Basic Research
Human thymic development and in vitro differentiation of human pluripotent cells into thymic epithelial cells
The thymus is an essential immune organ in which specialized thymic epithelial cells direct T cell maturation and specificity. Through positive and negative selection, the thymus gives rise to T cells with a diverse and self-tolerant T cell receptor repertoire. Thymic function naturally declines with age and is responsible for the waning immune competence (immunosenescence) and increased inflammation (inflammaging) in the elderly, which the COVID-19 pandemic exposed in tragic fashion. Congenital athymia caused by genetic defects of thymic development, e.g., in 22q11 Deletion Syndrome, causes life-threatening immunodeficiency and autoimmunity. Many more patients suffer from acquired thymic injury in the context of HSCT, chronic GVHD, cancer-directed therapies, or aging. Regenerating thymic function through iPSC-derived thymic epithelial cells would be a breakthrough medical advance for patients suffering from insufficient thymopoiesis, autoimmunity, and breach of central tolerance due to compromised thymic function for which no treatments exist to date. In addition, patients undergoing treatment with non-cell-based immunotherapies would greatly benefit from the synergistic effect of regenerative thymic tissues to boost T cell immunity and experience the benefits of, e.g. cancer vaccines, checkpoint blockade or T cell engagers to the fullest.
We have recently shown that type I and II interferons are indispensable for thymic epithelial cell differentiation by driving the expression of the proteasome, antigen-presenting molecules, and critical thymus-specific cytokines. These essential features of thymic epithelial maturity can be induced in culture (Mohammed et al., Science Immunology 2025). We have translated these insights into a differentiation platform that derives fully-functional thymic epithelial cells from iPSCs. When iPSC-derived thymic epithelial cells (iTECs) are transplanted into athymic NSG-Foxn1-/- mice that were “humanized” with human hematopoietic stem cells, iTECs promote the development of human abT cells, with a T cell receptor repertoire diversity that is indistinguishable from positive control animals transplanted with human fetal thymus. Moreover, iTECs promote the development of regulatory T cells, gdT cells, and induce tolerance to “self”. Therefore, iTECs execute all essential functions of the human thymus and have significant therapeutic potential that we seek to translate to patients with the highest urgency.
Metabolism during hematopoietic lineage development
An unresolved question in inborn errors of metabolism is why they lead to severe defects in some cells but not in others. One example is the primary immunodeficiency syndrome reticular dysgenesis, which presents with severe congenital neutropenia and T cell lymphopenia, while most hematopoietic stem and progenitor cells are well preserved. Reticular dysgenesis is caused by loss of mitochondrial adenylate kinase 2 (AK2) function. By coupling single-cell transcriptomics in reticular dysgenesis patient samples with metabolomic and isotope tracing studies in a human hematopoietic stem cell CRISPR model of this disorder, we have recently shown that the consequences of AK2 deficiency for the hematopoietic system are contingent on the engagement of metabolic checkpoints. The control of metabolic checkpoints such as mTOR, AMPK, and ACC is highly dynamic. In hematopoietic stem and progenitor cells, actively engaged metabolic checkpoints promote adaptive responses to AK2 deficiency such that mTOR signaling and anabolic pathways are reduced. This conserves nutrient homeostasis and maintains cell survival. In contrast, during late-stage granulopoiesis, metabolic checkpoints are ineffective, leading to a paradoxical upregulation of anabolic pathways in AK2-deficient cells. This results in nucleotide imbalance, including highly elevated AMP and IMP levels together with the depletion of essential substrates such as NAD+ and aspartate, ultimately causing proliferation arrest and demise of the granulocyte lineage. We have shown that even severe metabolic defects can be tolerated with the help of metabolic checkpoints, but that the failure of such checkpoints in differentiated cells results in a catastrophic loss of homeostasis (Wang et al. Blood, 2024).
Our ongoing work investigates metabolic checkpoint control during T cell development and how we can leverage this for therapeutic purposes.
