IgG autoantibodies bound to surfaces of necrotic cells and complement C4 comprise the phagocytosis promoting activity for necrotic cells of systemic lupus erythaematosus sera

Objective: Accumulation of dying and dead cells is thought to be involved in the etiopathogenesis of systemic lupus erythaematosus (SLE). Clearance has been described mainly for apoptotic cells; however, the knowledge of serum factors participating in the phagocytosis of necrotic cells is limited. Patients and methods: Sera from 18 patients with SLE and 10 normal healthy donors (NHD), and macrophages from 3 NHD were included. Autoantibodies and complement were measured by ELISA and phagocytosis by flow cytometry. Binding of serum IgG to necrotic cells was assessed by flow cytometry and confocal microscopy. Results: Sera from patients with SLE and NHD generally promoted the phagocytosis of necrotic cells by macrophages isolated from NHD. Five independent experiments with macrophages from three different NHD led to similar results. The sera from healthy controls displayed a homogeneous activity, whereas sera from patients with SLE showed a dichotomic behaviour. Only sera containing autoantibodies binding to the surfaces of necrotic cells and sufficient complement showed increased phagocytosis promoting activities. In SLE sera, C4 turned out to be the critical complement component in this process. Sera de-complemented by heat treatment strongly reduced phagocytosis of necrotic cells. Conclusions: Serum components influence the uptake of necrotic cells by phagocytosis competent macrophages from NHD. Complement is required for this process and autoantibodies binding to the surfaces of necrotic cells additionally promote their phagocytosis.