Our Research

Following periods of hematopoietic stress, patient outcomes are positively correlated with the degree of immune reconstitution, in particular of T cells, in various clinical conditions. The capacity of the T cells to mount and maintain effective responses to a wide variety of antigens depends on a large repertoire of unique T cell receptors (TCRs) generated in the thymus during the process of T cell development. This process is highly complex involving crosstalk between developing thymocytes and their supporting non-hematopoietic stromal microenvironment, which includes highly specialized thymic epithelial cells (TECs), endothelial cells, mesenchymal cells and hematopoietic cells (such as dendritic cells and macrophages). Paradoxically, to its crucial role in generating and maintaining a diverse immune repertoire, the thymus is particularly sensitive to acute injury due to common cancer therapies such as chemotherapy, immune or radiation therapy, infections, steroids and GVHD. These deleterious effects lead to disruption of stromal architecture, reduction of naive T cell output and peripheral T cell pool diversity. In addition, thymus undergoes progressive degeneration through life, which leads to degeneration of T-cell compartment and ultimately the restriction of the TCR repertoire diversity resulting in suboptimal immune responses, as observed in older individuals.

Therefore, the development of strategies to improve the regeneration of the immune system and reconstitution of the peripheral T cell pool represents a significant clinical challenge with the potential to improve overall outcome in immunocompromised recipients. Reconstitution of the hematopoietic system and, in particular of the process of the T cell development in the thymus, will increase TCR repertoire and improve patient responses to infections, vaccine and immunotherapy.

REGENERATION OF THYMIC FUNCTIONALITY

The thymus undergoes a severe involution with age. This decline has been in part accredited to the increase in sex hormones after puberty. Importantly, surgical or pharmacological ablation of sex hormones promotes thymic rejuvenation and immune reconstitution in transplanted patients and in mouse models of aging and acute immune insults. Previously, we have demonstrated that one of the mechanisms by which sex hormones regulate thymic function is through the inhibition of the Notch ligand Delta-like 4 (Dll4), a key gene involved in T cell commitment, proliferation and differentiation. Indeed, the administration of androgens decreases the expression of Dll4 in cortical thymic epithelial cells (cTECs). We also found that the androgen receptor (AR) directly binds to the Dll4 promoter and suppresses its expression. Consistent with this, pharmacological ablation of sex hormones, using a gonadotropin-releasing hormone antagonist, induces in cTECs the expression of Dll4 and of its downstream target genes Hes1, Ptcra and CD25 in the developing thymocytes. In addition, we also demonstrated that pharmacological hormonal ablation induces recovery of thymus and T cell immunity after radiation-exposure and murine models of allo-HSCT (Velardi et al. Journal of Experimental Medicine 2014 Nov 17;211(12):2341-9).

More recently, we characterized a novel pathway that mediates thymic regeneration after immunological damage. We can demonstrate that thymic endothelial cells (ECs) are actively involved in thymic reconstitution after insults through their production of Bone morphogenetic protein 4 (BMP4). We found that thymic ECs respond to thymic damage by increasing the production of BMP4, which triggers on thymic epithelial cells (TECs) the expression of Foxn1 – the master regulator of TEC proliferation and regeneration. These effects promote thymic recovery after radiation exposure (Wertheimer*, Velardi*, Tsai* et al Science Immunology. 2018 Jan 12;3(19)) .

BOOSTING HEMATOPOIETIC STEM CELL RECOVERY AFTER INJURIES

Recovery of adaptive and innate immunity is of paramount importance for survival after HSCT, and its failure contributes to the morbidity and mortality associated with transplantation. Although in recent years much has been learned on HSC transplantation, a better understanding on HSC response during periods of stress and on how the hematopoietic system is rebuilt post transplantation will help develop a safer and more effective therapy. There has been growing interest in how long-range effects of circulating sex hormones integrate HSC function with overall tissue physiology.

Recently, we identified a novel strategy to protect hematopoietic stem cells (HSCs) from exhaustion and promote hematopoietic regeneration after myeloablative treatments. We and others have shown that ablation of sex hormones promotes T and B lymphopoiesis in the bone marrow and thymus. In addition, it has been also shown that hormonal ablation promotes hematopoietic rejuvenation by mediating functional changes to HSC pool. We demonstrated that the expression of the receptor for the luteinizing hormone (LHR) is highly enriched in murine and human long term HSCs with nearly absent expression in downstream hematopoietic progenitor cells as well as in the stromal cell compartment. Functionally, we found that LH stimulation significantly enhanced mouse HSC colony formation in cobblestone area-forming cell and colony-forming cell assays. In addition, LH significantly expanded human HSCs in a stroma-free culture system, without impacting on their colony-forming potential. Given the direct effects of LH on HSCs, we investigated if blocking LH could attenuate HSC proliferation in two different models of hematopoietic stress that force HSCs out of their quiescent status. Pharmacological suppression of LH using a gonadotropin-releasing hormone antagonist inhibits HSCs from entry into cell cycle after Poly I:C and sub-lethal dose of total body irradiation exposure. Finally, we found that mice treated with the LHRH-Ant after lethal TBI exposure showed a significant increase in survival compared to control animals. (Velardi et al, Nature Medicine. 2018 Jan 8. doi: 10.1038/nm.4470).