Cellular and molecular factors in the pathogenesis of systemic autoimmunity and comorbidities

Abstract

Sjögren’s syndrome (SS) and systemic lupus erythematosus (SLE) are systemic rheumatic autoimmune diseases having a major impact on patient’s wellbeing. Besides fatigue, pain and direct damage to exocrine glands in SS, and various target organs, such as skin, joints, kidneys and brain in SLE, these diseases confer an increased risk of lymphoma development and can cause an autoimmune condition in the foetus of pregnant patients, termed neonatal lupus.

Several cell types and cytokines contribute to the autoimmune process, with central roles for B cells and autoantibodies. Mature B cells are attracted to the sites of inflammation by chemokine CXCR5. Recent studies identified CXCR5 as a susceptibility gene in SS, and a decrease in expression of CXCR5 on circulating B cells in SS patients, however, the link between the polymorphisms in the CXCR5 locus and B cell trafficking wasn’t addressed. We therefore explored the expression of the chemokine on lymphocytes in peripheral blood and minor salivary glands in SS patients. Confirming the previous findings, our results demonstrate reduced CXCR5 expression in circulating B cell pool, coinciding with the accumulation of CXCR5-positive cells in the salivary glands of the patients. We conclude that the observed decrease in CXCR5 expression in SS results from relocation of CXCR5-highly-positive B cells from blood stream to the salivary glands.

T follicular regulatory (TFR) cells also express CXCR5, and the frequency of circulating TFR was suggested a biomarker of degree of tissue inflammation and autoantibody production in SS and rheumatoid arthritis. We could confirm the elevated frequencies of TFR in the blood of SS patients, and show that most of them have a naïve phenotype. B cells are producers of the SS hallmark autoantibodies against Ro and La, that have been used for diagnostic purposes for decades. In this thesis we investigated the anti-inflammatory and anti-proliferative properties of TRIM21/Ro52, one of the targets of anti-Ro. We studied the role of Trim21 in the homeostasis of mouse B cells, and the relevance of TRIM21 as a prognostic marker in diffuse large B-cell lymphoma. In Trim21 knockout mice, B cells had a higher rate of proliferation, follicular B cells were expanded and higher levels of both IgM and IgG were generated after immunization with both thymus dependent and independent antigens engaging the B cell receptor. In accordance with these data, maintained TRIM21 expression was associated with a better prognosis in diffuse large B-cell lymphoma, independently of subtype and underlying autoimmune disease.

Another severe comorbidity in SS and SLE affects foetuses and newborns of the patients. Maternal Ro and La autoantibodies are transferred to the developing foetus, and in 2% cases this leads to a congenital heart block (CHB), a part of neonatal lupus. Here, we studied signs of inflammation in the cord blood of neonates exposed to maternal anti-Ro/La. We observed an elevation of serum type I interferon (IFN) levels and an IFN-gene signature in autoantibody-exposed neonates. Besides, neonatal PBMC proved to be capable to produce IFN in vitro, suggesting a possible foetal origin of the cytokine. Further, we show increased frequencies of circulating natural killer cells, and presence of type II IFN in the sera of some of these newborns. These novel findings contribute to the understanding of the mechanisms behind CHB.

In summary, we connect genetic variants of the CXCR5 gene locus to expression patterns and cell compartmentalization in SS, demonstrate a role of the autoantigen TRIM21 in B cell homeostasis and function and present evidence for innate immune activation in Ro and La autoantibody-exposed newborns.