Cyclic chemical castration with leuprolide followed by sub-threshold DHT add-back exploits a putative thymic AR suppr...

IF aged male C57BL/6 mice (20–24 months) are treated with a cyclic leuprolide acetate protocol — two sequential 4-week depot injection courses (75 µg/mouse subcutaneous sustained-release, separated by a 4-week off-period to allow partial gonadal recovery) — followed by calibrated sub-physiological dihydrotestosterone (DHT) add-back (delivered via 5 mg/kg/day subcutaneous micro-osmotic pump, targeting plasma DHT at ~10–15% of intact-male physiological levels, below the estimated thymic AR-suppressive threshold),

THEN thymic cellularity (total thymocyte counts, cortical and medullary TEC numbers by flow cytometry), thymic volume (micro-CT), peripheral naïve CD4⁺ and CD8⁺ T-cell output (sjTREC quantification by qPCR from peripheral blood), and TCR repertoire diversity (bulk TCRβ sequencing of splenocytes) will be significantly and durably elevated above age-matched intact controls at 12 weeks post-completion — with regeneration persisting through DHT add-back in at least 60% of the magnitude achieved at peak depletion — and systemic androgen-dependent endpoints (lean muscle mass, bone mineral density by DXA) will be partially restored compared to fully castrate controls,

BECAUSE of the following mechanistic chain:

1. Leuprolide depot induces pituitary GnRH receptor downregulation, causing sustained suppression of LH and FSH secretion, and consequently reducing testicular testosterone to castrate levels (<0.5 ng/mL) within 2–3 weeks of administration, as established in murine chemical castration models. (Activation of thymic regeneration in mice and humans following androgen blockade, Journal of Immunology, 2005 — referenced in Evidence Set)

2. Testosterone depletion removes tonic androgen receptor (AR) signaling in the thymic stromal compartment. AR is expressed selectively on thymic epithelial cells (TECs), fibroblasts, and endothelial cells — not on developing thymocytes — and its activation under normal physiology actively suppresses FOXN1 transcription, the master regulator of TEC identity and function, and downregulates Notch ligands including Delta-like 4 (Dll4) required for T-cell lineage commitment. (Evidence Set: Molecular Mechanisms section; Olsen et al., Immunology Today, 1998 — referenced in Evidence Set)

3. Relief of AR-mediated FOXN1 suppression permits thymic epithelial progenitor (TEP) re-activation and TEC regeneration. Following castration, TEPs proliferate and re-establish both cortical TEC (cTEC) and medullary TEC (mTEC) compartments, restoring the stromal microenvironment that supports sequential thymocyte maturation through DN → DP → SP stages. Absolute numbers of all thymocyte subsets recover toward young-mouse levels. (Reversal of age-related thymic involution with castration, Journal of Immunology — referenced in Evidence Set)

4. [SPECULATIVE] Cyclic depletion-recovery cycles cumulatively expand the TEP pool beyond what a single continuous depletion achieves. The 4-week off-period between leuproli...

SENS category: RepleniSENS