Complement Factor H in Human Platelets: Implications for Atypical HUS.

Atypical hemolytic uremic syndrome (aHUS) is a severe kidney disease that mainly affects children, where risks of progression to renal failure and death remain significant. aHUS pathology is associated with hyperactivity of the complement system and activation of platelet-dependent clotting leading to the formation of platelet-rich thromboemboli that occlude the glomerular microvasculature. These events have been linked to defects in the regulation of the alternative complement pathway, with >25% of pediatric aHUS cases being associated with mutations in the soluble complement activation inhibitor Factor H (CFH), or inhibitory autoantibodies. There is also evidence that interactions between platelets and the complement system are important in the progression of aHUS (

Immunoblot analysis detected the presence of CFH in washed cell lysates from serumfree cultured megakaryocytes, normal circulating platelets and platelets from an ARC syndrome patient lacking a-granules. CFH was also detected in supernatants recovered after in vitro activation of normal platelets with thrombin and other agonists. The location of CFH in resting platelets was determined via immunostaining of fixed cells followed by laser fluorescence confocal microscopic (LFCM) imaging. CFH staining was observed inside and on the surface of platelets and microparticles, and was not confined to granules or other secretory vesicles. Deconvolved LFCM images were used for colocalization analysis (Volocity software) with known platelet proteins. CFH showed significant colocalization (p<0.05; Pearson correlation coefficient) with actin (cytoplasmic), tubulin (cytoskeletal) and GPIIIa (CD61, cell surface receptor), but not with α-granule proteins Von Willebrand Factor and fibrinogen, or dense granule/lysosomal proteins CD63 (tetraspanin) and LAMP-1. CFH surface staining was observed to intensify uniformly during the early stages of platelet activation and then subside and disperse to patches on fully-activated platelets.

Blood samples were taken from an aHUS patient lacking CFH due to mutation before and after therapeutic fresh frozen plasma (FFP) infusion, and over the following 14 days. Immunoblot analysis of plasma and lysates from washed circulating platelets showed rapid platelet uptake of CFH in vivo after FFP treatment, with the level of platelet CFH declining in parallel with plasma CFH levels. Laboratory assays of blood clotting via thromboelastography showed blood samples from pediatric aHUS patients to have a consistent hypercoagulable tendency. Thrombin generation assays via calibrated automated thrombography of aHUS patient plasmas revealed an elevated level of platelet-dependent, but not platelet-independent, thrombin generation compared to normal.

We conclude that CFH in circulating platelets is likely derived from both megakaryocyte synthesis and plasma uptake. CFH uptake and secretion by platelets does not appear to be granule-dependent and may be surface-mediated, with platelets developing a transient increase in CFH affinity early in activation. Pediatric aHUS patients have a tendency towards hypercoagulability, which may be associated with constitutively elevated platelet-dependent thrombin generation. These observations support the hypothesis that loss of CFH-mediated complement inhibition contributes to increased platelet procoagulant activity, which may contribute to the progression of aHUS, and possibly other thrombosis-associated diseases.