Immunopathogenic mechanisms in primary Sjögren’s syndrome

Abstract

Primary Sjögren’s syndrome is a systemic autoimmune disease primarily affecting salivary and lacrimal glands, but patients may also have other organ involvement. Development of Sjögren’s syndrome has been associated with genetic polymorphisms and aberrances in innate and adaptive immunity, which include abnormal B cell functions with autoantibody production, and increased activity of the interferon (IFN) system. Microbial triggers have been suspected to partake in the pathogenesis. The overall aim of this thesis was to characterize exogenous and endogenous mechanisms contributing to the immunopathology of Sjögren’s syndrome.

Genetic variants associated with Sjögren’s syndrome convey low risk-increase for disease development, indicating that environmental factors must play an important role. However, at the start of the work leading up to this thesis, no environmental risk factors for Sjögren’s syndrome had been identified. Therefore, to assess the influence of smoking, we performed the largest questionnaire-based case-control study to date. We found that current and ever smoking was less frequent among patients compared to controls, however, smoking patterns were similar up until approximately 35 years before diagnosis and smoking thereafter declined in patients. We conclude that smoking does not appear to be a risk factor for development of Sjögren’s syndrome, but that early symptoms of dryness may influence behavior. Further, to define connections between previous infections and disease development, we extracted data on patients and matched controls from the Swedish National Patient Register. We found that a history of infection was associated with a higher risk of Sjögren’s syndrome, with overall stronger associations to anti- Ro/SSA and anti-La/SSB positive disease. Infections of the respiratory tract associated both with autoantibody positive and negative disease, and a dose-response relationship was present in autoantibody positive patients.

Previous studies have identified CXCR5 as a susceptibility locus for Sjögren’s syndrome. We describe a novel eQTL effect for CXCR5 in B cells. Further, decreased percentages of CXCR5+ cells with lower CXCR5 surface expression in the circulation coincided with higher CXCL13 plasma protein levels, and higher numbers of CXCR5+ cells were detected in salivary glands of patients compared to controls. We conclude that CXCR5+ cells likely relocate from the blood stream to the autoimmune target tissues in patients. By transcriptomic analysis of peripheral B cells, we found prominent type I and type II IFN signatures, as well as higher expression of chemokines and chemokine receptors in patients compared to controls, while inhibitors of cytokine signaling were downregulated. Our data add to the understanding of B cells in Sjögren’s syndrome and define potential candidates for future functional studies. Activation of the IFN system is often assessed by quantifying the expression of IFN regulated genes. However, RNA samples are not always available, wherefore alternative methods are needed. We describe two novel IFN scores calculated from protein levels in serum or plasma, and from levels of DNA methylation.

Responses to microbial antigens can be studied in vivo during vaccination. We therefore assessed serological and cellular responses to influenza vaccination in patients with Sjögren’s syndrome and SLE. Untreated patients with Sjögren’s syndrome and SLE patients receiving no or light treatment responded to the viral antigens with higher vaccine-specific antibody titers compared to controls. Augmented antibody responses were associated with upregulated markers of type I IFN system activation at the mRNA level in monocytes and at the protein level in the circulation before vaccination. Specific components associated with a higher serological response were identified, which warrant further study. Our data increase the comprehension of immune reactions toward microbial antigens in autoimmune disease. In conclusion, this thesis expands our current understanding of environmental factors and immunopathogenic processes in primary Sjögren’s syndrome.