Abstract A082: Investigating the impact of systemic immunosuppression on immunotherapy responses in melanoma

Abstract Combination immunotherapy (combi IO) has revolutionized the treatment of melanoma patients. Although combi IO is effective, with 87% of patients reaching 10-year melanoma specific survival, 62.6% of those treated will experience severe immune-related adverse events (irAEs) requiring treatment discontinuation and administration of systemic steroids, such as prednisolone (PRED). To determine the impact of systemic steroids on combi IO response, we established a melanoma mouse model. Mouse melanoma cells (Y3.3UVRc34) were subcutaneously injected into female C57BL/6J mice, and tumor growth was monitored. Once tumor volume reached 145mm3, combi IO (0.2 mg anti-CTLA4 and 0.3 mg anti-PD1 per mouse) was administered intraperitoneally every three days for 3 cycles, followed by oral vehicle control (DMSO) or PRED starting at 1 mg/kg daily and tapered by 0.1 mg/kg every three days until completion. Control mice received isotype antibodies ± PRED. Blood, lymph node and tumor tissues were collected prior to, during treatment and at endpoint of the study. As expected, 100% of isotype (8/8) and isotype + PRED treated mice (18/18) did not respond to treatment (non-responders, NRs). Of animals treated with combi IO + vehicle control, 74% (14/19 animals) showed a complete response (CR), and the remaining 26% (5/19) were NRs. In contrast, of mice treated with combi IO + PRED, 57% (12/21 animals) achieved CR, 14% (3/21) NR, and 29% (6/21) displayed delayed progression (DP), defined as initial ≥ 30% reduction in peak tumor volume followed by tumor outgrowth in the steroid-treatment phase. Changes in circulating T lymphocytes were investigated using flow cytometry. Early during treatment (EDT), PRED-treated animals displayed a reduction in proliferation of naïve CD4 T cells (down 28%, p = 0.011), naive CD8 (down 29%, p = 0.015), central memory CD8 (Tcm, CD44+CD62L+, down 30%, p = 0.048) and effector memory CD8 T cells (Tem, CD44+CD62L-, down 25%, p = 0.005). There was a profound reduction in highly proliferative and activated (ki67+ PD1+) CD8 Tcm (down 48%, p = 0.010) and CD8 Tem (down 41%, p = 0.002), compared to vehicle control treated mice. We next addressed the early effects of PRED on T cell phenotypes in lymph nodes (LN) and tumor microenvironment (TME). We found a reduction in proliferation (ki67+) of regulatory T cells (down 28% in the LN and 25% in the TME; p < 0.05), and CD8 Tcm (down 36% in the LN and 30% in the TME; p <0.001). In contrast, changes to CD8 Tem were limited to the LN, with a 39.4% reduction in proliferation (p = 0.034) and 38% reduction in PD1 expression (p = 0.023). There were no changes late during treatment timepoints. In summary, PRED administration following 3 cycles of combi IO allowed for tumor escape in 30% of animals, which was accompanied by a reduction in the activation and proliferation of multiple T cell subsets. Further research is needed to understand the mechanisms by which PRED affects CD8 Tcm and CD8 Tem cells, to optimize the management of severe irAEs and maintain immunotherapy response. Citation Format: Jesse R Brown, Bernadette Pedersen, Malama Irvine, Ashleigh Stewart, Georgina V Long, Su Yin Lim, Helen Rizos, Elena Shklovskaya. Investigating the impact of systemic immunosuppression on immunotherapy responses in melanoma [abstract]. In: Proceedings of the AACR IO Conference: Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2025 Feb 23-26; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2025;13(2 Suppl):Abstract nr A082.