PD-L1 is upregulated by TSLP and highly expressed on synovial CD1c myeloid dendritic cells. (A) Thymic stromal lymphopoietin (TSLP) stimulation of myeloid dendritic cells (mDCs) significantly upregulates PD-L1 mRNA expression (peripheral blood (PB)–derived and synovial fluid (SF)–derived mDCs of rheumatoid arthritis (RA) patients; circles represent unstimulated mDCs, squares represent TSLP-stimulated mDCs, n = 5) and (B) PD-L1 protein expression. Representative histrogram of PD-L1 expression on TSLP-stimulated mDCs (filled dark grey area) and unstimulated mDCs (transparent) from PB of one donor is shown. Dashed line represents autofluorescence. (C, D) PD-L1 is expressed to a higher extent on CD1c mDCs derived from SF (n = 8) compared with PB (n = 9) of RA patients. (C) Representative histogram of PD-L1 expression on PB mDCs (filled light grey area; solid line) and SF mDCs (transparent area; solid line) of one donor is shown. Dashed line represents autofluorescence for PB mDCs (filled light grey area) and SF mDCs (transparent). (D) Mean percentages and MFI of PD-L1 expression of PB and SF mDCs (n = 9 and n = 8, respectively). *P < 0.05, **P < 0.01, and ***P < 0.001 (statistically significant differences). MFI, Mean fluorescence intensity; PD-L1, Programmed death ligand 1.