Novel TGF-β inhibitors ameliorate oral squamous cell carcinoma progression and improve the anti-tumor immune response of anti-PD-L1 immunotherapy.

TGF-β is a key regulator of oral squamous cell carcinoma (OSCC) progression and its potential role as a therapeutic target has been investigated with a limited success. This study evaluates two novel TGF-β inhibitors as mono or combinatorial therapy with anti-PD-L1 antibodies (α-PD-L1 Abs) in a murine OSCC model. Immunocompetent C57BL/6 mice bearing malignant oral lesions induced by 4-nitroquinoline 1-oxide (4-NQO) were treated for 4 weeks with TGF-β inhibitors mRER (i.p., 50μg/d) or mmTGF-β2-7m (10μg/d delivered by osmotic pumps) alone or in combination with α-PD-L1 Abs (7x i.p. of 100μg/72h). Tumor progression and body weight was monitored. Levels of bioactive TGF-β in serum were quantified using a TGF-β bioassay. Tissues were analyzed by immunohistology and flow cytometry. Therapy with mRER or mmTGF-β2-7m reduced tumor burden (p<0.05) and decreased body weight loss compared to controls. In inhibitor-treated mice, levels of TGF-β in tumor tissue and serum were reduced (p<0.05), while they increased with tumor progression in controls. Both inhibitors enhanced CD8+ T cell infiltration into tumors and mRER reduced levels of myeloid-derived suppressor cells (p<0.001). In combination with α-PD-L1 Abs, tumor burden was not further reduced, however, mmTGF-β2-7m further reduced weight loss (p<0.05). The collagen-rich stroma was reduced by using combinatorial TGF-β/PD-L1 therapies (p<0.05), enabling an accelerated lymphocyte infiltration into tumor tissues. The blockade of TGF-β signaling by mRER or mmTGF-β2-7m ameliorated in vivo progression of established murine OSCC. The inhibitors promoted anti-tumor immune responses, alone and in combination with α-PD-L1 Abs.