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. © 2010 International Clinical Cytometry Society