The immunological factor of sterility, specifically the abnormal count and activity of uterine NK (uNK) cells, may represent one of the potential contributors affecting specific subgroups of sterile couples undergoing assisted reproductive treatment (ART). Therefore, the primary purpose of the present paper was to assess uNK cell count. A total of 387 endometrial biopsies from patients with recurrent implantation failure (RIF) or recurrent pregnancy loss (RPL) were analyzed to identify abnormalities in uNK cell count, using immunohistopathological evaluation. ANOVA analysis revealed a strong association with factor 0.161 with p-value < 0.01, indicating that higher uNK cell count is associated with the presence of clusters (multicellular aggregates of uNK cells). These results suggest that the formation of clusters and the spatial distribution of uNK cells are significant factors in the context of the aforementioned clinical questions. However, the actual translational potential to clinical practice has not yet been established due to several challenges, namely: 1. the constantly changing definitions and diagnostic criteria for RIF and RPL, 2. varying sampling approaches for uNK cells, and 3. the historical lack of clear differentiation between uterine and peripheral NK cells. When all these issues are resolved, the observed tendency of uNK cells to form clusters will need to be a central focus of future investigations addressing RIF and RPL, thus improving ART outcomes.

Faculty of Healthcare Studies University of Western Bohemia 301 00 Pilsen Czech Republic

Faculty of Medicine Institute of Anatomy and Anthropology Riga Stradins University LV 1007 Riga Latvia

Faculty of Medicine Institute of Histology and Embryology Comenius University 811 08 Bratislava Slovakia

Faculty of Medicine Institute of Pathology Comenius University 811 08 Bratislava Slovakia

Zobrazit více v PubMed

Nabekura T. Immunological memory in natural killer cells. Int. Immunol. 2025;37:435–443. doi: 10.1093/intimm/dxaf016. PubMed DOI

Roberts J.M., Burton G.J., Conrad K.P., Luke B., Mann M.R., Moffett A., Jancsura M.K. An overview of periconceptional pathology and pathophysiology: Where do we go next? Placenta. 2025;168:98–110. doi: 10.1016/j.placenta.2025.06.010. PubMed DOI

Lapides L., Varga I., Csöbönyeiová M., Klein M., Pavlíková L., Visnyaiová K., Babál P., Mikušová R. The Neglected Uterine NK Cells/Hamperl Cells/Endometrial Stromal Granular Cell, or K Cells: A Narrative Review from History through Histology and to Medical Education. Int. J. Mol. Sci. 2023;24:12693. doi: 10.3390/ijms241612693. PubMed DOI PMC

Guan D., Chen Z., Zhang Y., Sun W., Li L., Huang X. Dual Role of Natural Killer Cells in Early Pregnancy: Immunopathological Implications and Therapeutic Potential in Recurrent Spontaneous Abortion and Recurrent Implantation Failure. Cell Prolif. 2025;58:e70037. doi: 10.1111/cpr.70037. PubMed DOI PMC

Shreeve N., Colucci F. Mamma MIA! Decidual NK cells involved in fetal neurodevelopment. Immunity. 2025;58:1364–1366. doi: 10.1016/j.immuni.2025.05.010. PubMed DOI

Wyns C., De Geyter C., Calhaz-Jorge C., Kupka M.S., Motrenko T., Smeenk J., Bergh C., Tandler-Schneider A., Rugescu I.A., Vidakovic S., et al. ART in Europe, 2017: Results generated from European registries by ESHRE. Hum. Reprod. Open. 2021;2021:hoab026. doi: 10.1093/hropen/hoab026. PubMed DOI PMC

Kamel R.M. Assisted reproductive technology after the birth of louise brown. J. Reprod. Infertil. 2013;14:96–109. doi: 10.4172/2161-0932.1000156. PubMed DOI PMC

Saito S. Role of immune cells in the establishment of implantation and maintenance of pregnancy and immunomodulatory therapies for patients with repeated implantation failure and recurrent pregnancy loss. Reprod. Med. Biol. 2024;23:e12600. doi: 10.1002/rmb2.12600. PubMed DOI PMC

Vomstein K., Aulitzky A., Strobel L., Bohlmann M., Feil K., Rudnik-Schöneborn S., Zschocke J., Toth B. Recurrent Spontaneous Miscarriage: A Comparison of International Guidelines. Geburtshilfe Frauenheilkd. 2021;81:769–779. doi: 10.1055/a-1380-3657. PubMed DOI PMC

Stevens Brentjens L., Roumen R.J.E., Smits L., Derhaag J., Romano A., van Golde R.J.T., den Hartog J.E. Pregnancy rate and time to pregnancy after recurrent implantation failure (RIF)-a prospective cohort follow-up study. J. Assist. Reprod. Genet. 2024;41:3061–3070. doi: 10.1007/s10815-024-03257-9. PubMed DOI PMC

Cimadomo D., de Los Santos M.J., Griesinger G., Lainas G., Le Clef N., McLernon D.J., Montjean D., Toth B., Vermeulen N., Macklon N. ESHRE good practice recommendations on recurrent implantation failure. Hum. Reprod. Open. 2023;2023:hoad023. doi: 10.1093/hropen/hoad023. PubMed DOI PMC

Lapides L., Varga I., Klein M., Rybánska L., Belušáková V., Babál P. When Less Is More—Pipelle Endometrial Sampling for Quantification of Uterine Natural Killer Cells in Patients with Recurrent Implantation Failure or Habitual Abortion. Physiol. Res. 2022;71:S65–S73. doi: 10.33549/physiolres.934961. PubMed DOI PMC

Zhao Y., Chen X., Zhang T., Chan L.K.Y., Liu Y., Chung J.P., Kwong J., Li T.C. The use of multiplex staining to measure the density and clustering of four endometrial immune cells around the implantation period in women with recurrent miscarriage: Comparison with fertile controls. J. Mol. Histol. 2020;51:593–603. doi: 10.1007/s10735-020-09908-2. PubMed DOI

King A. Uterine leukocytes and decidualization. Hum. Reprod. Update. 2000;6:28–36. doi: 10.1093/humupd/6.1.28. PubMed DOI

Croy B.A., He H., Esadeg S., Wei Q., McCartney D., Zhang J., Borzychowski A., Ashkar A.A., Black G.P., Evans S.S., et al. Uterine natural killer cells: Insights into their cellular and molecular biology from mouse modelling. Reproduction. 2003;126:149–160. doi: 10.1530/rep.0.1260149. PubMed DOI PMC

Manaster I., Mandelboim O. The unique properties of uterine NK cells. Am. J. Reprod. Immunol. 2010;63:434–444. doi: 10.1111/j.1600-0897.2009.00794.x. PubMed DOI

Croy B.A., van den Heuvel M.J., Borzychowski A.M., Tayade C. Uterine natural killer cells: A specialized differentiation regulated by ovarian hormones. Immunol. Rev. 2006;214:161–185. doi: 10.1111/j.1600-065X.2006.00447.x. PubMed DOI

Tuckerman E., Mariee N., Prakash A., Li T.C., Laird S. Uterine natural killer cells in peri-implantation endometrium from women with repeated implantation failure after IVF. J. Reprod. Immunol. 2010;87:60–66. doi: 10.1016/j.jri.2010.07.001. PubMed DOI

Clifford K., Flanagan A.M., Regan L. Endometrial CD56+ natural killer cells in women with recurrent miscarriage: A histomorphometric study. Hum. Reprod. 1999;14:2727–2730. doi: 10.1093/humrep/14.11.2727. PubMed DOI

Quenby S., Nik H., Innes B., Lash G., Turner M., Drury J., Bulmer J. Uterine natural killer cells and angiogenesis in recurrent reproductive failure. Hum. Reprod. 2009;24:45–54. doi: 10.1093/humrep/den348. PubMed DOI

Chen X., Mariee N., Jiang L., Liu Y., Wang C.C., Li T.C., Laird S. Measurement of uterine natural killer cell percentage in the periimplantation endometrium from fertile women and women with recurrent reproductive failure: Establishment of a reference range. Am. J. Obstet. Gynecol. 2017;217:680.e681–680.e686. doi: 10.1016/j.ajog.2017.09.010. PubMed DOI

Babayeva G., Purut Y.E., Giray B., Oltulu P., Alakuş R., Çolakoğlu M.C. Endometrial CD56+ natural killer cells in women with recurrent implantation failure: An immunohistochemical study. Turk. J. Obstet. Gynecol. 2020;17:236–239. doi: 10.4274/tjod.galenos.2020.90359. PubMed DOI PMC

Lapides L., Klein M., Belušáková V., Csöbönyeiová M., Varga I., Babál P. Uterine Natural Killer Cells in the Context of Implantation: Immunohistochemical Analysis of Endometrial Samples from Women with Habitual Abortion and Recurrent Implantation Failure. Physiol. Res. 2022;71:S99–S105. doi: 10.33549/physiolres.935012. PubMed DOI PMC

Von Woon E., Greer O., Shah N., Nikolaou D., Johnson M., Male V. Number and function of uterine natural killer cells in recurrent miscarriage and implantation failure: A systematic review and meta-analysis. Hum. Reprod. Update. 2022;28:548–582. doi: 10.1093/humupd/dmac006. PubMed DOI PMC

Seshadri S., Sunkara S.K. Natural killer cells in female infertility and recurrent miscarriage: A systematic review and meta-analysis. Hum. Reprod. Update. 2014;20:429–438. doi: 10.1093/humupd/dmt056. PubMed DOI

Vento-Tormo R., Efremova M., Botting R.A., Turco M.Y., Vento-Tormo M., Meyer K.B., Park J.E., Stephenson E., Polański K., Goncalves A., et al. Single-cell reconstruction of the early maternal-fetal interface in humans. Nature. 2018;563:347–353. doi: 10.1038/s41586-018-0698-6. PubMed DOI PMC

Whettlock E.M., Woon E.V., Cuff A.O., Browne B., Johnson M.R., Male V. Dynamic Changes in Uterine NK Cell Subset Frequency and Function Over the Menstrual Cycle and Pregnancy. Front. Immunol. 2022;13:880438. doi: 10.3389/fimmu.2022.880438. PubMed DOI PMC

Kuon R.J., Weber M., Heger J., Santillán I., Vomstein K., Bär C., Strowitzki T., Markert U.R., Toth B. Uterine natural killer cells in patients with idiopathic recurrent miscarriage. Am. J. Reprod. Immunol. 2017;78:e12721. doi: 10.1111/aji.12721. PubMed DOI

Russell P., Sacks G., Tremellen K., Gee A. The distribution of immune cells and macrophages in the endometrium of women with recurrent reproductive failure. III: Further observations and reference ranges. Pathology. 2013;45:393–401. doi: 10.1097/PAT.0b013e328361429b. PubMed DOI

Zhao Y., Man G.C.W., Wang J., Liu Y., Kwong J., Zhang T., Chung J.P.W., Wang C.C., Chen X., Li T.-C. The identification of endometrial immune cell densities and clustering analysis in the mid-luteal phase as predictor for pregnancy outcomes after IVF-ET treatment. J. Reprod. Immunol. 2021;148:103431. doi: 10.1016/j.jri.2021.103431. PubMed DOI

Hazan A.D., Smith S.D., Jones R.L., Whittle W., Lye S.J., Dunk C.E. Vascular-leukocyte interactions: Mechanisms of human decidual spiral artery remodeling in vitro. Am. J. Pathol. 2010;177:1017–1030. doi: 10.2353/ajpath.2010.091105. PubMed DOI PMC

Kusakabe K., Ohmoto M., Okada T., Mukamoto M., Sasaki F., Kiso Y. Uterine NK cells produce epidermal growth factor in the murine pregnant uterus. J. Vet. Med. Sci. 1999;61:947–949. doi: 10.1292/jvms.61.947. PubMed DOI

Wang F., Qualls A.E., Marques-Fernandez L., Colucci F. Biology and pathology of the uterine microenvironment and its natural killer cells. Cell. Mol. Immunol. 2021;18:2101–2113. doi: 10.1038/s41423-021-00739-z. PubMed DOI PMC

Choudhury R.H., Dunk C.E., Lye S.J., Aplin J.D., Harris L.K., Jones R.L. Extravillous Trophoblast and Endothelial Cell Crosstalk Mediates Leukocyte Infiltration to the Early Remodeling Decidual Spiral Arteriole Wall. J. Immunol. 2017;198:4115–4128. doi: 10.4049/jimmunol.1601175. PubMed DOI

Vilotić A., Nacka-Aleksić M., Pirković A., Bojić-Trbojević Ž., Dekanski D., Jovanović Krivokuća M. IL-6 and IL-8: An Overview of Their Roles in Healthy and Pathological Pregnancies. Int. J. Mol. Sci. 2022;23:14574. doi: 10.3390/ijms232314574. PubMed DOI PMC

Dillenburg-Pilla P., Patel V., Mikelis C.M., Zárate-Bladés C.R., Doçi C.L., Amornphimoltham P., Wang Z., Martin D., Leelahavanichkul K., Dorsam R.T., et al. SDF-1/CXCL12 induces directional cell migration and spontaneous metastasis via a CXCR4/Gαi/mTORC1 axis. FASEB J. 2015;29:1056–1068. doi: 10.1096/fj.14-260083. PubMed DOI PMC

Hanna J., Wald O., Goldman-Wohl D., Prus D., Markel G., Gazit R., Katz G., Haimov-Kochman R., Fujii N., Yagel S., et al. CXCL12 expression by invasive trophoblasts induces the specific migration of CD16- human natural killer cells. Blood. 2003;102:1569–1577. doi: 10.1182/blood-2003-02-0517. PubMed DOI

Niessen C.M., Leckband D., Yap A.S. Tissue organization by cadherin adhesion molecules: Dynamic molecular and cellular mechanisms of morphogenetic regulation. Physiol. Rev. 2011;91:691–731. doi: 10.1152/physrev.00004.2010. PubMed DOI PMC

Burrows T.D., King A., Loke Y.W. The role of integrins in adhesion of decidual NK cells to extracellular matrix and decidual stromal cells. Cell Immunol. 1995;166:53–61. doi: 10.1006/cimm.1995.0007. PubMed DOI

Wallace A.E., Fraser R., Cartwright J.E. Extravillous trophoblast and decidual natural killer cells: A remodelling partnership. Hum. Reprod. Update. 2012;18:458–471. doi: 10.1093/humupd/dms015. PubMed DOI PMC

Xie M., Li Y., Meng Y.Z., Xu P., Yang Y.G., Dong S., He J., Hu Z. Uterine Natural Killer Cells: A Rising Star in Human Pregnancy Regulation. Front. Immunol. 2022;13:918550. doi: 10.3389/fimmu.2022.918550. PubMed DOI PMC

Croy B.A. Hasn’t the time come to replace the term metrial gland? J. Reprod. Immunol. 1999;42:127–129. discussion 131–134. PubMed

Pijnenborg R. The metrial gland is more than a mesometrial lymphoid aggregate of pregnancy. J. Reprod. Immunol. 2000;46:17–19. doi: 10.1016/S0165-0378(99)00054-6. PubMed DOI

Nitric Oxide Synthase Expression in Endometrium During Physiological Cycle