IL-6 Levels Correlate with Prognosis and Immunosuppressive Stromal Cells in Patients with Colorectal Cancer

The prognosis for patients with colorectal cancer (CRC) is determined by tumor characteristics as well as the host immune response. This study investigated the relationship between an immunosuppressive state and patient prognosis by evaluating the systemic and tumor microenvironment (TME) interleukin (IL)-6 levels.

Preoperative serum IL-6 levels were measured using an electrochemiluminescence assay. Expression of IL-6 in tumor and stromal cells was evaluated immunohistochemically in 209 patients with resected CRC. Single-cell analysis of tumor-infiltrating immune cells was performed using mass cytometry in 10 additional cases.

Elevated serum IL-6 levels were associated with elevated stromal IL-6 levels and a poor prognosis for patients with CRC. High IL-6 expression in stromal cells was associated with low-density subsets of CD3⁺ and CD4⁺ T cells as well as FOXP3⁺ cells. Mass cytometry analysis showed that IL-6⁺ cells among tumor-infiltrating immune cells were composed primarily of myeloid cells and rarely of lymphoid cells. In the high-IL-6-expression group, the percentages of myeloid-derived suppressor cells (MDSCs) and CD4⁺FOXP3^highCD45RA^- effector regulatory T cells (eTreg) were significantly higher than in the low-IL-6-expression group. Furthermore, the proportion of IL-10⁺ cells in MDSCs and that of IL-10⁺ or CTLA-4⁺ cells in eTregs correlated with IL-6 levels.

Elevated serum IL-6 levels were associated with stromal IL-6 levels in CRC. High IL-6 expression in tumor-infiltrating immune cells also was associated with accumulation of immunosuppressive cells in the TME.