The phagocytic function of macrophages during sepsis relies on their PD-1 expression, indicating the role of the PD-1-PD-L1 axis as a phagocytosis checkpoint in microbial clearance.232 Furthermore, PD-L1 expression is upregulated on synovial fluid myeloid DCs, T cells and macrophages in rheumatoid arthritis due to the high levels of IFN-γ and TNF-α in RA-derived synovial fluid.280,281. Here, PDCD1 is linked to rheumatoid arthritis.