CXCR2 and head and neck squamous cell carcinoma: Trials currently enrolling HNSCC patients combine checkpoint inhibitors with a number of agents designed to enhance the local anti-tumor immune microenvironment such as T-lymphocyte co-stimulatory agonists (CD27 agonist), chemokine receptor blockade (CXCR2, CSF1R and CCR4 blockade) and tumor targeting agents (EGFR targeting mAb and STAT3 blockade).