Whole-Thorax Irradiation Induces Persistent T Cell Clonal Dysregulation in Pediatric Rhesus Macaques

The thymus is critical for the development and selection of T cells with a diverse range of non-self-reactive antigen receptors. Both the thymus and circulating T cells can be damaged by acute exposure to ionizing radiation, leading to dose-dependent lymphopenia, a temporarily increased risk of infection that can be life-threatening, and long-term disruptions in T cell homeostasis and function. Currently, there are no biomedical countermeasures available to prevent radiation-induced T cell lymphopenia or other T cell defects caused by radiation. Therefore, preclinical models of radiation-induced thymic injury are necessary for testing countermeasures. Adult mice and non-human primates (NHP) that are subjected to whole-body or thorax irradiation are suitable models for this purpose. However, findings from these models may not directly apply to juveniles, given the significant changes in thymus size and function during childhood. To address this, we characterized the effects of 10 Gy whole-thorax irradiation on the thymus of pediatric rhesus macaque NHPs. Computed tomography (CT) assessments of thymic density and volume were used as in vivo indicators of thymic injury, but they did not correlate with the changes in thymic weight observed 19 weeks after irradiation. Histopathological staining revealed that whole-thorax irradiation caused disruption of thymic architecture, evident four months post-irradiation in some animals. Molecular analyses showed that radiation led to a decrease in thymic output, reduced diversity of T cell antigen receptors, and an over-representation of certain receptor sequences indicative of extensive clonal expansion. Overall, this work demonstrates the usefulness of the NHP whole-thorax irradiation model-commonly employed in lung radiobiology research-in studying radiation-induced thymic injury in children and in developing medical countermeasures.