IFNγ-induced PD-L1+MHC II+ macrophages and Tim-3+ tumor-reactive CD8+ T cells predict a response to anti-PD-1 therapy in tumor-bearing mice.

While immune checkpoint inhibitors have led to durable responses in various cancer types, a substantial proportion of patients do not respond to these interventions. To uncover potential factors associated with a positive response to immunotherapy, we used a bilateral tumor model using P815 mastocytoma implanted in DBA/2 mice. In this model, only a fraction of tumor-bearing mice responds to anti-PD-1 treatment. Thus, it provides a valuable model to explore the influence of the tumor microenvironment (TME) in determining the efficacy of immune checkpoint blockade (ICB)-based immunotherapies. It also allows for the analysis of a pretreatment tumor and inference of its treatment outcome based on the response observed in the contralateral tumor. Herein, we report that tumor-reactive CD8+ T-cell clones expressing high levels of Tim-3 were associated with a positive antitumor response following anti-PD-1 administration. Our study also revealed distinct differentiation dynamics in tumor-infiltrating myeloid cells in responding and non-responding mice. An IFNγ-enriched TME promoted the differentiation of monocytes into PD-L1posMHC IIhigh cells in mice responding to immunotherapy. Monocytes present in the TME of non-responding mice failed to reach the same final stage of differentiation trajectory, suggesting that an altered monocyte to macrophage differentiation route may hamper the response to ICB. These insights will direct future research towards a temporal analysis of tumor-associated macrophages (TAMs), aiming to identify factors responsible for transitions between differentiation states within the TME. This approach may pave the way to novel strategies to enhance the efficacy of PD-1 blockade.