TDO2 Deficiency Exacerbates the Immune Rejection Response in Rat Liver Transplantation via the Kyn-AHR Axis.

Background: The role of tryptophan 2,3-dioxygenase2 (TDO2), a key enzyme in the L-tryptophan (Trp)-kynurenine (Kyn) pathway, in liver transplant immunity is unclear. This study aims to explore the role of TDO2 in liver transplant rejection.

Methods: We used clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 to construct a TDO2 knockout rat model for liver transplant rejection. We validated the effects of TDO2 on acute rejection and survival, assessed TDO2 expression, and measured Trp and Kyn levels. We studied how TDO2 deficiency affects inflammatory cytokines, analyzed immune cell subtypes and their spatial distribution, and examined programmed death 1 and programmed cell death-ligand 1 (PDL1) spatial distribution and expression using multiplex immunohistochemistry. We also validated the regulatory mechanism of TDO2 on transplant-related immune cells in vivo and in vitro.

Results: TDO2 deficiency in the allograft liver worsens acute rejection and reduces survival rates. During transplant rejection, TDO2 expression increases, enhancing Trp metabolism and elevating serum Kyn levels. TDO2 knockout mitigates this process. The TDO2-Kyn-aryl hydrocarbon receptor pathway regulates acute rejection. TDO2 knockout reprograms immune cell distribution, decreasing regulatory T cells and M2 macrophages in the intermediate region while increasing CD8 + T cells and M1 macrophages in the portal area, leading to M1 polarization. Additionally, TDO2 deficiency raises programmed death 1 and programmed cell death-ligand 1 expression, varying with the spatial distribution and quantity of immune cells. TDO2 can regulate the proliferation and differentiation of various immune cells through the Kyn-aryl hydrocarbon receptor pathway.

Conclusions: Collectively, we elucidated the mechanism of TDO2 in liver transplant immune rejection and used spatial immunity to reveal the impact of TDO2 on liver transplantation.