Figure 5. Hypoactivity of the PD-1 (programmed cell death protein-1)/ PD-L1 (programmed cell death ligand-1) checkpoint in giant cell arteritis (GCA).

A functioning PD-1/PD-L1 checkpoint provides negative signals to end the T cell activation and expansion program. In GCA, antigen-presenting cells fail to express the inhibitory ligand PD-L1, preventing the transmission of inhibitory signals. At the same time, GCA T cells are highly responsive to co-stimulatory signals, favoring T cell activation, clonal expansion and effector differentiation. HLA indicates human leukocyte antigen; and TCR, T cell receptor.