Isolated Full Donor T-Cell Chimerism After Haploidentical TCRαβ/CD19 Depleted HSCT Maintains Remission of Familial HLH

Familial hemophagocytic lymphohistiocytosis (FHL) is a life-threatening disorder requiring hematopoietic stem cell transplantation (HSCT) for cure, often complicated by mixed donor chimerism (DC). We report a 5-month-old girl with FHL-3 treated with haploidentical TCRαβ+/CD19+ depleted HSCT, achieving isolated full CD3+ DC sufficient to maintain HLH remission despite impaired NK cell function. Post-transplant complications included autoimmune hemolytic anemia (ptAIHA) managed with steroids, Rituximab, and Sirolimus. This case highlights the critical role of healthy CD8+ T cells in preventing HLH recurrence, challenges of mixed DC, and the importance of serial monitoring to guide management decisions.