P–426 Uterine Natural Killer Cells in Recurrent Miscarriage and Implantation Failure: An Updated Systematic Review and Meta-analysis

Study question Do women with recurrent miscarriage (RM) or implantation failure (RIF) have different levels of uterine Natural Killer (NK) cells compared to fertile controls? Summary answer Women with RIF but not RM are associated with significantly higher levels of CD56+ uterine NK cells compared to controls. What is known already Uterine NK cells (uNK) are different from peripheral NK cells (pNK) and are important in early pregnancy for development of the placenta. The association between uNK and RM/RIF is less clear, but dysfunction of uNK is believed to result in early pregnancy failure. Previous systematic reviews by Seshadri (2014) and Tang (2013) on infertile and RM patients showed no significant difference in uNK levels and highlighted need for further studies. Since, many prospective studies have been published and therefore warrant an updated systematic review. On the other hand, evidence for correlation between uNK and pNK is sparse and needs clarification. Study design, size, duration We have conducted a systematic review and meta-analysis to evaluate three outcomes. The primary outcome was the difference of uNK level in RM/RIF compared to controls. The secondary outcome was livebirth rate in women with RM/RIF with high compared to normal uNK level, and the tertiary outcome was correlation between uNK and pNK in RM/RIF. Participants/materials, setting, methods The electronic database search included MEDLINE, EMBASE, Web of Science and bibliographies from included articles from inception to December 2020 using a combination of MESH and keywords. Search, screen, and data extraction were performed by two reviewers independently. Quality assessment was conducted with ROBINS-I and meta-analysis with Revman 5.3. Out of 4636 studies screened, 43 studies (2539 women) and 3 studies each (598 and 77 women) were analysed for primary, secondary and tertiary outcomes respectively. Main results and the role of chance Our meta-analysis showed that CD56+ uNK were significantly higher in women with RIF but not RM compared to controls (SMD 0.60; 95% CI 0.12–1.08]. Subgroup analysis in RM patients showed no significant difference whether definition of 2 or 3 previous RM was used, in primary/secondary RM compared to controls, or in primary versus secondary RM. CD56+ uNK were significantly higher in RM/RIF when sampled during mid-luteal phase [SMD 0.56; 95% CI 0.19–0.93] but not in the early pregnancy decidua. Interestingly, there was significant difference in CD56+ uNK when analysed by immunohistochemistry [SMD 0.50; CI 0.05–0.94] but not by flow cytometry, and when CD56+ uNK were reported as percentage over total endometrial cells [SMD 0.58; 95% CI 0.10–1.07]. Further subgroup analysis showed significant difference in CD16 + [SMD 0.54; 95% CI 0.18–0.89] but not in CD56+CD16-, CD56+CD16+ or CD57. For pregnancy outcome, there was no significant difference in livebirth rate in RM/RIF patients with high uNK compared to normal uNK [RR 1.06, 95% CI 0.86–1.30]. Mean uNK level in RM patients with subsequent miscarriage was not significantly higher than subsequent livebirth. Finally, the pooled correlation between CD56 pNK and CD56 uNK (r = 0.42; 95% CI –0.04–0.73] was not significant in RM/RIF patients. Limitations, reasons for caution The meta-analysis is limited by quality of some of the studies. Some data were presented in median that was transformed to mean which may result in data skew. Other confounding factors e.g. maternal age, fetal karyotype, number of previous miscarriages and variable definition of controls may contribute to bias. Wider implications of the findings: Clinical interpretation of uNK level needs to be treated with caution because there is significant heterogeneity in method of analysis. There may be a role for uNK measurement in RIF patients however further studies to understand pathophysiology underlying elevated uNK is warranted before recommending it as a diagnostic tool. Trial registration number N/A

Hyaluronan-NK cell Interaction Controls the Primary Vascular Barrier during Early Pregnancy

Successful implantation is associated with a unique spatial pattern of vascular remodeling, characterized by profound peripheral neo-vascularization surrounding a peri-embryo avascular niche. We hypothesized that hyaluronan controls the formation of the unique vascular pattern encompassing the embryo. This hypothesis was evaluated by genetic modification of hyaluronan metabolism specifically targeted to embryonic trophoblast cells. The outcome of altered hyaluronan deposition on uterine vascular remodeling and post-implantation development were analyzed by MRI, detailed histological examinations, and RNA-sequencing of uterine NK cells. Our experiments revealed that eliminating the anti-angiogenic hyaluronan, led to elevated expression of MMP-9, VEGF-A and its receptor VEGFR-2, accompanied by reduced recruitment of uterine NK cells. Further local decrease in VEGFR-3 resulted in impaired formation of vascular sinuous folds, ectopic angiogenesis and dysfunctional uterine NK cells. Conversely, enhanced deposition of hyaluronan caused the expansion of the maternal-embryo barrier, leading to an increased diffusion distance and aborted implantation. These results demonstrate a pivotal role for hyaluronan in successful pregnancy by fine-tuning the peri-embryo avascular niche and maternal vascular morphogenesis.

Natural killer cells: functional differences in recurrent spontaneous abortion†

Recurrent spontaneous abortion (RSA) is one of the major pregnancy disorders and poses a serious risk to both the mother and the fetus. Although a number of research efforts have been conducted, therapeutic advances for treating RSA have not lived up to their expectations. Hence, other treatments should be explored. The important role of natural killer (NK) cells in immunotherapy is attracting increasing attention, both as a pharmaceutical target and for cell therapies. NK cells are abundant in the endometrium and play a role in implantation and placentation in normal pregnancy. As research progresses, NK cells are increasingly regarded as playing essential roles in the emergence and development of RSA. In this article, I review recent findings on the role of uterine NK cells in the pathophysiology of RSA. These cells may become therapeutic NK cell-related targets. In conclusion, although several issues regarding NK cells in RSA remain unresolved and require further investigation, extensive evidence is available for the treatment of RSA.