BTLA-HVEM Checkpoint Axis Regulates Hepatic Homeostasis and Inflammation in a ConA-Induced Hepatitis Model in Zebrafish

The BTLA-HVEM checkpoint axis plays extensive roles in immunomodulation and diseases, including cancer and autoimmune disorders. However, the functions of this checkpoint axis in hepatitis remain limited. In this study, we explored the regulatory role of the Btla(-)Hvem axis in a ConA-induced hepatitis model in zebrafish. Results showed that Btla and Hvem were differentially expressed on intrahepatic Cd8(+) T cells and hepatocytes. Knockdown of Btla or Hvem significantly promoted hepatic inflammation. Btla was highly expressed in Cd8(+) T cells in healthy liver but was downregulated in inflamed liver, as evidenced by a disparate proportion of Cd8(+)Btla(+) and Cd8(+)Btla(-) T cells in individuals without or with ConA stimulation. Cd8(+)Btla(+) T cells showed minimal cytotoxicity to hepatocytes, whereas Cd8(+)Btla(-) T cells were strongly reactive. The depletion of Cd8(+)Btla(-) T cells reduced hepatitis, whereas their transfer enhanced hepatic inflammation. These observations indicate that Btla endowed Cd8(+)Btla(+) T cells with self-tolerance, thereby preventing them from attacking hepatocytes. Btla downregulation deprived this tolerization. Mechanistically, Btla-Hvem interaction contributed to Cd8(+)Btla(+) T cell tolerization, which was impaired by Hvem knockdown but rescued by soluble Hvem protein administration. Notably, Light was markedly upregulated on Cd8(+)Btla(-) T cells, accompanied by the transition of Cd8(+)Btla(+) Light(-) to Cd8(+)Btla(-)Light(+) T cells during hepatitis, which could be modulated by Cd4(+) T cells. Light blockade attenuated hepatitis, thereby suggesting the positive role of Light in hepatic inflammation. These findings provide insights into a previously unrecognized Btla-Hvem-Light regulatory network in hepatic homeostasis and inflammation, thus adding a new potential therapeutic intervention for hepatitis.