Tumour-associated macrophages (TAMs) play an important role in tumour progression, which is facilitated by their ability to respond to environmental cues. Here we report, using murine models of breast cancer, that TAMs expressing fibroblast activation protein alpha (FAP) and haem oxygenase-1 (HO-1), which are also found in human breast cancer, represent a macrophage phenotype similar to that observed during the wound healing response. Importantly, the expression of a wound-like cytokine response within the tumour is clinically associated with poor prognosis in a variety of cancers. We show that co-expression of FAP and HO-1 in macrophages results from an innate early regenerative response driven by IL-6, which both directly regulates HO-1 expression and licenses FAP expression in a skin-like collagen-rich environment. We show that tumours can exploit this response to facilitate transendothelial migration and metastatic spread of the disease, which can be pharmacologically targeted using a clinically relevant HO-1 inhibitor.

Unprecedented clinical outcomes have been achieved in a variety of cancers by targeting immune checkpoint molecules. This preclinical study investigates heme oxygenase-1 (HO-1), an immunosuppressive enzyme that is expressed in a wide variety of cancers, as a potential immune checkpoint target in the context of a chemotherapy-elicited antitumor immune response. We evaluate repurposing tin mesoporphyrin (SnMP), which has demonstrated safety and efficacy targeting hepatic HO in the clinic for the treatment of hyperbilirubinemia, as an immune checkpoint blockade therapy for the treatment of cancer. SnMP and genetic inactivation of myeloid HO-1 were evaluated alongside 5-fluorouracil in an aggressive spontaneous murine model of breast cancer (). Single-cell RNA sequencing analysis, tumor microarray, and clinical survival data from breast cancer patients were used to support the clinical relevance of our observations. We demonstrate that SnMP inhibits immune suppression of chemotherapy-elicited CD8 T cells by targeting myeloid HO-1 activity in the tumor microenvironment. Microarray and survival data from breast cancer patients reveal that HO-1 is a poor prognostic factor in patients receiving chemotherapy. Single-cell RNA-sequencing analysis suggests that the myeloid lineage is a significant source of HO-1 expression, and is co-expressed with the immune checkpoints PD-L1/2 in human breast tumors. we therapeutically compare the efficacy of targeting these two pathways alongside immune-stimulating chemotherapy, and demonstrate that the efficacy of SnMP compares favorably with PD-1 blockade in preclinical models. SnMP could represent a novel immune checkpoint therapy, which may improve the immunological response to chemotherapy. .

Highlights d A dual oxygen-sensing switch provides stringent hypoxiadependent regulation of a CAR d HypoxiCAR T cells deliver tumor-selective CAR expression and anti-tumor efficacy d HypoxiCAR T cells prevent on-target, off-tumor activation and cytokine release syndrome d HypoxiCAR provides a strategy to expand the CAR repertoire for solid malignancies

There has been significant interest in the prospects of chimeric antigen receptor (CAR) T-cell therapy in the treatment of solid malignancies, and multiple clinical trials are in progress1. However, the scope of these trials has been restricted by the lack of availability of tumorspecific targets to direct CAR binding. Tumor specificity is crucial as on-target off-tumor activation of CAR T-cells in healthy tissues can result in potentially lethal toxicities due to uncontrolled cytokine release syndrome2. Here we engineer a stringent hypoxia-sensing CAR T-cell system which achieves selective expression of a pan-ErbB-targeted CAR within a solid tumor, a microenvironment characterized by an inadequate oxygen supply. Using murine xenograft models, we demonstrate that despite widespread expression of ErbB receptors in healthy organs, the approach provides anti-tumor efficacy without off-tumor toxicity. This dynamic on/off oxygen-sensing safety switch has the potential to facilitate the unlimited expansion of the CAR T-cell target repertoire for treating solid malignancies.