Regulation of osteoclast differentiation and function by BiP

Immunoglobulin heavy-chain-binding protein (BiP): a stress protein that has the potential to be a novel therapy for rheumatoid arthritis

Immunoglobulin heavy-chain-binding protein (BiP) or glucose-regulated protein 78 (Grp78) is a vital ubiquitous resident of the endoplasmic reticulum (ER). As an intracellular chaperone, BiP correctly folds nascent polypeptides within the ER and regulates the unfolded protein response ensuring protection of the cell from denatured protein and reinforcing its anti-apoptotic role, when the cell is under stress. Additionally, BiP is a member of the heat-shock protein (HSP) 70 family and, as a stress protein, is up-regulated by conditions of reduced oxygen and glucose. Cell stress induces surface expression and secretion of BiP. Consequently, BiP is detectable in several bodily fluids including serum, synovial fluid (SF) and oviductal fluid. However, as an extracellular protein, BiP has additional properties that are quite distinct from the intracellular functions. Extracellular BiP is immunoregulatory and anti-inflammatory causing development of tolerogenic dendritic cells (DCs), induction of regulatory T-cells, abrogation of osteoclast development and function, induction of anti-inflammatory cytokine production, including interleukin (IL)-10, IL-1 receptor antagonist and soluble tumour necrosis factor (TNF)-receptor type II, and attenuation of TNFα and IL-6. Together, these functions help drive the resolution of inflammation. Disease models of inflammatory arthritis have helped to demonstrate the novel mode of action of BiP in which the pharmacokinetics and pharmacodynamics are dissociated. The three murine models to be discussed each show BiP induced long-term therapeutic protection and therefore has potential for long-lasting drug-free therapy in rheumatoid arthritis (RA).

Pro-resolution immunological networks: binding immunoglobulin protein and other resolution-associated molecular patterns

Appropriate regulation and subsequent resolution of acute inflammatory events is critical to the prevention of autoinflammatory diseases. Indeed, the chronic inflammation observed in diseases such as RA is at least partially consequent on the failure of endogenous immunoregulation. Current RA therapies (e.g. anti-TNF-α inhibitors and MTX) inhibit components of the inflammatory disease process without directly promoting the resolution of inflammation. We propose that the next generation of RA therapeutics will complement and augment endogenous immunoregulatory and pro-resolution immunological networks, thus promoting the definitive resolution of inflammation rather than temporary immunological control. Of particular interest with respect to this therapeutic approach is binding immunoglobulin protein [BiP; also known as glucose-regulated protein-78 (GRP78)], a member of the recently defined resolution-associated molecular pattern (RAMP) family of molecules. In this review, we consider the preclinical evidence from experiments in mouse and man that suggests BiP and other members of the RAMP family have the potential to herald a new generation of immunotherapeutics.

Resolution-associated molecular patterns (RAMP): RAMParts defending immunological homeostasis?

The resolution of inflammation is central to the maintenance of good health and immune homeostasis. Recently, several intracellular stress proteins have been described as having extracellular properties that are anti-inflammatory or favour the resolution of inflammation. We propose that these molecules should be defined as resolution-associated molecular patterns (RAMPs). RAMPs are released at times of cellular stress and help to counterbalance the inflammatory effects of pathogen-associated (PAMPs) and damage-associated (DAMPs) molecular patterns. We propose that heat shock protein 10 (HSP10), αB-crystallin (αBC), HSP27 and binding immunoglobulin protein (BiP) should be considered founding members of the RAMP family. A greater understanding of RAMP biology may herald the development of novel immunotherapies.