Emerging Roles of Eosinophils in Bone.

Purpose of the review: Eosinophils are traditionally known for their role in immune defense against parasites and their involvement in various immunopathologies, including eosinophilic airway diseases, eosinophilic dermatoses, and gastrointestinal disorders. However, recent findings from our group and other leading laboratories have broadened this perspective, revealing that eosinophils also play crucial roles in tissue development, homeostasis, and regeneration. This review aims to highlight the regulatory functions of eosinophils within the bone niche and emphasize the importance of further research into their role in bone biology.

Recent findings: Growing evidence suggests that eosinophils are key regulators of bone metabolism, extending beyond their established roles in immunity and inflammation. They contribute to bone homeostasis by inhibiting osteoclast differentiation, helping to prevent excessive bone resorption in osteoporosis and inflammatory arthritis. Additionally, eosinophils may promote osteoblast-mediated bone formation, modulate the mesenchymal and hematopoietic stem cell niche, and contribute to the bone microenvironment by affecting vascularization and extracellular matrix composition. However, their impact may vary under pathological conditions. Patients with eosinophilic disorders are often at an increased risk of osteoporosis and fragility fractures, though this is largely attributed to disease-related treatments rather than eosinophil activity itself. Despite emerging insights into the role of eosinophils in bone biology, the underlying mechanisms remain incompletely understood. Further research is essential to elucidate how eosinophils influence bone physiology and pathology.