William Ruff

Autoimmune host–microbiota interactions at barrier sites and beyond

The microbiota is considered to be an important factor influencing the pathogenesis of autoimmunity at both barrier sites and internal organs. Impinging on innate and adaptive immunity, commensals exert protective or detrimental effects on various autoimmune animal models. Human microbiome studies of autoimmunity remain largely descriptive, but suggest a role for dysbiosis in autoimmune disease. Humanized gnotobiotic approaches have advanced our understanding of immune-commensal interactions, but little is known about the mechanisms in autoimmunity. We propose that, similarly to infectious agents, the microbiota mediates autoimmunity via bystander activation, epitope spread, and, particularly under homeostatic conditions, via crossreactivity. This review presents an overview of the current literature concluding with outstanding questions in this field.

Translocation of a gut pathobiont drives autoimmunity in mice and humans

Bacterial involvement in autoimmunity The composition of the commensal microbiota is known to influence autoimmune disease development and persistence. Manfredo Vieira et al. identified a gut microbe, Enterococcus gallinarum, that translocates from the gut into the organs of mice with a genetic predisposition to lupus-like autoimmunity (see the Perspective by Citi). Molecular signatures of gut barrier disintegration and pathogenic T helper cells were evident in the gut, liver, and lymphoid organs during colonization with the pathobiont. The ensuing pathology could be reversed by vancomycin treatment and by vaccination against E. gallinarum. The same bug was also found in liver biopsies of autoimmune patients, but not in healthy controls. Science, this issue p. 1156; see also p. 1097 Enterococcus gallinarum is implicated in the exacerbation of autoimmune pathology in genetically predisposed mice and humans. Despite multiple associations between the microbiota and immune diseases, their role in autoimmunity is poorly understood. We found that translocation of a gut pathobiont, Enterococcus gallinarum, to the liver and other systemic tissues triggers autoimmune responses in a genetic background predisposing to autoimmunity. Antibiotic treatment prevented mortality in this model, suppressed growth of E. gallinarum in tissues, and eliminated pathogenic autoantibodies and T cells. Hepatocyte–E. gallinarum cocultures induced autoimmune-promoting factors. Pathobiont translocation in monocolonized and autoimmune-prone mice induced autoantibodies and caused mortality, which could be prevented by an intramuscular vaccine targeting the pathobiont. E. gallinarum–specific DNA was recovered from liver biopsies of autoimmune patients, and cocultures with human hepatocytes replicated the murine findings; hence, similar processes apparently occur in susceptible humans. These discoveries show that a gut pathobiont can translocate and promote autoimmunity in genetically predisposed hosts.

The Role of the Gut Microbiota in the Pathogenesis of Antiphospholipid Syndrome

Infectious triggers are associated with the induction of transient antiphospholipid antibodies. One therefore wonders if microbes that permanently colonize us play a role in the pathogenesis of antiphospholipid syndrome (APS). The microbiota represents the collection of all microorganisms colonizing humans and is necessary for normal host physiology. The microbiota, however, is a constant stress on the immune system, which is tasked with recognizing and eliminating pathogenic microbes while tolerating commensal populations. A growing body of literature supports a critical role for the commensal-immune axis in the development of autoimmunity against colonized barriers (e.g., gut or skin) and sterile organs (e.g., pancreas or joints). Whether these interactions affect the development and sustainment of autoreactive CD4+ T cells and pathogenic autoantibodies in APS is unknown. This review provides an overview of the current understanding of the commensal-immune axis in autoimmunity with a focus on the potential relevance to APS. Additionally, we discuss emerging findings supporting the involvement of the gut microbiota in a spontaneous model of APS, the (NZW × BXSB)F1 hybrid, and formalize hypotheses to explain how interactions between the immune system and the microbiota may influence human APS etiopathogenesis.

Commensal orthologs of the human autoantigen Ro60 as triggers of autoimmunity in lupus

Commensal bacterial orthologs of the human autoantigen Ro60 may trigger cross-reactive T and B cells that initiate and sustain chronic autoimmunity in lupus. Autoimmune initiation by bacterial antigens Lupus patients react to many self-proteins throughout the course of disease, with some of the earliest autoantibodies targeting the RNA binding protein Ro60. Greiling and colleagues sampled the microbiota of lupus patients and detected commensals with orthologs of human Ro60. These bacterial Ro60 proteins could be recognized by patient sera and stimulated patient T cells. Colonization of germ-free mice also led to human Ro60 reactivity and lupus-like symptoms, strongly indicating that molecular mimicry of the commensal Ro60 could be triggering autoreactivity and driving disease progression. These striking results have implications beyond lupus and could help uncover global mechanisms of autoimmune pathogenesis. The earliest autoantibodies in lupus are directed against the RNA binding autoantigen Ro60, but the triggers against this evolutionarily conserved antigen remain elusive. We identified Ro60 orthologs in a subset of human skin, oral, and gut commensal bacterial species and confirmed the presence of these orthologs in patients with lupus and healthy controls. Thus, we hypothesized that commensal Ro60 orthologs may trigger autoimmunity via cross-reactivity in genetically susceptible individuals. Sera from human anti-Ro60–positive lupus patients immunoprecipitated commensal Ro60 ribonucleoproteins. Human Ro60 autoantigen–specific CD4 memory T cell clones from lupus patients were activated by skin and mucosal Ro60-containing bacteria, supporting T cell cross-reactivity in humans. Further, germ-free mice spontaneously initiated anti-human Ro60 T and B cell responses and developed glomerular immune complex deposits after monocolonization with a Ro60 ortholog–containing gut commensal, linking anti-Ro60 commensal responses in vivo with the production of human Ro60 autoantibodies and signs of autoimmunity. Together, these data support that colonization with autoantigen ortholog-producing commensal species may initiate and sustain chronic autoimmunity in genetically predisposed individuals. The concept of commensal ortholog cross-reactivity may apply more broadly to autoimmune diseases and lead to novel treatment approaches aimed at defined commensal species.

64 Dysbiosis and gut barrier dysfunction in antiphospholipid syndrome as revealed by iga-seq profiling

Background and aims The antiphospholipid syndrome (APS) is an autoimmune thrombophilic non-gut disorder with high mortality. Various pathogens have been associated with transient antiphospholipid antibody production. We hypothesised that members of the gut microbiota in APS patients could represent a chronic trigger and exhibit heightened adaptive immune responses to the microbiota. The purpose of this study was to explore gut barrier function and faecal IgA-coated microbial composition in APS patients. Methods Stool from 15 APS patients, 5 non-autoimmune thrombotic states, and 12 normal donors (total of 17 controls) was collected. Faecal homogenates were analysed for the gut permeability marker calprotectin and, in parallel, stained stained with PE-conjugated anti-human IgA prior to cell sorting. Faecal DNA was isolated and PCR-amplified targeting the V4 region of the 16S rRNA gene. Samples were sequenced on the Illumina MiSeq platform. Results Faecal calprotectin and IgA-coated faecal bacterial levels were significantly higher in APS patients compared to controls (p<0.003; p<0.05). LEfSe analysis of IgA+ fractions showed that the strongest IgA-coated genus is Blautia in APS. Conclusions These data suggest gut barrier dysfunction and aberrant IgA coating of commensals in APS. Markedly enhanced bacterial IgA coating in several APS patients supports a stronger adaptive immune response to the microbiota. Increased IgA coating of Blautia might reflect altered gut homeostasis as a Blautia species was shown to be part of proinflammatory IgA+ consortium in a recent study in IBD. To our knowledge, this study represents the first 16S rRNA profiling of IgA-coated gut commensals in patients with non-gut autoimmunity.