Dissecting cell-specific CEACAM1 expression on immune cells in hepatic immune regulation

Introduction:

Autoimmune hepatitis results from a quantitative or functional imbalance between pro-inflammatory effector T cells and anti-inflammatory regulatory T cells (Treg) in the liver. CEACAM1 (carcinoembryonic antigen-related cell adhesion molecule 1) is a member of the glycoprotein CEA family and it is expressed by T cells, natural killer (NK) cells, as well as antigen-presenting cells. CEACAM1 acts as an immune co-receptor and a homophilic and heterophilic adhesion molecule; its two major isoforms with either a long (CEACAM1-L) or a short (CEACAM1-S) cytoplasmic tail control T cell activation. The long isoform contains two ITIMs (immunoreceptor tyrosine-based inhibitory motifs) and inhibits T cell receptor activation and inflammatory cytokine production via binding of the tyrosine phosphatase SHP-1. In contrast, the short isoform without the ITIMs acts as an independent T cell activator. Previously, we have shown that CEACAM1 expression in CD4+T cells augments IL-2 production, which is crucial for hepatic Treg induction and expansion. Ceacam1knockout mice show exacerbation in a mouse model for autoimmune hepatitis as a result of inappropriate expansion of T effector cells and concomitant reduction in Tregs. In the serum of human patients with obstructive or autoimmune liver disease, soluble CEACAM1 is elevated and could act as a modulator of the hepatic immune response. However, the functional relevance of CEACAM1-CEACAM1-interactions in hepatic immune regulation has not been defined so far.

Objectives:

To reveal the functional relevance of soluble and cell-bound CEACAM1 and manipulation of potential CEACAM1-CEACAM1-homophilic or heterophilic adhesion between antigen presenting cells and CD4+T cells in hepatic immune regulation.

Methods:

In co-cultures between murine CD4+T cells and dendritic cells from wild type (WT) and Ceacam1-/-mice, activation markers as well as cytokine production were analyzed in flow cytometry or ELISA. Furthermore, the kinetics of CEACAM1 expression on CD4+T cells and dendritic cells was analyzed by semiquantitative RT-PCR. In Western Blots, presence of soluble CEACAM1 was analyzed in human patients with AIH and PSC.

Results:

In human sera, soluble CEACAM1 was elevated most prominently in PSC. CEACAM1 was upregulated on murine CD4+T cells after homophilic binding of CEACAM1 between WT CD4+T cells and dendritic cells. Semiquantitative RT-PCR revealed CEACAM1-S as the predominantly expressed isoform on CD4+T cells and dendritic cells. Additionally, CEACAM1-deficient CD4+T cells showed a hyperproliferative and hyperactivated phenotype. Increased IL-12 production by CEACAM1-deficient dendritic cells in coculture with WT CD4+T cells but not CEACAM1-deficient CD4+T cells points towards heterophilic CEACAM1-dependent activation of DCs.

Conclusion and outlook:

CEACAM1 expression on CD4+T cells and T cell and DC activation are modulated in a CEACAM1-dependent manner. Soluble CEACAM1 is elevated in human patients with autoimmune liver disease. Currently, we are validating the effects of recombinant soluble CEACAM1 on T cell and DC activation in T cell and DC cocultures.