Chronic lymphocytic leukemia includes a tumor subset resembling memory B cells that develop early and persist
Intratumor heterogeneity (ITH) refers to the coexistence of distinct cancer cell subpopulations within a single tumor, each with unique molecular and functional properties. Understanding the dynamics and evolution of ITH is crucial for predicting tumor progression and the development of therapy resistance. Here, we conducted a comprehensive investigation of ITH in Chronic Lymphocytic Leukemia (CLL) and identified a subpopulation that carried the CLL B-cell receptor rearrangement but contrasted from the CLL main population by low CD5 and high CD20 expression and highly mutated Ig-genes. This CLL-related memory B-cell population shared somatic driver mutations with the main CLL population but also exhibited exclusive somatic mutations. Phylogenetic analysis suggested a pathogenically early generation of these CLL-related B cells before monoclonal B lymphocytosis or CLL manifestation. Our data indicated that CLL-related B cells have self-replenishing potential, as they diminish upon treatment but recover indistinguishably on relapse. This contrasts with the main CLL population, which mostly represents a selective and novel expansion of the CLL-related B cells. This differentiation capacity into conventional CLL cells, and the expression of leukemic stem cell signatures further supported their tumorigenic capacity. We propose that these CLL-related B cells represent a pool of highly diversified, early-stage CLL precursor cells, which persist in the shape of "malignant memory B cells". Longitudinal analyses of these CLL precursor cells suggested that they form a reservoir of malignant, leukemia-originating cells which contribute during disease progression to CLL outgrowth and clonal evolution.