Study on the immunological characteristics and suppressive function of CEACAM1-expressing intratumoral regulatory T cells

Abstract

Regulatory T cells (Tregs) exert immunosuppressive functions and hamper antitumor immune responses in the tumor microenvironment. Understanding the heterogeneity of intratumoral Tregs, and how it changes with tumor progression, will provide clues regarding novel target molecules of Treg-directed immunotherapies. Here I characterized the phenotypes and suppressive function of Tregs from tumor tissues of patients with renal cell carcinoma and ovarian cancer using multicolor flow cytometry, RNA sequencing, and in vitro functional assay, and CT26 mouse tumor model. I found that carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) was selectively expressed on intratumoral Tregs, while its expression on peripheral Tregs or other immune cells was low. In consequence, CEACAM1$^+$ intratumoral Tregs accumulated with tumor progression in both RCC patients and mouse tumor model, while the CEACAM1$^–$ subset did not. Notably, I found that CEACAM1 marked intratumoral Tregs that exhibited highly suppressive and activated phenotypes with substantial clonal expansion. CEACAM1$^+$ intratumoral Tregs exhibited increased suppressive capacity compared to CEACAM1– counterpart in both in vitro and in vivo. Depletion of CEACAM1-expressing cells from tumor-infiltrating leukocytes led to selective depletion of Tregs and increased effector functions of CD8$^+$ and CD4$^+$ T cells. Moreover, CEACAM1$^+$ cell depletion further enhanced anti-PD-1–mediated reinvigoration of exhausted CD8$^+$ T cells. These findings demonstrate that CEACAM1 is an optimal target for intratumoral Treg depletion.