Preclinical Canine Model of Graft-versus-Host Disease after In Utero Hematopoietic Cell Transplantation

•GVHD induced by IUHCT has a postnatal onset.•Its phenotype is similar to GVHD after bone marrow transplantation.•GVHD potential depends on T cell concentration of the donor graft.•Grafts containing 1% to 3% T cells allow long-term chimerism without GVHD.•This study demonstrates a safe therapeutic index for IUHCT. In utero hematopoietic cell transplantation (IUHCT) offers the potential to achieve allogeneic engraftment and associated donor-specific tolerance without the need for toxic conditioning, as we have previously demonstrated in the murine and canine models. This strategy holds great promise in the treatment of many hematopoietic disorders, including the hemoglobinopathies. Graft-versus-host disease (GVHD) represents the greatest theoretical risk of IUHCT and has never been characterized in the context of IUHCT. We recently described a preclinical canine model of IUHCT, allowing further study of the technique and its complications. We aimed to establish a threshold T cell dose for IUHCT-induced GVHD in the haploidentical canine model and to define the GVHD phenotype. Using a range of T cell concentrations within the donor inoculum, we were able to characterize the phenotype of IUHCT-induced GVHD and establish a clear threshold for its induction between 3% and 5% graft CD3+ cell content. Given the complete absence of GVHD at CD3 doses of 1% to 3% and the excellent engraftment with the lowest dose, there is a safe therapeutic index for a clinical trial of IUHCT.