• T cells
• thymus
• immune reconstitution
• zinc
• bone marrow transplant

Prolonged T cell reconstitution after allogeneic hematopoietic stem cell transplant (allo-HSCT) is an important contributor to transplant-related morbidity and mortality due to infection and malignant relapse. Therefore, strategies to enhance thymic reconstitution in allo-HSCT recipients are clinically desirable, although currently limited. Zinc, the second most abundant trace metal in the body, plays an important role in T cell homeostasis and thymic function. Zinc deficiency negatively impact intrathymic maturation of thymocytes and also impairs the spontaneous capacity of regeneration of the thymus after acute damage. In a mouse model of allo-HSCT, we demonstrated that zinc supplementation can significantly improve thymic regeneration. Importantly, these findings in thymus were translated to the periphery as mice that received zinc supplementation showed increased numbers of naïve T cells as well as increased recent thymic emigrants (demonstrated using RAG2-GFP BM donors) model 5-8 weeks after allo-HSCT. We have previously demonstrated that endothelial cells (EC), which are extremely resistant to damage, can promote endogenous thymic regeneration after acute injury via their production of BMP4, a growth factor that targets thymic epithelial cells (TECs), a key population crucial for T cell development. Interestingly, when stimulated in vitro for 24 hours with zinc sulphate, ECs could be directly induced to produce BMP4, but not when exposed to increased zinc import with the cell-permeable zinc pyrithione.
This latter finding suggests a role for extracellular zinc in stimulating the endogenous response to damage. To explore this, we first measured the content of zinc in whole mouse thymus by mass spectrometry. Interestingly, when we examined a lysate of the entire thymus, total zinc concentration sharply declined early after total body radiation (TBI), followed by a steady increase that mirrored the reconstitution of the thymic cellularity. However, if we looked at zinc in the extracellular fraction of thymic dissociation, we saw that zinc increased significantly after TBI, revealing an inverse correlation with thymic cellularity , and providing a rationale for how zinc might contribute to the endogenous regenerative response. Given these findings, we hypothesized that zinc is normally used and stored in the T-cell precursors, a population of highly-replicating cells that account for approximately 98% of thymic cellularity in young mice and require the import of intracellular zinc for their proliferation. T-cell precursors are radio-sensitive and might release zinc in the extracellular space after cell death due to TBI, thereby triggering the production of regenerating factors from radio resistant cells, such as EC. Zinc supplementation could help this loop by increasing endogenous zinc levels. This hypothesis was confirmed when we co-cultured EC in presence of thymic SN there was no difference in BMP4 expression in cocultures with SN from control and zinc-treated mice at day 0, whereas BMP4 increased in presence of SN harvested from mice that had previously received TBI and even more when mice also received zinc supplement.
In conclusion, our findings demonstrate that zinc supplementation can improve T-cell regeneration in mice receiving allo-HSCT by reinforcing endogenous mechanisms of thymic regeneration. These results could be readily clinically translated into better outcomes for recipients of allo-HCT.