Moving from peripheral blood to local uterine immunophenotype analysis in patients with poor reproductive history: pilot study of a novel technique.

Background: A complete reproductive immunophenotype is poorly described, with most focus on peripheral blood natural killer cells rather than uterine populations. There is debate regarding normal endometrial levels, with no consensus, and much controversy on correlation with implantation/miscarriage.

Aims: Development and validation of a rapid endometrial assessment flow cytometry (FCM) technique, allowing determination of local lymphocyte subset ranges, comparison to peripheral blood, and patient subgroup analysis.

Methods: Prospective pilot, assessing patients with prior implantation, failure offered endometrial biopsy before subsequent ART cycle, functioning as therapeutic scratch. HRT regime administered to standardise environment, and progesterone-primed mid-luteal biopsy (five completed days progestogen, P+5) analysed using comprehensive flow panel to identify lymphocyte subsets.

Results: Two hundred patients were recruited in a tertiary university-affiliated ART centre. FCM identified differing lymphocyte ranges between peripheral blood and biopsy. Uterine/decidual natural killer cells are the dominant endometrial subtype. Patients with repeated implantation failure had higher uNK levels (52.4 vs 43.7%, p = 0.01). Conversely, B lymphocytes (0.87 vs 0.72%, p = 0.032), pNK (1.21 vs 0.8%, p = 0.041), and NK-T (2.68 vs 2.26, p = 0.031) cells were higher in recurrent pregnancy loss.

Conclusion: FCM is widely used to assess cellular populations, but not typically employed for endometrial evaluation. FCM provides a rapid, detailed, and quantitative analysis and reduces inter-observer subjectivity bias. Detailed understanding of the normal endometrial immunophenotype, and associated deviations, may provide insight into the aetiology of infertile patients labelled "unexplained". Failure despite transfer of high grade, or proven euploid blastocysts, is a difficult problem, and endometrial profiling may help identify research areas to determine potential future therapeutic interventions for this difficult to treat population.