Kupffer cells in immune activation and tolerance toward HBV/HCV infection.

Kupffer cells (KCs) are macrophages that are found in the sinusoids of the liver. KCs are a crucial part of the innate immune system, acting as scavengers and phagocytes. KCs and sinusoidal endothelial cells together form the first immune barrier of the portal system. Studies show that KCs can not only maintain homeostasis in the immune response, but also facilitate the pathogenesis of type B and type C hepatitis (HBV/HCV) through their antigen-presenting function and secretion of soluble mediators. KCs can express toll-like receptors (TLRs), Fas ligand (FasL) and programmed cell death ligand 1 (PD-L1), and secrete large amounts of inflammatory factors leading to immune tolerance toward HBV/HCV. On the one hand, KCs contribute to the clearance of HBV/HCV due to their nature as innate immune cells. At the same time, KCs induce immune tolerance toward HBV/HCV, which leads to chronicity of the infection. The dual role of KCs in the immune response toward HBV/HCV means it is a gigantic challenge for scientists to illuminate the detailed mechanisms involved, but it also offers important potential therapeutic targets.