Characterization of PLNs in human spleen. (A) Representative photograph of Ki67 staining of spleens from 7 controls and 7 ITP patients showing GCs with LZ/DZ polarization and PLNs with a homogeneous distribution of proliferating cells. Representative photograph of IgM staining of spleens from 8 controls and 8 ITP patients showing GCs mostly lacking the expression of IgM and PLNs exhibiting the dense presence of IgM. Sections of 9 ITP and 10 control spleens were stained for expression of Bcl6, which was mostly limited to the DZ of the GCs and absent in PLNs. Visualization with a Zeiss Axio Imager Z1 fluorescence microscope (×200 magnification). The photomicrographs shown here are from ITP spleens. (B) GCs and PLNs were identified by either Ki67 or IgM stains and the total number of these structures/3.7 cm² of spleen sections was determined by counting 9 spleen sections from ITP and 11 sections from control. Samples of ITP spleens were divided into those who had received prednisone (Pred) and those who received IvIg just before splenectomy and the number of PLN determined. (C) Comparison of the expression of bcl6, AIDCA, and bcl2 mRNA in spleen from ITP sections versus normal spleen. Distilled water (H₂O) was used as a negative control of PCR. Representative RT-PCR of 8 sections from ITP spleen is shown. Each of these sections lacked identifiable GCs. (D) Serial sections of 10 ITP and 10 controls spleens were stained to evaluate the expression of Ki67, CD21, and CD23 in both PLNs (top panels) and GCs (bottom panels). The black arrows mark the dark zone (Ki67⁺, CD21^+/−, and CD23⁻) and the white arrows indicate the light zone (Ki67^+/−, CD21⁺, CD23⁺) of a GC. Visualization with Zeiss Axio Imager Z1 fluorescence microscope (×200 magnification). The photomicrographs shown here are from ITP spleens. (E) Comparison of the frequency of proliferating cells in splenic GC and PLN after staining with Ki67. Data are expressed as percentage of Ki67⁺ cells.