Robust Immune Reconstitution in Children with Severe Primary Immunodeficiency after Reduced-Intensity Conditioning Hematopoietic Stem Cell Transplantation

Hematopoietic stem cell transplantation (HSCT) is a potentially curative therapy for severe primary immunodeficiency diseases (PID), but it is still unclear what is the optimal transplant approach. To help answer this question, the stem cell transplant team at Ann & Robert H. Lurie Children's Hospital of Chicago (formerly Children's Memorial Hospital) evaluated the risks and benefits of a reduced-intensity conditioning (RIC) transplant approach for treatment of severe PID. Between 2000 and 2013, 42 children with severe PID were treated with allogeneic HSCT following a RIC regimen consisting of fludarabine (30mg/m²/day x 6), rabbit anti-thymocyte globulin (once daily x 4), and intravenous busulfan (once daily x 2). Total busulfan exposure, as measured by the plasma concentration-time area-under-the-curve (AUC), was targeted individually for each patient at 4000 µM*min/day in an earlier cohort (n=14), and 5000 in a later cohort (n=28). Patients were treated in an ambulatory setting and were hospitalized only for specific medical or social reasons. There were 31 male and 11 female patients. The median age at the time of transplant was 8.2 months (range 1 month-17.4 years). The median weight was 7.6 kg (range 2.5-81.5 kg). Patients were diagnosed with severe combined immunodeficiency (n=17), Wiscott-Aldrich syndrome (n=7), hyper-IgM syndrome (n=5), major histocompatibility complex II deficiency (n=3), X-linked lymphoproliferative disease (n=3), NEMO syndrome (n=2), Omenn syndrome, reticular dysgenesis, Chediak-Higashi syndrome, IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) syndrome, and IPEX-like syndrome (n=1 each). Fourteen patients had related donors and 28 had unrelated donors. Twenty-eight patients received peripheral blood HSC, 12 received cord blood, and 2 received bone marrow as the source of HSC. All patients tolerated the conditioning regimen with minimal nausea, vomiting, or pain. No severe mucositis, sinusoidal occlusion syndrome, seizure, grade III/IV acute GVHD or extensive chronic GVHD were seen. In most patients, the engraftment was rapid, with ANC ≥ 500 at a median of +15 days post-transplant, following a nadir at day +10 with a median ANC nadir of 60. In the AUC 4000 cohort, 2 patients (14%) developed primary graft failure and 2 patients (14%) developed immune-mediated secondary graft failure; in the AUC 5000 cohort, 2 patient (7%) developed primary graft failure and 1 patient developed secondary graft failure after primary EBV infection. Two patients died from complications associated with primary graft failure, 2 from complications caused by EBV infections, and 2 from complications related to their underlying diseases in the early post-transplant period. Kaplan–Meier analysis shows overall and event-free survival rates of 81.9% and 73.6%, respectively, at a median follow-up of 4.3 years. Most engrafted patients have stable donor chimerism over time. At a mean of 3.5 years post-transplant, the median donor chimerism in total white blood cells was 97% and the first and third quartiles were 77% and 99%, respectively. Engrafted patients demonstrated robust immune reconstitution with B and T cell counts normalized by 3 months and 9 months post-transplant, and CD45RA+ naive T cells by 1 year. Between 1 to 4 years post-transplant, the median absolute CD4+, CD8+ and CD19+ cell counts were 1388, 976 and 720 per mm³, respectively, percentage of CD45RA+ naive CD4 and CD8 cells were 68% and 81%, and endogenous IgG and IgM levels were 840 and 97 mg/dL. T cells showed regular responses to antigen and mitogen stimulation and exhibited normal TCRVβ repertoires. Endogenous IgG levels returned to normal by day +100, and no intravenous immunoglobulin infusion was required after day +100 in all but 1 patients. Positive vaccine-specific antibody responses were seen in patients after reimmunization at 12-24 months post-transplant. Clinically, patients have normal growth and development and do not exhibit increased susceptibility to infection. Our data showed that HSCT after RIC offers an optimal treatment for severe PID by providing robust post-transplant immune reconstitution while minimizing side effects. To reduce the risk of graft failure, we recommend a targeted busulfan AUC of 5000 µM*min/day for 2 days.