Regulation of PD‐1, PD‐L1, and PD‐L2 expression during normal and autoimmune responses

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Abstract
Newer members of the B7‐CD28 superfamily include the receptor PD‐1 and its two ligands, PD‐L1 and PD‐L2. Here, we characterize the expression of PD‐1, PD‐L1, and PD‐L2 in tissues of naive miceand in target organs from two models of autoimmunity, the pancreas from non‐obese diabetic (NOD) mice and brain from mice with experimental autoimmune encephalomyelitis (EAE). In naive mice, proteiexpression of PD‐1, PD‐L1, and PD‐L2 was detected in the thymus, while PD‐1 and PD‐L1 were detected in the spleen. PD‐L1, but not PD‐L2, was also detected at low levels on cardiac endothelium, pancreatic islets, and syncyciotrophoblasts in the placenta. In pre‐diabetic NOD mice, PD‐1 and PD‐L1 were expressed on infiltrating cells in the pancreatic islets. Furthermore, PD‐L1 was markedly up‐regulated on islet cells. In brains from mice with EAE, PD‐1, PD‐L1, and PD‐L2 were expressed on infiltrating inflammatory cells, and PD‐L1 was up‐regulated on endothelium within EAE brain. The distinct expression patterns of PD‐L1 and PD‐L2 led us to compare their transcriptional regulation in STAT4–/–, STAT6–/–, or NF‐κB p50–/–p65+/– dendritic cells (DC).PD‐L2, but not PD‐L1, expression was dramatically reduced in p50–/–p65+/– DC. Thus, PD‐L1 and PD‐L2 exhibit distinct expression patterns and are differentially regulated on the transcriptional level.