Rapid in vivo loss of non-T-cell binding capacities of ATLG Grafalon® and ATG Thymoglobulin® explains selective effect on T-cell reconstitution after hematopoietic stem cell transplantation

Anti-thymocyte globulin (ATG Thymoglobulin®) and anti-T-lymphocyte globulin (ATLG Grafalon®) are commonly used in hematopoietic stem cell transplantation (HSCT) for in vivo T-cell depletion, aiming to reduce graft failure and graft-versus-host disease (GvHD). Despite their widespread use, the precise mechanisms by which ATG/ATLG affect immune recovery post-transplantation remain partially understood. In this study, in 289 pediatric HSCT patients treated with ATG (n = 213) or ATLG (n = 76), the relationship between longitudinal serum levels of the active T-lymphocyte-binding component of ATG/ATLG and immune reconstitution was evaluated. High levels of active ATG/ATLG did not prevent the recovery of neutrophils or NK-cells; however, T-cell recovery was delayed until active ATG/ATLG concentrations fell below 1 AU/mL. This is in apparent contradiction with the fact that ATG/ATLG are known not only to contain antibodies against antigens present on T-cells but also to antigens present on other immune and non-immune cells, including B-, NK-cells, granulocytes, monocytes and hematopoietic stem and progenitor cells (HSPC). To address this, we assessed the binding capacities of ATG and ATLG to immune cell subsets and HSPC both in vitro directly from the vial and in vivo from patients serum samples taken at multiple time points pre- and post-HSCT. Shortly after infusion, we observed a rapid reduction of ATG and ATLG binding to all cell types except for T-cells. This real-life specificity of ATG and ATLG explains their selective impact on T-cell reconstitution. These findings enhance our understanding of the mechanisms of action of ATG and ATLG and may contribute to optimization of these therapies.