Complement activation by apoptotic cells occurs predominantly via IgM and is limited to late apoptotic (secondary necrotic) cells.
Apoptotic cells activate complement via various molecular mechanisms. It is not known which of these mechanisms predominate in a physiological environment. Using Jurkat cells as a model, we investigated complement deposition on vital, early and late apoptotic (secondary necrotic) cells in a physiological medium, human plasma, and established the main molecular mechanism involved in this activation. Upon incubation with recalcified plasma, binding of C3 and C4 to early apoptotic cells was similar to background binding on vital cells. In contrast, late apoptotic (secondary necrotic) cells consistently displayed substantial binding of C4 and C3 and low, but detectable, binding of C1q. Binding of C3 and C4 to the apoptotic cells was abolished by EDTA or Mg-EGTA, and also by C1-inhibitor or a monoclonal antibody that inhibits C1q binding, indicating that complement fixation by the apoptotic cells was mainly dependent on the classical pathway. Late apoptotic cells also consistently bound IgM, in which binding significantly correlated with that of C4 and C3. Depletion of plasma for IgM abolished most of the complement fixation by apoptotic cells, which was restored by supplementation with purified IgM. We conclude that complement binding by apoptotic cells in normal human plasma occurs mainly to late apoptotic, secondary necrotic cells, and that the dominant mechanism involves classical pathway activation by IgM.
Department of Immunopathology, Sanquin Research at Sanquin, Plesmanlaan 125, 1066 CX Amsterdam, The Netherlands.
Bas
B
Zwart
Caroline
C
Ciurana
Irma
I
Rensink
Rishi
R
Manoe
C Erik
CE
Hack
Lucien A
LA
Aarden
D000818:Animals; D017209:Apoptosis / Q000276:immunology / Q000502:physiology*; D015922:Complement C1q / Q000276:immunology / Q000502:physiology; D003176:Complement C3 / Q000276:immunology / Q000502:physiology; D003181:Complement C4 / Q000276:immunology / Q000502:physiology; D003165:Complement System Proteins / Q000276:immunology / Q000502:physiology*; D006801:Humans; D007075:Immunoglobulin M / Q000276:immunology / Q000502:physiology*; D051379:Mice; D010949:Plasma / Q000276:immunology / Q000502:physiology
D003176:Complement C3; D003181:Complement C4; D007075:Immunoglobulin M; D015922:Complement C1q; D003165:Complement System Proteins