Head and neck squamous cell carcinoma are highly innervated by peripheral sensory neurons. Local neurotransmitter release (e.g., calcitonin gene‐related peptide (CGRP)) from sensory neurons innervating cancer is linked to tumorigenesis. CGRP‐immunoreactive nerve presence comprised 9.53±1.9% of total nerve area across 11 HNSCC patients. A syngeneic tongue tumor transplant mouse model of oral cancer and a global Calca knockout mouse (CGRP^KO) are used to investigate the impact of CGRP signaling on tumor growth and the associated immune response in vivo. In tumor‐bearing CGRP^KO mice, there is a significant reduction in tumor size over time compared to wildtype mice using two different mouse oral cancer cell lines. Furthermore, tumor tissue from CGRP^KO mice had a significant increase in tumor‐infiltrating CD4⁺ T cells, cytotoxic CD8⁺ T cells, and NK1.1⁺ NK cells compared to wildtype. Fluorescent‐activated cell sorting and real‐time qPCR are used to confirm that CD4⁺ T cells are isolated from tumor‐bearing wildtype mice containing a high expression of Ramp1 compared to sham mice. These data suggest that sensory neurotransmitter CGRP may modulate oral cancer progression via tumor immunosurveillance. Understanding the relationship between sensory neurons and cancer will aid in repurposing clinically available nervous system drugs for the treatment of cancer.

Summary: The field of cancer neuroscience has begun to define the contributions of nerves to cancer initiation and progression; here, we highlight the future directions of basic and translational cancer neuroscience for malignancies arising outside of the central nervous system.

Sensory nerves form a crucial component of the tumor microenvironment (TME) that relays vital information to the central nervous system and modulates tumor progression via immunosurveillance. Afferent activity processed by the brain can sensitize brain circuitry and influence host behaviors. Peripheral sensory signaling (e.g., release of neuropeptides in the TME) can drive phenotypic changes in the tumor immune response, such as increased exhaustion markers and inhibited effector cell activity, which promote cancer progression. In this review we highlight the most recent evidence demonstrating the pivotal role of the sensory nervous system in cancer, with a focus on primary tumor pain, and we discuss the extent to which pain can influence cancer progression and treatment response, including immunotherapeutic strategies.

Oral squamous cell carcinoma (SCC) pain is more prevalent and severe than pain generated by any other form of cancer. We previously showed that protease-activated receptor-2 (PAR₂) contributes to oral SCC pain. Cathepsin S is a lysosomal cysteine protease released during injury and disease that can activate PAR₂. We report here a role for cathepsin S in PAR₂-dependent cancer pain. We report that cathepsin S was more active in human oral SCC than matched normal tissue, and in an orthotopic xenograft tongue cancer model than normal tongue. The multiplex immunolocalization of cathepsin S in human oral cancers suggests that carcinoma and macrophages generate cathepsin S in the oral cancer microenvironment. After cheek or paw injection, cathepsin S evoked nociception in wild-type mice but not in mice lacking PAR₂ in Na_v1.8-positive neurons (Par₂Na_v1.8), nor in mice treated with LY3000328 or an endogenous cathepsin S inhibitor (cystatin C). The human oral SCC cell line (HSC-3) with homozygous deletion of the gene for cathepsin S (CTSS) with CRISPR/Cas9 provoked significantly less mechanical allodynia and thermal hyperalgesia, as did those treated with LY3000328, compared to the control cancer mice. Our results indicate that cathepsin S is activated in oral SCC, and that cathepsin S contributes to cancer pain through PAR₂ on neurons.

Evolutionarily, the nervous system and immune cells have evolved to communicate with each other to control inflammation and host responses against injury. Recent findings in neuroimmune communication demonstrate that these mechanisms extend to cancer initiation and progression. Lymphoid structures and tumors, which are often associated with inflammatory infiltrate, are highly innervated by multiple nerve types (e.g. sympathetic, parasympathetic, sensory). Recent preclinical and clinical studies demonstrate that targeting the nervous system could be a therapeutic strategy to promote antitumor immunity while simultaneously reducing cancer‐associated neurological symptoms, such as chronic pain, fatigue and cognitive impairment. Sympathetic nerve activity is associated with physiological or psychological stress, which can be induced by tumor development and cancer diagnosis. Targeting the stress response through suppression of sympathetic activity or activation of parasympathetic activity has been shown to drive activation of effector T cells and inhibition of myeloid‐derived suppressor cells within the tumor. In addition, there is emerging evidence that sensory nerves may regulate tumor growth and metastasis by promoting or inhibiting immunosuppression in a tumor‐type specific manner. Because neural effects are often tumor‐type specific, further study is required to optimize clinical therapeutic strategies. This review examines the emerging evidence that neuroimmune communication can regulate antitumor immunity as well as contribute to development of cancer‐related neurological symptoms.