Microparticles as antigenic targets of antibodies to DNA and nucleosomes in systemic lupus erythematosus

Systemic lupus erythematosus is a prototypic autoimmune disease characterized by antibodies to DNA and other nuclear molecules. While these antibodies can form immune complexes, the mechanisms generating the bound nuclear antigens are not known. These studies investigated whether microparticles can form complexes with anti-DNA and other anti-nucleosomal antibodies. Microparticles are small membrane-bound vesicles released from dead and dying cells; these particles contain a variety of cellular components, including DNA. To assess antigenicity, microparticles generated in vitro from apoptotic cell lines were tested using murine monoclonal anti-DNA and anti-nucleosomal antibodies as well as plasma from lupus patients. Antibody binding was assessed by flow cytometry. As these studies showed, some but not all of the monoclonal antibodies bound to microparticles prepared from apoptotic HL-60, THP-1 and Jurkat cells. For HL-60 cells, both staurosporine and UV radiation led to the production of antigenically active particles. For the anti-DNA antibody with high particle binding, prior treatment of DNase reduced activity. With plasma from patients with SLE, antibody binding to microparticles was present although a clear relationship with anti-DNA antibody levels was not observed. To determine whether lupus plasma contains immune complexes with particle properties, particle preparations were tested for bound IgG by flow cytometry. These studies indicated that lupus plasma contains particles with IgG binding, with numbers correlated with anti-DNA levels. Together, these findings indicate that microparticles display DNA and nucleosomal molecules in an antigenic form and could represent a source of immune complexes in SLE.