Pure Red Cell Aplasia Associated With Recipient B‐Cell Mixed Chimerism Successfully Treated With Rituximab

BACKGROUND: Pure red cell aplasia (PRCA) is a rare but clinically significant complication of allogeneic hematopoietic stem cell transplantation (HSCT), particularly in patients with major ABO incompatibility. Although mixed chimerism is usually evaluated using whole-blood or T-cell-based assays, these approaches may fail to detect lineage-specific immune dysregulation relevant to posttransplant immune complications. METHODS: We report a pediatric case of late-onset PRCA after reduced-intensity allogeneic HSCT. Clinical course and lineage-specific chimerism analyses were retrospectively reviewed. RESULTS: Lineage-specific chimerism analysis demonstrated persistent recipient B-cell mixed chimerism, whereas T-cell and granulocyte fractions showed complete donor chimerism. Given the suspected pathogenic role of residual recipient B cells, rituximab was administered as a targeted B-cell-directed therapy. This resulted in reticulocyte recovery, resolution of transfusion dependence, and conversion to complete donor B-cell chimerism. CONCLUSION: This case suggests that recipient B-cell persistence may contribute to the pathogenesis of posttransplant PRCA and highlights the potential value of lineage-specific chimerism analysis in clarifying the underlying mechanism and guiding mechanism-based therapy, including rituximab.