Our Tanespimycin findings suggest that the abnormally high levels of cytokines in OA sera largely reflect overproduction of these cytokines in the joint, consistent with the finding that levels of high sensitivity C reactive protein in the serum of OA patients correlate with the degree of inflammatory infiltrate in the patients joints. Thus, OA is associated with low grade inflammation that may originate in the joints. Interestingly, 39 of the proteins we identified in OA synovial fluid are classically considered plasma pro teins. Indeed, plasma proteins form a large proportion of the proteins enriched in OA synovial fluid relative to healthy synovial fluid. What might these plasma proteins be doing in the OA joint Like cer tain products of ECM breakdown, the plasma pro tein fibrinogen can function as a DAMP and has been proposed to contribute to the pathogenesis of inflamma tory arthritis.
We therefore examined whether other plasma proteins in OA synovial fluid can function as immunostimulatory DAMPs that could contribute to the low grade inflammation Inhibitors,Modulators,Libraries associated with OA. Key players in OA associated inflammation are the macrophages. The cell infiltrate in human OA joints consists mainly of macrophages, and Inhibitors,Modulators,Libraries mice depleted of macrophages are relatively resistant to col lagenase induced OA. Macrophages from OA joints produce a number of growth factors, such as VEGF, and inflammatory cytokines, such as the major OA asso ciated cytokines IL 1b and TNF. We detected VEGF, IL 1b, and TNF in OA synovial fluid in our cyto kine screen and found that levels of VEGF and IL 1b are significantly higher in OA sera than in normal sera.
VEGF may promote OA pathology by inducing angiogenesis and by inducing matrix metallopro tease production. The cytokines produced by macrophages amplify the inflammation in the joints by inducing synovial cells to produce further cytokines and chemokines, as well as matrix metalloproteases. Moreover, macrophages express many Inhibitors,Modulators,Libraries of the receptors that Inhibitors,Modulators,Libraries mediate DAMP sig naling, Inhibitors,Modulators,Libraries and they can thus trigger an inflammatory cas cade in response to DAMPs present in OA synovial fluid. selleckchem Imatinib Mesylate We therefore assessed whether a subset of the identi fied plasma proteins could induce macrophages to pro duce TNF, a key cytokine that is thought to drive the inflammatory cascade in OA. We tested a1 micro globulin, a1 acid glycoprotein 1, a2 macroglobulin, Gc globulin, albumin, and haptoglobin, all of them plasma proteins detected in our survey of synovial fluid proteins and shown to be enriched in OA synovial fluid. With mouse macrophages, we found that a1m, a2m, and Gc globulin, at concentrations similar to those mea sured in synovial fluid, each dose dependently stimulated the production of TNF, whereas AGP 1, albumin, and haptoglobin did not.