How come our mothers’ immune systems do not reject us in the uterus?

One of the most basic questions in human biology still remains mostly unanswered

After a long journey down the fallopian tube, an embryo, now at approximately blastocyst stage, has finally reached the uterine cavity. Suddenly, this embryo is in a very different micro-environment: no more cilia that propel the embryo countercurrent toward the uterus; and, also, no more peristalsis that the embryo had utilized on this journey to float toward the uterus. Instead, an eerie quiet in what appears to be a much larger body with very different shape, different lining and filled by relatively thick mucus. But this is the organ that the embryo was destined to reach because only here normal access is to be granted to the Holy Grail of implantation.

Immune system of the mother has to reprogram itself to tolerate the implanting embryo in order for the embryo to survive and thrive. Image by Suhyeon Choi via Unsplash.

Let us, therefore, quickly recapitulate who this embryo is: It is the product of one egg and one sperm, each representing approximately half of the embryo’s nuclear genome (DNAn). The embryo’s mitochondrial DNA (DNAm), representing less than 1% of total DNA, is almost 100% derived from the mother. Overall, the mother, therefore, contributes slightly over half of the total DNA of the embryo but the father, still, adds almost a half and, thereby, converts the embryo into something “foreign,” trying, against all seeming rules of biology to gain access to the mother’s body.

“Foreigners” are, however, never supposed to be allowed access, – and, certainly, not through the endometrium, where embryos are supposed to implant. The reasons are obvious: Were the endometrium to permit easy access, bacteria, viruses and parasites would constantly infect the uterus. The endometrium at all times, therefore, must be hostile to all invaders because, otherwise, women would constantly become infected and probably not survive for very long. This is where the immune system comes into play, which in its sophisticated layering of defense mechanisms, maintains our species by protecting us from infections, survival of initial cancer cells and other dangers.

With the blastocyst-stage embryo now in the endometrial cavity, it, interestingly, does not implant right away. It, indeed, spends approximately 48 hours freely floating in the mucus-filled cavity before only a small minority of embryos that do reach the cavity does implant. A majority, simply, degenerates after being denied access by endometrium and the maternal immune system. Why only so few embryos implant is still unknown but very intriguing work by British colleagues in recent years tells a rather fascinating story: It appears that the endometrium has the ability “to sense” the quality of embryos that arrive and then opens the gates for implantation only if embryos fulfill certain (still largely unknown) quality criteria.

One, therefore, can assume the following series of events: An embryo reaches the endometrial cavity and, in some ways, must be able to announce its presence. It probably does so by sending one or more biological messages out to endometrium and immune system (separating between those two is probably an artificial divide), in response to which the endometrium/immune system starts “feeling out” the embryo in order to make a decision whether this given embryo should be allowed to implant or not.

Interestingly, it increasingly appears that embryo aneuploidy (i.e. chromosomal abnormalities) does not disqualify embryos from implanting. Pregnancy rates with embryos that were diagnosed with chromosomal abnormalities by preimplantation genetic testing for aneuploidy (PGT-A) have, indeed, been so unexpectedly high, that some biologists have proposed the hypothesis that at least some chromosomal abnormalities in blastocyst-stage embryos may favor implantation and establishment of pregnancy. They, in other words, are turning the old, and largely disproven hypothesis that embryos with chromosomal abnormalities should be excluded from transfer, on its head.

Once endometrium and the immune system identify an embryo as “acceptable” for implantation, the endometrium, usually highly hostile to all forms of invasion, now opens its gate to welcome this embryo. It does so, however, only with incredible selectivity. To maintain the analogy of opening a gate, that gate is only opened for this one specific embryo and it does not happen by the immune system shutting down for even only a split second (because that would be more than enough time to also let bacteria, viruses and other invaders through the door), but through induction of highly specific tolerance pathways, which allow entry only to this one embryo.

When a tolerance pathway is freshly induced, it means that the immune system reprograms itself from universal hostility to selective tolerance. If a new tolerance pathway is successfully established, so-protected embryos will be “invisible” to the mother’s immune system, even though they, as noted before, are almost 50% paternal and, therefore, “foreign” (also called “allogeneic”). As we know from solid organ transplantation science, allogeneic organs elicit an automatic allogeneic immune response from the recipient’s immune system, which in organ transplantation is battelled with immuno-suppressive drugs. Such drugs counteract the allogeneic immune response on a usually very broad basis. In recent years, transplantation medicine has become increasingly knowledgeable in how to battle these anti-allogeneic immune responses more selectively. In pregnancy, however, development of tolerance pathways by a normally functioning maternal immune system is highly selective and so specific that hardly any secondary effects of decreased immunity are observed. In other words, nature induces tolerance much better, and clinical medicine would greatly benefit from learning how to mimic nature’s processes in tolerating the semi-allograft of pregnancy.

Establishing tolerance for a transplanted organ would, indeed, appear to be a much easier task to accomplish than establishing tolerance for pregnancy because organ transplants (except for liver transplants) maintain size, while the fetus, of course, grows exponentially from microscopic size as an embryo to an over 7-pound (~3.5kg) term infant on average. The normal maternal immune system, therefore, not only maintains for 38 weeks tolerance toward the pregnancy but successfully maintains tolerance toward a very rapidly expanding semi-allograft.

In a large majority of pregnancies, all of this works perfectly well, as normal tolerance pathways are induced, allowing normal implantation for the embryo. A minority of women, however, have lost the ability to induce tolerance pathways in a timely and appropriate fashion, and these women experience difficulties in getting embryos “accepted” (i.e., have implantation failure) and/or because absence of appropriate tolerance pathways has not made those embryos “invisible” to the maternal immune system, the mothers mount an allogeneic immune response against their own pregnancy, resulting in miscarriages. Their immune system would, of course, have mounted the same immune response against a kidney transplant, for example, from their partner.

Those are the patients we see at CHR with a presumed diagnoses of implantation failure and/or repeated immunological pregnancy loss. We very purposefully note that diagnoses are only “presumed” because current limitations in diagnostic as well as therapeutic abilities in this arena must be honestly acknowledged. While through the organ transplantation literature significant progress has been made in understanding mechanisms involved, CHR is always very cautious in not over-representing current knowledge levels in reproductive immunology: At current knowledge levels, we really can never be certain that a patient, indeed, suffers from implantation failure or immunological pregnancy loss since there are no tests that can be ordered to confirm either diagnosis. CHR, indeed, has been quite critical of colleagues who have been chasing batteries of blood tests in attempts to claim these two diagnoses. The literature does not offer supportive published evidence that any number of tests, whether anti-phospholipid antibodies (APAs), NK cells, specific lymphokine levels or any other immune parameter are really diagnostic for either or both diagnoses. A lot of snake-oil has, unfortunately, been sold over the years to patients with immunological infertility, leading to perfectly warranted skepticism from many colleagues in the field.

This skepticism, however, unfortunately, has often led to the opposite extreme because to suggest that the immune system is irrelevant to female fertility also makes absolutely no sense, considering every pregnancy is a semi-allograft. Yet, this is unfortunately an opinion widely held among fertility centers and recently to a degree also supported in a formal opinion of ASRM’s Practice Committee, which concluded that, in association with IVF, none of the currently applied immune treatments have improved live birth rates or have been sufficiently studied to reach definite conclusions about their effectiveness [Fertil Steril 2018;110(3):378-400].

As a center that has never advocated excesses on either side and, indeed, at times strongly criticized both, CHR repeatedly pointed out that complete denial of importance of the immune system for reproductive success, is also a form of negativistic snake-oil, considering that the need for adequate maternal tolerance of the allogeneic products of conception is universally accepted in order for pregnancy to thrive and survive to term.

Who are then the women who are unable to induce proper tolerance at the onset of gestation? One recent study in the general immunology literature answered this question elegantly, even though clinical observations have suggested the answer for quite some time: Women who have hyper-active immune systems, i.e., women with autoimmunity, inflammation and/or severe allergies, are all at risk to have defective tolerance induction. Once this fact is understood, it will also immediately be apparent why CHR’s approach to immunological problems in infertile women differs from that of many, if not most, colleagues.

As already noted, CHR does not assume and/or believe that specific immunological abnormalities indicate risk for either implantation failure or increased miscarriage risks. Instead, CHR assumes and believes that women with hyperactive immune systems are at increased risk for developing inadequate tolerance toward embryos, thereby exposing embryos to allogeneic immune responses against their paternal components. Since we do not (yet) understand how to diagnose such an immunological attack, CHR does not search for evidence of the attack itself, but for the underlying condition that leads to the attack, which is inadequate development of tolerance. And since we do not (yet) have a way of measuring what represents adequate tolerance, we are looking for the evidence of hyperactive immune systems because that is a diagnosis that can be made with relative ease and accuracy (for details, please go to the article on immunologic “diagnosis” on page 6).

Once a woman is recognized to have a hyperactive immune system, she then is automatically suspected of being incapable of developing adequate tolerance in a timely fashion and, therefore, will receive immune-suppressive treatments in anticipation of a maternal allo-immune response toward the products of conception. Those treatments can vary depending on the severity of observed immune system hyperactivity (for further details, see “treatments” for immune infertility, on page 7).

Here are few final points in an attempt to demonstrate the importance of the maternal immune system for pregnancy success: Now that we established the importance of normal tolerance development for pregnancy, it will also appear obvious that a certain minimal level of tolerance must be established before the embryo even attempts to implant. The development of initial tolerance, therefore, must be induced very early, at the latest after arrival of the embryo into the uterine cavity. Furthermore, since the pregnancy grows, antigen loads the maternal immune system must tolerate increase logarithmically, meaning that tolerance development and expansion must be a continuous process. And, indeed, organ transplantation also demonstrated that tolerance evolves in phases, with a second phase involving the concept of microchimerism, characterized by passage of donor organ cells into the recipient and seeding of the donor organ with recipient cells. The same thing happens in pregnancy, with fetal cells (and cell-free DNA) entering into the maternal circulation and vice-versa. In human pregnancy, maternal microchimerism can be observed at as early as 7 weeks gestational age (in IVF ca. 5 weeks post embryo transfer).

The development of additional tolerance mechanisms is one reason why pregnancies that have successfully survived the early pregnancy loss period in the 1st trimester, in most cases are relatively safe, until they get into the 3rd trimester. In the last trimester and carrying over into the postpartum period for up to 3-4 months, these pregnancies can again significantly affect the maternal immune system because immune systems that have difficulties in establishing adequate tolerance at the beginning of pregnancy often also lose tolerance prematurely at the end of pregnancy.

Though what initiates labor is still not well understood, labor is increasingly viewed as the end of immunological tolerance, initiating certain inflammatory processes which, in turn, result in release of biological substances (cytokines and lymphokines) that initiate labor. With normal immune systems, tolerance usually ends at around 38 weeks following embryo transfer but in mothers with hyperactive immune systems, the end of tolerance often occurs prematurely. This is why, for example, with all autoimmune diseases, women almost uniformly deliver prematurely.

This is a part of the May 2019 CHR VOICE.

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.