10, 11 We have previously shown that FH binds to washed platelets via the C terminus. 9 The complement regulatory region in the N terminus displays cofactor activity and decay-accelerating activity and the C-terminal cell-binding region mediates host recognition by interacting with heparin, glucosaminoglycans, C3b, and endothelial cells. 8 FH consists of 20 short consensus repeats (SCRs). 7 In addition, it binds to C3b, preventing binding of factor B to the C3Bb convertase. It circulates as a 150-kDa protein at a concentration of approximately 500 μg/mL and acts as a cofactor for cleavage of C3b by factor I. FH is predominantly produced in the liver 6 and exhibits activity both in the fluid phase and on the cell surface. 3 Platelets and megakaryocytes also contain factor H (FH), 4, 5 the main fluid-phase regulator of the alternative pathway. Platelets express membrane-bound complement regulators CD46, CD55, 1, 2 and CD59. In summary, mutated FH enables complement activation on the surface of platelets and their activation, which may contribute to the development of thrombocytopenia in aHUS.Ĭomplement activation is regulated on cell surfaces by a combination of several cellular and fluid phase regulators. The purified FH-E1198Stop mutant added to FH-deficient plasma (complemented with C3) allowed considerable C3 deposition on washed platelets, in comparison to normal FH. Complement deposition and platelet activation were reduced when normal FH was preincubated with platelets and were minimal when using normal serum. Combining patient serum/plasma with normal washed platelets led to C3 and C9 deposition, CD40L and CD62P expression, aggregate formation, and generation of tissue factor-expressing microparticles. Washed platelets taken from the 4 patients with aHUS during remission exhibited C3 and C9 deposition, as well as CD40-ligand (CD40L) expression indicating platelet activation.

Purified FH-E1198Stop mutant exhibited reduced binding to normal washed platelets compared with normal FH, detected by flow cytometry. A total of 4 FH mutations, in SCR15 (C870R) and SCR20 (V1168E, E1198K, and E1198Stop) in patients with aHUS, were studied regarding their ability to allow complement activation on platelet surfaces. FH binds to platelets via the C-terminal as previously shown using a construct consisting of short consensus repeats (SCRs) 15 to 20.

Atypical hemolytic uremic syndrome (aHUS) may be associated with mutations in the C-terminal of factor H (FH).

Decreased platelets with normal h and h