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Orthopedic and Rehabilitation Engineering

Orthopedic and Rehabilitation Engineering - Poster Session E

Y26 - Modulation of Inflammatory Signaling by IL-15 in the Rotator Cuff is Dependent on Sex

Saturday, October 11, 2025

10:00 AM - 11:00 AM PDT

Location: Exhibit Hall F, G & H

Presenting Author(s)

JS

Jonathan Sim

Student
Boston University
Boston, Massachusetts, United States

Co-Author(s)

HK

Hana Kalčo

PhD student
Boston University, United States

Last Author(s)

Brianne Connizzo

Assistant Professor
Boston University
Boston, Massachusetts, United States

Primary Investigator(s)

Brianne Connizzo

Assistant Professor
Boston University
Boston, Massachusetts, United States

Introduction: : It is well established that muscle stress triggers the release of signaling proteins that influence surrounding musculoskeletal tissues, locally and through the bloodstream. Due to the unique anatomy of the rotator cuff, muscle-derived signals can directly influence the tendon and bone within the joint through the synovial fluid. We developed a novel in vitro model to study local interactions between muscle, tendon, and bone. In preliminary studies, we identified interleukin-15 (IL-15) as a key cytokine elevated in response to muscle stress. IL-15 has been implicated in modulating immune responses and muscle regeneration, but its effects on tendon and bone remain poorly understood. IL-15 production is also elevated in certain autoimmune and inflammatory disorders, such as rheumatoid arthritis (RA) [1]. Interestingly, these are also more prevalent in females, suggesting potential sex-specific differences in IL-15 signaling [2]. Therefore, this study aimed to examine how IL-15 impacts tendon, muscle, and bone health in the rotator cuff and whether these effects differ by sex. Based on our previous work, we hypothesized that the tissues would exhibit sex-specific responses to IL-15 treatment, with female tissues exhibiting a reduced secretory phenotype [3].

Materials and

Methods: : Flexor digitorum longus (FDL) tendon, humeral head bone, and supraspinatus muscle explants were harvested from young (4m) male and female C57BL/6J mice [4]. Explants were cultured in stress-deprived conditions with standard culture medium for 24h, then treated with exogenous IL-15 at 0.1 ng/mL for 24h. Relative gene expression was measured via qPCR, normalized to both β-Actin and untreated control explants. Culture media was analyzed using a custom multi-spot assay to quantify secreted protein levels of IFN-γ, IL-1β, IL-4, IL-6, IL-10, IL-15, KC/GRO, MCP-1, and TNF-α (Meso Scale Discovery). All data is presented as a fold-change relative to the control condition (tissue alone), such that < 1 would represent suppression of cytokine release and >1 would represent amplification. Statistical comparisons were made to untreated controls and between sexes, with significance set at *p < 0.05.

Results, Conclusions, and Discussions:: IL-15 treatment suppressed pro-inflammatory cytokine (TNF-α, CCL2, CXCL1, IL-6, and IL-1β) production in bone, tendon, and muscle (Fig. 1A-C). Female explants exhibited either no change or increased production of the same cytokines. For anti-inflammatory signals (IL-4, IL-15, IL-10), female tendons and muscles exhibited increased production; in males, production was suppressed. IL-15 also increased production of IL-6 in female explants but decreased production in male tissues.

IL-6 was differentially expressed in bone following IL-15 treatment, with higher levels in females compared to males (Fig. 2A). Other bone-associated genes did not show significant sex differences, although nearly all were upregulated relative to baseline. In tendon (Fig. 2B), IL-6, IL-15, TGF-β, and IGF-1 were upregulated with IL-15 treatment in males but not females. Bcl-2 was also elevated in males, although the effect size was small. IGF-1 and FNDC-5 (irisin) expression were significantly higher in male muscle (Fig. 2C). IGF-1, DCN, and MSTN were also upregulated relative to control in YM, but not YF.

IL-15 treatment resulted in consistent downregulation of pro-inflammatory cytokines across all tissues in young males, suggesting a broadly immunosuppressive role for IL-15 in male tissues. In contrast, young female tissues exhibited minimal suppression of all cytokines, and in some cases increased production, suggesting sex-specific regulation of cytokine profiles. Interestingly, IL-15 treatment stimulated tissue-specific turnover markers, and in some cases, growth, potentially related to the transition from inflammation to remodeling. This was generally much stronger in males than in females. However, it is important to note that all female studies were performed without hormonal stimulation; future studies will investigate interactions between cytokine production and sex hormones. Regardless, these findings highlight tissue- and sex-specific differences in cytokine responses to IL-15 treatment, particularly in tendon, suggesting complex regulatory mechanisms that may be influenced by sex.

Acknowledgements and/or References (Optional): : 1. Kurowska+2020. J Clin Med.
2. Firestein GS+ 2017. Immunity.
3. Kalčo+ 2025. bioRxiv 2025.06.10.658924.
4. Connizzo+2019. Connect Tissue Res.,