CD5-Negative Monoclonal B-Cell Lymphocytosis (MBL) Is Detectable In 9% of Adults Aged Over 40 but Phenotypes Associated with Clinically Common Non-Hodgkin Lymphomas Are Infrequent (<2%)

Monoclonal B-cells with a Chronic Lymphocytic Leukemia (CLL) phenotype are detectable in more than 10% of adults in the general population using high sensitivity flow cytometry assays designed to detect minimal residual disease after treatment. However, the prevalence of MBL with a phenotype corresponding to other B-lymphoproliferative disorders (B-LPD) is less than 2% of the general population even using the most sensitive assays. No studies have reported CD10+ MBL whereas several studies have demonstrated that the t(14;18) is frequently detectable in the general population at a level which should be detectable by flow cytometry. The lack of CD10+ MBL may indicate that the t(14;18) alone rarely results in the expansion of a clonal B-cell population in the blood, or that currently available assays are inadequate for detecting circulating follicular lymphoma. We have previously investigated 66 markers to determine the best candidates for diagnosis and monitoring B-LPD, which were then tested in over 1500 cases. We have developed a single-tube assay to screen for residual disease that can detect lymphoma cells when they represent as few as 1 in 10,000 leucocytes. The aim of this study was to asses the frequency with which lymphoma-phenotype monoclonal B-cells are detected in the general population using a high sensitivity assay.

Cells from 679 individuals (342 male, 337 female, median age 64, range 40–99) with a normal blood count and no current or prior history of cancer were incubated with antibodies to Kappa, Lambda, CD19, CD20, CD5, CD10, LAIR1, CXCR5 and 0.5 million cells were acquired using a BD FACSCanto II cytometer. In cases with detectable MBL further phenotyping was performed and B-cells were selected and stored for FISH and molecular clonality studies.

MBL was detected 129/679 cases (19.0%): CLL-type MBL in 86/679 cases (12.7%), non-CLL MBL in 60/679 cases (8.8%) with both CLL-type and non-CLL MBL were present in 17/679 cases (2.5%). Within the non-CLL MBL group, in 21/60 cases the monoclonal B cells had no additional features to confirm a neoplastic population and it was not possible to ascertain whether these were neoplastic cells or a reactive population with a highly skewed kappa/lambda ratio. Of the remaining 39 specimens: none showed evidence of germinal centre differentiation; 12 (1.7% of total) showed a phenotype most consistent with marginal zone lymphoma/lymphoplasmacytic lymphoma; and 27 cases expressed strong levels of LAIR1 coupled with strong CD19 and CD20 and an extended phenotype that is relatively rare in clinical B-LPD, restricted to hairy cell leukemia and a small proportion (<15%) of marginal zone lymphomas.

Using an assay designed for detection of residual disease in lymphoma it was possible to detect non-CLL phenotype MBL in 8.8% of individuals over 60 years of age with normal blood counts and no history of cancer. Only a small number of these cases had a phenotype comparable to follicular lymphoma or marginal zone lymphoma, which constitute the majority of clinically significant indolent B-LPD. The complete absence of cases with a germinal centre phenotype contrasts with the high frequency of detection of BCL2-IGH rearrangement by PCR in the peripheral blood of healthy individuals. The flow cytometry assay used in this study readily detects circulating cells with appropriate phenotype in patients known to have follicular lymphoma, diffuse large B-cell lymphoma or marginal zone lymphoma/lymphoplasmacytic lymphoma and the data indicate a high specificity for peripheral blood based diagnosis and monitoring in these conditions. The most clinically prevalent lymphomas are rarely detectable in peripheral blood in individuals without lymphadenopathy but there is a surprisingly frequent development in the general population of both CLL and a restricted subset of marginal zone lymphomas which may indicate a common developmental pathway.

No relevant conflicts of interest to declare.