15-Lipoxygenase-1 expression in rheumatoid arthritis synovial fluid cells and 15-hydroxyeicosatetraenoic acid production. Rheumatoid arthritis (RA) synovial fluid cells express 15-lipoxygenase-1 (15-LO-1) and produce 15-hydroxyeicosatetraenoic acid (15-HETE) upon stimulation with arachidonic acid (AA). (a, b) Cytospin preparation of synovial fluid cells shows brown (diaminobenzidine) staining for 15-LO-1. Inset: isotype control. (c) 15-HETE formation in control synovial fluid cells and synovial fluid cells incubated with AA. (d) Cellular composition of the RA synovial fluid showing the percentage of neutrophils, monocytes and lymphocytes in samples from five patients. Horizontal lines, median values; whiskers, range values. Original magnification: (a) ×500 and (b) ×800.