Up-regulation of activation-induced cytidine deaminase and its strong expression in extra-germinal centres in IgG4-related disease

Abstract

Immunoglobulin (Ig) G4-related disease (IgG4-RD) is a systemic disorder involving benign mass formation due to fibrosis and intense lymphoplasmacytosis; the chronic inflammation associated with the disease might also contribute to oncogenesis. Activation-induced cytidine deaminase (AID), normally expressed in germinal centre activated B-cells, is an enzyme that edits DNA/RNA and induces somatic hypermutation and Ig class switching. AID expression is strictly controlled under physiological conditions; however, chronic inflammation and some infectious agents induce its up-regulation. AID is overexpressed in various cancers and may be important in chronic inflammation-associated oncogenesis. We examined AID expression in IgG4-related sialadenitis (n = 14), sialolithiasis (non-specific inflammation, n = 13), and normal submandibular glands (n = 13) using immunohistochemistry and quantitative real-time polymerase chain reaction (qPCR). Immunohistochemistry revealed significantly more AID-expressing cells in IgG4-related sialadenitis than in sialolithiasis or normal submandibular gland samples (P = 0.02 and P < 0.01, respectively); qPCR yielded similar results. Thus, AID was significantly more up-regulated and had higher expression in extra-germinal centres in IgG4-RD than in non-specific inflammation or normal conditions. This report suggests that IgG4-RD has several specific causes of AID up-regulation in addition to inflammation. Furthermore, chronic inflammation-associated AID-mediated oncogenesis is possible in IgG4-RD.

Figure 1

Immunohistochemical staining of activation-induced cytidine deaminase (AID). (a) IgG4-related sialadenitis (×100), (b) IgG4-related sialadenitis (×400). Greater numbers of strongly AID-positive cells were noted in IgG4-related sialadenitis than in those from healthy controls or patients with sialolithiasis. Ductal epithelia in IgG4-related sialadenitis were also positive for AID. In addition to AID-positivity in the germinal centres of AID-positive cells, the interfollicular areas were also AID-positive. AID-positive interfollicular areas were observed in lymphoid cells, plasma cells, and plasmacytoid cells. (c) Sialolithiasis (×400). Although lymphoid cell infiltration was observed in patients with sialolithiasis, fewer AID-positive cells were observed compared with IgG4-related sialadenitis. (d) Normal submandibular gland (×400). No AID-positive cells were noted in normal tissue.

Figure 2

Intensity scores of specimens following immunohistochemical staining for activation-induced cytidine deaminase (AID). Compared with sialolithiasis and normal submandibular gland, AID was more strongly expressed in IgG4-related sialadenitis.

Figure 3

Quantitative analysis of activation-induced cytidine deaminase (AID) expression. AID expression in IgG4-related sialadenitis was significantly higher than in sialolithiasis and normal controls. *P < 0.05, **P < 0.01.

Figure 4

Intensity index of immunohistochemical staining for activation-induced cytidine deaminase (AID). (a) Strongly positive (3+), (b) moderately positive (2+), (c) weakly positive (1+), (d) negative (0). IgG4-related sialadenitis contained strongly AID-positive lymphoid and plasmacytoid cells in both the germinal centres and interfollicular areas (AID immunostaining, ×400 magnification).