For decades, the aryl hydrocarbon receptor (AHR) was studied for its role in environmental chemical toxicity i.e., as a quirk of nature and a mediator of unintended consequences of human pollution. During that period, it was not certain that the AHR had a “normal” physiological function. However, the ongoing accumulation of data from an ever-expanding variety of studies on cancer, cancer immunity, autoimmunity, organ development, and other areas bears witness to a staggering array of AHR-controlled normal and pathological activities. The objective of this review is to discuss how the AHR has gone from a likely contributor to genotoxic environmental carcinogen-induced cancer to a master regulator of malignant cell progression and cancer aggression. Particular focus is placed on the association between AHR activity and poor cancer outcomes, feedback loops that control chronic AHR activity in cancer, and the role of chronically active AHR in driving cancer cell invasion, migration, cancer stem cell characteristics, and survival.

Immune checkpoint inhibitors represent some of the most important cancer treatments developed in the last 20 y. However, existing immunotherapy approaches benefit only a minority of patients. Here, we provide evidence that the aryl hydrocarbon receptor (AhR) is a central player in the regulation of multiple immune checkpoints in oral squamous cell carcinoma (OSCC). Orthotopic transplant of mouse OSCC cells from which the AhR has been deleted (MOC1AhR-KO) results, within 1 wk, in the growth of small tumors that are then completely rejected within 2 wk, concomitant with an increase in activated T cells in tumor-draining lymph nodes (tdLNs) and T cell signaling within the tumor. By 2 wk, AhR+ control cells (MOC1Cas9), but not MOC1AhR-KO cells up-regulate exhaustion pathways in the tumor-infiltrating T cells and expression of checkpoint molecules on CD4+ T cells (PD-1, CTLA4, Lag3, and CD39) and macrophages, dendritic cells, and Ly6G+ myeloid cells (PD-L1 and CD39) in tdLNs. Notably, MOC1AhR-KO cell transplant renders mice 100% immune to later challenge with wild-type tumors. Analysis of altered signaling pathways within MOC1AhR-KO cells shows that the AhR controls baseline and IFNγ-induced Ido and PD-L1 expression, the latter of which occurs through direct transcriptional control. These observations 1) confirm the importance of malignant cell AhR in suppression of tumor immunity, 2) demonstrate the involvement of the AhR in IFNγ control of PD-L1 and IDO expression in the cancer context, and 3) suggest that the AhR is a viable target for modulation of multiple immune checkpoints.

Immunotherapy has shown dramatic results in treating cancer but only in a minority of patients, so there is unmet need to understand the regulation of immune suppression in tumors. We have shown that the aryl hydrocarbon receptor (AhR) is a central player in regulating immune checkpoints in oral squamous cell carcinoma (OSCC) and may play a similar role in lung cancer. Orthotopic transplant of mouse AhR-knockout (KO) OSCC (MOC1) cells led to complete rejection of tumor formation coupled with increased T cell activation in tumor draining lymph nodes (tdLNs) and T cell signaling in tongues, whereas wild type (WT) OSCC challenge led to tumor formation and upregulation of T cell exhaustion pathways and checkpoint molecules. Similar patterns of AhR-facilitated immune protection appear in our lung cancer model. On average, mice transplanted subcutaneously with CMT167AhRKO lung adenocarcinoma cells survive up to twice as long as their WT-challenged counterparts; up to 50% of CMT167AhRKO-transplanted mice never grow tumor and also were partially protected from challenge with CMT167WT cells. CMT167WT-transplanted mice exhibited increased numbers of CD4+ and CD8+ in tdLNs and decreased T cell trafficking to tumors while the proportion of PD1+, CTLA4+ and Lag3+ CD4+ and CD8+ T cells were significantly lower in CMT167AhRKO tumors. AhRKO in both cell lines decreases PD-L1 expression by 50%, suggesting a mechanism of immune escape by WT tumors. Thus, the immunological protection following transplantation of both AhRKO CMT167 and MOC1 cells suggests downregulation of PD-L1 on malignant cells is at least partially responsible for immune enhancement in AhRKO cells and supports AhR’s potential as a viable immunotherapeutic target in at least two cancers. Supported by grants from the NIH (R21 ES029624-01, R01 ES029136) as well as Find the Cause Breast Cancer Foundation and Johnson & Johnson Lung Cancer Initiative.

The aryl hydrocarbon receptor (AHR) is a sensor of low-molecular-weight molecule signals that originate from environmental exposures, the microbiome, and host metabolism. Building upon initial studies examining anthropogenic chemical exposures, the list of AHR ligands of microbial, diet, and host metabolism origin continues to grow and has provided important clues as to the function of this enigmatic receptor. The AHR has now been shown to be directly involved in numerous biochemical pathways that influence host homeostasis, chronic disease development, and responses to toxic insults. As this field of study has continued to grow, it has become apparent that the AHR is an important novel target for cancer, metabolic diseases, skin conditions, and autoimmune disease. This meeting attempted to cover the scope of basic and applied research being performed to address possible applications of our basic knowledge of this receptor on therapeutic outcomes.