Origin, phenotype and autoimmune potential of T cells in human immune system mice receiving neonatal human thymus tissue

Robust human immune system (HIS) mice are created using human fetal thymus tissue and hematopoietic stem cells (HSCs). A HIS mouse model using neonatal human thymus tissue and umbilical cord blood (CB) HSCs (NeoHu) was recently described. We improved the model by removing the native murine thymus, which can also generate human T cells, and demonstrated definitively the capacity of human T cells to develop in a grafted neonatal human thymus. Human T cells derived from the neonatal thymus tissue appeared in peripheral blood early post-transplantation and CB-derived T cells appeared later. Naïve T cells were demonstrated in peripheral blood but effector memory and T peripheral helper phenotypes predominated later, in association with development of autoimmunity in some animals. Treatment of thymus grafts with 2-deoxyglucose (2-DG) increased the proportion of stem cells derived from injected HSCs, delayed onset of autoimmune disease, reduced early T cell reconstitution, and reduced effector/memory T cell conversion. Younger neonatal human thymus tissue was associated with improved T cell reconstitution. While the NeoHu model bypasses the need for fetal tissue, it has yet to demonstrate equivalent reconstitution to fetal tissue, though 2-DG can improve results by removing native thymocytes prior to transplantation.

Fatty infiltration of the thymus in response to illness in the pediatric population

Under physiologic stress, glucocorticoids contribute to thymic involution. While steroids enhance fatty infiltration, this change has not been well studied in the pediatric thymus during illness. Evaluation of 130 thymuses from fetuses, infants and children determined the frequency of thymic fatty infiltration to be low (25%), particularly in fetal thymus (4%). In most cases, fatty infiltration was focal. There was a significant correlation with duration of illness, but not with patient age, gestational age, or underlying disease. There was significantly less fatty infiltration in fetal thymus compared to post-natal thymus, for the same degree of thymic involution. Only seven cases showed diffuse thymic fatty infiltration; all were post-natal associated with an infectious etiology. In contrast, fetal cases of chorioamnionitis seldom showed fatty infiltration and only focally, implying the stress response of fetal thymus differs post-natal, possibly related to the timing of adipose tissue development and fetal glucocorticoid response to stress.