Interaction of transforming growth factor-beta-1 with alpha-2-macroglobulin from normal and inflamed equine joints

Binding between equine plasma alpha-2-macroglobulin (alpha 2M) and several cytokines known to participate in inflammatory reactions in other species was initially examined. Plasma was obtained from 5 horses with various abnormalities. Samples, both untreated and after reaction with methylamine, were incubated with exogenous, radiolabeled, porcine-derived transforming growth factor-beta-1 (125I-TGF-beta 1), recombinant human interleukin-1-beta (125I-IL-1 beta), and recombinant human tumor necrosis factor-alpha (125I-rhTNF-alpha). They were then subjected to nondenaturing polyacrylamide gel electrophoresis (PAGE). Binding of the native (slow) and activated (fast) forms of alpha 2M to each cytokine was subjectively evaluated with autoradiography. Equine alpha 2M bound 125I-TGF-beta 1. However, poor or no binding was observed between alpha 2M and either of 125I-rhTNF-alpha or 125I-IL-1 beta. Synovial fluid was then obtained from 6 normal horses, 6 horses with septic arthritis, and 6 horses with degenerative joint disease. Untreated and methylamine-reacted samples were quantitatively examined for binding with 125I-TGF-beta 1, using the autoradiographic techniques described above and densitometry. Native and activated alpha 2M were also quantified by densitometry of PAGE gels. Native alpha 2M was significantly elevated in septic arthritis (6.4% to 29.5% of total protein detected) and degenerative joint disease (2.8% to 12.3%), compared to normal joints (0.9% to 4.2%). Activated alpha 2M, however, was not detected in untreated synovial fluid samples. In all plasma and joint fluid samples, whether untreated or reacted with methylamine, 125I-TGF-beta 1 bound predominantly to alpha 2M, and preferentially to the activated form of alpha 2M. In synovial fluid, the amount of 125I-TGF-beta 1 binding was proportional to the quantity of alpha 2M present. These results indicate that: 1) equine alpha 2M binds TGF-beta 1; 2) the native form of alpha 2M is present in both equine plasma and synovial fluid, and 3) alpha 2M is a major binding protein for TGF-beta 1 in equine synovial fluid. Therefore, alpha 2M may play a role in regulating this mediator of inflammation in equine joints.