Natural killer cells in reproduction: Helpful, harmful, or misunderstood?
Scott J. Morin, M.D., Jason M. Franasiak, M.D., T.S.
Reproductive Medicine Associates of New Jersey
Sidney Kimmel Medical College, Thomas Jefferson University
The study of reproductive immunology has always carried tremendous promise as a mechanism for exploring infertility and pregnancy pathology. The mere fact that pregnancy relies upon the measured permissiveness of the maternal host to a prolonged, semiallogenic exposure is proof that a finely balanced response by the immune system is essential to establish a healthy gestation. With this in mind, clinician scientists have long sought to identify how alterations in this complex system may result in a variety of pregnancy pathologies. Unfortunately, results have been varied and areas of proposed success have been difficult to replicate in more than one laboratory.
Perhaps the most familiar line of research in the context of IVF is the attempt to implicate peripheral blood natural killer (pbNK) cells in pregnancy loss. This theory was set forth soon after the discovery that the dominant leukocyte in the uterine mucosa featured the CD56 cell surface marker characteristic of pbNK cells (1). Given that pbNKs function by inducing cytotoxicity to virus-infected cells, it seemed plausible that overzealous cells of a similar lineage in the uterus may exhibit a cytotoxic function to invading trophoblast resulting in early pregnancy loss.
Early investigators utilized measurements of pbNKs from peripheral blood samples as a surrogate for the activity of CD56+ leukocytes (termed uterine natural killer cells or uNK) at the feto-maternal interface. These authors further postulated that a general reduction in pbNK load with immunomodulatory therapies, such as IVIg, could diminish a putative cytotoxic response to early pregnancy among patients with recurrent miscarriage (2). However, early studies were plagued with selection bias and have not been successfully replicated. Indeed, the largest meta-analysis evaluating the role of pbNK cells on IVF outcomes demonstrated no decrement in outcomes among women with elevated pbNK cell numbers or activity (3). Furthermore, a meta-analysis of 20 studies demonstrated no benefit following immunotherapy for patients with recurrent miscarriage (4).
The most likely reason for the failure of pbNK diagnostics or systemic immunomodulatory treatment is a fundamental misunderstanding of the biology. Obtaining a peripheral sample to evaluate pbNK cells is an attractive option for extrapolating to activity at the feto-maternal interface given its convenience and ease of measurement. However, despite sharing CD56, uNKs are in fact very different from pbNK cells. Uterine natural killer cells diverge radically from pbNK in other phenotypic markers (5). Furthermore, uNKs behave very differently in functional assays, where they exhibit a much weaker cytotoxic activity – the fundamental principle upon which the theory is based – than pbNKs (6). Thus, the available evidence suggests that uNKs arise completely independently of pbNKs.
As a result, efforts at evaluating the impact of uNKs on early pregnancy events by measuring pbNKs was bound to miss the mark from the beginning. However, the failure of these initial studies of pbNK based on a flawed biological foundation does not necessarily mean that uNKs are also unable to provide a better understanding of early pregnancy pathology. In fact, there are a variety of exciting data that suggest that uNKs have a major impact on the establishment of early pregnancy. It is now well established that a major function of uNKs is mediating the invading extravillous trophoblast’s (EVT) access the maternal vasculature. The success of maternal arteriole remodeling is in part dependent upon the interaction between HLA-C ligands on the EVT and their receptors on uNKs, killer immunoglobulin receptors (KIRs). While each patient’s uNKs feature multiple KIRs, they are classically categorized as primarily inhibitory (KIR A) or activating (KIR B). Conversely, HLA-C ligands are categorized as C1 or C2, depending on their amino acid structure. Multiple recent studies have observed that certain deleterious combinations (such as KIR A uterus / HLA-C2/C2 conceptus) are associated with an increased risk of pathology associated with abnormal placentation (miscarriage, growth restriction, preeclampsia) (7, 8).
While more study is needed, the modern era of research on natural killer cells in the context of reproduction will rely on two factors: 1) more sensitive diagnostic tools, such as molecular genotyping and 2) a better physiologic foundation for investigation. Similar to early efforts at preimplantation genetic screening (PGS), which also suffered from limited analytic tools (such as FISH) and a weak physiologic grounding (single blastomere biopsy underestimating the impact of mosaicism), the initial studies on natural killer cells were flawed from the start. And like PGS, ongoing work on uNKs will undoubtedly uncover increasing complexity and require careful recalibration as our understanding improves. It remains essential that high quality demonstrating benefit is necessary prior to the implementation of treatment.
- Bulmer JN, Lash GE. Human uterine natural killer cells: a reappraisal. Mol Immunol 2005;42:511-21.
- Kwak JY, Kwak FM, Ainbinder SW, Ruiz AM, Beer AE. Elevated peripheral blood natural killer cells are effectively downregulated by immunoglobulin G infusion in women with recurrent spontaneous abortions. Am J Reprod Immunol 1996;35(4):363-9.
- Seshadri S, Sunkara SK. Natural killer cells in female infertility and recurrent miscarriage: a systematic review and meta-anlaysis. Hum Reprod Update 2014;20(3):429-38.
- Wong LF, Porter TF, Scott JR. Immunotherapy for recurrent miscarriage. Cochrane Database Syst Rev 2014;10:CD000112.
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- Morin SJ, Treff NR, Tao X, Scott RT 3rd, Franasiak JM, Juneau CR, Maguire M, Scott RT. Combination of uterine natural killer cell immunoglobulin receptor haplotype and trophoblastic HLA-C ligand influences the risk of pregnancy loss: a retrospective cohort analysis of direct embryo genotyping data from euploid transfers. Fertil Steril 2017 Jan 6. pii: S0015-0282(16)63072-8. doi: 10.1016/j.fertnstert.2016.12.004.