Binding of CXCL12 to RA ECs is independent of CXCR4. (a) Rheumatoid arthritis endothelial cells (RA ECs) were incubated with 300 nM biotinylated CXCL12α and, after extensive washing to remove free chemokine, were labeled with fluorescein isothiocyanate-conjugated avidin. Where indicated, RA ECs were simultaneously incubated with 50 μg/ml of the CXCR4 antagonist T134 or not (untreated). (b) Surface or intracellular CXCR4 was detected with 12G5 mAb. Filled histograms show isotype control IgG. Results are representative of three independent experiments with RA ECs from different donors.