Distribution of IgG4- and/or IgG-Positive Plasma Cells in Hashimoto’s Thyroiditis: An Immunohistochemical Study

Background and Study Aim: Recently, immunohistochemistry has shown numerous IgG4-positive plasma cells in a subset of Hashimoto’s thyroiditis (HT), and this type of HT (IgG4 HT) appears to be a subtype of IgG4-related sclerosing disease. However, little is known about the distribution pattern of plasma cells in IgG4 thyroiditis. To clarify the distribution pattern of IgG4-positive plasma cells, 33 cases of HT demonstrating abundant lymphoplasmacytic infiltrate were studied. Methods: Using formalin-fixed paraffin-embedded sections, histological, immunohistochemistry and polymerase chain reaction were performed. Results: Fourteen cases were classified as IgG4 HT and 19 cases were non-IgG4 HT. Histologically, there was no significant difference between the 2 groups with regard to the degree of stromal fibrosis, lymphoid follicle formation, or the presence of phlebitis or fibrous thyroiditis. The present study demonstrated 2 distribution patterns of IgG4- and/or IgG-positive plasma cells, namely the interfollicular (n = 31) and intrafollicular + interfollicular patterns (n = 2). Interfollicular plasma cells were always polytypic intracytoplasmic immunoglobulin in all 33 cases. However, intrafollicular plasma cells in 2 lesions had monotypic kappa light chain by immunohistochemistry. Conclusion: The present study demonstrated that the majority of IgG4 HT and non-IgG HT cases showed an interfollicular distribution pattern of IgG4- and/or IgG-positive plasma cells.