Study Question: Does the presence of microbial pathogens in the endometrium affect the endometrial immune environment and ART outcomes?

Summary Answer: Endometrial pathogens were linked to poorer ART outcomes and increased inflammatory immune activity. Presence of Lactobacillus improved outcomes, potentially by modulating immune responses.

What Is Known Already:

Chronic endometritis and pathogen-dominant microbiota have been linked to infertility. Conversely, a Lactobacillus-dominant endometrial microbiota is associated with eubiosis and higher ART success. However, the impact on the immune compartment remains unclear.

Study Design:

• Retrospective cohort study involving 597 women undergoing ART between March 2016 and February 2022. Patients were distributed as follows:
  • Egg donation cycles (DEgg): 258 with PGT-A, 209 without PGT-A
  • Own-egg cycles (OEgg): 104 with PGT-A, 26 without PGT-A
• Endometrial samples were collected during the window of implantation (WOI) of an HRT cycle.
• qPCR was used to detect 18 microbial pathogens, 4 Lactobacillus species, and 29 immune mediators.
• Flow cytometry (n=151) assessed NK, B, and T cells.
• Microbiota classified as:
  • Undetected (U, n=196)
  • Lactobacillus only (L, n=263)
  • Lactobacillus + Pathogens (L+P, n=74)
  • Pathogens only (P, n=64)
• Clinical outcomes: βhCG+, clinical pregnancy (CPr), and live birth (LB) after euploid single embryo transfer (sET) in both PGT-A and non-PGT-A cycles.

Results:

1. In DEgg cycles, pathogen-only (P) patients had significantly lower outcomes than undetected (U) patients:
   1. βhCG+: 39.3% vs 66.7% (p=0.002)
   2. Clinical pregnancy: 32.1% vs 57.1% (p=0.008)
   3. Live birth: 25% vs 49.2% (p=0.01)
2. P group had higher:
   1. T cells (39.6% vs 34.3%)
   2. Activated NK cells (5.6% vs 4.4%)
   3. Proinflammatory mediators:
      1. Perforin 1 (+23%, p=0.01)
      2. Granzyme B (+38%, p=0.06)
      3. IL-1β (+110%, p=0.03)
   4. CXCL8 (+72.9%, p=0.06)
   5. IL-17A (+38.2%, p=0.09)
   6. IL-17F (+43%, p=0.09)
3. L+P patients had outcomes comparable to U patients:
   1. βhCG+: 57.8%
   2. CPr: 54.5%
   3. LB: 48.5%
   4. Lower IL-17A/F expression, indicating immunomodulatory role of Lactobacillus.

Limitations:

• Retrospective design; results need validation in RCTs.
• Transcervical sampling may carry minimal risk of contamination.

Wider Implications of the Findings: Presence of endometrial pathogens promotes inflammation and reduces ART success. Lactobacillus may help restore immune balance and improve reproductive outcomes in pathogen-positive women.

ESHRE 2025, June 29 - July 2, Paris