Dynamic variation of CD5 surface expression levels within individual chronic lymphocytic leukemia clones

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of clonally derived mature CD5^high B cells; however, the cellular origin of CLL is still unknown. Patients with CLL also harbor variable numbers of CD5^low B cells, but the clonal relationship of these cells to the bulk disease is unknown and can have important implications for monitoring, treating, and understanding the biology of CLL. Here, we use B-cell receptors (BCRs) as molecular barcodes to first show by single-cell BCR sequencing that the great majority of CD5^low B cells in the blood of CLL patients are clonally related to CD5^high CLL B cells. We investigate whether CD5 state switching was likely to occur continuously as a common event or as a rare event in CLL by tracking somatic BCR mutations in bulk CLL B cells and using them to reconstruct the phylogenetic relationships and evolutionary history of the CLL in four patients. Using statistical methods, we show that there is no parsimonious route from a single or low number of CD5^low switch events to the CD5^high population, but rather, large-scale and/or dynamic switching between these CD5 states is the most likely explanation. The overlapping BCR repertoires between CD5^high and CD5^low cells from CLL patient peripheral blood reveal that CLL exists in a continuum of CD5 expression. The major proportion of CD5^low B cells in patients are leukemic, thus identifying CD5^low B cells as an important component of CLL, with implications for CLL pathogenesis, clinical monitoring, and the development of anti-CD5-directed therapies.