Boosting Thymus Function in Old Mice Restores Immune Defenses Against Infection

Thymic involution — the progressive shrinkage of the thymus with age — is one of the most well-documented but poorly understood contributors to immunological aging. By middle age, the thymus has already lost most of its T-cell-producing capacity, leaving older individuals dependent on long-lived peripheral T-cell pools that become increasingly skewed, exhausted, and less diverse. While this decline is widely believed to increase susceptibility to infections and reduce vaccine efficacy in the elderly, direct experimental proof had been lacking — until now.

Researchers from the National Cancer Institute (NIH) and collaborating institutions developed two complementary mouse models to directly test whether enhancing thymic function in aged animals could restore immune competence. In the first model, the transcription factor Myc was constitutively expressed in thymic epithelial cells (TECs) in middle-aged mice (12 months), maintaining thymic size and function that would otherwise decline. In the second model, Myc expression was inducible, allowing researchers to reverse established thymic involution in already-aged animals. Both models preserved or restored cortical and medullary TEC compartments and increased thymic cellularity compared to age-matched wild-type controls.

Flow cytometric analyses of peripheral blood and spleen revealed that both models significantly increased numbers of naïve CD4+ and CD8+ T-cells. Constitutive Myc expression maintained naïve T-cell counts comparable to young mice, while inducible Myc expression partially recovered naïve T-cell numbers even after involution had occurred. Critically, the enhanced naïve T-cell pool was functionally competent: mice with improved thymic function mounted significantly stronger T-cell-dependent antibody responses following immunization, including higher titers of antigen-specific IgG, compared to age-matched controls.

The most striking functional result was survival following infection with Toxoplasma gondii, an intracellular parasite that requires robust T-cell-mediated immunity to control. Aged wild-type mice showed high mortality after infection, consistent with immunosenescence. In contrast, aged mice with enhanced thymic function — either constitutive or inducible Myc expression — showed significantly reduced T-cell-associated mortality, with survival curves dramatically shifted in their favor. This finding provides direct causal evidence that thymic output, not just peripheral T-cell homeostasis, governs immune protection in old age.

Beyond naïve T-cell numbers, the study examined qualitative changes in the T-cell compartment. In aged mice, the regulatory T-cell (Treg) pool showed age-associated imbalances — including an elevated Treg-to-conventional T-cell ratio — that were partially corrected by enhanced thymic function. Furthermore, single-cell transcriptomic analyses revealed that conventional T-cells from aged mice with improved thymic function retained stronger Th1 transcriptional signatures (including higher expression of T-bet and IFN-γ pathway genes), which are critical for defense against intracellular pathogens. Age-matched controls showed characteristic erosion of these signatures. Together, these findings demonstrate that TEC-focused thymic regeneration strategies can not only increase naïve T-cell output but also qualitatively rebalance the aging immune landscape, with direct implications for vaccine development, infection control, and potentially cancer immunosurveillance in elderly populations.