To investigate the sources of Flt3L in peripheral blood (PB) and SF we characterized the relative expression of Flt3L on CD14+ monocytes, CD19+ B cells, CD56+ NK cells, CD4+ and CD8+ T cells and both CD1c+ mDCs and CD304+ pDCs present in RA paired synovial fluid mononuclear cells and PBMC and in HI PBMC by flow cytometry. Here, NRP1 is linked to rheumatoid arthritis.