mast cells in human tumor settings is limited, but there remain excellent opportunities to attempt to modulate mast cell function as a new approach to therapy.

Interferon α2 and interferon γ induce the degranulation independent production of VEGF-A and IL-1 receptor antagonist and other mediators from human mast cells: Interferons induce VEGF and IL-1Ra from human mast cells

Mast cells are resident immune effector cells, often studied in the context of allergic disease. Found in substantial numbers at sites of potential infection they are increased at sites of angiogenesis and can be pivotal for the sensing and clearance of a variety of pathogens. Interferons (IFNs) are cytokines that are critical for host defence against intracellular pathogens. Increased levels of IFNs are observed during viral infection and in autoimmune diseases. IFNs are also widely used therapeutically and have been examined in the therapy of severe asthma.

Ranitidine Inhibition of Breast Tumor Growth Is B Cell Dependent and Associated With an Enhanced Antitumor Antibody Response

Background The histamine receptor 2 antagonist ranitidine is a commonly used, non-prescription, medication. It limits the development, growth, and metastasis of breast cancers in mouse models of disease. In this study, we examined the role of B cells in this response, the impact of ranitidine on the development of antitumor antibodies and subpopulations of natural killer cells using murine breast cancer models. Methods Peripheral blood granulocyte populations were assessed in both E0771-GFP and 4T1 orthotopic tumor-bearing mice by evaluation of stained blood smears. Antibody responses were assessed both in terms of the levels of anti-GFP antibodies detected by enzyme-linked immunosorbent assay and also by antibody binding to the surface of tumor cells evaluated by flow cytometry. B cell and NK cell populations were examined in the draining lymph nodes and spleens of tumor-bearing animals, by flow cytometry with and without ranitidine treatment. Results Oral ranitidine treatment was not associated with changes in peripheral blood granulocyte populations in tumor-bearing mice. However, ranitidine treatment was associated with the development of enhanced antitumor antibody responses. This was not limited to the tumor setting since ranitidine-treated mice immunized with ovalbumin also demonstrated increased IgG antibody responses. Analysis of B cell populations indicated that while B1 cell populations remained unchanged there was a significant decrease in B2 cells in the tumor-draining inguinal lymph nodes. Notably, ranitidine did not significantly inhibit primary tumor growth in B cell-deficient animals. Examination of NK cell populations revealed a significant decrease in the proportion of intermediately functionally mature NK cells populations (CD27+CD11b−) in ranitidine-treated tumor-bearing mice compared with untreated tumor-bearing controls. Conclusion These data demonstrate an important role for B cells in the enhanced antitumor immune response that occurs in response to ranitidine treatment. Our findings are consistent with a model, whereby ranitidine reduces tumor-associated immune suppression allowing for the development of more effective antitumor responses mediated by B cells which may include the participation of NK cells. These data underline the importance of considering widely used histamine receptor antagonists as modulators of antitumor immunity to breast cancer.

Local Toll-like receptor-mediated mast cell activation inhibits melanoma growth

Mast cells are abundant at the periphery of the majority of solid tumors where they typically promote angiogenesis and enhance tumor growth. However, there is some evidence from epidemiological studies that allergic disease reduces cancer risk. In the context of infection, mast cell activation with Toll-like receptor (TLR) agonists induces the production of multiple mediators which mobilize and activate known anti-tumor effector cells. TLR agonists can also be effective in tumor immunotherapy. We demonstrate that the TLR2 agonist Pam3CSK4 is tumor inhibitory and mediates its effects via mast cell activation and reversal of the normal pro-tumorigenic activity of mast cells. TLR2 agonist treatment decreased B16.F10 tumor growth in wild-type mice, but not in mast cell deficient Kit W-sh/W-sh mice. Tumor growth inhibition following TLR2 agonist administration was restored in Kit W-sh/W-sh mice by local reconstitution with wild-type, but not TLR2-deficient mast cells. Mast cell dependent tumor growth inhibition occurred independently of tumor necrosis factor (TNF) production and was not associated with direct tumor cytotoxicity or degranulation in vitro. Tumor growth inhibition was associated with mast cell dependent recruitment of natural killer (NK) and T cells. This study demonstrates that local mast cell activation provides a novel opportunity to modify the tumor microenvironment for successful cancer immunotherapy. Supported by the CCSRI and CIHR. S.A.O is a recipient of support from the Nova Scotia Health Research Foundation.