Altered frequency of FOXP3⁺ regulatory T cells is associated with development of inhibitors in patients with severe hemophilia A

The development of anti-factor VIII (FVIII) antibodies or "inhibitors" is a major complication following FVIII replacement therapy in patients with severe hemophilia A (HA), rendering the treatment inefficient. Data on the role of regulatory T cells (Tregs) in inhibitor formation in these patients are rare. Herein, we aimed to investigate whether a difference in the FOXP3+ Tregs is linked to the formation of the inhibitors in severe HA patients.In this cross-sectional study, 32 patients with severe HA (8 patients with inhibitors and 24 without inhibitors) and 24 healthy controls were enrolled. The frequency of FOXP3+ Tregs was determined using multicolor flow cytometry method.Our results showed that the median level of CD4+ CD25+ FOXP3+ Tregs did not significantly differ between HA patients and healthy controls and between HA patients with and without inhibitors (P > 0.05). However, patients with inhibitors had significantly lower amounts of CD4+ CD25- FOXP3+ Tregs compared to those without inhibitors as well as healthy controls (*P = 0.012 and *P = 0.004, respectively). The frequency of CD4+ CD25+ T cells was significantly higher in HA patients who developed inhibitors compared to the inhibitor-negative ones whereas they were lower in inhibitor-negative patients compared to the healthy controls (*P = 0.013 and *P = *0.029, respectively). The percentages of CD4+ CD25+ T cells were positively correlated with the levels of inhibitors in HA patients (r = 0.45, *P = 0.021).Our data demonstrated for the first time that the CD4+ CD25- FOXP3+ Tregs might be implicated in the prevention of inhibitor formation in severe HA patients.