B. Kaczkofsky1, L.A. Golden2, A. Shafer1, J. Wilkowski1, E. Kenna2, C.V. Angeles1,2 1University Of Michigan, Surgical Oncology, Ann Arbor, MI, USA 2University Of Michigan, Cancer Biology, Ann Arbor, MI, USA
Introduction: Despite recent advances, metastatic melanoma remains a lethal disease with over half of patients failing to respond to available immunotherapies. Studies suggest that melanoma patients who respond to immune checkpoint blockade (ICB) have intratumoral and skin resident memory CD8+ T cells (TRM). Patients with exceptional response to ICB, such as complete responders, have been shown to have a population of TRM characterized by high interferon gamma (IFNg) expression which predicted longer survival in melanoma patients. A key limitation to studying the role of TRM has been the fact that previous mouse models, such as B16, do not have intratumoral TRM. We now have newer melanoma mouse models that represent diverse subtypes of human cutaneous melanoma, including one in which we observe the establishment of TRM. We hypothesize that mouse melanomas with TRM have better responses to ICB.
Methods: We used B2905 and HCmel1274 melanoma cell lines. B2905 melanoma has a higher tumor mutation burden and is more sensitive to ICB than HCmel1274 melanoma. C57BL/6 mice (n=3 per cell line) were injected with cells for tumor formation. Flow cytometry was conducted on fresh tumors to identify T cell subsets based on expression of CD44+ (antigen experienced), cytotoxicity CD8+, resident memory markers CD62L-, CD103+, CD69+, and checkpoints PD-1 and LAG-3. C57BL/6 mice (n=22) injected with B2905 were further analyzed for intratumoral CD8+ IFNg expression. The t-test was used to compare proportions of T cell populations.
Results: Both B2905 and HCmel1274 derived tumors had similar amounts CD8+ (42% vs. 55%, p=0.2) and CD4+ T cells (48% vs. 35%, p=0.4), respectively. However, more CD69+CD103+CD8+ TRM were identified in B2905 derived tumors compared to minimal amounts in tumors derived from HCmel1274 (15% vs. 4%, p=0.03). CD103+CD69+ TRM had more PD-1 and LAG-3 expression compared to CD103+CD69- CD8+ T cells. When activated, 43% of CD103+CD69+ TRM compared to 34% of CD103-CD69+CD8+ non-TRM express IFNg (p=0.017).
Conclusion: These results suggest that melanomas with higher levels of TRM are more sensitive to ICB. This may be due to increased levels of immune checkpoint expression and ability to be activated and express IFNg. Future studies utilizing these newer models have the potential to advance the understanding of durable immunity to cancer and provide new avenues for immunotherapy development and improved patient survival.