Role of decay-accelerating factor in regulating complement activation on the erythrocyte surface as revealed by gene targeting

Decay-accelerating factor (DAF) is a glycosylphosphatidylinositol (GPI)-anc hored membrane protein that inhibits both the classical and the alternative pathways of complement activation. DAF has been studied extensively in hum ans under two clinical settings: when absent from the erythrocytes of parox ysmal nocturnal hemoglobinuria (PNH) patients, who suffer from complement-m ediated hemolytic anemia, and in transgenic pigs expressing human DAF, whic h have been developed to help overcome complement-mediated hyperacute rejec tion in xenotransplantation. Nevertheless, the exact role of DAF in regulat ing complement activation in vivo on the cell surface and the species speci ficity of this molecule remain to be fully characterized. To address these issues, we have used gene targeting to produce mice lacking GPI-anchored DA F, We found that erythrocytes from mice deficient in GPI-anchored DAF showe d no increase in spontaneous complement activation in vivo but exhibited im paired regulation of zymosan-initiated bystander and antibody-triggered cla ssical pathway complement activation in vitro, resulting in enhanced comple ment deposition. Despite a high level of C3 fixation, no homologous hemolys is occurred. It is noteworthy that GPI-linked DAF knockout erythrocytes, wh en tested with human and guinea pig sera, were more susceptible to heterolo gous complement lysis than were normal erythrocytes, These results suggest that DAF is capable of regulating homologous as well as heterologous comple ment activation via the alternative or the classical pathway. They also ind icate that DAF deficiency alone is not sufficient to cause homologous hemol ysis, In contrast, when the assembly of the membrane-attack complex is not properly regulated, as in the case of heterologous complement activation or in PNH patients, impaired erythrocyte DAF activity and enhanced C3 deposit ion could lead to increased hemolytic reaction.