Synovial mast cells: role in acute and chronic arthritis

Mast cells contribute to autoimmune inflammatory arthritis via their tryptase/heparin complexes

Although mast cells (MCs) often are abundant in the synovial tissues of patients with rheumatoid arthritis, the contribution of MCs to joint inflammation and cartilage loss remains poorly understood. MC-restricted tryptase/heparin complexes have proinflammatory activity, and significant amounts of human tryptase beta (hTryptase-beta) are present in rheumatoid arthritis synovial fluid. Mouse MC protease-6 (mMCP-6) is the ortholog of hTryptase-beta, and this serine protease is abundant in the synovium of arthritic mice. We now report that C57BL/6 (B6) mice lacking their tryptase/heparin complexes have attenuated arthritic responses, with mMCP-6 as the dominant tryptase responsible for augmenting neutrophil infiltration in the K/BxN mouse serum-transfer arthritis model. While inflammation in this experimental arthritis model was not dependent on protease-activated receptor-2, it was dependent on the chemokine receptor CXCR2. In support of the latter data, exposure of synovial fibroblasts to hTryptase-beta/heparin or mMCP-6/heparin complexes resulted in expression of the neutrophil chemotactic factors CXCL1/KC, CXCL5/LIX, and CXCL8/IL-8. Our proteomics, histochemistry, and immunohistochemistry data also revealed substantial loss of cartilage-derived aggrecan proteoglycans in the arthritic joints of wild-type B6 mice but not mMCP-6-null B6 mice. These observations demonstrate the functional contribution of MC-restricted tryptase/heparin complexes in the K/BxN mouse arthritis model and connect our mouse findings with rheumatoid arthritis pathophysiology.

CD153 in rheumatoid arthritis: detection of a soluble form in serum and synovial fluid, and expression by mast cells in the rheumatic synovium

OBJECTIVE

A CD30-CD153 mast cell axis has been described in skin inflammations and Hodgkin's lymphoma. We investigated if a soluble form of CD153 is present in the serum and synovial fluid (SF) of patients with rheumatoid arthritis (RA), and determined whether mast cells express CD153 in the synovium of these patients.

METHODS

Soluble forms of CD30 and CD153 were quantified in serum and SF of patients with RA by ELISA. Consecutive sections of synovial biopsies from 12 patients were stained against tryptase (mast-cell marker), CD30, and CD153.

RESULTS

Elevated concentrations of the soluble form of CD153 were found in serum from 14/15 RA patients. In the SF, 11/20 patients had detectable levels of soluble CD153. CD30 and CD153 were expressed in all biopsies that were studied. Mast cells were present in all the synovial biopsies, and expressed CD153 in one-third of the cases.

CONCLUSION

We observed that CD153 was expressed in the synovium of patients with RA and we were able to correlate the serum levels of soluble CD153 with SF levels in the same patients. Because CD30 can activate mast cells to release chemokines without degranulation, our finding that mast cells express CD153 in RA synovium raises the possibility that a CD30-CD153 axis may contribute to the activation of synovial mast cells in the absence of degranulation.

Our perception of the mast cell from Paul Ehrlich to now

Just over a century ago Paul Ehrlich received the Nobel Prize for his studies of immunity. This review describes one of his legacies, the histochemical description of the mast cell, and the research that has ensued since then. After a long period of largely descriptive studies, which revealed little about the biological role of the mast cell, the field was galvanized in the 1950s by the recognition that the mast cell was the main repository of histamine and a key participant in anaphylactic reactions. Although the mast cell was long-viewed in these terms, recent research has now shown that the mast cell also plays a key role in innate and adaptive immune responses, autoimmune disease, and possibly tissue homeostasis by virtue of its expression of a diverse array of receptors and biologically active products. In addition, the responsiveness of mast cells to immunological and pathological stimulants is highly modulated by the tissue cytokine environment and by synergistic, or inhibitory, interactions among the various mast cell receptor systems. This once enigmatic cell of Paul Ehrlich has proved to be both adaptable and multifunctional.

The controversial role of mast cells in tumor growth

Mast cells (MCs) were first described by Paul Ehrlich (Beiträge zur Theorie und Praxis der Histologischen Färbung, Thesis, Leipzig University, 1878). They have long been implicated in the pathogenesis of allergic reactions and protective responses to parasites. However, their functional role has been found to be complex and multifarious. MCs are also involved in various cell-mediated immune reactions and found in tissues from multiple disease sites, and as a component of the host reaction to bacteria, parasite, and even virus infections. They also participate in angiogenic and tissue repair processes after injury. The importance of a possible functional link between chronic inflammation and cancer has long been recognized. As most tumors contain inflammatory cell infiltrates, which often include plentiful MCs, a possible contribution of these cells to tumor development has emerged. In this review, general biology of mast cells, their development, anatomical distribution, and phenotype as well as their secretory products will first be discussed. The specific involvement of MCs in tumor biology and tumor fate will then be considered, with particular emphasis on their capacity to stimulate tumor growth by promoting angiogenesis and lymphangiogenesis. Finally, it is suggested that mast cells may serve as a novel therapeutic target for cancer treatment.

Mast cells as regulators of adaptive immunity to tumours

The observation that mast cells accumulate at the periphery of growing tumours is now well documented, and the loss of mast cells correlates with reduced tumour growth. The role of mast cells as innate regulators of both inflammatory and immunosuppressive responses slowly becomes clear as novel tools become available. This review will address the role of mast cells in tumours and how they can interact with the local immune environment to mediate immune suppression contributing to tumour escape.

The master switch: the role of mast cells in autoimmunity and tolerance

There are many parallels between allergic and autoimmune responses. Both are considered hypersensitivity responses: pathologies that are elicited by an exuberant reaction to antigens that do not pose any inherent danger to the organism. Although mast cells have long been recognized as central players in allergy, only recently has their role in autoimmunity become apparent. Because of the commonalities of these responses, much of what we have learned about the underlying mast cell-dependent mechanisms of inflammatory damage in allergy and asthma can be used to understand autoimmunity. Here we review mast cell biology in the context of autoimmune disease. We discuss the huge diversity in mast cell responses that can exert either proinflammatory or antiinflammatory activity. We also consider the myriad factors that cause one response to predominate over another in a particular immune setting.

Endothelins modulate inflammatory reaction in zymosan-induced arthritis: participation of LTB4, TNF-alpha, and CXCL-1

Endothelins (ETs) are involved in inflammatory events, including pain, fever, edema, and cell migration. ET-1 levels are increased in plasma and synovial membrane of rheumatoid arthritis (RA) patients, but the evidence that ETs participate in RA physiopathology is limited. The present study investigated the involvement of ETs in neutrophil accumulation and edema formation in the murine model of zymosan-induced arthritis. Intra-articular (i.a.) administration of selective ET(A) or ET(B) receptor antagonists (BQ-123 and BQ-788, respectively; 15 pmol/cavity) prior to i.a. zymosan injection (500 microg/cavity) markedly reduced knee-joint edema formation and neutrophil influx to the synovial cavity 6 h and 24 h after stimulation. Histological analysis showed that ET(A) or ET(B) receptor blockade suppressed zymosan-induced neutrophil accumulation in articular tissue at 6 h. Likewise, dual blockade of ET(A)/ET(B) with bosentan (10 mg/kg, i.v.) also reduced edema formation and neutrophil counts 6 h after zymosan stimulation. Pretreatment with BQ-123 or BQ-788 (i.a.; 15 pmol/cavity) also decreased zymosan-induced TNF-alpha production within 6 h, keratinocyte-derived chemokine/CXCL1 production within 24 h, and leukotriene B(4) at both time-points. Consistent with the demonstration that ET receptor antagonists inhibit zymosan-induced inflammation, i.a. injection of ET-1 (1-30 pmol/cavity) or sarafotoxin S6c (0.1-30 pmol/cavity) also triggered edema formation and neutrophil accumulation within 6 h. Moreover, knee-joint synovial tissue expressed ET(A) and ET(B) receptors. These findings suggest that endogenous ETs contribute to knee-joint inflammation, acting through ET(A) and ET(B) receptors and modulating edema formation, neutrophil recruitment, and production of inflammatory mediators.

Mast cells: versatile regulators of inflammation, tissue remodeling, host defense and homeostasis

The possible roles of mast cells in heath and disease have been a topic of interest for over 125 years. Many adaptive or pathological processes affecting the skin or other anatomical sites have been associated with morphological evidence of mast cell activation, and/or with changes in mast cell numbers or phenotype. Such observations, taken together with the known functions of the diverse mediators, cytokines and growth factors which can be secreted by mast cells, have suggested many potential functions for mast cells in health and disease. Definitively identifying the importance of mast cells in biological responses in humans is difficult. However, mutant mice which are profoundly mast cell-deficient, especially those which can undergo engraftment with wild-type or genetically altered mast cells, provide an opportunity to investigate the importance of mast cells, and specific mast cell functions or products, in various adaptive or pathological responses in mice. Such work has shown that mast cells can significantly influence multiple features of inflammatory or immune responses, through diverse effects that can either promote or, surprisingly, suppress, aspects of these responses. Through such functions, mast cells can significantly influence inflammation, tissue remodeling, host defense and homeostasis.

The Multitasking Mast Cell: Positive and Negative Roles in the Progression of Autoimmunity

Among the potential outcomes of an aberrantly functioning immune system are allergic disease and autoimmunity. Although it has been assumed that the underlying mechanisms mediating these conditions are completely different, recent evidence shows that mast cells provide a common link. Mast cells reside in most tissues, are particularly prevalent at sites of Ag entry, and act as sentinel cells of the immune system. They express many inflammatory mediators that affect both innate and adaptive cellular function. They contribute to pathologic allergic inflammation but also serve an important protective role in bacterial and parasite infections. Given the proinflammatory nature of autoimmune responses, it is not surprising that studies using murine models of autoimmunity clearly implicate mast cells in the initiation and/or progression of autoimmune disease. In this review, we discuss the defined and hypothesized mechanisms of mast cell influence on autoimmune diseases, including their surprising and newly discovered role as anti-inflammatory cells.