Post orthotopic liver transplantation (OLT) viral hepatitis is an immunological condition where immune cells induce hepatitis during conditions of immune-suppression. The immune-regulatory programmed death-1 (PD-1)/PD-ligand 1 system is acknowledged to play important roles in immune-mediated diseases. However, the PD-1/PD-L2 interaction is not well characterized, with PD-L2 also exhibiting an immunostimulatory function. We hypothesized that this atypical molecule could affect the recurrence of post-OLT hepatitis. To test this hypothesis, we conducted immunohistochemical staining analysis and in vitro analysis of PD-L2. Methods The expression of PD-L2 was evaluated in liver biopsy specimens from patients with chronic hepatitis B (n = 15), post-OLT hepatitis B (n = 8), chronic hepatitis C (n = 48), and post-OLT hepatitis C (CH-C-OLT) (n = 14). The effect of calcineurin inhibitors (CNIs) and hepatitis C virus (HCV) on PD-L2 expression was investigated in hepatoma cell lines. Results The PD-L2 was highly expressed on CH-C-OLT hepatocytes. Treatment of hepatoma cell lines with CNIs resulted in increased PD-L2 expression, especially in combination with HCV core or NS3 protein. Transfection of cell lines with PD-L2 containing plasmid resulted in high intercellular adhesion molecule-1 (ICAM-1) expression, which might enhance hepatitis activity. Conclusions The PD-L2 is highly expressed on CH-C-OLT hepatocytes, whereas HCV proteins, in combination with CNIs, induce high expression of PD-L2 resulting in elevated expression of ICAM-1. These findings demonstrate the effect of CNIs on inducing PD-L2 and subsequent ICAM-1 expression, effects that may produce inflammatory cell infiltration in post-OLT hepatitis C.
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