Molecular mimicry is an attractive mechanism for triggering autoimmunity. In this review, we explore the potential role of evolutionary conserved bacterial proteins in the production of autoantibodies with focus on granulomatosis with polyangiitis (GPA) and rheumatoid arthritis (RA). Seven autoantigens characterized in GPA and RA were BLASTed against a bacterial protein database. Of the seven autoantigens, proteinase 3, type II collagen, binding immunoglobulin protein, glucose-6-phosphate isomerase, alpha-enolase, and heterogeneous nuclear ribonuclear protein have well-conserved bacterial orthologs. Importantly, those bacterial orthologs are also found in human-associated bacteria. The wide distribution of the highly conserved stress proteins or enzymes among the members of the normal flora and common infectious microorganisms raises a new question on how cross-reactive autoantibodies are not produced during the immune response to these bacteria in most healthy people. Understanding the mechanisms that deselect auto-reactive B cell clones during the germinal center reaction to homologous foreign antigens may provide a novel strategy to treat autoimmune diseases.
Arthritis, Rheumatoid ; Autoantibodies ; Autoantigens ; Autoimmune Diseases* ; Autoimmunity ; Bacteria ; Bacterial Infections* ; Bacterial Proteins ; Clone Cells ; Collagen Type II ; Germinal Center ; Glucose-6-Phosphate Isomerase ; Heat-Shock Proteins ; Immunoglobulins ; Molecular Mimicry* ; Myeloblastin ; Phosphopyruvate Hydratase

The pathophysiology of glandular dysfunction in Sjögren's syndrome (SS) has not been fully elucidated. Previously, we reported the presence of autoantibodies to AQP-5 in patients with SS, which was associated with a low resting salivary flow. The purpose of this study was to investigate the presence of anti-AQP1 autoantibodies. To detect anti-AQP1 autoantibodies, cell-based indirect immunofluorescence assay was developed using MDCK cells that overexpressed human AQP1. By screening 112 SS and 52 control sera, anti-AQP1 autoantibodies were detected in 27.7% of the SS but in none of the control sera. Interestingly, the sera that were positive for anti-AQP1 autoantibodies also contained anti-AQP5 autoantibodies in the previous study. Different from anti-AQP5 autoantibodies, the presence of anti-AQP1 autoantibodies was not associated with the salivary flow rate. Although anti-AQP1 autoantibodies are not useful as a diagnostic marker, the presence of autoantibodies to AQP1 may be an obstacle to AQP1 gene therapy for SS.
Aquaporin 1 ; Autoantibodies* ; Fluorescent Antibody Technique ; Fluorescent Antibody Technique, Indirect ; Genetic Therapy ; Humans ; Madin Darby Canine Kidney Cells ; Mass Screening

PURPOSE: We previously reported that human serum significantly reduces the invasion of various oral bacterial species into gingival epithelial cells in vitro. The aims of the present study were to characterize the serum component(s) responsible for the inhibition of bacterial invasion of epithelial cells and to examine their effect on periodontitis induced in mice. METHODS: Immortalized human gingival epithelial (HOK-16B) cells were infected with various 5- (and 6-) carboxy-fluorescein diacetate succinimidyl ester-labeled oral bacteria, including Fusobacterium nucleatum, Provetella intermedia, Porphyromonas gingivalis, and Treponiema denticola, in the absence or presence of three major serum components (human serum albumin [HSA], pooled human IgG [phIgG] and alpha1-antitrypsin). Bacterial adhesion and invasion were determined by flow cytometry. The levels of intracellular reactive oxygen species (ROS) and activation of small GTPases were examined. Experimental periodontitis was induced by oral inoculation of P. gingivalis and T. denticola in Balb/c mice. RESULTS: HSA and phIgG, but not alpha1-antitrypsin, efficiently inhibited the invasion of various oral bacterial species into HOK-16B cells. HSA but not phIgG decreased the adhesion of F. nucleatum onto host cells and the levels of intracellular ROS in HOK-16B cells. N-acetylcysteine (NAC), a ROS scavenger, decreased both the levels of intracellular ROS and invasion of F. nucleatum into HOK-16B cells, confirming the role of ROS in bacterial invasion. Infection with F. nucleatum activated Rac1, a regulator of actin cytoskeleton dynamics. Not only HSA and NAC but also phIgG decreased the F. nucleatum-induced activation of Rac1. Furthermore, both HSA plus phIgG and NAC significantly reduced the alveolar bone loss in the experimental periodontitis induced by P. gingivalis and T. denticola in mice. CONCLUSIONS: NAC and the serum components HSA and phIgG, which inhibit bacterial invasion of oral epithelial cells in vitro, can successfully prevent experimental periodontitis.
Acetylcysteine* ; Actin Cytoskeleton ; Albumins ; Alveolar Bone Loss ; Animals ; Bacteria ; Bacterial Adhesion ; Epithelial Cells* ; Flow Cytometry ; Fusobacterium nucleatum ; Humans ; Immunoglobulin G ; Mice* ; Monomeric GTP-Binding Proteins ; Periodontitis* ; Porphyromonas gingivalis ; Reactive Oxygen Species ; Serum Albumin