Implantation, Still the Big Black Box in IVF

This article is a part of the November 2015 issue of CHR VOICE, our monthly newsletter.

It is generally assumed that most IVF cycles fail because of embryo quality. Most research in IVF is, therefore, concentrated on improving egg and embryo quality and selecting the best embryos for transfer. Common wisdom holds that only ca. 15% of IVF cycle failures are caused by implantation failure. The truth, however, is that we never know why an IVF cycle fails and nobody really knows how many cycles fail because of an embryo problem, and how many because the implantation process does not work correctly.

What we know about implantation is very limited. Mostly from animal models and artificial implantation models, it has become clear that the implanting embryo starts a “conversation” with the endometrium and, likely, the maternal immune system, while swimming in the endometrial cavity fluid for ca. 48 hours prior to implantation. This “conversation” is conducted via biological substances produced by both the embryo and the maternal host. The goal of the “conversation” is two-fold: First, to find a good implantation site (ca. 95% of implantation sites are in the fundal area of the uterus, which represents only a small portion of the endometrial surface) and, second, to reprogram the maternal immune system from immunological rejection to immunological tolerance. The second step is biologically very important because the implanting embryo is really “foreign” to the maternal immune system, as 50% of it is genetically paternal.

If a woman were to receive an organ transplant from her partner, she, likely, would violently reject it without appropriate anti-rejection treatments. Even with treatments, chances of acceptance of the graft would be poor if the organ donor was genetically not very similar. Yet, in a normal pregnancy, a woman “accepts” the implanting graft (i.e., embryo) without any signs of an immunological rejection taking place. Somehow, her immune system gets “reprogrammed” by this conversation between the implanting embryo and her immune system.

Malfunction of tolerance

We now understand that this “reprogramming” of the maternal immune system sometimes malfunctions. This frequently happens in women with hyperactive immune systems; for example, in presence of autoimmunity or if women are “hyper allergenic” (i.e., allergic to many different stimulants). In such women, their immune system still, at least partially, sees the implanting embryos as “foreign,” and attacks it like it would a foreign organ transplant. This is called an allogeneic immune response, directed against the “foreign” alloantigens of the graft, in this case those of the biological father. The consequence can be difficulties of implantation (i.e., implantation failure) and/or early miscarriages.

Indeed, we often cannot differentiate between implantation failure and very early miscarriages because even the most sensitive pregnancy tests we currently have at our disposition become positive only approximately one week after implantation. It, therefore, is very likely that in many IVF cycles, considered to have failed, embryos did indeed implant but were “rejected” by the maternal immune system before a positive pregnancy test could be obtained.

Such a rejection is also known to occur later: For example, only since IVF was developed, do we know that so-called chemical pregnancies are very frequent. These are very early pregnancies, which already result in a positive pregnancy test since the hormone human chorionic gonadotropin (hCG), produced by the implanting embryo, can be detected in the mother’s circulation. It, of course, can be found in the mother’s blood only if the embryo, almost like a cancer, dug itself deep into her endometrium (i.e., implanted) and made contact with her vasculature. Alloimmune responses can, however, also occur much later in pregnancy, and then they can be the cause of clinical miscarriages, also called immunological miscarriages because they are caused by failure of normal development of immunological tolerance in the mother.

Apparent failure to conceive, chemical pregnancies and immunological pregnancy losses, therefore, can represent one spectrum of reproductive failure, characterized by the failure of the maternal immune system to establish normal immunological tolerance.

Though we now (believe to) understand the existence of this problem, we currently do not have reliable tests to diagnose this condition. Moreover, even if we are reasonably certain that this problem exists, we do not have well established treatments to combat it. At CHR, we prescribe various immune-suppressive treatments in such situations, but because prospectively randomized studies in this area, for a variety of reasons, are almost impossible to conduct, none of them can, as of this moment, be considered well supported by a high enough level of evidence.

Implantation as an inflammatory process

From the preceding it will be obvious that implantation is an immunological process. It also is an inflammatory process. Indeed, most key processes in human reproduction are inflammatory processes, starting with ovulation over implantation and, finally, even the onset of labor.

Reproductive immunology has been one of the main research interests at CHR for decades. Indeed, Norbert Gleicher, MD, CHR’s Medical Director and Chief Scientist, was one of the first clinical reproductive immunologists in the world, one of the original founders of the American Society for Reproductive Immunology in 1980 and the first Editor-in-Chief of the American Journal of Reproductive Immunology. Inflammation in recent years has been an increasingly important research subject at CHR as we attempt to find markers of poor tolerance in peripheral blood of affected women.

After many years of investigations, researchers at CHR  have been able to identify some promising inflammatory markers, which appear strongly associated with female reproductive success and/or failure. In a number of manuscripts currently awaiting publication, they demonstrated that the well known inflammatory marker C-reactive protein (CRP) is strongly negatively associated with live birth after IVF, while the inflammatory marker interleukin 6 (IL-6) is strongly associated with miscarriages. Practically, this means that women who have elevated CRP have decreased live birth chances after IVF and those with elevated IL-6 demonstrate increased risk of miscarriages. Interestingly, these associations are linear, i.e., the higher the values of CRP and IL-6, the higher their associated risks and, to our surprise, they proved much stronger than associations with autoimmune markers, which have been known for decades.

All research at CHR is in principle translational. CHR researchers attempt to direct their investigations toward specific clinical problems. The research of inflammatory markers is a good example: It suggests that traditional treatments in women with implantation failure and/or excessively activated immune systems may in many cases have been ineffective because they primarily had been directed at autoimmune processes. CHR’s recent research, however, now suggests that in some patients treatments have to be directed at inflammatory rather than autoimmune pathways. Consequently, in such patients, treatment may have to be modified in that (auto)immune-suppressive treatments either have to be replaced by anti-inflammatory treatments or, even more likely, anti-inflammatory agents may have to be added to (auto)immune-suppressive treatments because autoimmune and inflammatory pathways often overlap and/or run in parallel.

Building on CHR’s recent research, CHR’s physicians have already started to integrate anti-inflammatory treatments in patients with obvious evidence of excessive inflammation, and are carefully studying their effects. Associations do not always reflect causations, but where such associations make physiological sense, they often do.

Chronic endometritis, an inflammatory disease?

voice_1511_endometritisOne aspect of CHR’s expanded investigation of the importance of inflammation in implantation failure has been the investigation of chronic endometritis. That this condition exists has been known for decades. Recently a number of studies associated the presence of this condition with implantation failure.
CHR investigators, led by Vitaly A. Kushnir, MD, Associate Scientist and Director of CHR’s Fertility Preservation Program, indeed, discovered that in a select group of women with implantation failure (i.e., multiple IVF cycle failures unexplained by other causes), almost half suffered from chronic endometritis when endometrial biopsies were performed (a manuscript reporting this and related findings is close to submission).

The typical histological picture of chronic endometritis is characterized by infiltrates of so-called plasma cells. Those are white blood cells (WBCs), which produce antibodies. They are not the typical WBCs that can be found in bacterial infections but usually are typical for autoimmune infiltrates in sterile inflammations of endocrine organs. Indeed, in every endocrine organ of the human body, from pituitary, over adrenals, pancreas, thyroid and ovary, such sterile autoimmune inflammations have been described, except for in the endometrium. Yet, for decades, ever since this condition was originally described, chronic endometritis has been treated with antibiotics as a bacterial infection.

Some cases of chronic endometritis, indeed, have an infectious etiology, and treatment with antibiotics will cure the condition.  Many others, however, as the recent CHR study again demonstrated, do not respond to antibiotics. CHR investigators, therefore, under the assumption of an inflammatory rather than direct autoimmune process, have started to treat affected patients with anti-inflammatory medications, and are investigating the effects of such treatments.

We, therefore, encourage patients with repeated implantation failure, or colleagues who have been unsuccessful in treating such patients, to contact us. We will gladly share our protocols and/or help in the treatment of such patients.

Implantation as a defense mechanism against implantation of abnormal embryos

In recent publications, some investigators have proposed an intriguing new theory that connects the concepts that embryo quality and/or implantation failure can be causes of implantation failure. In very well executed experiments, these investigators appear to demonstrate that the human endometrium has the ability to “judge” which embryo is “normal.” If confirmed, this model would suggest an explanation why most chromosomally abnormal embryos do not succeed to implant, at least for the long-term.

In addition, this hypothesis would explain why it is so difficult to differentiate between different causes of implantation failure. It would also close the circle with the above described recent recognition by investigators at CHR and elsewhere that many more embryos are mosaic than has previously been assumed. And it would, finally, explain why it takes so many eggs and embryos in IVF to establish a normal pregnancy that leads to live birth.

Norbert Gleicher, MD, leads CHR’s clinical and research efforts as Medical Director and Chief Scientist. A world-renowned reproductive endocrinologist, Dr. Gleicher has published hundreds of peer-reviewed papers and lectured globally while keeping an active clinical career focused on ovarian aging, immunological issues and other difficult cases of infertility.