Limitations of Bu/Flu-based preparative regimen in CTLA-4 haploinsufficiency to achieving disease phenotype correction

CTLA-4 haploinsufficiency (CTLA4-HI) is a broad immune dysregulation disorder characterized by multisystem autoimmunity and immune deficiency. In this report, we highlight the limitations of Busulfan-Fludarabine (Bu-Flu)-based conditioning regimens for HSCT in CTLA4-HI patients. Three patients with CTLA4-HI underwent HSCT at three centers between 2018–2019. Nine patients with nonmalignant disorders, without T cell (TC) dysregulation, who underwent allo-HSCT with Bu-Flu-based regimens were utilized as comparison. Further, we performed TC subset chimerism studies in CD4 recent thymic emigrants (RTE), CD4 and CD8 naïve TC cells, T regulatory cells (Tregs), circulating T follicular helper cells (cTfh), and CD4 and CD8 effector memory (EM). Two patients (P1, P2) underwent MSD and 1 patient, MUD BM HSCT (P3). Cumulative Busulfan AUC was 66–95 mg*h/L and Fludarabine was 160 mg/m2. Only P1 and P2 had Alemtuzumab 0.3–0.48 mg/kg. Neutrophil (Day +18) and platelet (Day +24) engraftment was achieved in all patients with 100% donor myeloid chimerism. None had aGVHD. Mean TC chimerism at engraftment, 3 and 6 months, 1 year, 2 years, and last follow-up in the CTLA4-HI group was 20%, 26%, 25%, 45%, 46%, and 49%, respectively; lower than the comparison cohort – 59%, 52%, 70%, 87%, 93%, and 97%, respectively. With low TC chimerisms, all three patients had relapse of autoimmune manifestations, necessitating immune-modulatory therapy. No advantage of donor T cells in Treg, cTfh, CD4, or CD8 EM compartments was noted. P1 died unexpectedly at 25 months post-HSCT while still on sirolimus. P2 continues on medications for cytopenias. P3 came off sirolimus 3 years post-HSCT without autoimmune manifestations. CD4 RTE remains mixed donor (69%) >3 years post-HSCT (P2), suggesting incomplete thymic niche clearance, and chimerisms were only 54% in Tregs and 52% in cTfh. Correspondingly, P2 immunophenotype showed CD4 and CD8 activation with expansion of cTfh and decreased Treg frequency, suggesting ongoing immune dysregulation. In our CTLA4-HI cohort, Bu-Flu-based conditioning resulted in inadequate immune ablation, low TC chimerisms, and incomplete or delayed disease phenotype correction. Moreover, no donor Treg benefit was noted in CTLA4-HI. Thus, in addition to augmenting immunoablation, strategies to enhance graft vs. immune dysregulation must be explored.