THU0024 Human Mast Cells Stimulated with IL-33 and Immune Complexes Down-Regulate Monocyte Activation in Rheumatoid Arthritis
Authors: F. Rivellese, J. Suurmond, K. Habets, A.L. Dorjée, A. de Paulis, G. Marone, C. Pitzalis, T.W. Huizinga, R.E. Toes
Journal: Annals of the Rheumatic Diseases (2015)

<h3>Background</h3> Mast cells have been implicated in the pathogenesis of Rheumatoid Arthritis (RA). In particular, activation of mast cells by IL-33 has been linked to the development of arthritis in animal models. Likewise, increased levels of IL-33 have been described in the sera and synovial fluid of RA patients. However, little is known about the influence of IL-33-triggered human mast cells on immune responses in RA. <h3>Objectives</h3> To evaluate the activation of mast cells by IL-33 and immune complexes and their ability to modulate monocyte-mediated immune responses in the context of Anti-Citrullinated Protein Antibody-positive (ACPA+) RA. <h3>Methods</h3> Human mast cells were stimulated with IL-33 combined with immune complexes (IC) formed by plate bound total IgG or IgG-ACPA bound to citrullinated fibrinogen. Mast cell activation was analysed by measuring cytokine production with ELISA and a multiplex assay. By immunofluorescence, the cellular interactions of mast cells were assessed in the synovial tissue of patients with established ACPA+ RA undergoing knee joint replacement. Finally, the influence of mast cells on LPS-induced monocyte activation was evaluated by measuring cytokine production by ELISA and membrane markers by flow cytometry. <h3>Results</h3> IL-33 enhanced the activation of mast cells by IC, including IgG-ACPA IC, as demonstrated by a significant increase of IL-8 production. Further analysis of mast cell supernatants showed a differential activation of mast cells, with IC preferentially inducing pro-inflammatory mediators (e.g. IL-8) and IL-33 inducing significantly higher amounts of potential immunomodulatory mediators (e.g. IL-10 and histamine). Additionally, the combined stimulation with IL-33 and IgG IC showed a significant synergistic effect exclusively for the IL-33-induced immunomodulatory mediators. Ex vivo, in the synovial membrane of RA patients, mast cells were frequently found in contact with CD14+ cells. Finally, mast cells activated with IL-33 and IC inhibited, mainly via the release of IL-10 and histamine, the LPS-induced production of the prototypical pro-inflammatory cytokine TNF-α and the upregulation of the co-stimulatory molecule CD80 by monocytes. <h3>Conclusions</h3> We here show that human mast cells acquire an immunomodulatory phenotype when exposed to IL-33, releasing regulatory mediators even when triggered with pro-inflammatory stimuli such as immune complexes. This finding was corroborated by the evidence of mast cell-monocyte interactions at synovial level and by the functional demonstration that mast cells triggered with IL-33 and IC are able to suppress monocyte pro-inflammatory activation. Overall, our study suggests a novel immunomodulatory role for human mast cells that might have a functional relevance in the pathogenesis of RA. <h3>References</h3> Galli SJ, Grimbaldeston M, Tsai M. Immunomodulatory mast cells: negative, as well as positive, regulators of immunity. Nature reviews Immunology. 2008;8(6):478-86. Brown MA, Hatfield JK. Mast Cells are Important Modifiers of Autoimmune Disease: With so Much Evidence, Why is There Still Controversy? Front Immunol. 2012;3:147. Liew FY. IL-33: a Janus cytokine. Ann Rheum Dis. 2012;71 Suppl 2:i101-4. <h3>Acknowledgements</h3> This research was conducted while F. Rivellese was an ARTICULUM Fellow. <h3>Disclosure of Interest</h3> None declared