Regeneration of the adult thymus is preceded by the expansion of K5+K8+ epithelial cell progenitors and by increased expression of Trp63, cMyc and Tcf3 transcription factors in the thymic stroma

Abstract

Studies of HIV-1-infected individuals on anti-retroviral therapies and of patients receiving lymphoablating treatments indicate that the thymus retains restorative capacity even in adults. The contributions of the thymic epithelial cells (TECs) to the regeneration of the thymus and the identity of epithelial cell progenitors were evaluated in murine models of transient thymic atrophy followed by a complete regeneration. Using microarray approach, we analyzed the pattern of gene expression in TECs sorted from mice that were depleted of thymocytes by steroid treatment or by irradiation. The initial analysis identified significant increases in the mRNA for cMyc, Trp63 and Tcf3 transcription factors known to be expressed in early epithelial cell progenitors in tissues other than the thymus. Immunohistochemistry showed that in involuted thymuses, the cMyc and Trp63 proteins were expressed in a subset of cortical thymic epithelial cells (cTECs) that were keratin 5 positive (K5(+)), typifying cTEC precursors. Importantly, confocal microscopy established that epithelial cells with the phenotype of putative TEC progenitors (i.e. K5(+)K8(+)) expressed the Trp63 protein and confirmed that K5(+)K8(+) TEC progenitors expanded significantly during atrophy and prior to the thymic regeneration. Thus, our data demonstrated for the first time that critical steps in the recovery of the adult thymus include expansion of TEC progenitors and elevated expression of Trp63, cMyc and Tcf3 transcription factors in the thymic stroma. These results suggest that TEC progenitors could be reactivated in the adult thymus and, therefore, reactivation of TEC progenitors could provide a new approach for thymic reconstitution.