Critical role of the C-terminal domains of factor H in regulating complement activation at cell surfaces.
The plasma protein factor H primarily controls the activation of the alternative pathway of complement. The C-terminal of factor H is known to be involved in protection of host cells from complement attack. In the present study, we show that domains 19-20 alone are capable of discriminating between host-like and complement-activating cells. Furthermore, although factor H possesses three binding sites for C3b, binding to cell-bound C3b can be almost completely inhibited by the single site located in domains 19-20. All of the regulatory activities of factor H are expressed by the N-terminal four domains, but these activities toward cell-bound C3b are inhibited by isolated recombinant domains 19-20 (rH 19-20). Direct competition with the N-terminal site is unlikely to explain this because regulation of fluid phase C3b is unaffected by domains 19-20. Finally, we show that addition of isolated rH 19-20 to normal human serum leads to aggressive complement-mediated lysis of normally nonactivating sheep erythrocytes and moderate lysis of human erythrocytes, which possess membrane-bound regulators of complement. Taken together, the results highlight the importance of the cell surface protective functions exhibited by factor H compared with other complement regulatory proteins. The results may also explain why atypical hemolytic uremic syndrome patients with mutations affecting domains 19-20 can maintain complement homeostasis in plasma while their complement system attacks erythrocytes, platelets, endothelial cells, and kidney tissue.
Ferreira
Viviana P
VP
Department of Biochemistry, Center for Biomedical Research, University of Texas, Health Science Center, Tyler, TX 75708, USA.
Herbert
Andrew P
AP
Hocking
Henry G
HG
Barlow
Paul N
PN
Pangburn
Michael K
MK
D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D003167:Complement Activation; D003179:Complement C3b / Q000378:metabolism*; D017242:Complement Factor H / Q000235:genetics / Q000378:metabolism*; D004910:Erythrocyte Membrane / Q000276:immunology*; D004912:Erythrocytes / Q000276:immunology; D006461:Hemolysis / Q000276:immunology; D006801:Humans; D017434:Protein Structure, Tertiary / Q000235:genetics / Q000502:physiology; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism
D011994:Recombinant Proteins; D003179:Complement C3b; D017242:Complement Factor H