Thymic Epithelial Cells And Dendritic Cells Mediate Thymic Renewal Through CCL25 Production

Impaired thymopoiesis contributes to immune deficiency following hematopoeitic stem cell transplantation (HSCT). Clinical data suggest that thymic epithelial cell (TEC) damage from the preparative regimen may compromise thymic recovery. We have previously shown that TEC are critical for thymic renewal in the setting of androgen withdrawal. These UEA+ medullary TEC provide a niche for early thymic progenitor (ETP) entry and thymocyte development by increasing the production of CCL25. Following androgen withdrawal, CCL25 accelerates thymocyte development and enhances ETP entry. Blockade of this ligand completely abrogates these affects, preventing thymic renewal. We now demonstrate that cytotoxic preparative regimens may impair thymic reconstitution by damaging stroma, decreasing CCL25 availability. In this study, we examined the effects of cyclophosphamide (CY) (240 mg/kg/mouse) or total body irradiation in female mice on TEC and medullary dendritic cell (mDC) populations and CCL25 production. At ½ myeloablative dose of 750 cGy, using flow cytometry of TEC, we show that both CY and radiation significantly deplete UEA+ medullary TEC while sparing Ly51+ CD45- cells. However, the effects of these two agents differed with regards to the proportions of TEC within the Ly51+ fraction. Radiation led to an increase in fibroblasts (CD45- Ly51+ MHCII-) with a marked reduction of cortical TEC (CD45- Ly51+ MHCII+), while this TEC subset was spared following CY administration. Furthermore, mDC were decreased following radiation while unaffected by CY. After irradiation, these alterations significantly impair CCL25 production as shown by decreased mRNA production of TEC enriched preparations and thymocyte fractions (in which only mDC produce CCL25). In contrast, CCL25 production was unaffected following CY administration, consistent with the hypothesis that the spared TEC populations preserve CCL25 production. These data suggest that cytotoxic preparative regimens may impair thymic renewal by destroying TEC and mDC populations thereby reducing available CCL25, a ligand necessary and essential for thymic recovery. Furthermore, these data suggest that different preparative regimens may impair thymic recovery to varying degrees due to discrimination of injured TEC and mDC populations, and the preservation of thymic CCL25 production. Investigation of alternative preparative regimen agents may present an opportunity to improve thymic renewal following HSCT.