Immune Checkpoints and the Immunology of Liver Fibrosis

Liver fibrosis is a very complicated dynamic process where several immune cells are involved. Both innate and adaptive immunity are implicated, and their interplay is always present. Multi-directional interactions between liver macrophages, hepatic stellate cells (HSCs), immune cells, and several cytokines are important for the induction and perpetuation of liver fibrosis. Detailed studies of proteomics and transcriptomics have produced new evidence for the role of individual cells in the process of liver fibrosis and cirrhosis. Most of these cells are controlled by the various immune checkpoints whose main function is to maintain the homeostasis of the implicated immune cells. Recent evidence indicates that several immune checkpoints are involved in liver fibrosis. In particular, the role of the programmed cell death protein 1 (PD-1), the programmed death-ligand 1 (PD-L1), and the role of the cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) have been investigated, particularly after the availability of checkpoint inhibitors. Their activation leads to the exhaustion of CD4+ve and CD8+ve T cells and the promotion of liver fibrosis. In this review, the current pathogenesis of liver fibrosis and the immunological abnormalities are discussed. The recent data on the involvement of immune checkpoints are identified as possible targets of future interventions.