Activated M2 Macrophages Contribute to the Pathogenesis of IgG4-Related Disease via Toll-like Receptor 7/Interleukin-33 Signaling

Objective: IgG4-related disease (IgG4-RD) is a unique inflammatory disorder in which Th2 cytokines promote IgG4 production. In addition, recent studies have implicated the Toll-like Receptor (TLR) pathway. This study was undertaken to examine the expression of TLRs in salivary glands (SGs) from patients with IgG4-RD.

Methods: SGs from 15 patients with IgG4-RD, 15 patients with Sjögren's syndrome (SS), 10 patients with chronic sialadenitis, and 10 healthy controls were examined histologically. TLR family gene expression (TLR-1 through TLR-10) was analyzed by DNA microarray in the submandibular glands (SMGs). Up-regulation of TLRs was confirmed in SGs from patients with IgG4-RD. Finally, the phenotype of human TLR-7 (huTLR-7)-transgenic C57BL/6 mice was assessed before and after stimulation with TLR agonist.

Results: In patients with IgG4-RD, TLR-4, TLR-7, TLR-8, and TLR-9 were overexpressed. Polymerase chain reaction validated the up-regulation of TLR-7 in IgG4-RD compared with the other groups. Immunohistochemical analysis confirmed strong infiltration of TLR-7-positive cells in the SGs of patients with IgG4-RD. Double immunohistochemical staining showed that TLR-7 expression colocalized with CD163+ M2 macrophages. After in vitro stimulation with a TLR-7 agonist, CD163+ M2 macrophages produced higher levels of interleukin-33 (IL-33), which is a Th2-activating cytokine. In huTLR-7-transgenic mice, the focus and fibrosis scores in SMGs, pancreas, and lungs were significantly higher than those in wild-type mice (P < 0.05). Moreover, the concentration of serum IgG, IgG1, and IL-33 in huTLR-7-transgenic mice was distinctly increased upon stimulation with a TLR-7 agonist (P < 0.05).

Conclusion: TLR-7-expressing M2 macrophages may promote the activation of Th2 immune responses via IL-33 secretion in IgG4-RD.