B cells in Sjögren's syndrome: indications for disturbed selection and differentiation in ectopic lymphoid tissue

Abstract

Primary Sjögren's syndrome (pSS) is an autoimmune disorder characterized by specific pathological features. A hallmark of pSS is B-cell hyperactivity as manifested by the production of autoantibodies, hypergammaglobulinemia, formation of ectopic lymphoid structures within the inflamed tissues, and enhanced risk of B-cell lymphoma. Changes in the distribution of peripheral B-cell subsets and differences in post-recombination processes of immunoglobulin variable region (IgV) gene usage are also characteristic features of pSS. Comparison of B cells from the peripheral blood and salivary glands of patients with pSS with regard to their expression of the chemokine receptors CXCR4 and CXCR5, and their migratory capacity towards the corresponding ligands, CXCL12 and CXCL13, provide a mechanism for the prominent accumulation of CXCR4+ CXCR5+ memory B cells in the inflamed glands. Glandular B cells expressing distinct features of IgV light and heavy chain rearrangements, (re)circulating B cells with increased mutations of cmu transcripts in both CD27- and CD27+ memory B-cell subsets, and enhanced frequencies of individual peripheral B cells containing IgV heavy chain transcripts of multiple isotypes indicate disordered selection and incomplete differentiation processes of B cells in the inflamed tissues in pSS. This may possibly be related to a lack of appropriate censoring mechanisms or different B-cell activation pathways within the ectopic lymphoid structures of the inflamed tissues. These findings add to our understanding of the pathogenesis of this autoimmune inflammatory disorder and may result in new therapeutic approaches.

Figure 1

Hypothetical scheme of B-cell differentiation pathways in ectopic lymphoid tissues in primary Sjögren's syndrome. Preactivated peripheral B cells are recruited by chemokines into the microenvironment of chronically inflamed tissues. This microenvironment represents a 'niche' where B cells may escape from peripheral check points against autoreactivity but are abnormally stimulated, proliferate and incompletely differentiate via T-cell-dependent or T-cell-independent pathways into memory B cells and plasma cells. Rheumatoid factor-expressing B cells may be abnormally stimulated by local BAFF excess and locally secreted (auto)antibodies. Abnormal stimulation and impaired censoring mechanisms enhance the risk for malignant transformation of B cells. Emigration and recirculation of B cells that had incomplete differentiation processes contribute to peripheral B-cell disturbances. EC, epithelial cell; FDC, follicular dendritic cell; PC, plasma cell; BAFF, B-cell activating factor; Ig, immunoglobulin.