Abstract
Binding of the serum protein complement component C1q to the surface of dying cells facilitates their clearance by phagocytes in a process termed efferocytosis. Here, we investigate during which phase of apoptotic cell death progression C1q binding takes place. Purified C1q was found to bind to all dying cells and, albeit weaker, also to viable cells. The presence of serum abrogated completely the binding to viable cells. In addition, C1q binding to dying cells was limited to a specific subpopulation of late apoptotic/secondary necrotic cells. Co-culturing serum-treated apoptotic cells with human monocytes revealed a much higher phagocytosis of C1q-positive than of C1q-negative late apoptotic/secondary necrotic cells. But this phagocytosis-promoting activity could not be observed with purified C1q. Serum-treated C1q-positive late apoptotic/secondary necrotic cells exhibited a similar volume, a similar degraded protein composition, but a much lower DNA content in comparison with the remaining late apoptotic/secondary necrotic cells. This was mediated by a serum-bound nuclease activity that could be abrogated by G-actin, which is a specific inhibitor of serum DNase I. These results show that serum factors are involved in the prevention of C1q binding to viable cells and in the processing of late apoptotic/secondary necrotic cells promoting cell death progression toward apoptotic bodies. This process leads to the exposure of C1q-binding structures and facilitates efferocytosis.
Highlights
Signals to attract phagocytes without induction of inflammation.[3]
To characterize C1q binding to dying cells, we incubated viable and apoptotic cells for 1 h in either 20 mg/ml purified C1q or 25% normal human serum (NHS), which contained an equivalent concentration of C1q (Supplementary Figure 3)
This study revealed that C1q binds to a distinct subpopulation of secondary necrotic cells, which is formed in the presence of serum under involvement of DNase I
Summary
Signals to attract phagocytes without induction of inflammation.[3]. Late apoptotic/secondary necrotic cells as well as primary necrotic cells release additional pro-inflammatory ‘danger signals’ such as HMGB1.4–6 dying cells expose ‘eat me’ signals such as phosphatidylserine, oxidized phospholipids, nucleic acids and histones. In a previous proteomic analysis of breast cancer patients responding to an epirubicin/docetaxel combination therapy, we found significant changes in the plasma levels of C1q and of several activation fragments of C3 and C4.19 In addition, an increase of HMGB1 was observed.[20] In non-responders, the complement system as well as HMGB1 remained unaffected. This indicates that successful chemotherapy results in apoptotic tumor cells, which are opsonized by the complement system. This processing was found to be essential for the efferocytosis-promoting activity of C1q
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