Hypothesis

Age‑associated hypermethylation of the NKG2D ligand genes MICA and MICB in endometrial stromal cells blunts uNK‑cell surveillance, allowing senescent cells to evade clearance and transition from acute, receptivity‑promoting senescence to chronic, fibrotic senescence.

Mechanistic Rationale

1. uNK clearance relies on NKG2D – In receptive endometrium, uterine NK (uNK) cells eliminate senescent decidual stromal cells via granule exocytosis triggered by engagement of the activating receptor NKG2D with stress‑induced ligands MICA/B on target cells [4].
2. Senescence‑associated secretory phenotype (SASP) normally upregulates MICA/B – Acute senescence driven by FOXO1/IL‑8 transiently raises MICA/B expression, providing an “eat‑me” signal that couples SASP to immune clearance [1].
3. Epigenetic silencing uncouples SASP from immune recognition – Chronic stromal senescence in recurrent implantation failure (RIF) exhibits persistent SASP components (e.g., LGALS9, TGF‑β) but markedly reduced MICA/B transcript and protein levels, as seen in GSE160702‑derived in‑vitro aged stromal cells [2]. Promoter‑region CpG islands of MICA/MICB become hypermethylated with age, a change not captured by generic senescence markers but detectable with a custom stromal‑specific signature.
4. Consequences of evaded clearance – Unremoved senescent stromal cells accumulate, secrete profibrotic factors (COL1A1, ACTA2) and sustain PI3K‑AKT‑FOXO1 hyperactivation, driving fibrosis and impairing decidualization despite normal estrogen‑progesterone receptivity markers [3,6].
5. Therapeutic implication – Re‑expressing MICA/B should restore uNK‑mediated clearance, break the fibrosis loop, and rescue receptivity without abolishing beneficial acute senescence.

Testable Predictions

• Prediction 1: In endometrial biopsies from RIF patients, MICA/MICB mRNA and surface protein will be significantly lower than in fertile controls, even when SASP markers (IL‑6, LGALS9) are elevated. Validation via flow cytometry and qPCR on matched proliferative‑phase samples.
• Prediction 2: Treating aged stromal organoids with a low dose of the DNA‑demethylating agent 5‑aza‑2′‑deoxycytidine (or a targeted HDAC inhibitor) will increase MICA/B surface expression, enhance uNK degranulation (CD107a assay), and reduce senescent cell burden (SA‑β‑gal, p16) in co‑culture models.
• Prediction 3: In a murine model, stromal‑specific knockout of Mica1/Mica2 (orthologs of human MICA/B) will lead to increased stromal senescence, uterine fibrosis, and decreased litter size; stromal‑specific overexpression of MICA/B will reverse these phenotypes.

Falsifiability

If MICA/B expression is not reduced in RIF endometrium, or if demethylating treatment fails to improve uNK clearance and fibrosis, the hypothesis would be refuted. Conversely, confirmation would support a novel immune‑evasion mechanism underlying the acute‑to‑chronic senescence switch and provide a clear path for stromal‑targeted senotherapeutics.