Myeloid cell in FVIII immune response and their possible contribution in inhibitor formation

The major complication in Hemophilia A (HA) therapy is inhibitor formation against infused FVIII. While this is actively studied, the full mechanism remains unknown. Prior work from our lab suggests a role for plasmacytoid dendritic cells (pDCs) in inhibitor development. pDCs are traditionally known for type I interferon responses, but their antigen-presenting functions are under investigation. Another therapeutic option is Emicizumab, effective in HA patients with or without inhibitors, though few studies compare its use with FVIII in pediatric patients. This study analyzes the immunological and hematological profiles of pediatric HA patients on these treatments and examines the role of myeloid cells, focusing on pDCs and macrophages, in FVIII responses in both human and murine models. FVIII-treated patients had higher circulating CD11c+ cells, with also different activation status (higher HLA-DR expression on CD11c+ on Emicizumab-treated and CD14+ cells on HA compared to healthy). Plasma cytokine analysis revealed elevated IL-12p40, TNF-α, CCL-22, IL-18, and CCL-4 in HA patients, indicating a dysregulated myeloid compartment. HA pDCs exhibited non-conventional activation to FVIII in vitro, with stronger signatures in Emicizumab-treated and previously untreated patients. Murine HA pDCs showed enhanced activation and pro-inflammatory gene expression after in vitro FVIII stimulation. In vivo, FVIII increased pDC percentages in lymphoid organs and MHC-II and CD62-L expression on HA pDCs. Adoptive transfer of HA pDCs showed that donor cells trafficked mainly to lungs and spleen post-FVIII injection, excluding the lymph nodes. Patient-derived macrophages from patients in FVIII prophylaxis showed M1-like polarization, while Emicizumab-treated macrophages exhibited immune-regulatory features. These findings reveal distinct myeloid activation patterns in HA and provide insight into immune responses under different prophylactic regimens in pediatric patients.