Cytokines and cytokine-directed intervention in experimental arthritis

Abstract

Rheumatoid arthritis (RA) is a common chronic autoimmune and inflammatory disease. Although the etiology of RA remains unknown, a general understanding of the pathogenesis is steadily increasing. The macrophage products tumor necrosis factor (TNF) and interleukin 1 (IL-1) are recognized as pivotal mediators in the arthritic process. Therapeutic approaches targeting the action of these cytokines represent the most successful achievements since the discovery of corticosteroids. The introduction of anti-TNF directed treatment with infliximab (TNF-antibodies) or etanercept (soluble TNF-receptor) has revolutionized the therapeutic efficacy for many patients with previously therapy-resistant chronic arthritis.

My studies have dealt with the anti-inflammatory potential of a new macrophage deactivating compound CNI-1493, which is a tetravalent guanylhydrazone. CNI-1493 exerts its macrophage suppressive actions by inhibition of TNF and IL-1 synthesis at a translational level. In vitro studies revealed that the inhibitory effects of CNI-1493 was monocyte specific, with a downmodulation of the overall monokine production, but especially of TNF. These cytokine-suppressive effects were retained even in the presence of IFN-y, which is known to block the inhibitory effects mediated by glucocorticoids. T cell derived cytokine production was in contrast unaffected by CNI-1493. In vivo effects of CNI-1493 intervention were studied in collagen-induced arthritis (CIA) in the DA rat strain, where CNI-1493 successfully ameliorated clinical signs of arthritis.

Immunohistochemical staining methods were developed for detection of intracellular rat cytokines. These methods were employed in a longitudinal study of CIA in DA rats for characterization of synovial expression of the cytokines TNF, IL-I beta and TGF-beta, as compared to that in animals treated with CNI-1493. These studies revealed an early, previously unrecognized TNF and IL-10 synthesis appearing in the synovial lining layer more than a week before disease onset. The findings thus further underline the importance of these two inflammatory cytokines in the pathogenesis of arthritis. After onset of clinical disease, the number of TNF producing cells clearly exceeded that of IL-1beta, especially in the acute phase of inflammation but also in the chronic phase. The quantitative TNF dominance compared to IL-1 beta in CIA is the opposite result to that recorded in human RA using the same methodology. There was a marked down-regulation of TNF as well as of IL-1 in the joints of animals treated with CNI-1493. Suppression of both TNF and IL-1 is a highly desirable anti- rheumatic strategy supporting future clinical trials with CNI-1493 in RA.

Another new finding was the demonstration of high mobility group-1 protein (HMG-1) to act as a cytokine with potent proinflammatory properties. HMG-1 is a well characterized nuclear DNA- binding protein that in addition has been shown to be produced as a secreted protein by activated macrophages. HMG-1 is almost as potent as bacterial LPS in stimulating monokine synthesis, but with a different kinetic response, making HMG-1 the most powerful endogenous macrophage activating factor ever reported. These findings suggest that this protein warrants investigation as a therapeutic target in inflammatory diseases.