Margaret A. Morris Fears
$1,072,951
Michael A Pulsipher
National Marrow Donor Program
Minnesota
National Institute of Allergy and Infectious Diseases (NIAID)
Severe combined immunodeficiency (SCID) is a group of genetic disorders that abrogate T cell development and function. Allogeneic hematopoietic cell transplantation (HCT) is the standard treatment for the disease and can be performed successfully in SCID patients without the high dose pre-HCT busulfan conditioning due to the unique capacity for progenitors to engraft in the empty thymus and reconstitute T cell development. Without conditioning, lineages other than T cells remain of host origin. The CSIDE protocol was funded with an earlier grant to test the efficacy and safety of a regimen of low dose, individualized targeted busulfan compared to moderate dose in SCID patients at risk of poor humoral outcome undergoing non-matched sibling donor HCT. We hypothesize that patients randomized to receive low dose busulfan will achieve similar outcomes compared to those receiving moderate dose (myeloablative) busulfan, achieving both T and B cell immune reconstitution. Due to COVID and competition with gene therapy, accrual slowed, but with 50 centers open and a redesigned approach, additional enrollment facilitated by this grant will ensure that the trial reaches meaningful conclusions. In Aim 1, patients have been randomized to cumulative area-under-the-curve (cAUC) exposure of busulfan of 30 mg*h/L versus 60 mg*h/L. IL2RG/JAK3 patients also receive rATG, while RAG1/2 patients receive rATG, fludarabine, and thiotepa. Stem cell sources include unrelated and haploidentical related donor products that have been TCRαβ+/CD19+ depleted with no post-HCT GVHD prophylaxis. The safety profile of the trial has been excellent to date. The original primary endpoint was protective antibody response to tetanus by 2 years post-HCT. Because of enrollment challenges, we redesigned the primary endpoint into an ordinal ranked win comparison, which allows higher power even if we cannot fully enroll. The primary outcome will center around the IL2RG/JAK3 cohort, which should achieve full accrual, randomizing 32 patients. The RAG1/2 cohort will close once the IL2RG/JAK3 cohort closes, likely accruing up 18-20 patients which we will analyze descriptively. In Aim 2, We hypothesize that donor HSC engraftment measured by the surrogate of myeloid donor chimerism will be associated with superior quality of T cell reconstitution and improved adaptive immune responses to vaccination. We hypothesize that T cell exhaustion and poor T cell receptor (TRB) diversity seen in patients undergoing HCT in the absence of conditioning will be diminished or absent in CSIDE participants due to improvements in thymic output associated with engraftment of donor-derived HSC. We hypothesize that IL2RG/JAK3 patients receiving moderate dose busulfan and/or with high level donor chimerism will exhibit multiple in vitro biomarkers of IL-21 response, as this cytokine signals via IL2RG/JAK3. We hypothesize that vaccine response will correlate with normalization of IGH CDR3 diversity in RAG1/2 patients with mixed chimerism due to strong selective advantage for antigen-specific B cells. We hypothesize that T cell tolerance will occur by different mechanisms (central deletion versus peripheral regulation) according to donor type. Finally, we hypothesize that analysis of pK samples for elements of our preparative approaches (rATG, thiotepa, and fludarabine) will allow targeted treatment of infants undergoing HCT for SCID or other disorders moving forward.