Turning the clock: mRNA-encoded thymic factors restore aged immunity

In a recent study published in Nature by Friedrich et al. demonstrate that mRNA-encoded delivery of DLL1, FLT3L, and IL-7 to the liver rejuvenates aged immunity, improving vaccine responses and antitumor immunity in mice. 1 This work introduces a safe, transient approach to reversing immunosenescence by reconstituting a youthful signaling niche without inducing autoimmunity as shown in Fig. 1.Ageing weakens adaptive immunity through thymic involution, diminished naïve T cell output, and accumulation of dysfunctional memory and exhausted T cell states, leading to increased susceptibility to infections, poor vaccine responses, and cancer immunotherapy failure. 2 Recent studies have rigorously established that age-related thymic dysfunction is a primary driver of declining T cell immunity: enhancing thymic function through RANKL treatment or via Myc-driven thymic epithelial cell expansion restores T cell production and improves immune responses in aged mice. 3,4Previous strategies to reverse immune ageing, such as hormone therapy, cytokine administration, or heterochronic parabiosis, have been limited by toxicity, modest effects, or poor clinical feasibility.A central challenge has been how to safely and transiently restore the youthful thymic signaling milieu systemically without inducing autoimmunity or chronic inflammation. 5 Friedrich et al. first mapped age-dependent immune signaling by performing single-cell and spatial transcriptomics of thymic tissue and peripheral blood T cells across the murine lifespan.They identified a pronounced decline in interactions mediated by DLL1, FLT3L, and IL-7 factors critical for T cell development, dendritic cell maintenance, and lymphocyte survival.As a result, intrathymic and circulating levels of these factors declined with age. 1 In immunological homeostasis, DLL1, FLT3L, and IL-7 play different but complimentary roles.Early T cell development is hampered by the age-related reduction of DLL1, a canonical Notch ligand that guides T cell lineage commitment from hematopoietic progenitors within the thymus.Common lymphoid progenitors (CLPs) and conventional type 1 dendritic cells (cDC1s), which are crucial for antigen cross-presentation, proliferate when FLT3L acts on bone marrow progenitors. 2IL-7, a non-redundant homeostatic cytokine, supports the survival and proliferation of developing thymocytes and peripheral naïve T cells, and its reduced bioavailability with age compromises both thymic output and peripheral T cell maintenance.The coordinated decline of these three factors provided a rational basis for the combinatorial DFI intervention.To reconstitute these signals without relying on the involuted thymus, 6 the authors repurposed the liver as a transient protein