# Spontaneous cSCC Murine Model Shows Limited Response to PD-1 Blockade and Radiation Combination Therapy

## TL;DR

A mouse model of skin cancer showed limited response to immunotherapy and radiation, but some immune cell activity was observed.

## Contribution

A new spontaneous murine model of cSCC was developed to study limited responses to PD-1 blockade and radiation.

## Key findings

- Anti-PD1 therapy showed a trend toward slowed tumor growth but was not statistically significant.
- CD8+ T cell infiltration increased with anti-PD1 treatment and was further enhanced by radiation.
- Irradiated tumors exhibited fibroblastic spindle-like cell morphology.

## Abstract

The prevalence of cutaneous squamous cell carcinoma (cSCC) globally has increased significantly in the last 40 years. Specifically, locally advanced, metastatic, and high-risk cSCC tumors are associated with a worse prognosis and require multimodality therapy. This study investigated the effectiveness of treatment on spontaneously generated cSCC with both anti-PD1 monotherapy and the combinatorial anti-PD1 immunotherapy with radiation. Anti-PD1 therapy showed a trend toward a slowed progression of cSCC and increased CD8+ expression in preliminary single-specimen analysis.

Background/Objectives: Non-melanoma skin cancer, which includes cutaneous squamous cell carcinoma (cSCC), ranks as the 5th most common cancer globally with high morbidity and more total deaths than melanoma despite having a lower mortality rate. While most cSCC cases can be treated with surgery, locally advanced, metastatic, and high-risk cSCC tumors are associated with a worse prognosis with higher rates of recurrence and require multimodality therapy. However, there is limited data on animal models of cutaneous squamous cell carcinoma for the use of combinatory immunotherapy and radiation. Methods: In this study, spontaneously generated tumors using DMBA/TPA were treated over three weeks with either IgG control, anti-PD1 antibody monotherapy, 8 Gy of localized radiation, or a combination of anti-PD1 and 8 Gy of radiation followed by anti-PD1 therapy. Results: We found that while anti-PD1 therapy showed a trend toward slowed tumor growth compared to controls, this difference was not statistically significant (p = 0.0775), with most mice showing continued tumor progression. Preliminary histological analysis suggested that anti-PD1 treatment increased CD8+ T cell infiltration, and the addition of radiation further enhanced CD8+ responses but added greater variability. A pathologic review revealed that irradiated tumors were associated with fibroblastic spindle-like cell morphology. Conclusions: This animal model represents a potential preclinical model for studying CSCC with limited responses to immunotherapy to understand potential mechanisms of resistance.