CEACAM1 regulates TIM-3-mediated tolerance and exhaustion

Abstract

T-cell immunoglobulin domain and mucin domain-3 (TIM-3, also known as HAVCR2) is an activation-induced inhibitory molecule involved in tolerance and shown to induce T-cell exhaustion in chronic viral infection and cancers1–5. Under some conditions, TIM-3 expression has also been shown to be stimulatory. Considering that TIM-3, like cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed death 1 (PD-1), is being targeted for cancer immunotherapy, it is important to identify the circumstances under which TIM-3 can inhibit and activate T-cell responses. Here we show that TIM-3 is co-expressed and forms a heterodimer with carcinoembryonic antigen cell adhesion molecule 1 (CEACAM1), another well-known molecule expressed on activated T cells and involved in T-cell inhibition6–10. Biochemical, biophysical and X-ray crystallography studies show that the membrane-distal immunoglobulin-variable (IgV)-like amino-terminal domain of each is crucial to these interactions. The presence of CEACAM1 endows TIM-3 with inhibitory function. CEACAM1 facilitates the maturation and cell surface expression of TIM-3 by forming a heterodimeric interaction in cis through the highly related membrane-distal N-terminal domains of each molecule. CEACAM1 and TIM-3 also bind in trans through their N-terminal domains. Both cis and trans interactions between CEACAM1 and TIM-3 determine the tolerance-inducing function of TIM-3. In a mouse adoptive transfer colitis model, CEACAM1-deficient T cells are hyper-inflammatory with reduced cell surface expression of TIM-3 and regulatory cytokines, and this is restored by T-cell-specific CEACAM1 expression. During chronic viral infection and in a tumour environment, CEACAM1 and TIM-3 mark exhausted T cells. Co-blockade of CEACAM1 and TIM-3 leads to enhancement of anti-tumour immune responses with improved elimination of tumours in mouse colorectal cancer models. Thus, CEACAM1 serves as a heterophilic ligand for TIM-3 that is required for its ability to mediate T-cell inhibition, and this interaction has a crucial role in regulating autoimmunity and anti-tumour immunity.

Glycosylation Alters Dimerization Properties of a Cell-surface Signaling Protein, Carcinoembryonic Antigen-related Cell Adhesion Molecule 1 (CEACAM1)

“…These interactions are quite numerous, including those that are physiologically important and those that involve pathogen proteins. Recently, it has been found that the Ig V domain can interact with an immune regulator domain (T-cell immunoglobin domain and mucin domain 3, TIM-3), which shares a similar tertiary structure with CEACAM1-Ig V (24). Although the initial structure of a heterodimer reported has been withdrawn, NMR and SPR data confirm the existence of an interaction (59).…”

“…The asymmetric unit in an earlier crystal structure led to the suggestion that dimerization occurred via an interface involving the ABED face (25). More recent structures suggest dimerization occurs via the GFCCЈCЉ face (24). Supporting this more recent suggestion, interactions involving the GFCCЈCЉ side have been reported to be crucial for various types of biological response, including protection from killing by NK cells.…”

“…Val-39 and Asp-40 mutations in the CCЈ loop have also been shown to diminish homophilic dimer formation in fulllength CEACAM1 (32), and this same loop region also seems to be required for intercellular interaction with certain other family members (33). The recent crystal structure, Protein Data Bank (PDB) code 4QXW, as well as 4WHD, provide direct evidence that contacts between the GF loop and adjacent strands, along with CCЈ loop contacts, can form an interface (24). However, involvement of the ABED interface in dimer formation, as observed in the earlier crystal structure, PDB code 2GK2 (25), also has support.…”

“…It is likely that this complex set of effects arises as a result of the numerous homophilic and heterophilic interactions that CEACAM1 can have with itself and other members of the CEACAM superfamily. The N-terminal Ig variable (Ig V ) domain of CEACAM1 has been suggested to be the basis of cis and trans homo-dimer formation (20), as well as interactions with other molecules (21)(22)(23)(24). There are crystal structures of the Ig V domain expressed in Escherichia coli, which have led to the suggestion of two distinct dimerization interfaces (24,25).…”

“…The N-terminal Ig variable (Ig V ) domain of CEACAM1 has been suggested to be the basis of cis and trans homo-dimer formation (20), as well as interactions with other molecules (21)(22)(23)(24). There are crystal structures of the Ig V domain expressed in Escherichia coli, which have led to the suggestion of two distinct dimerization interfaces (24,25). However, examination of the interfaces suggests that the extensive glycosylation found in native material would inhibit dimer formation in one of these cases.…”