Skeletal muscle-derived IL-33 mediates muscle-to-bone crosstalk and regulates bone metabolism via CD8+ T cell-secreted CCL5

Background
Skeletal muscle-to-bone crosstalk may play critical roles in the association between muscle atrophy and bone loss. Here, we report a myokine, IL-33, which may mediate muscle-to-bone crosstalk.

Methods
A cohort study was conducted to elucidate the associations among circulating IL-33, osteoporosis, and sarcopenia in older adults. To delineate the role of IL-33 in muscle-to-bone crosstalk, we established rat models combining weighted-bearing exercise (WBE), botulinum neurotoxin A (BoNT/A)-induced muscle atrophy, and ovariectomy (OVX), and modulated muscle IL-33 levels both non-specifically and in a muscle-specific manner. Single-cell RNA sequencing was employed to identify the mechanisms by which IL-33 influences bone metabolism, and the downstream pathway was interrogated with anti-CD8β antibody, recombinant CCL5, and a selective CCR3 antagonist.

Findings
In humans, lower levels of IL-33 were linked to an increased risk of co-occurrence of sarcopenia and osteoporosis. In rats, strengthening muscle increased IL-33 levels and improved bone formation, while muscle atrophy had the opposite effect. These muscle-regulated effects can be manipulated by IL-33 supplementation or inhibition, or by skeletal muscle-specific IL-33 knockdown or overregulation. Single-cell sequencing and in vivo experiments confirm that IL-33 can regulate bone metabolism by promoting the release of CCL5 from CD8+ T cells via the ST2 receptor. The bone formation abilities improved by strengthening the muscle could be impaired by CD8+ T cell deletion and then rescued by CCL5 supplementation. CCL5 stimulated osteoclastogenesis through CCR3 and downstream ERK signalling, and blocking the CCR3 receptor can interrupt CCL5's osteogenic effect. Lastly, by improving the levels of IL-33 and CCL5, we may alleviate the bone loss in rats caused by OVX.

Interpretation
Our research has uncovered a muscle-bone communication mechanism primarily mediated by IL-33, linking muscle dysfunction to bone loss. This interaction could be a promising therapeutic target for musculoskeletal disorders and osteoporosis.

Funding
This work was supported by the Leading Talents Training Program of Pudong New Area Health Commission (Grant ID: PWR 12020-06), Health Science and Technology Project of Shanghai Pudong New Area Municipal Health Commission (Grant ID: PW2024B-07), and Shanghai Key Laboratory of Peripheral Nerve and Microsurgery/National Health Commission Key Laboratory of Hand Reconstruction (Fudan University) [Grant ID: 20DZ2270200].