Increased polyclonal CD5⁺ B1a lymphocytes in a haploidentical stem cell transplant recipient

Background: Atypical lymphocyte populations may be seen in the peritransplant setting. In this case report, we describe an unusually high number of CD5⁺ B-cells (B1a cells) following transplant. Methods: B1a cells identified during routine follow-up by immunophenotypic analysis in a middle-aged man who had a haploidentical stem cell transplant for acute myeloid leukemia were compared with a reference set of post-transplant samples. Results: Increased but polyclonal B1a cells were identified with 100% donor chimerism. Conclusions: Our case demonstrates that a high absolute number of B1a cells may be seen post-transplant and should not be confused with an atypical CD5 ⁺ lymphoproliferative disorder. Furthermore, the population of polyclonal CD5⁺ B lymphocytes from the patient's donor is prominent 7 months post-transplant. This suggests that the maintenance of CD5⁺ B1 cells prior to conversion to adult-type CD5^- B2 cells is not hindered by the recipient adult stromal environment.