Myeloid Cells and Inhibitory Immune Checkpoints in Colorectal Cancer Patients

Myeloid-derived suppressor cells (MDSCs) and upregulation of inhibitory immune checkpoints (ICs) are two of the key mechanisms utilized by tumor cells to induce immunosuppression and associated tumorigenesis. Despite the progress achieved in investigating MDSCs and ICs in cancer, the expression of different inhibitory ICs on distinct immune cells including MDSCs remains ambiguous. In this study, we investigated levels of myeloid cell subsets and IC expression on these myeloid subsets and other immune cells, using flow cytometry, in unfractionated whole blood (WB), peripheral blood mononuclear cells (PBMCs), normal adjacent tissue (NT), and tumor tissue (TT) of colorectal cancer (CRC) patients. We found monocytic myeloid cells (MMCs) are significantly expanded in PBMCs of CRC patients, compared to healthy donors (HDs). Contrary to the periphery, levels of granulocytic myeloid cells (GMCs) and immature myeloid cells (IMCs) were higher than MMCs in the tumor microenvironment (TME). Additionally, the TME had a significantly higher level of GMCs compared to NT. TIM-3 was mainly expressed on MMCs and antigen-presenting cells (APCs) in the periphery, but only on APCs in NT, and on T cells and APCs in the TME. Moreover, circulating TIM-3⁺ APCs showed a significantly higher level of PD-1 compared to their TIM-3^- counterparts. TIM-3^- T cells in NT expressed higher levels of PD-1 than TIM-3⁺ T cells. In contrast, TIM-3⁺ T cells in the TME showed a significantly higher expression of PD-1. This is the first study to investigate the differential expression of TIM-3 in CRC patients. Our results help in advancing the current knowledge on the immunosuppressive milieu in the periphery and TME of CRC patients which can aid in the improvement and development of immunotherapeutic agents for cancer patients.