2930 - Impacts of Combining Anti-PD-1 Immunotherapy and PLDR on the Tumour Immune Microenvironment in a Murine Lung Cancer Model

P. Zhang¹, and S. Yin²; ¹Department of Radiation Oncology, Sichuan Cancer Hospital& Institution, Sichuan Cancer Center, School of Medicine, University of Electronic Science and Technology of China, Radiation oncology Key Laboratory of Sichuan Province, Chengdu, China, ²Graduate School, Chengdu Medical College, chengdu, China

Purpose/Objective(s): Pulsed low dose rate radiotherapy (PLDR) irradiation has emerged as a strategy to effectively control tumors and reduce radiation-induced toxicity compared to conventional radiotherapy, the immunomodulatory properties of PLDR irradiation remain unknown. Here this study investigated the inhibitory effect of PLDR combined with anti-PD-1 antibody on lung cancer in mice and its impact on tumor immune microenvironment.

Materials/

Methods: Lewis lung cancer tumors were established in the right leg of C57BL/6 mice. Animals received irradiation with daily 2-Gy fractions or in ten 0.2Gy pulses separated by 3-min intervals given continuously 5days. Anti-PD-1 and isotype control were administered intraperitoneally once every three days at a dose of 5mg/kg, a total of three times. For cellular and cytokine depletion experiments, either 250ug aCD8 mAb or aCD4 mAb, per mouse was delivered four times by i.p. injection every 3 days. All antibody treatments were started from the day of RT. Tumour growth delay and body weight were analyzed. The tumor?peripheral blood and tumor-draining lymph nodes were harvested 7 days after treatment, and a single cell suspension was prepared for flow cytometry to analyze the changes in the immune microenvironment and the expression of PD-L1?PD-1 and the activated systemic immune response.

Results: In vivo, addition of anti-PD-1 to PLDR significantly improved tumor response in LLC(P<0.0001), prolonged the survival time of mice and did not show any significant side effects. CD8+T cells depleted, the survival time of mice was significantly shortened, with no difference compared to the control. Furthermore, radiosensitization after PD-1 blockade was associated with reduced CD11b+Gr1+ myeloid cell infiltration and enhanced CD45+CD8+T cell infiltration with concomitant up-regulation of T-cell activation, activate the activity of DCs cells at 7 days post-combined treatment. In the PLDR combination group, it was observed that the proportion of MDSCs rapidly increased to the same level as the control group after depletion of CD8+T cells, and compared to the RT combined group, the PLDR combined group is more significant.

Conclusion: PD-L1 was up-regulated dependent on CD8+T cells in vivo and in vitro after PLDR. PLDR combined with anti-PD-1 can increase the infiltration of CD8+T cells into tumor tissue, effectively stimulate the maturation and activation of DCs in lymph nodes, reduce the accumulation of MDSCs, thereby changing the tumor immune microenvironment, achieving the synergistic effect of inhibiting the growth of mouse lung cancer, and extending the survival period. Administration of anti-PD-1 enhanced the efficacy of PLDR through a cytotoxic T cell-dependent mechanism which is an effective and safe strategy in a preclinical models of Lung cancer. Our data provide evidence for a close interaction between PLDR, T cells, and the PD-L1/PD-1 axis and establish a basis for the rational design of combination therapy with immune modulators and radiotherapy.