Chapter 3

Pathogenetic Aspects of Primary Sjögren’s

Syndrome

Athanasios G. Tzioufas, Efstathia K. Kapsogeorgou, Menelaos N. Manoussakis, and Haralampos M. Moutsopoulos

Contents

3.1Introduction

Sjögren’s syndrome (SjS) is an ideal model for dissecting the pathogenetic aspects of autoimmune disorders because the affected organs, the labial minor salivary glands, are easily accessible with minimal morbidity to the patient, and patient’s sera are rich with autoantibodies directed against organ-specific and non-organ specific antigens. Two major biologic phenomena underlie the autoimmune nature of SjS: (1) the peri-epithelial lymphocytic infiltration of the affected tissues; and (2) B lymphocyte hyperreactivity. Several studies in the past several years have pointed to the central role of the epithelial cell in the pathogenesis of the disease, suggesting

A.G. Tzioufas (*) • E.K. Kapsogeorgou • M.N. Manoussakis • H.M. Moutsopoulos Department of Pathophysiology, School of Medicine, National

University of Athens, Athens, Greece

that the condition, in effect, is an “autoimmune epithelitis” [1]. The importance of B cell hyperreactivity is demonstrated by the presence of hypergammaglobulinemia and the large array of autoantibodies associated with SjS.

The extraglandular organ involvement in primary SjS can be categorized into two major groups, peri-epithelial disease and extra-epithelial disease. The periepithelial organ involvement, which includes interstitial nephritis, liver involvement, and obstructive bronchiolitis, is the result of lymphocytic invasion into the epithelial tissues of organs beyond the exocrine glands. These clinical features appear early in the disease and usually have a benign course. In contrast, the extra-epithelial manifestations, palpable purpura, glomerulonephritis, and peripheral neuropathy, result from immune complex deposition that is a consequence of ongoing B cell hyperactivity. These disease complications are associated with increased morbidity and risk for lymphoma development.

The etiopathogenic factors that lead to the loss of the immune balance and the massive infiltration of the exocrine glands in SjS are unknown. Incessant activation, defective regulation, or inherent defects of the immune system may all participate. The development of SjS can be conceptualized in three steps. First, autoimmunity is triggered by a given environmental factor or factors acting upon a particular genetic background. Second, the autoimmune response is augmented, becoming chronic through aberrant immune regulatory mechanisms. And third, the lymphoepithelial lesion and eventually tissue damage occur. These are the consequences of the ongoing inflammatory process.

3.2Characteristics of Autoimmune Lesions

The immunopathology of SjS has been studied extensively in the minor salivary glands. The histopathologic lesions of the exocrine glands consist of lymphocytic infiltrates that tend to develop around ducts and display variable intensity. They extend from mild, focal infiltrates that do not significantly affect the gland organization to diffuse, severe lesions associated with concomitant loss of epithelial structures and tissue architecture [2]. The lymphocytic infiltrates within salivary glands often organize around ectopic structures that resemble germinal centers [3]. T and B lymphocytes comprise the vast majority of infiltrating mononuclear cells in minor salivary gland lesions, whereas macrophages, dendritic cells, and natural killer (NK) cells comprise only a small proportion (approximately 5–10%) [4]. Most of T lymphocytes bear the CD4 phenotype (50–70% of total T cells) [4]. The incidence of T and B cells, macrophages, and interdigitating dendritic cells varies according to the severity of the lesion (Fig. 3.1 and Table 3.1) [4]. T cells predominate in mild lesions (up to 60% of total infiltrating mononuclear cells), whereas B cells predominate in advanced ones (up to 50% of total infiltrating mononuclear cells) [4]. The frequency of macrophages increase, whereas that of interdigitating dendritic cells decrease with lesion severity. The numbers of infiltrating T cells and interdigitating dendritic cells correlate inversely with infiltration severity [4]. In contrast, the numbers of B cells and macrophages correlate directly.

36 A.G. Tzioufas et al.

Table 3.1 Frequency of the inflammatory mononuclear cell (MNCs) types (mean values ± SE) in minor salivary gland tissues of SjS patients with variable infiltration grade

Table 3.2 Features indicative of the activation of T and B lymphocytes that infiltrate the minor salivary glands of SjS patients

The decline of T cell population with lesion severity is attributable to a decrease in CD4+ T cells because the frequency of infiltrating CD8+ T cells remains relatively stable [4]. In addition, regulatory T (Treg) cells, which represent a subpopulation of CD4+-T cells with pivotal suppressive role in the regulation of immune responses [5], are differentially distributed in minor salivary gland lesions of distinct severity: a higher incidence of Treg cells is observed in lesions of intermediate severity as opposed to mild or severe infiltrates (Fig. 3.1) [6].

The majority of infiltrating T lymphocytes (77%) in the inflammatory SjS lesions express the CD45-Ro memory helper/inducer marker and are activated, as judged by the expression of HLA class-II molecules, interleukin-2 receptor (IL-2R/CD25), and lymphocyte function-associated antigen-1 (LFA-1), as well as by the production of interleukin-2 (IL-2) (Table 3.2) [7, 8]. The T cell receptor (TCR) repertoire

of the infiltrating T lymphocytes is not restricted, but certain TCR variable (V) region genes (Va2, Va11.1, Va17.1, Vb2 and Vb13) are predominant in expression, suggesting limited heterogeneity of the infiltrating T cells [8–11]. These findings, along with the detection of clonal expansion of T lymphocytes [9] and the presence within minor salivary gland tissues of Ro(SSA)-52 kDa reactive T cells [12], support an antigen-driven proliferation of T cells at this site of disease.

Clinical and laboratory features including hypergammaglobulinemia, circulating immune complexes apparently seen in some patients as small vessel vasculitis, the plethora of autoantibodies, along with the altered distribution of peripheral B cell subpopulations, the oligoclonal B cell expansion, and the increased risk of B cell lymphomas indicate the occurrence of B cell disturbances in SjS (Table 3.2) [3]. In the minor salivary glands of SjS patients, an accumulation of memory B cells (CD20+/ CD27+) has been observed, whereas the infiltrating B lymphocytes are hyperreactive, as indicated by the expression of elevated levels of immunoglobulins (Ig) with autoantibody activity [3, 13, 14]. In fact, the distribution of the plasma cells producing the distinct Ig isotypes is altered within the inflammatory lesions of SjS patients, where the IgGand IgM-producing plasma cells predominate [15–17]. In normal salivary glands, the main plasma cell isotype bears IgA.

The increased numbers of salivary gland IgG-producing plasma cells correlated with increasing IgG concentrations in the serum [16]. In addition, B lymphocytes producing rheumatoid factors or antibodies reactive against the ribonucleoproteins Ro/SS-A and La/SS-B, the major targets of SjS autoimmune responses, have been identified within the salivary glands of SjS patients [18–21].

Several pieces of data suggest that antigen driven monoclonal or oligoclonal B cell expansion may occur within the salivary glands of patients with SjS and it appears to be associated with the development of B cell lymphomas [3, 22–25]. Among others, the elevated expression of B cell-activating factor (BAFF) by infiltrating mononuclear and epithelial cells in minor salivary gland tissues has been implicated in the expansion of autoreactive B lymphocytes, the altered B cell differentiation and distribution, the formation of ectopic germinal centers, and the lymphoma transformation [26–28].

The association between the local T and B lymphocyte activation in the salivary glands and the production of autoantibodies or the development of lymphoma suggests that the local immune responses are linked to the systemic manifestations of the disorder. This is indicated further by the correlations between certain types of infiltrating immune cells and disease parameters. Thus, the low incidence of Tregs and the high incidence of IL-18 producing macrophages correlate with persistent salivary gland enlargement and C4-hypocomplementemia [6, 29]. As discussed elsewhere in this book, these features are adverse prognostic indicators of a heightened risk of lymphoma development [30–34]. Furthermore, elevated levels of macrophage infiltration have been observed in the minor salivary gland lesions of SjS patients with MALT lymphoma [4], and intense salivary gland inflammation is associated with extraglandular systemic manifestations (such as Raynaud’s phenomenon, vasculitis, lymph node/spleen enlargement, and leucopenia) [35].