Impact of ATLG on CD4⁺ T-cell reconstitution after HSCT in children: a detailed immune profiling study

Graft versus host disease (GvHD) prophylaxis impacts on post-hematopoietic stem cell transplantation (HSCT) immune reconstitution (IR). Specifically, anti-T lymphocyte globulin (ATLG) delays CD4⁺ T cell reconstitution. However, the mechanism by which ATLG alters the composition of CD4⁺ T-cell compartments remains elusive.

Dissect the impact of ATLG on CD4⁺ T cells by integrating advanced and highly standardized flow cytometric panels, unsupervised clustering analysis, machine learning approaches, and molecular biology techniques.

We analyzed post-HSCT IR in 94 pediatric patients with a machine learning approach. ATLG exposure resulted in the most relevant variable affecting CD4⁺ T-cell counts. Severe combined immunodeficiency/recent thymic emigrant (SCID/RTE) and CD4 PERISCOPE EuroFlow antibody panels were applied to a sub-cohort of patients with acute lymphoblastic leukemia. Unsupervised clustering analysis showed that in the first months, post-HSCT CD4⁺ naïve T cells and RTEs were significantly reduced in ATLG-treated compared to no-ATLG patients, including haploidentical graft recipients receiving post-transplant cyclophosphamide. In no-ATLG patients, there was a significant contribution of residual recipient-derived cells to the RTE compartment, whereas in ATLG-treated patients RTEs were of donor origin. In addition, in no-ATLG patients, early RTEs and CD4⁺ T-cell counts correlated with infused CD3⁺ T cells/kg. Bone marrow and T-cell receptor excision circle (TREC) analysis showed that ATLG treatment does not affect early T-cell maturation. In vitro testing showed increased differentiation of naïve/RTE CD4⁺ into effector subsets upon ATLG exposure, confirmed by flow cytometry and RNA sequencing.

Our data show that ATLG induces concomitant differentiation and depletion of the peripheral CD4⁺ naïve/RTE T-cell compartment. Together, these findings suggest that naïve T-cell reconstitution after ATLG is delayed since it only relies on thymic maturation, differently from no-ATLG treated patients.