Complement Activation by Apoptotic Cells Occurs Predominantly via IgM and is Limited to Late Apoptotic (Secondary Necrotic) Cells

Abstract

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.

C4b-binding Protein and Factor H Compensate for the Loss of Membrane-bound Complement Inhibitors to Protect Apoptotic Cells against Excessive Complement Attack

“…In addition, apoptotic cells have been reported to bind complement-initiating molecules such as mannose-binding lectin and C1q that enhance uptake by phagocytes (8 -10). Some discrepancies exist in reports of the stage of cell death necessary for binding of such recognition molecules, which can partly be explained by differences in semantics, experimental design, and readout (9,(11)(12)(13)(14).The complement system is an integral part of the innate immune defense and is also involved in the instruction of adaptive immunity and clearance of waste such as dead cells and immune complexes (15,16). This potent enzyme cascade system has both membrane-bound and fluid phase inhibitors that protect host surfaces and prevent systemic depletion of complement activity caused by auto-activation (15, 16).…”

Do not let death do us part: ‘find-me’ signals in communication between dying cells and the phagocytes

“…91 These effects are dependent on different receptors as well as cell types, but not yet fully understood. LPC can also act as an indirect eat-me signal by promoting the LPC-dependent binding of IgM to apoptotic, 92 late apoptotic 93,94 and necrotic cells, 95 ultimately leading to clearance. LPC-IgM-dependent binding to different apoptotic and necrotic states may serve as a backup to normal recognition in situations where large numbers of cells are undergoing apoptosis.…”

Apaf-1-independent programmed cell death in mouse development

“…The complement system is another candidate for the engulfment of late apoptotic or necrotic cells. 43 It will be interesting to study the engulfment of necrotic cells by crossing the Apaf-1 À/À mice with mice deficient in the engulfment of apoptotic or necrotic cells. 44,45 Unengulfed apoptotic cells are difficult to find in mouse tissues.…”