IgE-mediated mast cell activation promotes inflammation and cartilage destruction in osteoarthritis

Osteoarthritis is characterized by articular cartilage breakdown, and emerging evidence suggests that dysregulated innate immunity is likely involved. Here, we performed proteomic, transcriptomic, and electron microscopic analyses to demonstrate that mast cells are aberrantly activated in human and murine osteoarthritic joint tissues. Using genetic models of mast cell deficiency, we demonstrate that lack of mast cells attenuates osteoarthritis in mice. Using genetic and pharmacologic approaches, we show that the IgE/FcεRI/Syk signaling axis is critical for the development of osteoarthritis. We find that mast cell-derived tryptase induces inflammation, chondrocyte apoptosis, and cartilage breakdown. Our findings demonstrate a central role for IgE-dependent mast cell activation in the pathogenesis of osteoarthritis, suggesting that targeting mast cells could provide therapeutic benefit in human osteoarthritis. Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).


Sample-size estimation
• You should state whether an appropriate sample size was computed when the study was being designed • You should state the statistical method of sample size computation and any required assumptions • If no explicit power analysis was used, you should describe how you decided what sample (replicate) size (number) to use Please outline where this information can be found within the submission (e.g., sections or figure legends), or explain why this information doesn't apply to your submission:

Replicates
• You should report how often each experiment was performed • You should include a definition of biological versus technical replication • The data obtained should be provided and sufficient information should be provided to indicate the number of independent biological and/or technical replicates • If you encountered any outliers, you should describe how these were handled • Criteria for exclusion/inclusion of data should be clearly stated Determination of sample sizes for analysis of human synovial fluid samples are described in the "Measurement of active tryptase in synovial fluids" sub-section of the Materials and Methods. Sample sizes are provided in the legend for Figure 1.
Determination of sample sizes for analysis of cartilage degradation, osteophyte formation and synovitis following destabilization of the medial meniscus in mice were determined using power calculations as is described in the "Surgical induction of osteoarthritis in mice" sub-section of the Materials and Methods. Sample sizes used in each experiment are provided in the legends of Figures 2, 3, 4 and 5. qPCR analysis of inflammatory/degradative enzyme expression was performed on mouse joint tissue samples derived from other experiments presented in the current manuscript, in which sample sizes were determined by power calculations as described above.
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Statistical reporting
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Please outline where this information can be found within the submission (e.g., sections or figure legends), or explain why this information doesn't apply to your submission: Methods of statistical analysis of expression of inflammatory and degradative mediators in cartilage and synovial fibroblasts treated with tryptase are provided in the legend of Figure 3 and "Statistics" sub-section of the Materials and Methods.

Group allocation
• Indicate how samples were allocated into experimental groups (in the case of clinical studies, please specify allocation to treatment method); if randomization was used, please also state if restricted randomization was applied • Indicate if masking was used during group allocation, data collection and/or data analysis Please outline where this information can be found within the submission (e.g., sections or figure legends), or explain why this information doesn't apply to your submission: Additional data files ("source data") • We encourage you to upload relevant additional data files, such as numerical data that are represented as a graph in a figure, or as a summary table • Where provided, these should be in the most useful format, and they can be uploaded as "Source data" files linked to a main figure or table • Include model definition files including the full list of parameters used • Include code used for data analysis (e.g., R, MatLab) • Avoid stating that data files are "available upon request" Please indicate the figures or tables for which source data files have been provided: Description of masking for assessment of cartilage damage, synovial thickening and osteophyte formation for destabilization of medial meniscus experiments is described in the "Histologic assessment of osteoarthritic development in mice" sub-section of the Materials and Methods.
Masking was not applicable for other experiments because of the use of unbiased data collection methods.
No additional data or source data is available for this manuscript.