Next-Generation IgVH Sequencing of Monoclonal B-Cell Lymphocytosis Reveals Clonal Heterogeneity, Ongoing Mutation, and Repertoire Differences Relative to Chronic Lymphocytic Leukemia

Background: Chronic lymphocytic leukemia (CLL) usually develops from asymptomatic monoclonal expansions of CD5 positive B-cells termed monoclonal B-cell lymphocytosis (MBL), present in the peripheral blood (PB) of approximately 5% of otherwise healthy older individuals. Although MBL only occasionally progresses to CLL, cases that do progress typically have higher MBL cell counts in the 1500-4000/µL range. Although antigen selection appears to play a central role in the development CLL, it is unclear whether this occurs at an early MBL stage or primarily during the progression of MBL to CLL. One prior study has reported clonal heterogeneity in MBL finding it in 4 of 6 low count MBL cases from familial CLL kindreds using a single cell PCR technique (Leukemia 2010,24:133-140). In this study, we assessed the VH repertoire and degree of clonal heterogeneity in sporadic MBL cases using next-generation sequencing (NGS) of the rearranged immunoglobulin heavy chain (IgH) locus.

Methods: The 35 cases selected for sequencing represented residual, cryopreserved material from PB specimens submitted to ARUP for clinical phenotyping studies. All contained polytypic CD5 negative B-cells in addition to MBL/CLL phenotype cells, and had 2 or more vials for analysis. The majority (80%) had counts of MBL cells below 1000/µL (mean 294/, range 795-30 cells/µL). FACS purification of MBL cells (CD20+CD5+) and CD5 negative B-cells was performed on all samples. The IgH repertoire from the unsorted and two sorted populations was determined by NGS using the LymphoSIGHT method.

Results: Five cases could not be analyzed due to insufficient numbers of MBL cells. Clonal VDJ rearrangements or clonotypes were identified in the remaining 30 based on their high frequency within the B-cell repertoire of the unsorted sample, and having a higher frequency in the sorted MBL cells relative to the sorted CD5 negative B-cells. Functional clonotypes were identified in 29 of these 30 cases. Interestingly, 5 cases had 2 functional unrelated clonotypes using different D and/or J segments that also employed different V segments. Of the 5 cases with 2 unrelated clonotypes, 3 had MBL cell counts below 1000/µL (32, 275, and 865) and 2 above (1640, 2600). Moreover, 1 of the clones in the case with 865 cells/µL represented only 25% of the MBL cells or 220 cells/µL, while 1 clone in the case with 2600 MBL cells/µL represented 18% of the MBL cells or 470 cells/µL. By flow cytometry, the CD5+ CD20+ cells in 2 of the cases with 2 functional clonotypes showed polytypic kappa/lambda expression (ratios near 1), 2 cases had uniform dim monotypic kappa expression, and 1 case showed 90% dim kappa and 10% dim lambda expression. The most frequently used VH segments were V4-34 in 6/34 or 18% of functional clonotypes, followed by V3-23 (11%), and V3-21 (9%). The V1-69 segment was used by only 1/34 (3%) functional clonotypes. The VH segments in 72% of cases with functional clonotypes were mutated (homology to germline < 98%), with 6 cases showing clear evidence of ongoing mutation by having 2 or more related clones.

Conclusions: We demonstrate that MBL exhibits considerable clonal heterogeneity, with 2 distinct unrelated clones identified in 17% of 30 analyzed cases. Finding 2 distinct clones cannot be explained by a lack of allelic exclusion or the presence of 1 cell with 2 productive IgH rearrangements since each clone had different frequencies within the sorted MBL cell repertoire. This is further supported by finding the ratios of the two MBL clones in 2 cases being different in the unsorted compared to the MBL sorted cells. Clonal heterogeneity appears to occur at an early stage since the majority of clones (6/10) had cell counts below 500 cells/µL. We also found that clonal heterogeneity of MBL may not be detectable by flow cytometry or may appear as polytypic CD5+CD20+ B-cells. To our knowledge, this represents the first report of clonal heterogeneity in sporadic MBL. Our identification of infrequent use of V1-69 (1/34) supports prior studies indicating the VH repertoire of MBL is different than CLL which frequently employs V1-69. Finding evidence of ongoing VH mutation suggests antigen selection may occur in early MBL. Overall, our findings are consistent with recent observations (Cancer Cell 2011, 20;246-259) suggesting that hematopoietic stem cells from CLL patients can generate mono-or oligoclonal MBL phenotype cells that can then be selected through antigen binding for expansion.