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Blood, 23 April 2009, Vol. 113, No. 17, pp. 4094-4100. Prepublished online as a Blood First Edition Paper on January 29, 2009; DOI 10.1182/blood-2008-11-189944.
RED CELLS, IRON, AND ERYTHROPOIESIS Complement fraction 3 binding on erythrocytes as additional mechanism of disease in paroxysmal nocturnal hemoglobinuria patients treated by eculizumab1 Hematology, Department of Biochemistry and Medical Biotechnologies, Federico II University, Naples; 2 Laboratory of Genetics and Gene Transfer, Core Research Laboratory–Istituto Toscano Tumori (CRL-ITT), Florence; 3 Hematology, University of Florence, Florence; 4 Department of Cellular Biotechnologies and Hematology, La Sapienza University, Rome; 5 Department of Hematology, Istituto di Ricovero e Cura a Carattere Scientifico (IRCCS) Fondazione Ospedale Maggiore Policlinico, Mangiagalli e Regina Elena, Milan; 6 Department of Cell Therapy and Hematology, Hemophilia and Thrombosis Center, San Bortolo Hospital, Vicenza; 7 Immunohematology and Transfusion Medicine, Federico II University, Naples; 8 Department of Nuclear Medicine, IRCCS Hospital Casa Sollievo Della Sofferenza, San Giovanni Rotondo; and 9 Istituto Toscano Tumori (ITT), Florence, Italy In paroxysmal nocturnal hemoglobinuria (PNH) hemolytic anemia is due mainly to deficiency of the complement regulator CD59 on the surface of red blood cells (RBCs). Eculizumab, an antibody that targets complement fraction 5 (C5), has proven highly effective in abolishing complement-mediated intravascular hemolysis in PNH; however, the hematologic benefit varies considerably among patients. In the aim to understand the basis for this variable response, we have investigated by flow cytometry the binding of complement fraction 3 (C3) on RBCs from PNH patients before and during eculizumab treatment. There was no evidence of C3 on RBCs of untreated PNH patients; by contrast, in all patients on eculizumab (n = 41) a substantial fraction of RBCs had C3 bound on their surface, and this was entirely restricted to RBCs with the PNH phenotype (CD59–). The proportion of C3+ RBCs correlated significantly with the reticulocyte count and with the hematologic response to eculizumab. In 3 patients in whom 51Cr labeling of RBCs was carried out while on eculizumab, we have demonstrated reduced RBC half-life in vivo, with excess 51Cr uptake in spleen and in liver. Binding of C3 by PNH RBCs may constitute an additional disease mechanism in PNH, strongly enhanced by eculizumab treatment and producing a variable degree of extravascular hemolysis.
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