Purpose/Objective(s): Immune checkpoint inhibitors (ICIs) improve outcomes of patients with some solid tumors such as melanoma and lung cancer, however the efficacy of ICI alone showed limited efficacy in many other tumors such as breast cancer. Radiation therapy (RT) is a promising combination partner of ICI as a strong immune stimulator; however, it also can increase immune suppressive repertoires.TIGIT (T cell Immunoglobulin and ITIM domain) is an inhibitory receptor expressed on activated lymphocytes. We evaluated the effects of TIGIT blockade in addition to local RT and PD-1 blockade as a strategy to overcome therapeutic resistance of ICI in a syngenic murine breast cancer model. Materials/
Methods: 4T1-Luc tumors were treated with various strategies including control, RT, anti-PD-1, anti-TIGIT, RT+anti-PD-1, RT+anti-TIGIT, anti-PD-1+anti-TIGIT, and RT+anti-PD-1+anti-TIGIT. A total of 24 Gy in 3 fractions at 1 week was delivered to the tumor at hindlimb (primary tumor) while the tumor at flank (secondary tumor) was left unirradiated. Anti-PD-1 blocking antibody (10 mg/kg) and anti-TIGIT blocking antibody (10 mg/kg) were intraperitoneally injected for 6 times for 2 weeks. Flow cytometry, IHC staining, and ELISA were performed to assess the immunologic response after each treatment modality.
Results: The triple combination therapy (TCT) group showed the most superior growth delay of primary and secondary tumor among treatments. The number of metastatic lung nodule was significantly reduced by TCT as well. Plasma levels of interferon-ß and interferon-? were the highest after TCT. Proportion of CD8+ dendritic cells among immune cells in spleen as well as tumor-draining lymph node was significantly increased by TCT. Moreover, TCT significantly increased effector-memory (CD44+CD62L-) cells among splenic Foxp3- non-regulatory CD4+ and CD8+ T cells, while decreasing proportion of regulatory T cells (Tregs). TCT also increased the infiltration of CD8+ T cells in primary and secondary tumors. Furthermore, increased expression of CD226, an activating counter-receptor of TIGIT, on CD8+ T cells following TIGIT blockade was observed in both spleen and primary tumor, possibly suggesting the activation of anti-tumor immune response by addition of TIGIT blockade to local RT and PD-1 blockade. Additionally, intratumoral myeloid-derived suppressor cells were significantly decreased by TCT. TCGA data revealed that expression of TIGIT was associated with worse survival outcomes in triple-negative breast cancer, suggesting the possible immunosuppressive role of TIGIT.
Conclusion: TIGIT blockade elicits local and systemic antitumor immune responses in a murine triple-negative breast cancer model. The tumor growth delay and inhibition of lung metastasis were most prominent in TCT. These results suggest that TIGIT blockade could be a viable approach to increase the efficacy of RT and ICIs in breast cancer which is a relatively immunologically cold tumor.
