Cooperation between C1q and DNase I in the clearance of necrotic cell–derived chromatin

Objective The efficient uptake of dying cells by phagocytes is essential to the avoidance of chronic inflammation. Some human autoimmune responses are thought to be driven by autoantigens from apoptotic or necrotic cells. We analyzed the role of C1q and DNase I in the disposal of necrotic cell–derived chromatin because deficiencies in these serum factors predispose to the development of systemic autoimmune disorders, such as systemic lupus erythematosus. Methods Human necrotic peripheral blood lymphocytes were incubated in cell culture medium supplemented with various sera or serum components. Chromatin degradation was monitored by measuring the residual DNA content by flow cytometry. The uptake of necrotic cell–derived nuclei was analyzed by in vitro phagocytosis assays. Results Reconstitution of C1q‐depleted serum with C1q strongly increased its ability to degrade necrotic cell–derived chromatin. Although C1q itself displayed no DNase activity, it strongly augmented the activity of serum DNase I. Whereas an excess of recombinant DNase I degraded chromatin in the absence of C1q, efficient uptake of the predigested material by monocyte‐derived phagocytes required the presence of C1q. These data show that C1q and DNase I cooperate in the degradation of chromatin from necrotic cells. Furthermore, C1q was found to be necessary for effective uptake of degraded chromatin by monocyte‐derived phagocytes. Conclusion C1q or DNase I deficiencies may precipitate autoimmunity in humans by a mechanism similar to that of other molecules that are involved in the safe, efficient, and rapid disposal of dying cells.