Clonal B-Cell Lymphocytosis of Marginal Zone Origin: Presenting Features, Clinical Evolution and Prognostic Factors

Background/Objective: During the last two decades, several cases presenting with circulating clonal B-cells with features consistent with possible marginal zone (MZ) derivation have been described under different terminologies. The present study aims to shed more light onto the main disease characteristics and determine prognostic factors for outcome in 98 consecutive cases with clonal B-cell lymphocytosis of marginal zone origin (CBL-MZ). Methods: This is a multicenter retrospective analysis including 98 consecutive CD5(-) CBL cases referred to our Departments between 1999 and 2017. These cases were selected based on the presence of circulating CD5(-) clonal B-cells, irrespectively of their absolute number, without B-symptoms, organomegaly, lymphadenopathy or cytopenias or any other features consistent with a known lymphoproliferative disorder. Clinical, morphologic, biochemical, immunophenotypic, histologic and molecular features of CBL-MZ cases were analyzed. Results: The median absolute lymphocyte counts (ALCs) and circulating CBLs were 6.7 × 10⁹/L and 3.447 × 10⁹/L, respectively. Paraproteinemia was found in 38%. Bone marrow (ΒΜ) was involved in all but one case. MYD-88L265P mutation was positive in 11%. Two subcategories of CBL-MZ were identified: One was characterized by lower ALC/CBL, paraproteinemia, CD38 expression, BM lymphoplasmacytic morphology and more frequent MYD-88L265P mutation. The second category displayed a leukemic picture, higher frequency of CD11c expression, hypogammaglobulinemia and elevated LDH. Treatment-free survival (TFS) was 91%, and median freedom from progression (FFP) was 95.6 months. For TFS, two factors proved significant using multivariate analysis: BM infiltration ≥ 50% and elevated LDH (RR 5.6 and 5.4, respectively). Evolution to splenic marginal zone lymphoma was a rare event (5%). A novel pattern of progression emerged, namely development of cytopenias due to extensive BM infiltration without any other disease localization. Conclusions: CBL-MZ is an indolent lymphoproliferative disorder with excellent outcome and low probability of progression. Based on our findings, CBL-MZ represents a heterogeneous entity where the vast majority of cases remain stable or develop increasing lymphocytosis. Clinically, our data also highlight that the extent of BM infiltration and elevated LDH levels appear to be the most notable predictors for introducing therapy.