Possible Involvement of TLR7 in the Pathogenesis of IgG4-related Disease

Objectives: IgG4-related dacryoadenitis and sialoadenitis (IgG4-DS) is a unique inflammatory disorder characterized by the elevation of serum IgG4 and infiltration of IgG4-positive plasma cells in lacrimal and salivary glands (SGs). Since it is well known that IgG4 is induced by T helper type 2 (Th2) cytokines such as IL-4 and IL-13, IgG4-DS is speculated to be a Th2-dominant disease. In addition, recent studies suggested that the activation of innate immunity also plays a key role in the IgG4 production upon stimulation with toll-like receptor (TLR) ligands. In this study, we thus examined the expression of TLRs in SGs from patients with IgG4-DS.
Methods: Gene expression was analyzed by DNA microarray in submandibular glands (SMGs) from patients with IgG4-DS (n=6), chronic sialoadenitis (CS) (n=3), and controls (n=3). TLR family (TLR1-10) was validated by real-time PCR and immunohistochemical staining in SGs from patient with IgG4-DS (n=15), Sjögren’s syndrome (SS) (n=15), CS (n=9), and controls (n=9). Finally, we assessed the phenotype (lymphocytic infiltration, fibrosis, and weight of the affected organs) of human TLR7 (huTLR7)-transgenic mice, compared with that of wild-type C57BL/6 mice.
Results: In IgG4-DS, 5 genes of TLR family (TLR4, TLR7-10) were overexpressed by DNA microarray analysis. PCR validated significantly higher expression of TLR7 in IgG4-DS than that in the other groups. Immunohistochemical analysis confirmed that the expression pattern of TLR7 was similar to that of the M2 macrophage marker CD163. Recent studies demonstrated that TLR7 agonist stimulates macropages to production of IL-33, which is identified as a cytokine that activates Th2 immune responses. Therefore, we focused on the relationship between TLR7 an IL-33 in IgG4-DS. The results showed that the mRNA expression of TLR7 was positively correlated with that of IL-33 in only IgG4-DS. In huTLR7-transgenic C57BL/6 mice, the number of infiltrating lymphocytes and fibrosis score in the SMGs and pancreas were significantly higher than those in wild-type mice.
Conclusions: Our current data suggest that TLR7-expressing M2 macrophages might promote the local inflammation in IgG4-DS.