Figure 1. TIM-3/Galectin-9 signaling pathway
T-cell immunoglobulin and mucin-domain containing-3 (TIM-3), otherwise known as Hepatitis A virus cellular receptor 2 (HAVCR2), is a protein that in humans is encoded by the HAVCR2 gene. TIM-3 belongs to TIM family cell surface receptor proteins. These proteins share a similar structure, in which the extracellular region consists of membrane distal single variable immunoglobulin domain (IgV) and a glycosylated mucin domain of variable length located closer to the membrane. Intracellular domain of TIM-3 is called C-terminal cytoplasmic tail. It contains five conserved tyrosine residues that interact with multiple components of T-cell receptor (TCR) complex and negatively regulate its function. TIM-3 was first described in 2002, and then the researchers found that it expressed on IFNγ producing CD4+ Th1 and CD8+ Tc1 cells. Later, the expression was found in Th17 cells, regulatory cells(Tregs), and innate immune cells (dendritic cells, NK cells, monocytes). Galectin-9 is one of the most studied ligand for TIM-3(HAVCR2). In human, it is encoded by the LGALS9 gene. And it has N- and C- terminal carbohydrate-binding domains connected by a link peptide. Multiple alternatively spliced transcript variants have been found for this gene. From the existing research, we can see that Galectin-9 can interact with CLEC7A, CD137, CD40 and regulate different function which independent TIM-3. For example, interaction with CD40 on T-cells induces their proliferation inhibition and cell death.
The function of pathway
TIM-3 is an immune checkpoint and together with other inhibitory receptors including programmed cell death protein 1 (PD-1) and lymphocyte activation gene 3 protein (LAG3) mediate the CD8+ T cell exhaustion. TIM-3 has also been demonstrated as a CD4+ Th1-specific cell surface protein that regulates macrophage activation and enhances the severity of experimental autoimmune encephalomyelitis in mice. TIM-3 is mainly activated by galectin-9. In the absence of ligand binding, TIM-3 is associated with Bat3, protecting the cell from TIM-3–mediated inhibition and allowing for greater activation. However, once TIM-3 binds to Galectin-9, Y265 (TIM-3 signaling is dependent on Y265 phosphorylation by inducible T-cell kinase) is phosphorylated and the interaction with Bat3 is disrupted, allowing TIM-3 to deliver inhibitory signals to the T cell. And then these processes will causes stimulation of an influx of calcium to intracellular space and induction of programmed cell death, apoptosis. As a consequence, a suppression of Th1 and Th17 responses and induction of immune tolerance occurs. In addition to galectin-9, a couple other ligands have been identified, such as phospatidyl serine (PtdSer), High Mobility Group Protein 1 (HMGB1) and Carcinoembryonic Antigen Related Cell Adhesion Molecule 1 (CEACAM1). The binding of PtdSer has been shown to cause an uptake of apoptotic cells and reduced cross presentation of dying cell-associated antigens by dendritic cells. The binding of HMGB1 can interfere with nucleic acid stimulation and suppress activation of the innate immune response. The role of CEACAM1 engagement has not yet been clear.
The importance of the immune system in protection against cancer was originally proposed in the theory of cancer immunosurveillance. This theory holds that the immune system can recognize cancerous cells as they arise and can mount both innate and adaptive immune responses to eliminate them. TIM-3/galectin-9 signaling pathway can be target to treat cancer by regulating the immune responses. According to previous study, the expression of galectin-9 has been detected on various hematological malignancies such as CLL(Chronic Lymphocytic Leukemia), MDS(Myelodysplastic Syndromes), Hodgkin and Non-Hodgkin lymphomas, AML(Acute Myeloid Leukemia) or solid tumors, such as lung cancer, breast cancer, and hepatocellular carcinoma. And its receptor is also upregulated in tumor-infiltrating lymphocytes in lung, gastric, head and neck cancer, schwannoma, melanoma and follicular B-cell non-Hodgkin lymphoma. And we know that TIM-3/ galectin-9 interaction attenuates T-cell expansion and effectors function in tumor microenviroment and chronic infections. From the existing research results, this pathway may act as a supporting role for the PD-1 pathway, such as the study found TIM-3/ galectin-9 pathway may interact with the PD-1 pathway in the dysfunctional CD8+ T cells and Tregs in cancer. TIM-3 is mainly expressed on activated CD8+ T cells and suppresses macrophage activation following PD-1 inhibition and TIM-3 upregulate in tumors progressing after anti-PD-1 therapy. This appears to be a form of adaptive resistance to immunotherapy. Multiple phase 1/2 clinical trials with anti-TIM-3 monoclonal antibodies in combination with anti-PD-1 or anti-PD-L1 therapies are ongoing. This benefits the efficiency of treatment of anti-PD-1 or anti-PD-L1 therapies. Besides, galectin-9 contributes to tumorigenesis by tumor cell transformation, cell-cycle regulation, angiogenesis, and cell adhesion. The correlative studies analyzing the expression of galectin-9 and malignant clinical features showed controversial results. This can be explained as that galectin-9 can promote tumor immune escape as well as inhibiting metastasis by promoting endothelial adhesion. Therefore many factors such as tumor type, stage, and the involvement of different galectins should be taken into consideration when correlating the expression level and the malignancy.