Sülzen, H.; Tulmin, H.; Hajek, M.; Cermak, J.; Kadlecova, A.; Pompach, P.; Votrubova, J.; Zoltner, M.; Zoll, S.

The complement system, a critical component of the human innate immune system, enhances the ability to clear microbes and damaged cells. Dysregulation of this system, particularly the alternative pathway (AP), can lead to several rare blood disorders such as paroxysmal nocturnal hemoglobinuria (PNH). This study introduces SH-01, a blood parasite-derived, novel recombinant protein which selectively inhibits the AP. We show that SH-01 effectively prevents the lysis of erythrocytes isolated from PNH patients. Unlike current treatments such as eculizumab, SH-01 targets the AP without impairing the classical or lectin pathways, reducing the risk of infections and extravascular hemolysis. SH-01 functions through a unique two-stage mechanism, preventing C3b deposition and inhibiting AP C5 convertase activity while maintaining the amplification loop\'s functionality. Immunization studies in mice showed no significant immune response against SH-01, and the protein exhibited high stability and no acute toxicity. These findings suggest SH-01 as a promising candidate for treatment of PNH and other diseases characterized by AP hyperactivation, offering a more targeted therapeutic and thus safer approach.