Rising prevalence of systemic autoimmune rheumatic disease: increased awareness, increased disease or increased survival?

Abstract

Lupus develops when genetically predisposed people encounter environmental agents that initiate flares. Current evidence indicates that the environmental contribution is mediated by T-cell DNA demethylation. DNA methylation patterns are established during differentiation, and silence inappropriate or unnecessary genes by promoting a condensed chromatin configuration that is inaccessible to transcription factors. The methylation patterns are then replicated each time a cell divides by DNA methyltransferase 1 (Dnmt1). Dnmt1 is upregulated during mitosis, binds the replication fork, and catalyzes transfer of the methyl group from S-adenosylmethionine (SAM) to dC bases in the daughter DNA strand only where the parent strand is methylated. Environmental agents that block ERK pathway signaling prevent Dnmt1 upregulation, and low Dnmt1 levels synergize with dietary micronutrient deficiencies that decrease SAM pools to impair methylation of the daughter strand. This activates genes silenced only by DNA methylation. Inhibiting T-cell DNA methylation converts helper CD4 + T cells into autoreactive, cytotoxic, proinflammatory cells that cause lupus-like autoimmunity in mice. Similar changes in CD4 + T-cell DNA methylation and gene expression are found in patients with active lupus. Procainamide and hydralazine, which cause ANAs in a majority of patients and lupus in a genetically predisposed subset, also inhibit T-cell DNA methylation. The lupus T-cell DNA methylation defect has been traced to low Dnmt1 levels caused by decreased ERK pathway signaling, and the signaling defect has now been traced to PKCδ inactivation caused by oxidative damage. The importance of decreased ERK pathway signaling was confirmed by generating a transgenic mouse with an inducible dominant negative MEK. Inducing the signaling defect selectively in T cells decreases Dnmt1, causing anti-DNA antibodies in mice without lupus genes, and higher anti-DNA antibody levels and an immune complex glomerulonephritis in mice with lupus genes. Autoantibody levels and kidney disease are suppressed by dietary transmethylation micronutrient supplementation in these mice. Epigenetic mechanisms also contribute to the gender dimorphism in lupus. Immune genes on the normally silenced X chromosome demethylate in women with active lupus, contributing to flare severity. In contrast, men with only one X chromosome require a greater genetic predisposition and/ or greater degree of DNA demethylation to develop a lupus flare equal in severity to women. Together, these studies indicate that environmental agents including oxidative stress and diet combine to inhibit T-cell DNA methylation, and that the epigenetically modified cells cause lupus-like autoimmunity in genetically predisposed people and mice. Background: CD4 T cells help B cells produce antibodies following antigen challenge. This response classically occurs in germinal centers (GC) located in B-cell follicles of secondary lymphoid organs (SLO), a site of immunoglobulin isotype switching and affinity maturation. GC formation requires specialized CD4 T cells, T-follicular helper (Tfh) cells, which localize to follicles and provide B cells with survival and differentiation signals that are essential for B-cell maturation into memory and long-lived plasma cells. Pathogenic autoantibodies in human and murine lupus arise in a like manner. Although Tfh cells are critical for GC development, their genesis in humans, role in promotion of autoimmunity, and potential as therapeutic targets in SLE are incompletely understood. To address these issues, we dissected Tfh cell development and function, defining their transcriptional regulation, migration, and function in vivo in normal and lupus-prone mice and ex vivo in normal humans and patients with SLE. Methods: We used a combination of approaches -flow cytometry, confocal microscopy, microarrays, quantitative chromatin immunoprecipitation and DNA sequencing (ChIP-seq), retroviral overexpression, and T-cell-B-cell helper assays -to characterize Tfh cells in normal mice and in lupus-prone strains, and from the tonsils of normal humans and the blood of patients with SLE. Results: We found that the transcription factor Bcl6 (B-cell CLL/lymphoma 6) is necessary and sufficient for Tfh development and function, via genetic control of Tfh proteins that are essential for their migration to B-cell follicles and GC and subsequent B-cell maturation. We dissected steps in Tfh development within SLO, beginning with their genesis in the T-cell zone followed by emigration to sites of B-cell interaction outside the B-cell follicle, where we have shown that B cells serve to provide signals for continued Tfh expansion and follicular migration. We have now begun to tease apart the factors that mediate T-cell-B-cell collaboration in the follicle; these represent therapeutic targets in SLE. Finally, we have shown that patients with SLE have expansion of Tfh cells in the blood, a finding that highlights their potential role in the pathogenesis of SLE and as likely therapeutic targets. Conclusion: These studies help define the developmental pathways for Tfh cells, and the steps that enable these cells to function in the B-cell follicle to promote immunoglobulin and autoantibody production. They have also helped define markers of Tfh cells in normals and autoimmune individuals, and suggest that they are a promising therapeutic target in patients.