The dual role of cellular senescence in decidualization—transient, FOXO1-DIO2-driven senescence essential for early remodeling versus chronic senescence driving fibrosis—presents a core paradox [mednexus.org/doi/10.1097/RD9.0000000000000076]. Current models attribute age-related decidualization failure primarily to stromal cell-intrinsic defects: H3K27ac depletion, PI3K-AKT-FOXO1 overactivation, and stromal subpopulations like DIO2-low cells that resist differentiation [pubmed.ncbi.nlm.nih.gov/40394215, pmc.ncbi.nlm.nih.gov/articles/PMC11969247, pmc.ncbi.nlm.nih.gov/articles/PMC12964726]. However, this overlooks a critical extrinsic regulator: uNK cells, the dominant immune population in the late secretory phase. I propose that aging reprograms uNK cells from acute senescence clearers to chronic senescence preservers, creating a pathogenic positive feedback loop.

Core Hypothesis: In young endometrium, uNK cells recognize and selectively clear transiently senescent stromal cells during the narrow window of implantation (WOI, days 20-24) via NKG2D receptor engagement of stress ligands (e.g., MICA/B, ULBP1-6). This timely clearance is essential for resolving the acute senescence program and enabling progressive decidualization. With endometrial aging, a chronological mismatch occurs: the physiological spike in uNK cell recruitment is delayed or blunted, while the onset of stromal senescence is advanced due to accumulated DNA damage and epigenetic drift. Consequently, senescent cells persist beyond the WOI. Critically, these persistent senescent cells undergo a secretory phenotype shift (SASP evolution), downregulating "eat-me" ligands (e.g., calreticulin) while upregulating "don't-eat-me" signals (e.g., CD47) and pro-fibrotic cytokines (e.g., TGF-β1, CTGF) [pmc.ncbi.nlm.nih.gov/articles/PMC12071859].

Novel Mechanistic Reasoning: The proposed reprogramming of uNK cells is not passive but active. Chronic exposure to an aged, inflammatory microenvironment (e.g., elevated IL-15, IL-18) polarizes uNK cells towards a decidual-like, non-cytotoxic state with reduced perforin/granzyme expression and increased VEGF and angiogenin production. These aged, "decidualized" uNK cells then adopt a pro-fibrotic role, secreting factors that further suppress the regenerative IGFBP3+ stromal compartment [pubmed.ncbi.nlm.nih.gov/41496196] and promote myofibroblast differentiation. This creates a vicious cycle: uNK cells fail to clear senescent stroma, the resulting pro-fibrotic milieu inhibits uNK cell cytotoxicity, and senescent cells accumulate, driving fibrosis and loss of receptivity.

This hypothesis reframes the open question of "uNK cell clearance" by specifying that the defect is not in uNK cell numbers per se, but in their temporal and functional coordination with stromal senescence. It also provides a unifying link between immune-stromal crosstalk defects in thin endometrium [pubmed.ncbi.nlm.nih.gov/41496196] and persistent stromal senescence in IUA [pmc.ncbi.nlm.nih.gov/articles/PMC12071859].

Testable Predictions:

1. Single-cell Trajectory Analysis: Re-analysis of dataset GSE215968 will reveal that in IUA (aged model), uNK cell clusters exhibit reduced expression of cytotoxic genes (PRF1, GZMB, NKG2D) and increased expression of pro-fibrotic genes (TGFB1, CTGF, VEGFA) compared to normal WOI samples.
2. Senescence Ligand Profiling: Senescent stromal cells from aged vs. young endometrium (organoid or primary culture) will show a quantitative shift in surface ligand profile: reduced NKG2D ligands and calreticulin, increased CD47 and PD-L1.
3. Functional Co-culture: Co-culturing young uNK cells with aged senescent stromal cells will result in impaired clearance compared to young senescent stromal cells. Conversely, aged uNK cells will show impaired clearance of any senescent stromal cell.
4. Timing Intervention: In vivo (mouse model) or in vitro (organoid), artificially advancing uNK cell recruitment (e.g., via timed IL-15 administration) in aged endometrium should improve senescent cell clearance and partially rescue decidualization markers (e.g., PRL, IGFBP1).

This model predicts that senomorphic treatments [pubmed.ncbi.nlm.nih.gov/41699681] may fail if administered without correcting uNK cell timing and function. Successful therapeutic strategies will need to be combinatorial: clearing chronic senescent cells (e.g., via senolytics) and restoring the youthful, cytotoxic uNK cell phenotype within the correct temporal window.