Histopathologic and Molecular Features of Persistent Polyclonal B-Cell Lymphocytosis Challenging the Benign Nature of the Disorder.
Authors: Giudice Ilaria Del , Pileri Stefano A. , Rossi Maura , Sabattini Elena , Campidelli Cristina , Della Starza Miss Irene , De Propris Stefania M. , Mancini Francesca , Mauro Francesca R. , Tafuri Agostino , Bizzoni Luisa , Rago Angela , Amendola Angela , Guarini Anna , Foa Robert 
Published: April 2020

Abstract
Persistent polyclonal B-cell lymphocytosis (PPBL) is an unusual expansion of B lymphocytes, whose immunophenotype resembles that of memory B cells. The physiopathology of PPBL is unknown and whether this syndrome represents a pre-malignant or benign disease is unclear. Aiming at further defining the histological features of BM involvement and at challenging the polyclonal nature of PPBL, we reviewed 6 cases of PPBL who underwent BM biopsy. There were 3 males and 3 females, with a median age at diagnosis of 35 years (range 17–46). The median lymphocyte count was 5.3 × 109/L (5.1–8.3). One patient had been splenectomized 10 years earlier and 2 were splenomegalic. During the follow-up (median 20 months, range 5–138), 1 showed progressive spleen enlargement and 2 additional patients developed splenomegaly. Screening for hepatitis B, C and HIV was negative. PB morphology showed 2 to 32% of cleaved lymphocytes. Immunophenotype documented 21 to 70% B lymphocytes in both PB and BM aspirate, all positive for CD19, CD22, CD79b, FMC7, CD18 and CD11a, negative for CD5, CD23, CD10, with no Ig light chain restriction. CD25 was expressed in 4 cases. PCR analysis of the IgH gene configuration of PB and BM cells showed a polyclonal picture in all cases. On Giemsa-stained sections of BM trephines, BM was normocellular in 5 cases and slightly hypocellular in 1. A mild lymphoid infiltrate was visible, mainly interstitial with occasional small lymphoid aggregates. An intrasinusoidal pattern was seen in 4/6 cases. Immunohistochemistry confirmed the interstitial infiltration by B cells (CD79a+, CD20+, IgM+, IgD+, DBA44−/+, CD5−, AnnexinD1−) in all cases and highlighted the intrasinusoidal pattern in 4 in which a splenic marginal zone lymphoma (MZL)-like morphology was described. By IgH gene PCR on BM tissue, a clonal rearrangement was detected in 2/4 cases with intrasinusoidal infiltration; in 2 cases DNA quality was poor. The 4 cases showed splenomegaly at diagnosis or during the follow-up. Splenectomy was performed in 1 patient due to progressive enlargement of the spleen over 9 years. White pulp was normal and red pulp was expanded by an infiltration of small lymphocytes with an intrasinusoidal distribution. Splenic B lymphocytes were CD79a+, CD20+, IgM+, IgD+, DBA44−, as in the BM biopsy, with low proliferation activity and proven clonality by IgH-PCR. In the remaining 2 cases, in which no intrasinusoidal lymphoid pattern was evident, clonal IgH rearrangement was absent and no splenomegaly has developed. We confirm the interstitial lymphoid infiltration of BM trephine in all cases of PPBL, with an evident intrasinusoidal component in some. The detection of IgH rearrangements on lymphocytes infiltrating the BM from bioptic samples and the absence of clonality in PB lymphocytes and in the BM aspirate, possibly due to the intrasinusoidal distribution, have not been described. These findings suggest that at least some of the cases described as PPBL are pre-malignant disorders close to splenic MZL, justifying the occurrence of progressive splenomegaly over time. Prolonged follow-up of patients with PPBL and an accurate biologic work-up of the BM, are therefore warranted.