Clonal evolution is a prognostic factor for the clinical progression of monoclonal B-cell lymphocytosis

Monoclonal B-cell lymphocytosis (MBL) has attracted intensive research as the prelude of chronic lymphoproliferative disorders, mainly chronic lymphocytic leukemia (CLL).1 According to current criteria, MBL is a preclinical condition characterized by monoclonal B-cell expansions at small concentrations (<5 × 109 cells/l) in the peripheral blood of otherwise healthy individuals.2, 3 It is now obvious that MBL is a highly heterogeneous entity regarding the immunophenotypic characteristics and the B-cell clone burden.4, 5, 6 In immunophenotypic terms, MBL is distinguished into three main categories: (i) CLL-like (CD5+CD20dimCD23+sIglow) accounting for 70–75% of all cases; (ii) atypical (CD5+CD20bright/+), mainly CD23dim/−; and (iii) CD5neg MBL.2, 3 Based on the number of monoclonal B-cells, MBL is divided into low and high-count, each with a clearly different clinical course.5, 6, 7 Low-count MBL is a non-progressive entity with a normal absolute B-cell count, whereas high-count or ‘clinical’ MBL (cMBL) is characterized by absolute lymphocytosis and progresses to CLL at a rate of ~1–2% per year.8, 9