The extracoagulative role of Factor VIII on bone cells and on the activation of CD8 T Cell proliferation

Hemophilia A (HA) is a genetic bleeding disorder caused by coagulation factor VIII (FVIII) deficiency. While treatment focuses on FVIII replacement to prevent hemorrhages, patients also suffer from non-bleeding complications whose causes are unclear. This thesis investigates the extracoagulative functions of FVIII, specifically its roles in bone remodeling and immune regulation. Project 1 examined FVIII’s role in bone homeostasis. HA patients show reduced bone mineral density, previously attributed to joint bleeds and immobility. However, our study demonstrates that FVIII deficiency intrinsically impairs bone architecture from early development. Long-term FVIII gene therapy restored bone structure, suggesting a direct role in bone maintenance. In vitro, HA osteoblasts showed defective formation and mineralization, while osteoclast differentiation was enhanced. FVIII supplementation rescued osteoblast function but not osteoclast overactivity. Coculture and secretome analyses revealed that FVIII-deficient osteoblasts fail to release key signaling molecules regulating osteoblast-osteoclast interaction, contributing to bone fragility. Project 2 explored FVIII’s impact on CD8 T cells. We observed that HA CD8 T cells proliferate more than wild-type (WT) cells both in vitro and in vivo. Transcriptomic profiling revealed that HA CD8 T cells are transcriptionally primed for activation and proliferation. This hyper-reactivity may underlie the heightened risk of developing anti-FVIII inhibitors during therapy. Beyond coagulation, FVIII plays essential roles in bone and immune regulation. FVIII deficiency directly disrupts osteoblast-osteoclast communication and promotes CD8 T cell hyperactivity, providing new insights into HA complications and guiding future therapeutic strategies.