Immune checkpoint inhibitors (ICIs) are effective in clinical settings; however, they present immune-related adverse effects and financial burden. Although dose reduction of ICIs may mitigate these limitations, it could compromise therapeutic efficacy. Using two adjuvants (poly(I:C) and LAG-3-Ig) combined with three neoantigen peptides (Comb), we examined whether Comb could enhance the efficacy of reduced dose of αPD-1 monoclonal antibody (RD-αPD-1 mAb), which has limited efficacy. In a murine colorectal cancer model using an MC38 cell line, Comb addition to RD-αPD-1 mAb enhanced treatment efficacy. Analysis of the tumour microenvironment (TME) in mice treated with Comb using flow cytometry and single-cell RNA sequencing revealed decreased macrophages with highly expressing immunosuppressive genes and increased plasmacytoid dendritic cells with highly expressing antigen-presenting genes. A potent infiltration of CD8+ tumour-infiltrating lymphocytes (TILs) with an effector profile was only observed in RD-αPD-1 mAb with Comb. Additionally, single-cell T cell receptor repertoire analysis underscored an oligoclonal expansion of CD8+ TILs following treatment with RD-αPD-1 mAb with Comb. This novel immune drug combination may be a promising strategy for reducing αPD-1 mAb dosage while preserving antitumour efficacy through modulating the TME.
