Kinetics of B-cell reconstitution after HSCT reveals the early appearance of CD21^lo transitional B cells. (A) PBMCs were collected from patients at different times after HSCT and then stained with anti-CD20, -CD27, -CD10, and CD21 to determine the B-cell reconstitution profile. CD20⁺ B cells were gated (top panel) and divided into transitional, naive, and memory B-cell subsets based on differential expression of CD10 and CD27 (middle panel). The transitional B cells were further divided into CD21^lo and CD21^hi populations (bottom panel). The numbers indicate the percentage of total B cells (top panel) and the corresponding subsets (middle and bottom panels). (B) Summary of the reconstitution of naive and transitional B-cell subsets after HSCT in the patient presented (A) as a percentage of all B cells. (C) Combined B-cell reconstitution profile in 12 post-HSCT patients.