T cell costimulation blockade promotes transplantation tolerance in combination with sirolimus and post-transplantation cyclophosphamide for haploidentical transplantation in children with severe aplastic anemia.

We conducted a pilot study employing extended T cell costimulation blockade (COSBL) with Abatacept along with sirolimus and post-transplantation cyclophosphamide (PTCy) in 10 patients (median age 12) with severe aplastic anemia (SAA). Nine patients engrafted in the COSBL group, compared to all 10 patients (median 14 vs 13days) treated on PTCy protocols without abatacept (CONTROL group). The incidence of acute graft-versus-host disease (GVHD) was 10.5% in the COSBL group compared to 50% in the CONTROL group (p=0.04). Chronic GVHD (12.5% vs 56%, p=0.02) and CMV reactivation (30% vs 80%, p=0.03) were also reduced in the COSBL group. T and NK cell subset analysis revealed higher CD56bright CD16- NK cells in the CONTROL group (p=0.004), but similar CD56dim CD16+ NK cells in both groups at day+30. Tregs (CD4+ CD25+ CD127dim/- FoxP3+) were markedly higher in the COSBL group at day+30 (8.4% vs 1.1%) and the trend was maintained through day+90 (p<0.01). The GVHD and Disease-free survival at one year in the COSBL group was 80% vs. 30% in the CONTROL group (p=0.05). Our preliminary findings suggest that COSBL in combination with PTCy and sirolimus might augment transplantation tolerance in children with SAA, probably due to synergistic effect on early recovery of Tregs.