IDDA-Scored Abatacept-to-HSCT Course in Pediatric LRBA Deficiency

Abstract LRBA deficiency is a rare inborn error of immunity in which impaired CTLA-4 recycling causes immune dysregulation; haematopoietic stem cell transplantation (HSCT) is curative, but no standardised measure of pre-transplant disease activity exists and the role of abatacept as a bridge to HSCT remains ill-defined. We report a 4-year-old girl with compound heterozygous LRBA variants in whom abatacept, given as a planned bridge to transplantation, reduced disease activity within four months (Immune Deficiency and Dysregulation Activity [IDDA] score 54.5 to 22.6), maintained control until transplant, and was followed by an IDDA of 1.0 at 22 months after fully matched unrelated-donor HSCT. Retrospective IDDA scoring was feasible in 32 of 46 published LRBA HSCT patients (22 with and 10 without pre-HSCT abatacept; baseline scores 11.1 vs 11.5), with descriptively higher survival in abatacept-bridged patients (95.8% vs 86.4%; P = 0.34). The IDDA provides a practical metric to quantify disease activity and to evaluate the contribution of pre-HSCT abatacept in LRBA deficiency.