Thymic regeneration in female mice following short-term rapamycin administration

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#1

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A team of Chinese researchers has discovered that a short burst of the longevity drug rapamycin temporarily shrinks the thymus—the immune system’s training ground—but then triggers an unexpected wave of regeneration that leaves the organ larger and seemingly more youthful than before. The findings, published in the European Journal of Pharmacology, could help explain rapamycin’s anti-aging reputation and point toward new ways to rejuvenate the immune system.

The thymus, located just behind the breastbone, is where immature immune cells mature into T cells that defend the body against infection and cancer. The gland naturally atrophies with age, contributing to weakened immunity in older adults. In the new study, scientists at Zhejiang Chinese Medical University treated young female mice with low-dose rapamycin (1 mg/kg per day for three days), a compound known to inhibit the mTOR signaling pathway and extend lifespan in many species.

Immediately after treatment, the thymus shrank by nearly half. Microscopy revealed loss of the tissue’s intricate architecture—particularly the medullary and subcapsular thymic epithelial cells that guide T-cell development. The number of developing T cells dropped sharply, and telomeres, the protective DNA caps associated with cellular youth, grew shorter.

But within two weeks of stopping rapamycin, the thymus bounced back—exceeding its original size by about 30 percent. Regenerating glands showed renewed cellular organization, lengthened thymocyte telomeres, and restored populations of double-positive and CD8-single-positive T cells. Gene analyses indicated renewed activity of Foxn1 and Klf6, transcription factors linked to thymic epithelial repair, along with elevated Sirt3 and Sirt6, two longevity-associated genes.

The researchers also tested whether two other compounds with anti-aging reputations—interleukin-7 (IL-7) and metformin—could enhance this recovery. While both agents improved certain molecular or immune markers, neither accelerated overall thymic regrowth. IL-7 lengthened telomeres and improved peripheral T-cell balance, whereas metformin boosted mitochondrial health and the positive selection of maturing T cells.

Taken together, the data suggest that rapamycin induces a kind of “controlled stress” in the thymus, prompting the organ to rebuild itself stronger and potentially rejuvenate immune function. The study was limited to young female mice and a short 14-day observation window, so it remains unclear whether similar effects occur in aged animals or humans. Still, the work adds a striking piece to the puzzle of how mTOR inhibitors might not just slow aging—but reset parts of the body’s immune machinery to a more youthful state.

Source Paper (Paywalled): Kinetics of thymic regeneration in female mice following short-term rapamycin administration

European Journal of Pharmacology, Volume 1003, 15 September 2025, 177960

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#2

And a more in-depth analysis of the full paper:

Summary

This 2025 study, “Kinetics of thymic regeneration in female mice following short-term rapamycin administration,” by Zhang et al. (Zhejiang Chinese Medical University) investigates how brief exposure to rapamycin (RAPA) influences thymic structure, function, and subsequent regeneration. Female BALB/c mice received 1 mg/kg/day RAPA intraperitoneally for 3 days, and the thymus was analyzed during the involution phase (Day 3) and regeneration phases (Days 3 and 14 after withdrawal). The study also examined whether IL-7 or metformin (MET) could augment thymic recovery.

Key findings:

  • RAPA caused acute thymic involution, with ~46% weight loss and cortical/medullary disruption, loss of medullary (mTECs) and subcapsular (sTECs) epithelial cells, reduced double-positive (DP CD4⁺CD8⁺) thymocytes, and shortened thymocyte telomeres.
  • During recovery, the thymus over-regenerated to ~130% of baseline mass within 14 days, showing medullary expansion, DP and CD8⁺ SP T-cell rebound, normalized Foxn1/Klf6 expression, and extended thymocyte telomeres.
  • IL-7 modestly promoted telomere elongation and peripheral T-cell recovery, but did not accelerate volumetric thymic regeneration.
  • MET enhanced positive selection (increased Gimap4, Sirt3 expression) and mitochondrial homeostasis, but similarly failed to boost regeneration speed.
  • The authors conclude that RAPA triggers transient thymic suppression followed by intrinsic regeneration, potentially explaining its systemic immune-rejuvenating and anti-aging effects.

Novelty

  1. Temporal kinetics of thymic remodeling: Previous studies established rapamycin’s general immunomodulatory or rejuvenating effects, but this is the first systematic mapping of thymic involution–regeneration dynamics following short-term RAPA exposure. The demonstration that the thymus rebounds beyond baseline mass (~130%) within two weeks is a novel quantitative finding.
  2. Linking telomere dynamics to thymic recovery: The observation that RAPA initially shortens thymocyte telomeres but that regeneration later extends them—especially under IL-7 co-treatment—is a new mechanistic layer connecting mTOR inhibition to cellular aging and DNA maintenance.
  3. IL-7 and metformin as modulators of post-RAPA recovery: The combined exploration of IL-7 and MET interventions, showing distinct mechanistic roles (IL-7 → telomere and T-cell homeostasis; MET → mitochondrial function and selection gene expression), adds translational relevance for thymic rejuvenation strategies.
  4. Identification of transcriptional markers of regeneration: The integration of Foxn1, Klf6, Gimap4, and Sirt3/Sirt6 expression data provides a molecular timeline for epithelial-thymocyte crosstalk during regeneration.

Critique

Strengths:

  • Comprehensive multimodal design: Combines histology, immunofluorescence, flow cytometry, TEM, and gene expression profiling.
  • Mechanistic integration: The paper links mTOR signaling, TEC biology, telomere integrity, and peripheral immune profiles coherently.
  • Clear temporal resolution: The Day 3 vs Day 14 framework effectively demonstrates biphasic thymic behavior (acute suppression → rebound).

Limitations:

  1. Short observation window: Only 14 days of recovery were tracked. It remains unknown whether the over-regeneration is sustained, or followed by secondary involution—important for assessing long-term anti-aging relevance.
  2. Single sex and age group: All subjects were 6–8-week female mice; results may not generalize to males or aged cohorts where thymic atrophy is more severe and regenerative potential lower.
  3. Functional immunity not directly tested: The study infers improved immune competence from thymic and peripheral markers, but did not measure antigen-specific T-cell responses, infection resistance, or vaccination outcomes.
  4. Pharmacologic realism: The RAPA dose (1 mg/kg × 3 days i.p.) produces higher systemic exposure than typical human low-dose regimens (e.g., 3–6 mg weekly orally). Translational extrapolation to clinical dosing remains uncertain.
  5. Mechanistic ambiguity: While mRNA data suggest up-/down-regulation of Foxn1, Klf6, Sirt3/6, etc., causality is not demonstrated; loss-of-function or knockdown models would strengthen conclusions about these pathways.
  6. Lack of systemic mTOR activity readouts: The study never confirmed mTORC1/2 inhibition in thymic tissue (e.g., via p-S6 or p-Akt assays), which would clarify whether effects were directly mTOR-mediated or secondary to metabolic stress.

Overall Evaluation

This paper provides the first kinetic framework for thymic degeneration and rebound following short-term rapamycin, demonstrating that temporary suppression can paradoxically lead to durable structural and molecular rejuvenation. Its translational implication—that transient mTOR inhibition might “reset” thymic architecture—is conceptually valuable for immune rejuvenation and anti-aging therapy design.

However, the lack of long-term follow-up, male/aged cohorts, and functional immune testing limit its clinical extrapolation. Future work should integrate lineage-tracing, phospho-mTOR readouts, and functional assays of T-cell repertoire diversity and pathogen response.

The paper does not explicitly state the total number of mice used, but it provides sample sizes (“n”) for each experimental comparison, from which the approximate total can be inferred.

From the Methods (Section 2.1) , there were 13 experimental groups :

Nor, Rap, RR1, RR2, RI1, I1, RI2, I2, RML1, RML2, RMH1, RMH2, ML1, ML2 .

Each group corresponds to one treatment/timepoint combination (RAPA ± IL-7 ± MET at 3 days or 14 days).

Across the Results sections, the figures consistently report:

  • n = 6 mice for most histological, mRNA, and telomere assays (Figs 1 – 6)
  • n = 3 for flow-cytometry subsets (T-cell phenotyping) and some peripheral-blood assays.

Given that multiple endpoints were measured from the same animals per group (not separate cohorts), the main analysis likely used ~6 mice per group.

:white_check_mark: Approximate total:

13 groups × 6 mice ≈ 78 mice total,

with some sub-assays (flow cytometry) using 3 mice per group.

So, while the paper never provides a single total-animal count, the data indicate that around 70–80 female BALB/c micewere used in total.

Journal quality snapshot

The article appears in European Journal of Pharmacology (Elsevier). Current public metrics (2024/25) place it in a solid, field-standard tier:

  • Journal Impact Factor (Web of Science, 2024): ~4.7.
  • SCImago SJR (2024): 1.197; Quartile: Q1 in Pharmacology & Pharmacy.
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#3

Nice find. TBH, I am a bit disappointed they just did only female mice? What the hell? Seems you could have some male mice in tandem.

Of course, to some degree, one would assume some benefits to males too, but why not include in the test - 50/50 male and female. We know in rapamycin trials with male mice - that they respond on a heavier dose of rapamycin… maybe even better than female mice.

Makes me wonder some about Greg Fahy’s TRIMM Trials.

Fahy felt in his interview with Matt Kaeberlein that rapamycin would have a negative affect on thymus regeneraton - this would point to evidence not so.

I am in my 5th year of rapamycin and now HGH for 9 months. Bio-markers are saying all is cool with both medications as of July 2025.

I will be doing a total work blood work up in January 2026 - Cornonary Calcium Scan, DEXA body index and bone density. Glycan inflammation and DNA Methylation. Will be curious to see where I am at.

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#4

Biting tongue hard not to hit send on my snarky comment about the poor neglected white straight man being left out of science :kissing_heart: :kissing_heart:

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#5

Agreed, go back 10-20 years and pretty much ALL animal studies were only in males.

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#6

Kinetics of thymic regeneration in female mice following short-term rapamycin administration1-compressed.pdf (4.2 MB)

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#8

Another promising result in rodent medicine. Looking forward to follow-up studies in humans.

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#9

it is VERY interesting. Especially, it would make the basic 6mg once a week protocol totally obsolete. We would need more dose, maybe over a week, and then stop for a long time to all the thymus regenerate fully ?

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#10

1 day for mice equals a week for humans, assuming of course that the mice in the trial were injected once a day.

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#11

Yes… mice age 30 years to 1 human year.
I thought the same thing on this study initially. Treated 3 days… hardly a start…but that is equivalent to 3 human months.

14 days later retest after stopping is a year and 2 months.

When I do my full blood work this January. … and total body scans for everything… I will decide if I need to tweak my protocol based on results.

#12

So Jason, are you still taking the Rapa whilst doing this HGH experiment?

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#13

Hey Rob,

Yes, I am doing rapamycin and HGH. At 9 months on HGH and 4 years 3 months on rapamycin.

After the Kaeberlein and Fahy Optispan interview… I was with Matt on rapamycin shouldn’t be an issue on the thymus. Given this latest paper on mice and the thymus… rapamycin does great work.

Probably why Joan Mannick’s human trial worked on the immune system.

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#14

actually I continue to warn here that it is NOT what people are doing here. Once a week does not allow the thymus to recover and people are probably shriking more their thymus. I also know it was shown like that in studies. What works is the intense short term treatment followed by a long recovery

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#15

Thanks Tom,

I see what you mean… especially in view of the paper… it is the equivalent of a human year later post dosing that the improvement occurs.

Just hesitant to stop rapamycin for all the other good stuff it does for me based on Bio-markers. My inflammation is very low and Methylation DNA is also 15 years younger.

I will see if I can get a thymus measurement and T- cell generation reading in January with my other testing.

Might be why Attia is taking a break.

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#16

N=1: My thymus is fine, and I have been taking weekly high-dose rapamycin for years.

Human intermittent/weekly studies show no thymic harm signal.

  • Rapalog everolimus (low dose) improved influenza vaccine responses and lowered PD-1 on T cells in elderly adults—the opposite of a “damaged thymus” phenotype. Thymic injury was not reported. PubMed
  • Trials using once-weekly sirolimus (e.g., 5–10 mg/week for 13–48 weeks) were designed to assess safety and healthspan metrics; protocols and early reports do not identify thymic toxicity as a concern or observed signal. (Registered protocols and reports.)

“In mice, the higher the dose, the longer lifespan [6, 12, 35, 43]. Therefore, in humans, the highest dose that does not yet cause unacceptable side effects (maximal tolerated dose) may be optimal for longevity. If (unacceptable) side effects develop, the dose should be decreased. In other words, anti-aging doses are maximal doses without side effects in a particular person [34]. Then anti-aging doses are individual and side-effect-free by definition.”

“Rapamycin transiently slows thymocyte proliferation by inhibiting mTORC1; with continuous drug levels, this can shrink the thymic cortex in mice. With intermittent (weekly) dosing, drug is cleared for several half-lives between doses, allowing normal thymic cycling and avoiding sustained suppression—consistent with the reversibility seen in animal models and the lack of harm signals in human weekly studies.”

Bottom line

“The specific claim that once-weekly 5–29 mg rapamycin damages the thymus is not supported by current evidence. Signals of thymic involution come from daily exposure models and are reversible; human weekly protocols (5–10 mg/week studied so far) show no evidence of thymic damage and rapalogs have even enhanced immune responsiveness in older adults.”

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#17

From your summary I can see the thymus over-regenerated to ~130% of baseline mass within 14 days. Can someone with access to the full paper also share the thymus measurements from day 3 of the regeneration phase?

After converting time periods for mice to humans I think that (day 3 mouse) data would be more relevant to most of our intermittent dosing schedules compared to 14 days after mouse withdrawal.

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#18

The full paper is included in this thread - above these posts. Just scroll up and you’ll find it.

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#19

Here is what ChatGPT 5 suggests is the human equivalent dosing:

What the mice got

  • Dose & schedule: 1 mg/kg/day, intraperitoneal (i.p.), for 3 consecutive days.
  • That’s a parenteral regimen with near-complete systemic delivery.

Simple body-surface–area (BSA) scaling → Human-Equivalent Dose (HED)

Use the standard Km factors (mouse = 3, human = 37):

HED (mg/kg) = 1 × (3/37) ≈ 0.081 mg/kg/day.

Per-day HED by body weight:

  • 60 kg: ~4.9 mg/day
  • 70 kg: ~5.7 mg/day
  • 80 kg: ~6.5 mg/day

If you copy the mouse schedule literally, that’s ~5–7 mg/day for 3 days in a 60–80 kg adult (systemic-equivalent, notaccounting for route).

Route matters (i.p. vs. oral)

  • Mice received i.p. (high systemic availability).
  • Humans take oral sirolimus/rapamycin , which has low and variable bioavailability (~10–20%) and a long human half-life (~60 h) .
  • If you tried to match systemic exposure from the BSA-scaled 5–7 mg/day using oral tablets, you’d theoretically need a much higher oral dose (because F is low). That quickly exceeds customary clinical “longevity” pulses and isn’t sensible.

Timing equivalence (the practical bit)

Because the human half-life is long, three daily mouse injections don’t map 1:1 to three daily human oral doses. After a single human oral dose, levels persist for several days.

Reasonable human analogs people use to approximate the pattern (not exact AUC):

  1. Single-pulse approach (most conservative)
  • ~6 mg once orally (70 kg reference).
  • Rationale: matches the BSA-scaled daily amount, and the human half-life lets it cover several days of exposure without stacking daily doses.
  1. Split-pulse (closer to “3-day” feel, but still reasonable)
  • ~6 mg on Day 0, ~4 mg on Day 2.
  • Keeps total exposure modest while stretching the signal over ~3–5 days thanks to slow decay.
  1. Literal BSA schedule (rarely advisable)
  • ~5–7 mg/day for 3 consecutive days.
  • This stacks on a 60-hour half-life and will overshoot in many people. It’s closer to the mouse pattern but not to real-world PK prudence.

Bottom line

  • Mathematical HED: ~0.081 mg/kg/day ⇒ ~5–7 mg/day for 3 days (60–80 kg adult).
  • Translation with PK reality: prefer one pulse (~6 mg) or split pulse (6 mg then 4 mg 48 h later) rather than three consecutive daily oral doses.
  • Observe effects over ~2 weeks (the mouse study watched 14 days post-course).

This is a pharmacologic approximation, not medical advice. Individual PK varies (CYP3A4, P-gp, food effect), so clinicians often start lower and titrate to target troughs/exposure if they’re aiming for a research-grade match.

My New Prompt after this last one:

While I know its not sensible or recommended, I’m just curious about what the equivalent systemic exposure dose would be from the BSA-scaled 5–7 mg/day using oral tablets, you’d theoretically need a much higher oral dose (because F is low). What is that theoretical higher dose calculated to be?

CGPT5 Response:

If you want to match the systemic exposure of the BSA-scaled i.p. dose (~5–7 mg/day reaching the bloodstream), then the oral dose must be divided by oral bioavailability (F). Using typical sirolimus F ≈ 10–20% (huge person-to-person spread), the math is:

Required oral dose = systemic target ÷ F

  • F = 10%: 5 mg ÷ 0.10 = 50 mg/day 7 mg ÷ 0.10 = 70 mg/day
  • F = 15%: 5 mg ÷ 0.15 ≈ 33 mg/day 7 mg ÷ 0.15 ≈ 47 mg/day
  • F = 20%: 5 mg ÷ 0.20 = 25 mg/day 7 mg ÷ 0.20 = 35 mg/day

If you want finer granularity given published variability:

  • F = 12%: 5/0.12 ≈ 41.7 mg/day ; 7/0.12 ≈ 58.3 mg/day
  • F = 18%: 5/0.18 ≈ 27.8 mg/day ; 7/0.18 ≈ 38.9 mg/day

So the theoretical oral range to mimic 5–7 mg/day systemic is roughly 25–70 mg/day, with most people landing around 33–47 mg/day if you assume F ≈ 15%.

Two reality checks (not advice, just pharmacokinetics):

  • Stacking: Human half-life ≈ 60 h. Daily dosing for 3 days accumulates, so exposure will overshoot the same “per-day” systemic target unless you space doses.
  • F is unstable: CYP3A4/P-gp interactions, fed vs. fasted state, and tablet vs. solution can swing AUC by >2×. That’s why literal translation from i.p. mice to oral humans is inherently noisy.

But purely on the math you asked for: ~25–70 mg oral sirolimus per day for three days would be the theoretical exposure-matching band for the BSA-scaled 5–7 mg/day i.p. equivalent.

Related Reading:

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#20

Huge fail right there. No idea why peer reviewers didn’t ask that.

Unfortunately I can’t get too excited, other than this is an interesting phenomenon/observation.

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#21

If this study pans out we have a great biomarker to look at. The thymus can be evaluated by imaging right? Longevity studies take a long time—even in mice. But a study of various doses and dose intervals in dogs or marmosets might only take a couple of months to give important results on the thymus. It should not be necessary to sacrifice the animals either,

We don’t know the ideal dose or dose interval of Rapamycin for treating sarcopenia. My guess is that the magic happens during the trough period. When mtor signaling has been restored, normal growth also resumes.

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