PDCD1 and hepatitis A virus infection: For example, upon murine coronavirus strain 3 infection, PD-1 is highly expressed on T cells, NK cells, and macrophages, and its deficiency is associated with overproduction of IFN-γ and TNF-α, which may enhance viral clearance, but also with increased fibrinogen deposition, worse hepatitis, and higher mortality compared with WT mice (5).