Double labelling of CXCL12/HSPGs and CXCL12/MECA-79 of rheumatoid arthritis (RA) and osteoarthritis (OA) synovial tissues. Frozen RA or OA sections were labeled simultaneously with anti-CXCL12 K15C mAb and fluorescein isothiocyanate (FITC)- conjugated anti-heparan sulfate proteoglycan (HSPG) mAb (green fluorescence) or MECA-79 mAb (red fluorescence). In CXCL12/HSPG double-labeled sections, CXCL12 was developed with immunoperoxidase (brown color) and in CXCL12/MECA-79 double-labeled sections with a secondary FITC-labeled antibody. Arrows indicate colocalization of CXCL12 to HSPG-labeled RA vessels. The same section, sequentially photographed under appropriate optics, is shown in parallel left and right panels. Original magnification × 400.