Thymus

The thymus is the primary lymphoid organ where T-cells mature. It sits in the anterior mediastinum and is the sole site of new naive-T-cell production from bone-marrow-derived precursors, via positive and negative selection on thymic epithelial cells (TECs).

Thymic involution — early, dramatic organ aging

Thymic involution — the progressive replacement of functional thymic epithelial parenchyma with adipose tissue, beginning at puberty — is one of the earliest and most dramatic organ-aging processes and the central mechanistic driver of immunosenescence. As naive-T-cell output falls, the adaptive repertoire narrows, impairing responses to new antigens and vaccines. The molecular driver is sex-steroid suppression of TEC proliferation (androgen-receptor signaling represses FOXN1, the master TEC transcription factor); thymic contribution to the naive-T-cell pool falls from ~16% in young adults to <1% in old age (see disabled-adaptive-immunity § Thymic involution for the mechanistic detail, conserved-in-humans table, and regeneration interventions). Thymic regeneration is an active rejuvenation target.

Thymic health is prognostic for mortality and cancer outcomes

The adult thymus was long dismissed as a metabolically inert post-childhood remnant. Two 2026 Nature companion papers (Bernatz, Aerts, Birkbak et al.) overturned this by training a deep-learning model to score “thymic health” from routine chest CT (size, shape, and the parenchyma-vs-fat composition) and validating it at population scale ¹²:

• In ~27,600 asymptomatic adults (National Lung Screening Trial + Framingham Heart Study), higher thymic health was associated with ~50% lower all-cause mortality, ~63% lower cardiovascular mortality, and ~36% lower lung-cancer incidence (adjusted for age, sex, smoking, comorbidities). The cardiovascular-mortality association replicated in the independent Framingham cohort. gap/no-fulltext-access
• In a pan-cancer immune-checkpoint-inhibitor cohort (n≈3,476), higher thymic health predicted better immunotherapy outcomes in NSCLC, independent of PD-L1 and tumor mutational burden — and correlated with TCR repertoire diversity and T-cell-receptor excision circles (TRECs) in TRACERx, confirming the CT score indexes genuine thymic function. gap/no-fulltext-access

This repositions the thymus as a measurable axis of immune aging with hard-endpoint relevance, and introduces a candidate opportunistic-imaging biomarker — see thymic-health-score. Associations are observational (not causal): a healthier thymus may partly mark better systemic health rather than drive it. gap/no-mechanism

See also

• thymic-health-score — the CT-derived imaging biomarker introduced by the 2026 papers
• immune-system · immunosenescence · disabled-adaptive-immunity
• bernatz-2026-thymic-health-adults · bernatz-2026-thymic-health-immunotherapy

Footnotes

1. bernatz-2026-thymic-health-adults · n≈27,612 (NLST + Framingham) · observational imaging cohort · model: deep-learning thymic-health scoring on chest CT; humans · ~50% lower all-cause / ~63% lower CV mortality, ~36% lower lung-cancer incidence (high vs low) · closed-access; unverified against full text gap/no-fulltext-access ↩

2. bernatz-2026-thymic-health-immunotherapy · n≈3,476 pan-cancer ICB cohort + TRACERx validation · observational · model: deep-learning on CT; humans · thymic health predicts NSCLC immunotherapy outcomes independent of PD-L1/TMB; correlates with TCR diversity + TRECs · closed-access; unverified against full text gap/no-fulltext-access ↩