Polyclonal CD5+/CD19+ B1a lymphocytes after allogeneic stem cell transplantation: a potential diagnostic pitfall.

Area of Science:

• Immunology
• Hematology

Background:

• B-lymphocytes are key to adaptive immunity, with B2 cells being the majority in circulation.
• B1 cells, a distinct subset, possess innate immune characteristics and include B1a (CD5+) and B1b (CD5-) subtypes.
• Limited data suggests a potential link between allogeneic bone marrow transplantation and elevated B1a cell counts.

Observation:

• A 17-year-old female with homozygous sickle cell disease (HbSS) underwent hematopoietic stem cell transplant (HSCT).
• Seven months post-HSCT, peripheral blood analysis revealed 16% immature mononuclear cells.
• Flow cytometry identified these as polyclonal CD5+/CD20+ B-cells, constituting 66% of lymphocytes.

Findings:

• The identified CD5+/CD20+ B-cells were characterized as B1a cells.
• No evidence of lymphoproliferative disorders was found despite the high B1a cell count.
• This represents a rare, non-neoplastic observation of increased circulating B1a cells post-stem cell transplant.

Implications:

• Accurate identification of B1a cells is vital to avoid misdiagnosis of atypical CD5+ lymphoproliferative disorders.
• This case expands the understanding of B1a cell dynamics in non-hematological malignancy patients post-HSCT.
• Further research is warranted to elucidate the mechanisms and clinical significance of increased B1a cells after stem cell transplantation.