Different biology and clinical outcome according to the absolute numbers of clonal B-cells in monoclonal B-cell lymphocytosis (MBL)

The biological and clinical relationship between Chronic Lymphocytic Leukaemia (CLL) and Monoclonal B-cell Lymphocytosis (MBL) has now been reported in some detail. This review investigates associations between biology and disease activity as they relate to the absolute numbers of abnormal cells. The clonal B-cells in CLL-type MBL are indistinguishable from CLL with respect to surface phenotype and the presence of chromosomal abnormalities. However, the majority of CLL-type MBL cases in the general population have very low numbers of clonal B-cells, typically in the range 0.1-10 per μL, and such cases use different IGHV genes than higher-count CLL-type MBL cases and often show intraclonal heterogeneity. Cases with higher counts are biologically similar to CLL although there is a relationship between the CLL cell count at presentation and the likelihood of further clonal expansion. Individuals presenting with CLL cell counts above 2,000 per μL are more likely to have gradually increasing B-cell counts over time and although the risk of requiring treatment for progressive CLL remains low there may be impaired normal B-cell activity.