Characterization of the Immunophenotype and Functional Properties of Fibroblast-like Synoviocytes in Comparison to Skin Fibroblasts and Umbilical Vein Endothelial Cells

We characterized the immunophenotype as well as functional properties — phagocytosis, the uptake of acetylated LDL, and the expression of HLA class II antigens, adhesion molecules, and cytokine mRNA — of fibroblast-like synoviocytes from rheumatoid arthritis synovium. Skin fibroblasts (FB) and umbilical vein endothelial cells (HUVEC) were studied in parallel. Cytofluorometric immunophenotyping by use of 84 mAb and 2 lectins and immunofluorescence microscopy indicated a high degree of homology between the three cell types. Only staining with mAb to von Willebrand factor (vWF) and CD31 and the lectin UEA-I appeared specific to HUVEC, whereas the mAb 5B5 to prolyl 4-hydroxylase that has been reported to be specific to FB stained HUVEC as well as synoviocytes and FB. All of the cells phagocytosed fluorescent latex beads of 1.7 and 2.6 μm in size. The uptake of acetylated LDL could be shown by HUVEC and, surprisingly, by synoviocytes, but not by FB. The induction of HLA-DR, -DP, and -DQ by IFN-γ on the three cell types showed a similar dose-dependence. The upregulation of ICAM-1 by IL-1α, TNF-α, and IFN-γ appeared similar, whereas the induction of VCAM-1 by IL-1α, IL-4, TNF-α, and IFN-γ showed differences between the three cell types. ELAM-1 was expressed only on HUVEC after treatment with IL-1α and TNF-α. The capacity of the cells to produce cytokines was studied at the level of mRNA by reverse transcription and PCR. All three cell types expressed the mRNA of IL-1α, IL-6, IL-8, GM-CSF, and TGF-β1 spontaneously or after LPS stimulation, but never TNF-α mRNA. Our results indicate a high degree of relationship between the three cell types. In contrast to HUVEC, none of the markers and functional properties investigated appear specific to FB. Therefore, the issue of the origin of fibroblast-like synoviocytes and the role of vascular endothelial cells in the inflamed synovium is discussed.