Under physiological conditions, PD-L1 is expressed constitutively at a low level on T cells, B cells, DCs, monocytes, mesenchymal stems cells, bone marrow derived mast cells, vascular endothelial cells, keratinocytes, pancreatic islet cells, astrocytes as well as various immune privileged tissues and organs (such as the placenta, testis and the anterior chamber of the eye), where exogenous antigens are tolerated without induction of inflammation and/or infection (145, 188, 189). The gene discussed is CD274; the disease is infection.