Post-translational modifications, subcellular relocation and release in apoptotic microparticles: apoptosis turns nuclear proteins into autoantigens

Induction of NF-κB inflammatory pathway in monocytes by microparticles from patients with systemic lupus erythematosus

Background

Elevated levels of circulating microparticles (MPs) and molecules of the complement system have been reported in patients with systemic lupus erythematosus (SLE). Moreover, microparticles isolated from patients with SLE (SLE-MPs) contain higher levels of damage-associated molecular patterns (DAMPs) than MPs from healthy controls (CMPs). We hypothesize that the uptake of MPs by monocytes could contribute to the chronic inflammatory processes observed in patients with SLE. Therefore, the aim of this study was to evaluate the expression of activation markers, production of proinflammatory mediators, and activation of the NF-κB signaling pathway in monocytes treated with CMPs and SLE-MPs.

Methodology

Monocytes isolated from healthy individuals were pretreated or not with pyrrolidine dithiocarbamate (PDTC) and cultured with CMPs and SLE-MPs. The cell surface expression of CD69 and HLA-DR were evaluated by flow cytometry; cytokine and eicosanoid levels were quantified in culture supernatants by Cytokine Bead Array and ELISA, respectively; and the NF-κB activation was evaluated by Western blot and epifluorescence microscopy.

Results

The cell surface expression of HLA-DR and CD69, and the supernatant levels of IL-6, IL-1β, PGE2, and LTB4 were higher in cultures of monocytes treated with SLE-MPs than CMPs. These responses were blocked in the presence of PDTC, a pharmacological inhibitor of the NF-κB pathway, with concomitant reduction of IκBα and cytoplasmic p65, and increased nuclear translocation of p65.

Conclusions

The present findings indicate that significant uptake of SLE-MPs by monocytes results in activation, production of inflammatory mediators, and triggering of the NF-κB signaling pathway.

Viral Impact in Autoimmune Diseases: Expanding the "X Chromosome-Nucleolus Nexus" Hypothesis

Viruses are suspected of significant roles in autoimmune diseases but the mechanisms are unclear. We get some insight by considering demands a virus places on host cells. Viruses not only require production of their own proteins, RNA and/or DNA, but also production of additional cellular machinery, such as ribosomes, to handle the increased demands. Since the nucleolus is a major site of RNA processing and ribonucleoprotein assembly, nucleoli are targeted by viruses, directly when viral RNA and proteins enter the nucleolus and indirectly when viruses induce increased expression of cellular polyamine genes. Polyamines are at high levels in nucleoli to assist in RNA folding. The size and activity of nucleoli increase directly with increases in polyamines. Nucleolar expansion due to abnormal increases in polyamines could disrupt nearby chromatin, such as the inactive X chromosome, leading to expression of previously sequestered DNA. Sudden expression of a large concentration of Alu elements from the disrupted inactive X can compete with RNA transcripts containing intronic Alu sequences that normally maintain nucleolar structural integrity. Such disruption of nucleolar activity can lead to misfolded RNAs, misassembled ribonucleoprotein complexes, and fragmentation of the nucleolus. Many autoantigens in lupus are, at least transiently, components of the nucleolus. Considering these effects of viruses, the “X chromosome–nucleolus nexus” hypothesis, which proposed disruption of the inactive X by the nucleolus during stress, is now expanded here to propose subsequent disruption of the nucleolus by previously sequestered Alu elements, which can fragment the nucleolus, leading to generation of autoantigens.

Elevated Transglutaminase Activity Triggers Angiotensin Receptor Activating Autoantibody Production and Pathophysiology of Preeclampsia

BACKGROUND

Preeclampsia (PE) is a life-threatening hypertensive disorder of pregnancy associated with autoantibodies, termed AT1-AA, that activate the AT1 angiotensin receptor. Although the pathogenic nature of these autoantibodies has been extensively studied, little is known about the molecular cause of their generation.

METHODS AND RESULTS

Here we show that tissue transglutaminase (TG2), an enzyme that conducts posttranslational modification of target proteins, directly modified the 7-amino acid (7-aa) epitope peptide that localizes to the second extracellular loop of the AT1 receptor. These findings led us to further discover that plasma transglutaminase activity was induced and contributed to the production of AT1-AA and disease development in an experimental model of PE induced by injection of LIGHT, a tumor necrosis factor superfamily member. Key features of PE were regenerated by adoptive transfer of purified IgG from LIGHT-injected pregnant mice and blocked by the 7-amino acid epitope peptide. Translating our mouse research to humans, we found that plasma transglutaminase activity was significantly elevated in PE patients and was positively correlated with AT1-AA levels and PE features.

CONCLUSIONS

Overall, we provide compelling mouse and human evidence that elevated transglutaminase underlies AT1-AA production in PE and highlight novel pathogenic biomarkers and innovative therapeutic possibilities for the disease.

Microparticles That Form Immune Complexes as Modulatory Structures in Autoimmune Responses

Microparticles (MPs) are induced during apoptosis, cell activation, and even “spontaneous” release. Initially MPs were considered to be inert cellular products with no biological function. However, an extensive research and functional characterization have shown that the molecular composition and the effects of MPs depend upon the cellular background and the mechanism inducing them. They possess a wide spectrum of biological effects on intercellular communication by transferring different molecules able to modulate other cells. MPs interact with their target cells through different mechanisms: membrane fusion, macropinocytosis, and receptor-mediated endocytosis. However, when MPs remain in the extracellular milieu, they undergo modifications such as citrullination, glycosylation, and partial proteolysis, among others, becoming a source of neoantigens. In rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE), reports indicated elevated levels of MPs with different composition, content, and effects compared with those isolated from healthy individuals. MPs can also form immune complexes amplifying the proinflammatory response and tissue damage. Their early detection and characterization could facilitate an appropriate diagnosis optimizing the pharmacological strategies, in different diseases including cancer, infection, and autoimmunity. This review focuses on the current knowledge about MPs and their involvement in the immunopathogenesis of SLE and RA.

Epigenetics of autoantigens: new opportunities for therapy of autoimmune diseases

The field of epigenetics requires that traditional divisions between scientific disciplines give way to cross-fertilization of concepts and ideas from different areas of investigation. Such is the case with research in autoimmunity. Recent discoveries of stimuli that induce autoimmunity reveal that epigenetic marks of autoantigens are recognized by autoreactive B and T cell receptors. Thus, insights into the initiation of autoimmunity, its prevention and therapy will arise from understanding the biochemistry, cell biology and microbiology of autoantigen epigenetics. Here, we highlight potential benefits from the inhibition of a histone modifying enzyme and the administration of a phosphorylated, spliceosome-derived peptide, in the treatment of autoimmunity.