IgG4-related disease in sclerosing and chronic nonspecific sialadenitis, and sialolithiasis

Chronic sclerosing sialadenitis (CSS) occurring in the submandibular gland has widely been considered a manifestation of the systemic entity of immunoglobulin G4-related disease (IgG4-RD). IgG4-RD causes tumorlike swelling and progressive fibrosis of the affected organ and can develop manifestations at multiple sites. Due to the systemic nature and high risk of recurrence, patients diagnosed with IgG4-RD require additional examinations and follow-up. The etiology of IgG4-RD is yet unknown, but aberrant activation of innate and acquired immune responses are implied in the pathogenesis, with autoimmunity and infections as suspected triggering factors. Based on our previous clinical observations, the association between CSS and IgG4-RD is not unequivocal. Both CSS and chronic nonspecific sialadenitis are often associated with sialolithiasis, and both share histological features such as fibrosis and dense lymphoplasmacytic infiltration. This thesis consists of four studies. The first aimed to clarify whether CSS always represents a manifestation of IgG4-RD. For this purpose, we collected tissue specimens from submandibular glands removed due to CSS between 2000–2017 (n = 51). The tissue specimens were histologically re-evaluated, and the number of IgG4-positive plasma cells was quantified based on immunohistochemical staining. A total of 12 patients (24%) had IgG4-positive tissue specimens but only two (4%) fulfilled the criteria for definite IgG4-RD. Based on histological re-evaluation, 33% of specimens showed features corresponding with chronic nonspecific sialadenitis rather than CSS. Both patients confirmed as IgG4-RD belonged to the nonspecific sialadenitis group. Based on these findings we formulated a hypothesis of an inflammatory continuum in the submandibular gland, where nonspecific sialadenitis may over time develop features of CSS and even overexpression of IgG4. Study II examined the hypothesized continuum and aimed to assess the prevalence of IgG4 overexpression and genuine IgG4-RD in chronic nonspecific inflammation of the submandibular gland. Tissue specimens were collected from submandibular glands excised due to chronic nonspecific sialadenitis, sialolithiasis, or both (n = 165). Histological features of CSS, infiltration of IgG4-positive plasma cells, and the criteria of IgG4-RD were assessed. Based on histological re-evaluation, CSS features were found in 20 specimens (12%). Immunohistochemical staining classified 19 specimens (11%) as IgG4-positive, of which one (0.6%) represented definite IgG4-RD. Studies III and IV aimed to investigate the role of bacterial or viral factors in the etiology of IgG4-RD and other IgG4 overexpression in the submandibular gland. A sample from the patient series of studies I and II, including all confirmed IgG4-RDs and all other IgG4-positive specimens, was collected for analysis. An equivalent number of consecutive IgG4-negative samples were included for comparison. In study III, the presence of 21 DNA viruses was examined in salivary gland tissue using quantitative polymerase chain reaction. DNA from seven different viruses was present in 80% of the specimens. No differences in the virus profiles between the IgG4-positive and IgG4-negative groups were detected. Study IV examined immunohistochemical expression of antibody markers for Porphyromonas gingivalis, as well as other gram-negative bacteria, and of TLRs 2 and 4, which recognize bacterial antigens. Among the investigated markers for bacterial infection, the expression of TLR2 and TLR4 was pronounced in IgG4-positive specimens. In conclusion, contrary to the current consensus, CSS rarely represents a manifestation of IgG4-RD in the Finnish population. Therefore, further examinations and follow-up are not warranted in this population, even if histological features of CSS are found in surgically removed submandibular glands. IgG4 overexpression may occur in chronic nonspecific sialadenitis but rarely represents genuine IgG4-RD. Additionally, the overlap of histological features of CSS, chronic nonspecific sialadenitis, and sialolithiasis suggests an inflammatory continuum, where more severe histological findings represent different stages of prolonged inflammation rather than separate disease entities. Distinguishing true IgG4-RD from other inflammatory IgG4 overexpression requires additional examinations. Gram-negative bacteria or DNA viruses do not appear to act as triggering factors for IgG4 overexpression in sialadenitis. However, the pronounced expression of TLR2 and TLR4, without concomitant bacterial infection, may indicate activation of these TLRs in IgG4-RD and other IgG4 overexpressing inflammatory processes.