Effects of burosumab on macrophage polarization and TRPV1 expression in children with XLH.

PURPOSE: X-linked hypophosphatemia (XLH) is a rare genetic disorder caused by PHEX mutations, leading to hypophosphatemia and impaired bone mineralization. Burosumab, a monoclonal antibody targeting FGF23, improves phosphate levels and bone health. Although burosumab improves musculoskeletal pain, it remains unclear whether this benefit results solely from restoration of phosphate homeostasis or also involves immunomodulatory mechanisms. Given the role of FGF23 in modulating macrophage function and the involvement of TRPV1 - expressed by macrophages - in pain transmission, we investigated the effects of burosumab on macrophage polarisation and TRPV1 expression in children with XLH. METHODS: Macrophages were isolated from untreated XLH patients (n=4), burosumab-treated XLH patients (n=8), and healthy donors (n=5). Western blot was used to assess M1 (CCR7, CD86, iNOS) and M2 (CD206, p-STAT6) markers, as well as TRPV1 expression. To evaluate burosumab's direct effects, macrophages from healthy donors were stimulated with LPS and treated with 1.5 or 3 μg/mL burosumab, followed by the same analyses. RESULTS: Untreated XLH macrophages predominantly exhibited an M1 phenotype with higher TRPV1 protein levels compared to healthy controls. In contrast, macrophages from burosumab-treated patients showed a tendency to acquire an M2-like phenotype and reduced TRPV1 expression. Similar effects were observed in vitro, with LPS-stimulated healthy macrophages shifting toward the M2 phenotype and showing decreased TRPV1 after burosumab exposure. CONCLUSION: In untreated XLH patients, macrophages exhibit a pro-inflammatory M1 phenotype with upregulated TRPV1 expression. Burosumab reverses this profile, driving M2 polarization and reducing TRPV1 levels, thereby suggesting that it exerts therapeutic effects beyond phosphate homeostasis through immunomodulation. PLAIN SUMMARY: X-linked hypophosphatemia (XLH) causes rickets and chronic pain. We found that immune cells from untreated patients show a pro-inflammatory profile. Treatment with burosumab shifted these immune cells toward an anti-inflammatory state and reduced the expression of TRPV1, a molecule involved in neuroimmune signaling, suggesting that burosumab may exert biological effects beyond bone health.