Abstract 1531: TIL analysis demonstrates different mechanisms underlying PD-1 resistance in animal models

Abstract Despite the tremendous success of PD-1 immunotherapy in treating cancer, resistance to the therapy is an inevitable risk. One of the major mechanisms underlying this resistance is alterations in the tumor microenvironment, including tumor-infiltrating lymphocyte (TIL) profiling changes after anti-PD1 treatment. To better understand the mechanism of PD-1 resistance, characterization of TILs in the tumor microenvironment was explored. First, immune cell profiling was performed in syngeneic tumor models established by 19 cancer cell lines without anti-PD-1 treatment. Depending on the tumor model, absolute numbers of CD45+ cells and immune cell subsets varied. Moreover, stratifying the analysis based on tumor size (100 mm3 to 3000 mm3) revealed alterations in the proportion of immune cell percentages in MC38 tumors, which implies alterations in TIL frequency during cancer progression. Next, we investigated the TIL profile in two PD-1 resistant models: a hPD-1 antibody-induced resistant model and a syngeneic model in aged mice. The hPD-1 antibody resistant model was induced by multiple rounds of hPD-1 antibody retreatment on MC38 tumor-bearing humanized B-hPD-1/hPD-L1 mice. Efficacy results from this model further validated their resistance to hPD-1 treatment (0% tumor growth inhibition (TGI) ), while the control group, which was not serially treated, showed substantial tumor inhibition (82. 2% TGI). Next, mPD-1 antibody resistance was evaluated in aged wild-type mice. While mPD-1 efficacy on MC38-bearing young mice was approximately 85. 5% TGI, the efficacy of this treatment in aged mice yielded only 6% TGI. TIL analysis in these models was subsequently performed: in the hPD-1-treated control mice that were responsive to anti-PD1 treatment, the numbers of total T cells, cytotoxic T cells (CD8+) and MHC II positive cells were increased, while the anti-tumor effector cells in the serially treated resistant mice were either unchanged (total T cells and MHC II positive cells) or decreased (CD8+ cells) ; strikingly, an increase in inhibitory myeloid-derived suppressor cell (MDSC) populations was also observed in the resistant model post-treatment. In the anti-PD1 treated aged mice, the frequency of TILs was unchanged upon mPD-1 treatment, suggesting that resistance in aged mice may be due to non-responsiveness of the immune cells. In sum, our results demonstrate different mechanisms underlying anti-PD-1 resistance, and suggest that a more thorough analysis of the TILs present in the tumor microenvironment could predict the responsiveness to PD-1 therapy and inform treatment strategies. Citation Format: Sha Qiao, Yujia Liu, Wei Liu, Wenqian Pan, Xiaofei Zhou, Linlin Wang, Mengya Chai, Yujie Liu, Hao Yang, Zhaoxue Yu. TIL analysis demonstrates different mechanisms underlying PD-1 resistance in animal models abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 1531.