Lymphocytic infiltration leads to degradation of lacrimal gland extracellular matrix structures in NOD mice exhibiting a Sjögren's syndrome-like exocrinopathy

Abstract

We previously reported that lacrimal glands (LGs) of male non-obese diabetic (NOD) mice, an established mouse model of autoimmune inflammatory LG disease that displays many features of human LGs in patients afflicted with Sjögren’s syndrome (SjS), exhibit significant degradation of extracellular matrix (ECM) structures as well as increased expression of matrix metalloproteinases (MMPs). The purpose of the current study was to expand the spectrum of proteases identified, to identify their probable origin as well as to clarify the contribution of these changes to disease pathogenesis. We explored in depth the changes in ECM structures and ECM protease expression at the onset of disease (6 weeks) versus late stage disease (18 weeks) in male NOD mouse LGs, relative to age-matched male NOD-scid, a severely immunocompromised congenic strain, and healthy BALB/c mice. LG tissues were examined using routine histological, immunohistochemical, Western Blot and gene expression analyses, as well as with novel multiphoton imaging technologies. We further characterized the profile of infiltrating immune cells under each condition using flow cytometry. Our results show that the initial infiltrating cells at 6 weeks of age are responsible for increased MMP and cathepsin H expression and therefore initiate the LG ECM degradation in NOD mice. More importantly, NOD-scid mice exhibited normal LG ECM structures, indicating the lymphocytes seen in the LGs of NOD mice are responsible for the degradation of the LG ECM. The disease-related remodeling of LG ECM structures may play a crucial role in altering the acinar signaling environment, disrupting the signaling scaffolds within the cells, which are required to mobilize the exocytotic trafficking machinery, ultimately leading to a loss of LG function in patients afflicted with SjS.

Role of Matrix Metalloproteinases 2 and 9 in Lacrimal Gland Disease in Animal Models of Sjögren's Syndrome

“…57 Furthermore, along with the ECM turnover described in NOD mouse lacrimal glands, MMP-2 and -9 protein expression levels as well as activities were upregulated, suggesting involvement in ECM remodeling. 22,23 Moreover, involvement of MMP-9 activity in EMT has been recently demonstrated in studies reporting that decreased expression of MMP-9 downregulates activation of EMT. [58][59][60] There is currently no universal animal model that mirrors precisely the complexity and chronicity of human Sjögren's syndrome.…”

“…However, maturation of the ECM and epithelial cell differentiation stay incomplete, and lacrimal gland acinar structure remains unrestored. 15,23 Alterations in the composition of the ECM in the salivary glands of Sjögren's syndrome patients as well as in animal models are well documented. [24][25][26][27][28] Thus it has been postulated that during diseased/inflamed conditions, abnormal ECM maturation, expression of cell matrix and adhesion molecules, and/or epithelial-ECM signaling hinder tissue repair in the lacrimal gland.…”

“…In spite of its critical role in lacrimal gland development and function, and even though changes in ECM might exert pathological effects, until now, very few reports 22,23 have characterized the changes in the ECM that accompany Sjögren's syndrome in the lacrimal gland. Previous studies in patients with Sjögren's syndrome suggested that increased activity and expression of MMP-2, MMP-9, and MMP-3 trigger the destruction of the lacrimal and salivary glands.…”

“…21 Increased degradation of ECM structures of the lacrimal gland has been implicated in the pathogenesis of Sjögren's syndrome. 22,23 In diseased glands (during chronic lacrimal gland inflammation), the ECM is remodeled and epithelial cell proliferation is initiated. However, maturation of the ECM and epithelial cell differentiation stay incomplete, and lacrimal gland acinar structure remains unrestored.…”

“…[29][30][31][32] Some of the proinflammatory mediators increased during dry eye disease include matrix metalloproteinases (MMPs), cytokines, elastases, and other proteolytic enzymes that destroy components of the ECM and damage the growth factors and their receptors, which are essential for tissue repair and cell differentiation. 22,23,33 Matrix metalloproteinases belong to a family of zinc-requiring neutral endopeptidases that are involved in remodeling of ECM and basement membrane during various physiological and pathological processes. 34 Matrix metalloproteinases are tightly regulated by tissue inhibitors of metalloproteinases (TIMPs), and disturbances in the delicate balance between MMPs and TIMPs lead to tissue destruction in various inflammatory and malignant diseases.…”