Potential Increase in Graft Failure Risk after Reduced Intensity Conditioning Stem Cell Transplant (RIT) Using Umbilical Cord Blood for Children with Primary Immunodeficiency.

Allogeneic stem cell transplantation is curative for patients with primary immunodeficiency. A unique reduced intensity conditioning regimen has been developed to maximize cure rate and minimize transplant-related toxicity. Between 2000 and 2007, we performed 16 RIT in patients with hyper-IgM syndrome (n=2), severe combined immune deficiency (SCID) (n=10), immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) (n=2), Wiskott-Aldrich syndrome (n=1), and X-linked lymphoproliferative disease (n=1). There were 12 males and 4 females, with a median age at the time of RIT of 8.0 months (range 1.1 months to 8.9 years). Donor sources included peripheral blood stem cells (PBSCs) from matched related donors (n=7) or unrelated donors (n=2), and cord blood units (CBUs) (n=7). The median cell dose was 8.55 × 10⁸ total nucleated cells (TNC)/kg and 6.7 × 10⁶ CD34+cells/kg for the PBSC group, and 1.99 × 10⁸ TNC/kg and 0.71 × 10⁶ CD34+cells/kg for the CBU group. The conditioning regimen consisted of Fludarabine (Flu), days -10 through -5, intravenous Busulfan (Bu), days -5 and -4, and Anti-thymocyte globulin (ATG), days -4 through -1. GVHD prophylaxis included tacrolimus/prednisone (n=1), cyclosporine A (CSA) alone (n=2), and CSA/mycophenolate mofetil (n=13). All patients who received PBSCs from related and unrelated donors engrafted (9/9), whereas only 4/7 (57%) patients who received CBUs engrafted. The 3 patients who experienced primary graft failure had the following diagnoses; IPEX, T⁻B⁻NK⁺ SCID and T⁻B⁺NK⁺ SCID. Their cord blood doses were 0.78, 1.19 and 1.99 × 10⁸ TNC/kg, and 0.11, 0.69, and 0.55 × 10⁶ CD34+cells/kg, respectively. For the 13 patients who engrafted, median time to absolute neutrophil count (ANC) >1000 was 19 days (range 4 to 53) and median time to platelets >50K was 23 days (range 14 to 90). The ANC never dropped <500 for 8/13 (62%) patients, and platelets never dropped <20K for 9/13 (69%) patients who engrafted. VNTR analysis of donor cell contribution showed that full donor chimerism was achieved in 8/16 patients (50%; 6 received PBSCs and 2 received CBUs), and, partial donor chimerism was achieved in 5/16 patients (31%; 3 received PBSCs and 2 received CBUs). Toxicities within 100 days post-RIT included bacteremia (n=8), candidemia (n=1), and viral infection (n=6). All infections were effectively treated and patients fully recovered. No episodes of seizure or veno-occlusive disease were experienced. No mucositis or severe nausea/vomiting was reported. No grade III/IV acute graft-versus-host disease (GVHD) or chronic GVHD was seen. Three patients died within 100 days post-RIT from causes related to primary or secondary graft failure. The overall survival was 81.0% at 2 year post-RIT (95% CI 89.5–71.5). All deceased patients received CBUs as the donor source. One of these patients were considered high-risk with pre-RIT Lansky score =30%. If this patient was excluded from analysis, the 100 day RIT related mortality was 13.3%. This retrospective analysis revealed that RIT with Flu/Bu/ATG conditioning is well tolerated in children with primary immunodeficiency. The use of CBUs, however, appeared to increase the risk of graft failure. A larger study of the use of RIT in primary immunodeficiency could further examine this hypothesis.