Conjugated Bilirubin Upregulates TIM-3 Expression on CD4 + CD25 + T Cells: Anti-Inflammatory Implications for Hepatitis A Virus Infection.

Bilirubin (BR), a metabolite with increased concentrations in plasma during viral hepatitis, has been recognized as a potential immune-modulator. We recently reported that conjugated BR (CB) augments regulatory T cell (Treg) suppressor activity during acute hepatitis A virus (HAV) infection. However, the mechanisms related to the effects of CB on Treg function in the course of hepatotropic viral diseases have not been elucidated. T cell immunoglobulin domain and mucin domain 3 (TIM-3), via its interactions with galectin-9 (GAL-9), is a receptor associated with enhanced Treg function. Thus, TIM-3 expression may be related to the crosstalk between CB and Tregs during HAV infection. Herein, in vitro treatment with high concentrations of CB upregulated TIM-3 expression on Tregs from healthy donors. CB treatment in vitro did not induce de novo Treg generation, and in vitro stimulation with TGF-β, which shows increased secretion during HAV infection, resulted in a trend toward increased TIM-3 expression on Tregs and CD4 ⁺ T lymphocytes (TLs) from healthy donors. Interestingly, an upregulation of TIM-3 expression on CD4 ⁺ CD25 ⁺ T cells and an increase in the proportion of CD4 ⁺ TLs expressing GAL-9 were found in HAV-infected patients with abnormal CB values relative to healthy controls. In addition, a statistically significantly reduction in IL-17F production was observed after treatment of CD4 ⁺ TLs from healthy donors with high doses of CB in vitro. In summary, our results suggest that CB might regulate Treg activity via a TIM-3-mediated mechanism, ultimately leading to an anti-inflammatory hepatoprotective effect.