Acute Endotoxin-Induced Thymic Atrophy Is Characterized By Intrathymic Inflammatory and Wound Healing Responses

BACKGROUND: Productive thymopoiesis is essential for a robust and healthy immune system. Thymus unfortunately is acutely sensitive to stress resulting in involution and decreased T cell production. Thymic involution is a complication of many clinical settings, including infection, malnutrition, starvation, and irradiation or immunosuppressive therapies. Systemic rises in glucocorticoids and inflammatory cytokines are known to contribute to thymic atrophy. Little is known, however, about intrathymic mechanisms that may actively contribute to thymus atrophy or initiate thymic recovery following stress events. METHODOLOGY/PRINCIPAL FINDINGS: Phenotypic, histologic and transcriptome/pathway analysis of murine thymic tissue during the early stages of endotoxemia-induced thymic involution was performed to identify putative mechanisms that drive thymic involution during stress. Thymus atrophy in this murine model was confirmed by down-regulation of genes involved in T cell development, cell activation, and cell cycle progression, correlating with observed phenotypic and histologic thymus involution. Significant gene changes support the hypothesis that multiple key intrathymic pathways are differentially activated during stress-induced thymic involution. These included direct activation of thymus tissue by LPS through TLR signaling, local expression of inflammatory cytokines, inhibition of T cell signaling, and induction of wound healing/tissue remodeling. CONCLUSIONS/SIGNIFICANCE: Taken together, these observations demonstrated that in addition to the classic systemic response, a direct intrathymic response to endotoxin challenge concurrently contributes to thymic involution during endotoxemia. These findings are a substantial advancement over current understanding of thymus response to stress and may lead to the development of novel therapeutic approaches to ameliorate immune deficiency associated with stress events.