Thymic Reconstitution after Damage Is Influenced By Zinc Status

Abstract

T cell reconstitution, which is dependent on thymus function, is significantly delayed after hematopoietic stem cell transplant; leading to morbidity and mortality due to infection and possibly even malignant relapse. Zinc, the second most abundant trace metal in the body, is crucial for thymic function: in fact, zinc deficiency of all causes lead to thymic atrophy with a consequent reduction in the number of circulating recent thymic emigrants (RTEs). However, the role of zinc in thymic reconstitution after chemotherapy and hematopoietic stem cell transplantation (HSCT) has not been investigated. Using a murine model of thymic function after acute damage, we found that mice showed reduced thymic cellularity after three weeks of a zinc deficient diet, and lower regenerative capacity after a sublethal dose of total body irradiation (SL-TBI) (Fig. a). Conversely, zinc supplementation in normal dietary condition caused increased thymic regeneration at time-points after a mouse model of allogeneic-HSCT (Fig. b). As a result, mice that received zinc supplementation demonstrated more naive T cells and more recent thymic emigrants in peripheral blood (Fig. c) after allo-HSCT. Mechanistically, we found that zinc supplementation could stimulates the proliferation of thymic epithelial cells (TECs), which are the main cell type responsible for supporting for T-cell development, likely via induction in the production of BMP4, a pro-regenerative cytokine produced by endothelial cells (EC) (Fig. d). In fact, treatment with the BMP-receptor antagonist dorsomorphin abrogated the effect of zinc supplementation (Fig. e), and ex vivo propagated ECs (exECs) were directly induced to produce BMP4 when stimulated in vitro for 24 hours with zinc sulfate (Fig. f). Following damage there is a switch in the availability of zinc from inside the cellular fraction to the extracellular fraction (Fig g), suggesting that zinc is normally used and stored in developing T-cells but after damage they release zinc in the extracellular space after cell death due to TBI. This release is enough to trigger the production of regenerating factors from radio resistant cells, such as EC. Consistent with this, we found higher levels of intracellular zinc in thymocytes from zinc-supplemented mice before TBI and higher levels of zinc in the extracellular fraction after damage (Fig h). Moreover, while there was no difference in BMP4 expression in cocultures of exEC in presence of SN from control and zinc-treated mice at day 0, BMP4 levels increased in the presence of SN harvested from mice that had previously received TBI (Fig i). In conclusion, we herein demonstrate that zinc is important for thymic regeneration, and zinc supplementation offers an innovative therapeutic strategy to improve T cell reconstitution in patients receiving allo-HSCT.