Soluble CEACAM1 induces suppressive Tregs and binds to CD5 in a heterophilic manner

Introduction CEACAM1 (carcinoembryonic antigen-related cell adhesion molecule 1) is an immune checkpoint regulator that controls immunity via self- and heteroligation. Soluble CEACAM1 (sCC1) was originally discovered as a serum marker in human patients with obstructive and autoimmune liver disease. In murine autoimmune hepatitis (Concanavalin A-induced hepatitis), CEACAM1 induces Tregs, and enhances Treg stability. Ceacam1-/- mice exhibit hyperinflammation and persistence of liver injury. IL-2-dependent Treg induction requires signaling via CEACAM1-S (CEACAM1 with a short cytoplasmic domain), on CD4+ T cells. Contrary, the long CEACAM1 isoform (CEACAM1-L) inhibits immune signaling via two ITIMs. It interacts with co-inhibitory immune receptors (e.g. TIM-3) and limits T cell activation. CD5 is a Treg marker that is implicated in Treg signaling and stability.

Objectives The role of CEACAM1-ligation and sCC1 in hepatic immune regulation is unknown. The role of CEACAM1/sCC1 in co-cultures of CD4+ T cells and antigen-presenting cells (dendritic cells, DCs) for T cell activation and Treg induction is investigated. Furthermore, heteroligands for CEACAM1, such as CD5, are validated regarding CEACAM1-dependent Treg induction.

Materials & methods In sera from human patients and mice, CEACAM1 was detected in Western Blots. Cocultures from bone-marrow derived, FACS-sorted DCs and MACS-sorted T cells from WT and Ceacam1-/- mice were analyzed with and without addition of sCC1 in FACS. Binding of sCC1 to CD5 was identified after affinity chromatography in mass spectrometry and transfection studies. Suppressive capacity of Tregs was analyzed in a suppression assay.

Results In sera of patients with advanced PSC and in sera of WT mice sCC1 was detectable. In cocultures of CD4+ T cells and dendritic cells, addition of sCC1 inhibits production of IL-2 by CD4+ T cells and IL-12 by DCs, regardless of their CEACAM1 expression status. sCC1 strongly binds to DC-activated CD4+CD25+ T cells which results in phosphorylation of STAT5 and upregulation of Foxp3. Preliminary data showed that sCC1-induced Foxp3 cells were capable of suppressing proliferation of effector T cells. Furthermore, sCC1-binding to CD5 on CD5-transfected HEK293 cells was confirmed.

Conclusion Addition of sCC1 to CD/T cell cocultures supports Treg induction. Currently, the relevance of CEACAM1-CD5 interaction in the context of Treg homeostasis is under validation.

Publication History

Article published online:
04 January 2021