[Hyper-IL-6 syndrome mimicking IgG4-related disease].

Distinguishing between IgG4-related disease (IgG4-RD) and hyper-interleukin (IL) -6 syndrome, such as immune mediated conditions, autoimmune diseases, and idiopathic multicentric Castleman disease (iMCD) is challenging. Here, we report the case of a 69-year-old man with cervical lymphadenopathy who was admitted to our hospital and histologically diagnosed with hyper-IL-6 syndrome mimicking IgG4-RD phenotypically. Laboratory data detected polyclonal hypergammaglobulinemia comprising IgG, including IgG4 (2,350 mg/dl). Computed tomography revealed presence of systemic lymphadenopathy, enlarged bilateral submandibular glands, and infiltrative shadow in the right lower lung. Magnetic resonance imaging revealed diffusely enlarged pancreas the size of a sausage and hypointense rim on T2, suggesting autoimmune pancreatitis as part of IgG4-RD. Biopsy of the cervical lymph node revealed proliferation of IL-6-positive mature plasma cells in the expanded interfollicular area with an elevated IgG4+/IgG+ cell ratio (approximately 70%). These histological findings were consistent with hyper-IL-6 syndrome rather than IgG4-RD; however, the serum IL-6 level was slightly elevated. Bone marrow aspiration detected both IgG4- and IL-6-positive mature plasma cells. Although this case cannot be diagnosed as IgG4-RD because it failed to meet its diagnostic criteria, administration of oral prednisolone (0.5 mg/kg) resulted in rapidly improved lymphadenopathy, enlarged pancreas, and serological findings. This report can be helpful for the diagnostic assessment of polyclonal hypergammaglobulinemia conditions.