Elevated Serum Lysophosphatidylcholine in Patients with Systemic Lupus Erythematosus Impairs Phagocytosis of Necrotic Cells In Vitro.

OBJECTIVES: Impaired clearance of dying and dead cells by professional and amateur phagocytes plays a crucial role in the etiology of systemic lupus erythematosus (SLE). While dying, cells expose and release a plethora of eat-me and find-me signals to ensure their timely removal before entering the dangerous stage of secondary necrosis. A well-described chemoattractant for macrophages is dying cell-derived lysophosphatidylcholine (LPC). However, its implications for and/or its association with SLE disease, so far, have not been examined. In the present study, we analyzed the LPC serum concentrations of patients with SLE and rheumatoid arthritis (RA). Subsequently, we examined if and to which extent the measured serum concentrations of LPC and an LPC-rich environment can impact the phagocytosis of necrotic cells. METHODS: Sera from patients with SLE, RA, and normal healthy donors (NHD) were characterized for several parameters, including LPC concentrations. Phagocytosis of dead cells by human macrophages in the presence of SLE and NHD sera was quantified. Additionally, the impact of exogenously added, purified LPC on phagocytosis was analyzed. RESULTS: Patients with SLE had significantly increased LPC serum levels, and high serum LPC of SLE patients correlated significantly with impaired phagocytosis of dead cells in the presence of heat-inactivated serum. Phagocytosis in the presence of sera from NHD showed no correlation to LPC levels, but exogenous addition of purified LPC in the range as measured in SLE patients' sera led to a concentration-dependent decrease. CONCLUSION: Our data show that high levels of LPC as observed in the sera of SLE patients have a negative impact on the clearance of dead cells by macrophages. Chemoattraction requires a concentration gradient. The higher the LPC concentration surrounding a dying or dead cell, the smaller the achievable gradient upon LPC release will be. Thus, it is feasible to assume that elevated LPC levels can interfere with the build-up of a local LPC gradient during cell death, and hence might play a role in the establishment and/or perpetuation of SLE disease.

Remnants of secondarily necrotic cells fuel inflammation in systemic lupus erythematosus.

OBJECTIVE: Patients with systemic lupus erythematosus (SLE) are often characterized by cellular as well as humoral deficiencies in the recognition and phagocytosis of dead and dying cells. The aim of this study was to investigate whether the remnants of apoptotic cells are involved in the induction of inflammatory cytokines in blood-borne phagocytes. METHODS: We used ex vivo phagocytosis assays comprising cellular and humoral components and phagocytosis assays with isolated granulocytes and monocytes to study the phagocytosis of secondarily necrotic cell-derived material (SNEC). Cytokines were measured by multiplex bead array technology. RESULTS: We confirmed the impaired uptake of various particulate targets, including immunoglobulin-opsonized beads, by granulocytes and monocytes from patients with SLE compared with healthy control subjects. Surprisingly, blood-borne phagocytes from two-thirds of the patients with SLE took up SNEC, which was rarely phagocytosed by phagocytes from healthy control subjects or patients with rheumatoid arthritis. Supplementation of healthy donor blood with IgG fractions derived from patients with SLE transferred the capability to take up SNEC to the phagocytes of healthy donors. Phagocytosis-promoting immune globulins also induced secretion of huge amounts of cytokines by blood-borne phagocytes following uptake of SNEC. CONCLUSION: Opsonization of SNEC by autoantibodies from patients with SLE fosters its uptake by blood-borne monocytes and granulocytes. Autoantibody-mediated phagocytosis of SNEC is accompanied by secretion of inflammatory cytokines, fueling the inflammation that contributes to the perpetuation of autoimmunity in SLE.

Removal of dying cells and systemic lupus erythematosus.

Systemic lupus erythematosus (SLE) is a very heterogeneous systemic autoimmune disease, in which autoantibody synthesis against nuclear constituents is the main immunological characteristic. These autoantibodies underwent affinity maturation and isotype switching. Additionally, T-cell tolerance against nuclear autoantigens should be affected in these autoimmune patients. Nuclear material derived from apoptotic and/or necrotic cells may serve as an important source of autoantigens. However, dead and dying cells as well as cellular debris are rapidly removed from tissues by phagocytes without eliciting inflammation or immune responses under healthy conditions. During apoptosis nuclear components are strongly modified through enzymatic reactions. If these cells are not timely cleared, those autoantigens may be released, taken up, and presented by dendritic cells in tissues or presented by follicular dendritic cells in lymph nodes to T and B cells, respectively. This could be a mechanism for breaking the peripheral self-tolerance. In this article we focus on the deficient clearance of apoptotic cells in SLE patients and its importance in development of this autoimmune disease.