Altered bioavailability of platelet-derived factor VIII during thrombocytosis reverses phenotypic efficacy in haemophilic mice

Ectopic delivery of factor VIII (FVIII) to megakaryocytes (Mk) represents a viable approach for localized tenase generation by concentrating the FVIIIa/FIXa enzyme-cofactor complex onto activated platelet membranes. We utilized a core rat platelet factor 4 (PF4) promoter for Mk/platelet-restricted expression of human B-domain-deleted (hBDD) FVIII within the background of a haemophilia A mouse (rPF4/hBDD/FVIII^-/-). Platelets from rPF4/hBDD/FVIII^-/- mice contained ∼122 mU FVIII:C/1×10 ⁹ platelets/ml with no detectable plasmatic FVIII:C, and with no effect on α-granule-derived platelet factor V/Va function. Paired tenase assays (± thrombin) confirmed that platelet (pt) FVIII (unlike platelet FV) required thrombin cleavage for complete activation. rPF4/hBDD/FVIII ^-/- mice exposed to a thrombocytotic stimulus (thrombopoietin, TPO) demonstrated a statistically-significant 66% reduction in molar ptFVIII activity with a non-significant reduction in total ptFVIII biomass. Decreased molar ptFVIII concentration correlated with loss of phenotypic correction as evaluated using a haemostatic tail-snip assay. Comparative studies using a transgenic mouse expressing human amyloid-β-precursor protein (hAβPP) from the rPF4 promoter confirmed diminished hAβPP expression without affecting endogenous α-granule PF4, establishing generalizability of these observations. While Mk/platelet-released ptFVIII (unlike pFV) is proteolytically inactive, we also conclude that thrombocytotic stimuli negatively affect ptFVIII bioavailability and phenotypic efficacy, results which correlate best with molar ptFVIII concentration, and not systemically available ptFVIII.