OBJECTIVE: To investigate the role of the major equine acute phase protein serum amyloid A (SAA) in inflammation of equine intraarticular tissues.

SAMPLE: Articular chondrocytes and fibroblast-like synoviocytes (FLSs) from 8 horses (4 horses/cell type).

PROCEDURES: Chondrocytes and FLSs were stimulated in vitro for various periods up to 48 hours with cytokines (recombinant interleukin [IL]-1β, IL-6, tumor necrosis factor-α, or a combination of all 3 [IIT]) or with recombinant SAA. Gene expression of SAA, IL-6, matrix metalloproteinases (MMP)-1 and −3, and cartilage-derived retinoic acid-sensitive protein were assessed by quantitative real-time PCR assay; SAA protein was evaluated by immunoturbidimetry and denaturing isoelectric focusing and western blotting.

RESULTS: All cytokine stimulation protocols increased expression of SAA mRNA and resulted in detectable SAA protein production in chondrocytes and FLSs. Isoforms of SAA in lysed chondrocytes and their culture medium corresponded to those previously detected in synovial fluid from horses with joint disease. When exposed to SAA, chondrocytes and FLSs had increased expression of IL-6, SAA, and MMP3, and chondrocytes had increased expression of MMP-1. Chondrocytes had decreased expression of cartilage-derived retinoic acid-sensitive protein.

CONCLUSIONS AND CLINICAL RELEVANCE: Upregulation of SAA in chondrocytes and FLSs stimulated with proinflammatory cytokines and the proinflammatory effects of SAA suggested that SAA may be involved in key aspects of pathogenesis of the joint inflammation in horses.