Activation of the zinc-sensing receptor GPR39 promotes T-cell reconstitution after hematopoietic cell transplant in mice.

Prolonged lymphopenia represents a major clinical problem after cytoreductive therapies such as chemotherapy and the conditioning required for hematopoietic stem cell transplant (HCT), contributing to the risk of infections and malignant relapse. Restoration of T-cell immunity depends on tissue regeneration in the thymus, the primary site of T-cell development, although the capacity of the thymus to repair itself diminishes over its lifespan. However, although boosting thymic function and T-cell reconstitution is of considerable clinical importance, there are currently no approved therapies for treating lymphopenia. Here we found that zinc (Zn) is critically important for both normal T-cell development and repair after acute damage. Accumulated Zn in thymocytes during development was released into the extracellular milieu after HCT conditioning, where it triggered regeneration by stimulating endothelial cell production of BMP4 via the cell surface receptor GPR39. Dietary supplementation of Zn was sufficient to promote thymic function in a mouse model of allogeneic HCT, including enhancing the number of recent thymic emigrants in circulation although direct targeting of GPR39 with a small molecule agonist enhanced thymic function without the need for prior Zn accumulation in thymocytes. Together, these findings not only define an important pathway underlying tissue regeneration but also offer an innovative preclinical approach to treat lymphopenia in HCT recipients.

Damage-induced pyroptosis drives endog thymic regeneration via induction of Foxn1 by purinergic receptor activation.

Endogenous thymic regeneration is a crucial process that allows for the renewal of immune competence following stress, infection or cytoreductive conditioning. Fully understanding the molecular mechanisms driving regeneration will uncover therapeutic targets to enhance regeneration. We previously demonstrated that high levels of homeostatic apoptosis suppress regeneration and that a reduction in the presence of damage-induced apoptotic thymocytes facilitates regeneration. Here we identified that cell-specific metabolic remodeling after ionizing radiation steers thymocytes towards mitochondrial-driven pyroptotic cell death. We further identified that a key damage-associated molecular pattern (DAMP), ATP, stimulates the cell surface purinergic receptor P2Y2 on cortical thymic epithelial cells (cTECs) acutely after damage, enhancing expression of Foxn1, the critical thymic transcription factor. Targeting the P2Y2 receptor with the agonist UTPγS promotes rapid regeneration of the thymus in vivo following acute damage. Together these data demonstrate that intrinsic metabolic regulation of pyruvate processing is a critical process driving thymus repair and identifies the P2Y2 receptor as a novel molecular therapeutic target to enhance thymus regeneration.

Attenuation of apoptotic cell detection triggers thymic regeneration after damage.

The thymus, which is the primary site of T cell development, is particularly sensitive to insult but also has a remarkable capacity for repair. However, the mechanisms orchestrating regeneration are poorly understood, and delayed repair is common after cytoreductive therapies. Here, we demonstrate a trigger of thymic regeneration, centered on detecting the loss of dying thymocytes that are abundant during steady-state T cell development. Specifically, apoptotic thymocytes suppressed production of the regenerative factors IL-23 and BMP4 via TAM receptor signaling and activation of the Rho-GTPase Rac1, the intracellular pattern recognition receptor NOD2, and micro-RNA-29c. However, after damage, when profound thymocyte depletion occurs, this TAM-Rac1-NOD2-miR29c pathway is attenuated, increasing production of IL-23 and BMP4. Notably, pharmacological inhibition of Rac1-GTPase enhanced thymic function after acute damage. These findings identify a complex trigger of tissue regeneration and offer a regenerative strategy for restoring immune competence in patients whose thymic function has been compromised.