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Objective. To investigate whether the established impaired phagocyte function in systemic lupus erythem-atosus (SLE) patients also affects apoptotic cell clearance. Accumulation of apoptotic waste as a source for autoantigens that induce and maintain autoimmune responses is discussed. Methods. Apoptosis was detected by morphology and propidium iodide staining. In vitro phagocytosis of autologous apoptotic cells in cultured peripheral blood mononuclear cells was evaluated microscopically. Cross-feeding experiments were performed to investigate phagocytosis of heterologous apoptotic cells by in vitro-differentiated macrophages. Furthermore, the effect of annexin V on the phagocytosis of apoptotic cells was investigated. Results. Reduced clearance of apoptotic cells in SLE patients was observed. The defective clearance appeared to reflect phagocyte dysfunction and not an abnormal execution of apoptosis. A similar picture was seen when in vitro-differentiated macrophages from control populations were treated with annexin V. Conclusion. Noninflammatory engulfment phago-cytosis of apoptotic cells is decreased in SLE patients. Persistently circulating apoptotic waste may encounter inflammatory removal pathways and serve as immuno-gen for the induction of autoreactive lymphocytes and as antigen for immune complex formation.

Autoantibody-mediated diseases like myasthenia gravis, autoimmune hemolytic anemia and systemic lupus erythematosus represent a therapeutic challenge. In particular, long-lived plasma cells producing autoantibodies resist current therapeutic and experimental approaches. Recently, we showed that the sensitivity of myeloma cells toward proteasome inhibitors directly correlates with their immunoglobulin synthesis rates. Therefore, we hypothesized that normal plasma cells are also hypersensitive to proteasome inhibition owing to their extremely high amount of protein biosynthesis. Here we show that the proteasome inhibitor bortezomib, which is approved for the treatment of multiple myeloma, eliminates both short- and long-lived plasma cells by activation of the terminal unfolded protein response. Treatment with bortezomib depleted plasma cells producing antibodies to double-stranded DNA, eliminated autoantibody production, ameliorated glomerulonephritis and prolonged survival of two mouse strains with lupus-like disease, NZB/W F1 and MRL/lpr mice. Hence, the elimination of autoreactive plasma cells by proteasome inhibitors might represent a new treatment strategy for antibody-mediated diseases.

Autoantibodies against double-stranded DNA (dsDNA) and nucleosomes represent a hallmark of systemic lupus erythematosus (SLE). However, the mechanisms involved in breaking the immunological tolerance against these poorly immunogenic nuclear components are not fully understood. Impaired phagocytosis of apoptotic cells with consecutive release of nuclear antigens may contribute to the immune pathogenesis. The architectural chromosomal protein and proinflammatory mediator high mobility group box protein 1 (HMGB1) is tightly attached to the chromatin of apoptotic cells. We demonstrate that HMGB1 remains bound to nucleosomes released from late apoptotic cells in vitro. HMGB1–nucleosome complexes were also detected in plasma from SLE patients. HMGB1-containing nucleosomes from apoptotic cells induced secretion of interleukin (IL) 1β, IL-6, IL-10, and tumor necrosis factor (TNF) α and expression of costimulatory molecules in macrophages and dendritic cells (DC), respectively. Neither HMGB1-free nucleosomes from viable cells nor nucleosomes from apoptotic cells lacking HMGB1 induced cytokine production or DC activation. HMGB1-containing nucleosomes from apoptotic cells induced anti-dsDNA and antihistone IgG responses in a Toll-like receptor (TLR) 2–dependent manner, whereas nucleosomes from living cells did not. In conclusion, HMGB1–nucleosome complexes activate antigen presenting cells and, thereby, may crucially contribute to the pathogenesis of SLE via breaking the immunological tolerance against nucleosomes/dsDNA.