The early evolution of chronic clonal lymphocytosis

Mature B-cell neoplasms often present with chronic clonal lymphocytosis, yet their earliest evolutionary stages and precise origins remain unclear. Using single-cell RNA and paired BCR sequencing with targeted DNA analysis across B-cell differentiation, we identified distinct pathological B-cell states—memory, aged, CLL, and aged-HCL—that only partially aligned with clinical diagnoses. Notably, CLL-like and aged-like expansions were also detected in healthy individuals, revealing both classical MBL and a previously unrecognized “aged-like” MBL. A central finding of this study is that CLL-like programs arise in mono-, oligo-, and polyclonal B cells, indicating that the malignant state emerges before clonal selection. This manuscript uncovers two mechanisms explaining this unusual oligo/polyclonal origin: (1) evidence of autoreactivity through BCR editing in CLL cells, and (2) somatic mutations in hematopoietic stem and progenitor cells that generate oligo/polyclonal B-cell outputs. These results establish pre-leukemic oligo/polyclonal evolution as a widespread precursor to chronic B-cell malignancies.