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The tumor microenvironment affects the outcome of radiotherapy against head and neck squamous cell carcinoma (HNSCC). We recently found that tolerogenic myeloid cells accumulate in the circulation of HNSCC patients undergoing radiotherapy. Here, we analyzed tumor-containing lymph node biopsies collected from these patients. After 2 weeks of radiotherapy, we found an increase in tumor-associated macrophages (TAMs) with activated STAT3, while CD[8.sup.+] T cells were reduced as detected using multiplex IHC. Gene expression profiling indicated upregulation of M2 macrophage-related genes (CD163, CD206), immunosuppressive mediators (ARG1, LIF, TGFB1), and Th2 cytokines (IL4, IL5) in irradiated tumors. We next validated STAT3 as a potential target in human HNSCC-associated TAMs, using UM-SCC1 xenotransplants in humanized mice. Local injections of myeloid cell-targeted STAT3 antisense oligonucleotide (CpG-STAT3ASO) activated human DCs/macrophages and promoted CD[8.sup.+] T cell recruitment, thereby arresting UM-SCC1 tumor growth. Furthermore, CpG-STAT3ASO synergized with tumor irradiation against syngeneic HP[V.sup.+] mEERL and HP[V.sup.-] MOC2 HNSCC tumors in mice, triggering tumor regression and/or extending animal survival. The antitumor immune responses were CD[8.sup.+] and CD[4.sup.+] T cell dependent and associated with the activation of antigen-presenting cells (DCs/M1 macrophages) and increased CD[8.sup.+] to regulatory T cell ratio. Our observations suggest that targeted inhibition of STAT3 in tumor-associated myeloid cells augments the efficacy of radiotherapy against HNSCC., Introduction Head and neck squamous cell carcinoma (HNSCC) is the sixth leading malignancy by incidence worldwide, with less than half of patients surviving for 5 years (1). HPV infection is [...]