Hypothesis

In aged T cells, progressive loss of TET enzymatic activity reduces 5hmC at recombination signal sequences (RSS) flanking V, D, and J gene segments. This loss permits DNMT3A‑mediated de novo methylation of CpG motifs within RSS, impairing RAG1/2 binding and skewing the TCR repertoire toward autoreactive clones. Restoring TET‑generated 5hmC—either by genetic rescue or vitamin C supplementation—will preserve RSS hypomethylation, maintain diverse TCR repertoires, and mitigate age‑related immune dysfunction.

Mechanistic Model

• TET as a protective oxidant: TET1/2/3 convert 5mC to 5hmC at RSS, creating a steric and electrostatic barrier that inhibits DNMT3A access to adjacent CpGs (based on the observation that immune tissues are globally low‑5hmC yet retain locus‑specific 5hmC marks) {[https://pubmed.ncbi.nlm.nih.gov/34158086/]}.
• DNMT3A encroachment: When TET activity wanes, DNMT3A methylates CpGs within the heptamer and nonamer of RSS, a modification known to reduce RAG binding affinity in vitro. This links the observed increase in global DNA methylation during T‑cell aging to a specific defect in V(D)J recombination.
• Non‑catalytic scaffolding: TETs may also recruit FOXO1/RUNX1 to RSS regions, stabilizing an open chromatin conformation; loss of TET disrupts this complex, compounding the effect of DNA methylation {[https://pubmed.ncbi.nlm.nih.gov/38146185/]}.
• Vitamin C rescue: As a cofactor for TET dioxygenase activity, ascorbate can restore 5hmC levels even in catalytically compromised TETs, thereby preventing DNMT3A methylation and preserving RSS accessibility {[https://www.stjude.org/media-resources/news-releases/2024-medicine-science-news/immune-cell-epigenetic-clock-ticks-independently-of-organism-lifespan.html]}.

Experimental Design

1. Mouse models: Generate Tet2/3 conditional knockout (cKO) in mature T cells using Cd4‑CreERT2; compare to wild‑type (WT) littermates at 6 mo (young) and 20 mo (aged).
2. Vitamin C treatment: Provide aged mice with 2 g/L ascorbic acid in drinking water for 8 weeks.
3. Readouts:
   • 5hmC mapping: Oxidative bisulfite sequencing (oxBS‑seq) focused on RSS of TCRβ V, D, J segments.
   • DNA methylation: Parallel BS‑seq to quantify CpG methylation at the same RSS.
   • RAG occupancy: CUT&RUN for RAG1 to assess binding efficiency.
   • TCR repertoire: High‑throughput V(D)J sequencing to evaluate clonality, diversity indices, and frequency of autoreactive clonotypes (predicted by self‑peptide–MHC binding).
   • Functional assays: In vitro stimulation for cytokine production; in vivo challenge with Listeria monocytogenes to gauge infection‑induced expansion.
4. Predictions:
   • Aged Tet2/3 cKO will show increased RSS methylation, decreased 5hmC, reduced RAG1 binding, and a contracted TCR repertoire with enrichment of clones bearing putative self‑reactivity.
   • Vitamin C‑treated aged WT mice will retain RSS 5hmC levels comparable to young mice, maintain RAG1 occupancy, and preserve repertoire diversity.
   • Rescue of TET activity (via vitamin C) in Tet2/3 cKO will partially restore RSS hypomethylation and improve TCR diversity, confirming that the effect is mediated through TET‑dependent 5hmC rather than off‑target antioxidant actions.

Falsifiability

If RSS methylation does not increase with age or TET loss, or if vitamin C fails to alter 5hmC levels at RSS without rescuing TCR diversity, the hypothesis would be refuted. Conversely, confirming the predicted epigenetic and repertoire changes would support a direct link between TET‑mediated 5hmC dynamics, RSS integrity, and age‑associated immune decline.