HMBG1 as a proinflammatory mediator in arthritis

Abstract

Proinflammatory cytokines are central in the pathogenesis of chronic arthritis and a new generation of therapeutics blocking TNF or IL-1 has improved outcome for many patients with inflammatory joint diseases. This thesis addresses the role of HMGB1 (High Mobility Group Box Chromosomal protein 1) in inflammation and as a potential novel therapeutic target molecule in arthritis.

HMGB1 is historically well-known as a ubiquitous, nuclear DNA-binding protein, extremely conserved among species. Unexpectedly, recent studies also identified extracellular HMGB1 as a potent proinflammatory mediator. HMGB1 can be secreted via an endolysosomal pathway by activated macrophages. In addition, HMGB1 is extracellularly released by any cell undergoing unprogrammed cell death, functioning as a major stimulus of necrosis-induced inflammation.

By performing in vitro studies with human blood mononuclear cells or rodent macrophages I have demonstrated that HMGB1 is not only a product released from activated macrophages/ monocytes, but is also by itself a powerful mediator of macrophage activation. HMGB1-stimulated TNF synthesis in human monocytes was biphasic and delayed in comparison to that following LPS stimulation. HMGB1 induced a generalized proinflammatory phenotype in rodent macrophages through transient intracellular phosphorylation of MAP kinases and a nuclear translocation of NF-kappa-B. Results with cultured macrophages from RAGE-/- mice indicated the receptor for advanced glycated end products (RAGE) as the major functional receptor for HMGB1-induced cytokine production. Truncation of HMGB1 into individual structural domains showed that the cytokine-inducing ability of the molecule resides in the B box domain. Administration of neutralizing anti-B box domain antibodies rescued mice from lethal sepsis, even when therapy was delayed 24 hours after onset of disease.

Immunohistochemical studies established HMGB1 aberrantly expressed in synovial tissue in both human and experimental arthritis. Tissue sections from human inflamed joints demonstrated cytoplasmic and extracellular HMGB1 apart from the expected nuclear presence, which was also apparent in healthy controls. Synovial fluid from patients with rheumatoid arthritis contained high levels of HMGB1. In synovial sections from rats with collagen-induced arthritis HMGB1 expression preceded clinical onset of disease, and extranuclear HMGB1 expression increased during progress of disease to levels comparable to that of TNF and IL-1beta. HMGB1-targeted intervention with either neutralizing antibodies or the antagonizing A-box domain of HMGB1 ameliorated collagen-induced arthritis in both mice and rats and inhibited the synovial expression of IL-IP.

Taken together, these results indicate extracellular HMGB1 as a proinflammatory cytokine of general interest for arthritis research. Activated macrophages, ischemia-induced necrosis and activated complement are all prominent features of chronic arthritis and these events will lead to harmful, intra- articular HMGB1 release, which can be therapeutically targeted.