Background/Purpose: Mast cells (MCs) are potent innate immune cells that frequently accumulate in chronically inflamed tissues, including the arthritic synovium. The factors that regulate MC persistence and survival in such tissues are unknown. Recent studies have found that the mesenchymal-derived cytokine IL-33 exerts potent effects on MC phenotype and function. Knowing that IL-33 can inhibit apoptosis in cardiac myocytes and hepatocytes, we tested the hypothesis that IL-33 might also regulate MC proliferation and survival, including the development of synovial mastocytosis in the context of inflammatory arthritis.

Methods: Murine bone marrow derived MCs (mBMMCs) were generated from wild-type (WT) mice and animals lacking the IL-33 receptor IL1RL1. Cell viability, proliferation and apoptosis were examined after exposure to IL-33, and compared with exposure to IL-3, a known survival factor for MCs.  Expression of the anti-apoptotic molecules Bcl-2 and Bcl-X_L were determined both as mRNA and as protein. To examine the role of IL-33 in vivo, fluorescently-labeled mixed WT and IL1RL1^-/- mBMMCs were transferred into the peritoneum of IL1RL1^-/- mice. Following 6 days of treatment with IL-33 (100ng/d/mouse, i.p.), peritoneal MCs were harvested and analyzed using flow cytometry.  WT and IL1RL1^-/- mBMMCs were sorted and the levels of Bcl-2 and Bcl-X_L mRNA were examined by qRT-PCR. Synovial tissue MCs were enumerated in WT and IL1RL1^-/-animals at baseline and after induction of K/BxN serum transfer arthritis.

Results:

While the viability of WT and IL1RL1^-/- mBMMCs was similar in IL-3-supported culture, exogenous IL-33 was able to support the survival of WT but not IL1RL1^-/- mBMMCs after IL-3 withdrawal. CFSE dilution studies confirmed that this effect did not reflect enhanced proliferation, but rather arose via inhibited apoptosis as evident by a reduction in the number of Annexin V positive cells. Exploring the mechanism of this effect, we found that IL-33 increased expression of Bcl-X_Lbut not Bcl-2, unlike IL-3 which supported mBMMC survival via enhanced Bcl-2 but not Bcl-X_L. Correspondingly, WT mBMMC persisted better than IL1RL1^-/- BMMC in IL-33-treated recipients, an effect again mediated not by proliferation but by impaired apoptosis and Bcl-X_L expression. While WT and IL1RL1^-/- demonstrated no difference in the number of synovial MC at baseline, after K/BxN serum transfer the number of MC in ankle synovium was significantly reduced in IL1RL1^-/- compared with WT mice. However, since IL-1RL1^-/-mice also exhibited less inflammation, the density of MC was similar in both strains, suggesting the IL-33 is not uniquely required for maintaining these cell populations in vivo.

Conclusion: These findings identify a novel role for IL‑33 as a promoter of murine MC viability, an effect likely mediated by enhanced expression of the anti-apoptotic protein Bcl-X_L. This effect could represent a new mechanism by which IL-33-producing cells, such as fibroblasts, support the maintenance of tissue mastocytosis.  However, at least in the short term K/BxN arthritis model, other pathways appear sufficient to enable and maintain tissue MCs even when this mechanism is interrupted by genetic deletion of the IL-33 receptor IL1RL1.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/il-33-promotes-mast-cell-survivial-via-inhibition-of-apoptosis-associated-with-enhanced-expression-of-bcl-xl/