All lymph node and lung specimens were fixed in 10% formaldehyde and embedded in paraffin. These paraffin‐embedded tissue blocks were sliced into 3 µm thick sections, which were stained with hematoxylin and eosin (H&E).
Immunohistochemical staining was performed on an automated Bond Max instrument (Leica Biosystems, Wetzlar, Germany) using the following primary antibodies: HHV‐8 (13B10, 1:40; LifeSpan Biosciences, Seattle, USA); IgG4 (MC011, 1:10 000; The Binding Site, Birmingham, UK); IgG (RWP49, 1:600; Novocastra, Newcastle, UK), CD20 (L26; 1:200; Novocastra, Newcastle, UK), and CD3 (PS1; 1:50; Novocastra, Newcastle, UK). In situ hybridization was also performed for the κ and λ light chains (Leica Biosystems, Wetzlar, Germany).
The number of IgG‐ and IgG4‐positive cells was estimated in the area with the highest density of IgG4‐positive cells. The numbers of cells in three different HPFs (eyepiece, 10×, and lens, 40×) were counted, and the average was calculated to obtain the number of IgG4‐positive cells and the IgG4‐/IgG‐positive cell ratio.
Immunohistochemical staining was performed on an automated Bond Max instrument (Leica Biosystems, Wetzlar, Germany) using the following primary antibodies: HHV‐8 (13B10, 1:40; LifeSpan Biosciences, Seattle, USA); IgG4 (MC011, 1:10 000; The Binding Site, Birmingham, UK); IgG (RWP49, 1:600; Novocastra, Newcastle, UK), CD20 (L26; 1:200; Novocastra, Newcastle, UK), and CD3 (PS1; 1:50; Novocastra, Newcastle, UK). In situ hybridization was also performed for the κ and λ light chains (Leica Biosystems, Wetzlar, Germany).
The number of IgG‐ and IgG4‐positive cells was estimated in the area with the highest density of IgG4‐positive cells. The numbers of cells in three different HPFs (eyepiece, 10×, and lens, 40×) were counted, and the average was calculated to obtain the number of IgG4‐positive cells and the IgG4‐/IgG‐positive cell ratio.