Mast cells are unique tissue-resident immune cells of the myeloid lineage that have long been implicated in the pathogenesis of allergic and autoimmune disorders. More recently, mast cells have been recognized as key orchestrators of anti-tumor immunity, modulators of the cancer stroma, and have also been implicated in cancer cell intrinsic properties. As such, mast cells are an underrecognized but very promising target for cancer immunotherapy. In this review, we discuss the role of mast cells in shaping cancer and its microenvironment, the interaction between mast cells and cancer therapies, and strategies to target mast cells to improve cancer outcomes. Specifically, we address (1) decreasing cell numbers through c-KIT inhibition, (2) modulating mast cell activation and phenotype (through mast cell stabilizers, FcεR1 signaling pathway activators/inhibitors, antibodies targeting inhibitory receptors and ligands, toll like receptor agonists), and (3) altering secreted mast cell mediators and their downstream effects. Finally, we discuss the importance of translational research using patient samples to advance the field of mast cell targeting to optimally improve patient outcomes. As we aim to expand the successes of existing cancer immunotherapies, focused clinical and translational studies targeting mast cells in different cancer contexts are now warranted.

Gut microbiota, specifically gut bacteria, are critical for effective immune checkpoint blockade therapy (ICT) for cancer. The mechanisms by which gut microbiota augment extraintestinal anticancer immune responses, however, are largely unknown. Here, we find that ICT induces the translocation of specific endogenous gut bacteria into secondary lymphoid organs and subcutaneous melanoma tumors. Mechanistically, ICT induces lymph node remodeling and dendritic cell (DC) activation, which facilitates the translocation of a selective subset of gut bacteria to extraintestinal tissues to promote optimal antitumor T cell responses in both the tumor-draining lymph nodes (TDLNs) and the primary tumor. Antibiotic treatment results in decreased gut microbiota translocation into mesenteric lymph nodes (MLNs) and TDLNs, diminished DC and effector CD8 ⁺ T cell responses, and attenuated responses to ICT. Our findings illuminate a key mechanism by which gut microbiota promote extraintestinal anticancer immunity.

Renal cell carcinoma (RCC), especially clear cell RCC, is generally considered an immunotherapy-responsive cancer. Recently, the prognosis for patients with locally advanced and metastatic RCC has significantly improved with the regulatory approvals of anti-PD-1/PD-L1/CTLA-4 immune checkpoint inhibitor (ICI)-based regimens. Yet in most cases, RCC will remain initially unresponsive to treatment or will develop resistance over time. Hence, there remains an unmet need to understand what leads to ICI resistance and to develop novel immune and nonimmune treatments to enhance the response to ICIs. In this review, we highlight recently published studies and the latest clinical studies investigating the next generation of immune approaches to locally advanced and metastatic RCC beyond traditional ICIs. These trials include cytokines, gut microbiota-based therapies, novel immune checkpoint agents, vaccines, and chimeric antigen receptor T cells. These agents are being evaluated as monotherapy or in combination with traditional ICIs and will hopefully provide improved outcomes to patients with RCC soon.

Circadian clocks regulate immunity, yet how they shape the tumor immune microenvironment and influence cancer immunotherapy remains unclear. Here, we show that tumor immune infiltration and immune checkpoint inhibitor efficacy vary by time of day in mice, driven by intrinsic clocks in dendritic cells and CD8 ⁺ T cells. Time-of-day modulates the abundance, spatial organization, and cytokine–chemokine production of tumor-infiltrating immune cells. Mechanistically, dendritic cell clocks control expression of Cx3cl1 , driving recruitment of CX3CR1 ⁺ CD8 ⁺ T cells and thereby reshaping the tumor immune microenvironment to enhance immunotherapy efficacy. Disruption of this axis abolishes time-of-day-dependent differences in treatment response. These findings identify a circadian mechanism of immune cell recruitment to tumors and provide mechanistic insight into clinical observations linking treatment timing to immunotherapy outcomes. One sentence summary: Time of day determines cancer immunotherapy efficacy through a circadian clock–dependent chemokine axis.

E23012 Background: Pulmonary metastases still remains the leading cause of mortality in osteosarcoma, with a 5-year survival rate of less than 30%. 17-β estradiol and testosterone are known to promote tumorigenesis and metastasis in other cancers, but its role in osteosarcoma tumorigenesis and metastasis is unclear. An understanding of the mechanism that these hormones have in osteosarcoma tumorigenesis and metastasis will lead to new therapeutic strategies using currently available targeted therapies used in breast and prostate cancer. Methods: We began by assessing the in vitro characteristics of two human osteosarcoma cell lines treated with 17-β estradiol or testosterone via proliferation and invasion assays. For the in vivo experiments, NCR nu/nu mice were injected with two luciferase-tagged human osteosarcoma cell lines and treated with 17-β estradiol and testosterone slow-release pellets. We first injected the mice subcutaneously and monitored tumor growth via caliper measurements. In the second experiment we injected the cell lines intratibially and monitored primary tumor growth in the bone and monitored metastasis formation in the lungs via IVIS bioluminescent imaging. Quantification of luciferase signal (photons/second) in the lungs was performed via the Xenogen Living Image software. H&E and immunohistochemistry for luciferase expression was performed on lung tissue to quantify tumor burden in the lungs. Statistical analysis was performed using Graphpad Prism. Results: Treatment with 17-β estradiol and testosterone increased cell proliferation and invasion in a dose-dependent manner. We observed increased tumor growth with both 17-β estradiol and testosterone in the subcutaneous model. In the intratibial experiment, testosterone increases primary tumor growth, but has no effect on metastasis. 17-β estradiol decreases primary tumor growth, but increases metastasis in a cell line independent manner. Conclusions: This study demonstrates a novel role for the puberty-related sex steroid hormones, 17-β estradiol and testosterone, in osteosarcoma tumor growth and metastasis. Uncovering the mechanism behind this phenomenon will uncover new therapeutic options for this devastating disease.

Gut microbiota are critical determinants of effective immune checkpoint therapy (ICT), yet the microbial mediators and host mechanisms that enhance antitumor immunity remain poorly understood. Here, we identify the microbiota-derived bile acid taurodeoxycholic acid (TDCA) as a metabolite associated with immune checkpoint therapy (ICT) response. TDCA administration alone is sufficient to overcome antibiotic-induced ICT hyporesponsiveness across multiple murine tumor models. Mechanistically, TDCA directly enhances CD8⁺ T cell–mediated antitumor immunity, increasing cytotoxicity. These effects required signaling through the bile acid receptor TGR5. Together, these findings reveal TDCA as a gut microbial metabolite that restores ICT efficacy after antibiotic disruption by directly augmenting CD8⁺ T cell anti-tumor activity. This work supports metabolite replacement as a therapeutic strategy to mitigate antibiotic-associated loss of cancer immunotherapy response. Significance: TDCA is a microbiota-derived metabolite that restores immune checkpoint therapy efficacy after antibiotic disruption by directly enhancing CD8⁺ T-cell–mediated anti-tumor immunity through bile acid receptor TGR5 signaling. Our findings suggest that supplementation with defined microbial metabolites can mitigate antibiotic-associated loss of immunotherapy response without requiring broader microbiome reconstitution.