Patients with chronic inflammatory bowel disease (IBD) suffer from chronic relapsing inflammation of the digestive tract. The intestinal mucosa of the affected patients is inflamed, which is caused by a dysregulation of the immune system. However, it remains largely unclear which mechanisms drive this process and why there is a dysregulation of the immune system and an excessive immune response in IBD patients. T cells, which are part of the immune system, play a key role in the development of IBD. An excess of pro-inflammatory T cells (effector T cells) promotes an overactive immune response. In this context, co-inhibitory immune checkpoint receptors play a crucial role. These receptors are primarily located on the surface of T cells and, when activated, dampen the immune response of the T cells. Previous studies have already shown that an impaired regulation of checkpoint receptors can lead to an uncontrolled and excessive immune response and, as a result, to autoimmune diseases. Therefore, the role of the immune checkpoint receptor T cell immunoglobulin and mucin domain-containing protein 3 (TIM3) on effector T cells in the context of chronic inflammatory bowel diseases should be clarified in this study. In order to decipher the role of TIM3 in IBD, human material was analysed for the TIM3 expression in this study. The study included intestinal sections, blood, and biopsies primarily from IBD patients and controls, which were characterised using flow cytometry, immunofluorescence staining, untargeted metabolomics, and further methods. Moreover, human recombinant galectin-9 was used in vitro in T cell culture experiments to investigate the effects of the interaction between TIM3 and its ligand galectin-9. Higher TIM3 transcript and protein levels were found on T cells in the colon of IBD patients, while the number of TIM3 receptors was significantly increased on mucosal TH17 cells in particular. An increased number of TIM3 molecules was also detected in inflamed intestinal tissue compared to non inflamed intestinal tissue. In vitro, after T cell receptor stimulation, higher TIM3 protein expression could be measured on immune cells from the blood of IBD patients compared to healthy patients. A notable finding is that a deficiency in the expression of the TIM3 ligand galectin-9 was detected at the gene and protein level in the intestines of colitis patients. This represents a relevant novel finding, as galectin-9 is necessary for the function of TIM3 and binds to the TIM3 receptor to activate its inhibitory effect. Elevated TIM3 protein levels combined with a simultaneous deficiency of galectin-9 result in a galectin-9/TIM3 imbalance in the intestines of IBD patients compared to controls. Thus, the aim was to activate mechanisms to restore the galectin-9/TIM3 balance in T cells and to investigate the in vitro effects of exogenous galectin-9 administration on metabolism and cell activity in T cells. The results of the untargeted metabolomics analysis indicated that TIM3 functions as an immunometabolic switch and that the activation of TIM3 by galectin-9 led to an inhibition of the enzyme adenosine deaminase and, consequently, to a decrease of the purine degradation. Furthermore, external administration of galectin-9 to stimulated T cells in culture resulted in a decrease in T cell activity, reduced IFN-γ, IL-17A, and IL-17F cytokine secretion, and a decrease in the number of TH17 cells. Exogenous administration of galectin-9 to stimulated TIM3 positive T cells prevented excessive T cell activity. In conclusion, the activation of TIM3 resulted in an inhibition of the enzyme adenosine deaminase, which led to the suppression of the purine metabolism and a reduced T cell activity. As a consequence, TIM3 plays an important role in the immune metabolism of T cells in IBD patients. A restoration of the galectin-9/TIM3 balance through external administration of galectin-9 or an agonistic α-TIM3 antibody represents a promising potential approach for the treatment of chronic inflammatory bowel diseases in the future.
Michael Gabel (Thu,) studied this question.