Therapeutic efficacy of external beam radiotherapy combined with anti-PD-L1 inhibition in a preclinical syngeneic head and neck cancer model

Abstract

Treating high-grade head and neck squamous cell carcinoma (HNSCC) has recently combined immunotherapy with conventional therapies. However, optimizing the scheduling of anti-PD-L1 with external beam radiation therapy (EBRT) requires further research to improve efficacies.

Methods

In vitro, MTCQ1, MOCL1, and MOCL2 murine HNSCC cell responses to 2 Gy x 6 X-ray EBRT were assessed in metabolic, clonogenic and γH2AX assays. Ex vivo, syngeneic tumors in C57BL/6 mice were stained for haematoxylin and eosin (H&E) and immune cell infiltration. Combination therapeutic in vivo studies using MTCQ1 tumors treated with 2 Gy x 6 or 8 Gy x 1 EBRT with sequential and/or concurrent dosing with anti-PD-L1 were also performed.

Results

MTCQ1 cells exhibited the most marked responses to EBRT in vitro. H&E analysis revealed highest cellular density and most disperse extracellular matrix in MTCQ1 tumors with infiltration of CD8a+ T cells in tumor centres and macrophages predominantly peripheral. The 2 Gy x 6 EBRT regimen slowed tumor progression; average tumor volumes were 129.2 ± 49.0 mm³ on day 10, compared to 234.1 ± 130.7 mm³ in the CT-only control (P = 0.039). However, there was also a reduced CD8a+ T cell infiltration on day 3 post complete treatment, with 0.19 ± 0.17 % CD8a+ T cell area compared to 0.91 ± 0.31 % in the CT-only control. Combining EBRT with anti-PD-L1 (delivered either concurrently or sequentially), resulted in greater median survival compared to the CT-only control (33 and 32 days versus 28 days, respectively). Similarly, statistically insignificant improved survival with 8 Gy x 1 EBRT regimen combined with concurrent anti-PD-L1 was observed.

Conclusion

These results reveal spatial distribution of immune cells in the tumor microenvironment and underscore the role of EBRT regimens in modulating immune cell dynamics. This highlights the importance of optimising radiation protocols to inform combination therapy designs in preclinical models.