Comprehensive immunophenotyping reveals distinct tumor microenvironment alterations in anti-PD-1 sensitive and resistant syngeneic mouse model.

The advent of immune checkpoint inhibitors targeting the PD-1/PD-L1 pathway has revolutionized cancer treatment, resulting in improved clinical outcomes. However, resistance remains a critical challenge. This study aimed to comparatively elucidate immunophenotypic changes in syngeneic mouse models sensitive (MC-38) or resistant (LLC1) to anti-PD-1 monoclonal antibody (mAb) treatment. In the sensitive MC-38 model, anti-PD-1 therapy increased dendritic cells (DCs) and macrophages, while decreasing myeloid-derived suppressor cells (MDSCs) within the tumor microenvironment. Enhanced expression of antigen presentation molecules (MHC I/II) and costimulatory molecules (CD80/CD86) was observed on tumor-associated DCs and macrophages. Tumor-infiltrating CD4T, CD8T, regulatory T, NK, and NKT cells also significantly increased. Importantly, treatment boosted lymphocyte cytotoxic potential, with perforin identified as a key marker of efficacy. Notably, perforin expression in CD4T and NKT cells strongly negatively correlated with tumor volume. In contrast, the resistant LLC1 model exhibited minimal immunophenotypic changes upon treatment. These findings highlight critical immune modifications induced by anti-PD-1 therapy, particularly the role of perforin, and the DC/MDSC ratio in predicting therapeutic outcomes. This research offers valuable insights into potential predictive biomarkers and informs strategies to overcome resistance, emphasizing the complex interplay between anti-PD-1 treatment and the tumor microenvironment, ultimately aiming to improve immunotherapy response rates.