Therapeutically targeting perivascular macrophage effector function improves CD8+ T-cell infiltration and response to chemotherapeutic drugs in cancer

The tumor microenvironment (TME) plays a crucial role in cancer progression and represents an appealing target for cancer treatment. Tumor-associated macrophages (TAMs) are an abundant stromal cell type in the TME and, due to their plasticity, TAMs can be exploited to promote a pro-tumoral response. TAMs have been implicated in facilitating a variety of pro-tumoral pathways including resistance to anti-cancer therapeutics, immune suppression, angiogenesis, and metastasis. However, pan-TAM depleting approaches have not delivered robust clinical outcomes as single agents. This might be explained due to their non-selective targeting of both pro- and anti-tumoral macrophage subsets. As such, therapeutically targeting key pro-tumoral TAM subsets may be a more desirable and efficacious strategy. Our group has recently characterised a subpopulation of pro-angiogenic and immune-suppressive TAMs which can be identified by their expression of the lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1). LYVE-1+ TAMs reside spatially proximal to blood vasculature and adopt a collaborative multi-cellular ‘nest’ arrangement which supports their biological activity. LYVE-1+ TAMs selectively express high levels of the immunomodulatory enzyme heme oxygenase-1 (HO-1), an enzyme which catabolizes heme to generate the biologically active catabolites carbon monoxide (CO), biliverdin and ferrous iron. Genetic inactivation of HO-1 in LYVE-1+ TAMs improve anti-tumor CD8+ T-cell infiltration into the TME and enhances the immune-stimulating effects of chemotherapy, resulting in sustain control of tumor growth in a spontaneous MMTV-PyMT murine model of breast cancer. This highlights that HO-1 represents a key effector pathway of these cells that prevents immune-elimination of tumors. Considering translation of these observations, unfortunately, current HO-1 inhibitors are not orally bioavailable, which limits their utility for chronic diseases, such as cancer, where daily administration would be required. In response to this hurdle, we developed a next generation HO-1 inhibitor called KCL-HO-1i. We demonstrate in pharmacokinetic studies that KCL-HO-1i is orally bioavailable in murine models with a serum half-life of around 3hours. Furthermore, oral delivery of KCL-HO-1i alongside standard of care chemotherapy delivers durable tumor control in MMTV-PyMT mice. We have characterised the response of the TME to KCL-HO-1i using flow cytometry, immunofluorescence and RNA sequencing approaches and demonstrate that KCL-HO-1i targeting perivascular LYVE-1+ TAM function supports a broader switch from an immunological ‘cold’ to ‘hot’ TME which provides a more favourable immune landscape for improving the response to chemotherapeutic drugs. Taken together, our data support the use of KCL-HO-1i as novel immunotherapeutic for targeting the function of perivascular TAMs in cancer.