Kinetics of endogenous thymic regeneration following chemotherapy-induced involution

Abstract The thymus is a primary lymphoid organ where T cell development occurs. Besides the naturally transpiring age-dependent involution, the thymus also undergoes acute involution from cytotoxic stimuli. Establishing thymic regeneration strategies would, thus, be immensely critical for pediatric cancer and transplantation patients receiving cytoablative chemotherapies. Several signaling pathways (e.g., KGF, BMP4) promote thymic regeneration, albeit with suboptimal therapeutic potential, mainly due to poor insight of the detailed mechanisms of endogenous thymic repair. To address this issue, we developed a mouse model of cyclophosphamide-induced thymic involution and monitored thymic reconstitution for up to 5 weeks post-treatment. Macroscopic assessment via thymic weight index and MRI-based volumetric analysis, along with histopathology, revealed that endogenous thymic regeneration is a non-linear, multistep process, consisting of a: (a) Reconstitution phase, during which recuperation occurs at an exponential rate, (b) Rebound phase, in which abnormal thymic hyperplasia is observed, and (c) Recoil phase, where thymic homeostasis ensues. Quantification of peripheral blood lymphocytes, however, revealed that thymic hyperplasia at Rebound does not correlate with restored thymic functions and output. Detailed investigation via flow cytometry revealed the stalling of T cell development at the Double Positive (CD4 +CD8 +) stage, implying cortical defects during thymic repair. Currently, we employ single-cell RNA sequencing with single cell resolution imaging, to define the molecular pathways disrupting cortical Thymic Epithelial Cell (cTEC) renewal post-chemotherapy, leading to impaired thymopoiesis. Supported by AAI's Career in Immunology Fellowship PI's funding supported by Albert Einstein College of Medicine: New Investigator Start-up funds