Identification of Small Heat Shock Protein B8 (HSP22) as a Novel TLR4 Ligand and Potential Involvement in the Pathogenesis of Rheumatoid Arthritis

Dendritic cells (DCs) are specialized APCs that can be activated upon pathogen recognition as well as recognition of endogenous ligands, which are released during inflammation and cell stress. The recognition of exogenous and endogenous ligands depends on TLRs, which are abundantly expressed in synovial tissue from rheumatoid arthritis (RA) patients. Furthermore TLR ligands are found to be present in RA serum and synovial fluid and are significantly increased, compared with serum and synovial fluid from healthy volunteers and patients with systemic sclerosis and systemic lupus erythematosus. Identification of novel endogenous TLR ligands might contribute to the elucidation of the role of TLRs in RA and other autoimmune diseases. In this study, we investigated whether five members of the small heat shock protein (HSP) family were involved in TLR4-mediated DC activation and whether these small HSPs were present in RA synovial tissue. In vitro, monocyte-derived DCs were stimulated with recombinant alphaA crystallin, alphaB crystallin, HSP20, HSPB8, and HSP27. Using flow cytometry and multiplex cytokine assays, we showed that both alphaA crystallin and HSPB8 were able to activate DCs and that this activation was TLR4 dependent. Furthermore, Western blot and immunohistochemistry showed that HSPB8 was abundantly expressed in synovial tissue from patients with RA. With these experiments, we identified sHSP alphaA crystallin and HSPB8 as two new endogenous TLR4 ligands from which HSPB8 is abundantly expressed in RA synovial tissue. These findings suggest a role for HSPB8 during the inflammatory process in autoimmune diseases such as RA.