Adaptation and Modulation of Memory and Regulatory T Cells in Pregnancy
Kieffer, Tom Eduard Christiaan
DOI:
10.33612/diss.97355536
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Kieffer, T. E. C. (2019). Adaptation and Modulation of Memory and Regulatory T Cells in Pregnancy. Rijksuniversiteit Groningen. https://doi.org/10.33612/diss.97355536
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8
Immunomodulators to treat
recurrent miscarriage
European Journal of Obstetrics, Gynecology,
and Reproductive Biology 2014; 181:334-337
Jelmer R. Prins
1Tom E.C. Kieffer
1Sicco A. Scherjon
11Department of Obstetrics and Gynaecology, University Medical Center Groningen, University of Groningen, Groningen,
ABSTRACT
Recurrent miscarriage is a reproductive disorder affecting many couples. Although several factors are associated with recurrent miscarriage, in more than 50% of the cases the cause is unknown. Maladaptation of the maternal immune system is associated with recurrent miscarriage and could explain part of its pathophysiology. Modulating the maternal immune system towards pregnancy tolerance could benefit pregnancy outcome. Although there is a clear scientific rationale that modulating the maternal immune system could benefit recurrent miscarriage, only a few studies suggest possible beneficial effects of immune modulators as a therapy for recurrent miscarriage. Therapies skewing the maternal immune response to a tolerating regu-latory T cell rich environment seem especially promising, however more research is needed to find effective and safe maternal immune modulators for reproductive pathologies as recurrent miscarriage. Moreover, the possible side effects on maternal, fetal, and neonatal immune function are essentially unknown and its elucidation is crucial before any possible therapeutic strategies could be clinically implemented.
INTRODUCTION
Recurrent miscarriage is a reproductive disorder distressing many couples. Depen-ding on the definition used, it is estimated that about 1-3% of couples seeking to reproduce are affected by it1,2. In contrast to a sporadic miscarriage which is often
caused by a conceptus having structural malformations or chromosomal abnormalities, the causes of recurrent miscarriage are far from completely understood1. Although
some factors are clearly associated with recurrent miscarriage, most cases are idio-pathic, and no possible cause is evident.
The maternal immune system plays an important role in the success of pregnancy. Optimal regulation of the maternal immune system is essential to tolerate the allogeneic fetus. Improper regulation leads to several reproductive disorders, such as recurrent miscarriage1. The complex mechanisms behind the tolerance towards the fetus are not
fully elucidated3. The important regulating role of the maternal immune system gives
the female reproductive system the possibility to accommodate or reject a conceptus depending on maternal, environmental, and fetal factors4.
Since the maternal immune system is implicated in the pathogenesis of recurrent miscarriage, modulation of the maternal immune system holds potential, and gives hope for treatment and prevention of recurrent miscarriage. Immune modulators being
able to skew the maternal immune system towards fetal tolerance could benefit preg-nancy outcome. However, skewing the maternal immune system towards tolerance could also result in tolerance of fetuses which would have been rejected rightfully by the maternal quality control system. Moreover, modulation of a pregnancy immune environment could very well influence fetal and neonatal development and have a role in the priming of the fetal immune system5. Furthermore, modulation of the
maternal immune system could cause long term effects, causing the mother to be vulnerable for immune associated diseases in later live. Although, no evidence of long term maternal or fetal effects has been found yet, caution is essential before implying immune modulating therapies in clinical settings without a proven and clear beneficial effect, and without evidence of safety. This short review aims to examine and highlight some of the possibilities of modulation of the maternal immune system as a treatment option for recurrent miscarriage.
METHODS
For this review, papers were included using combinations of the following search terms in Pubmed: ‘recurrent miscarriage, recurrent abortions, deciduas, immune, placenta, pregnancy, abortion, regulatory T cells, Treg cells, paternal, NK cells, cytokines, Th17 cells, Th1 cells, progesterone, G-CSF, GM-CSF, (immune) therapy, and prevention’. Systematic reviews were used for additional search through their reference list. Most (immune) therapies for recurrent miscarriage have been reviewed separately recently in meta-analyses, these Cochrane meta-analyses were also used in this review.
RESULTS
Recurrent miscarriage
Recurrent miscarriage is a disorder which is not consistently defined within existing literature. Most studies define recurrent miscarriage as three or more consecutive mis-carriages before 20 weeks of gestation1,6. Depending on the definition it is estimated
that 1-3% of couples seeking to reproduce are affected by it1,2. Although several
causes are associated with recurrent miscarriage including anatomical anomalies, genetic abnormalities, thrombophilia, lifestyle, and hormonal dysfunction, in more than 50% of the couples suffering from recurrent miscarriage no such associations are found1,7,8. Most likely, a part of these unexplained miscarriages is caused by
malfunctioning of the maternal immune regulation.
Immunology during pregnancy
It is known that regulation of the maternal immune system is essential for preg-nancy success to prevent rejection of the fetus. Several mechanisms are involved in the induction of maternal tolerance, and as adequate regulation is important, most likely redundant mechanisms protect the rejection of the fetus. Besides the immune changes seen locally at the fetal maternal interface, peripheral immune responses are also altered during pregnancy9.
Regulatory T (Treg) cells are a key player within the regulation of maternal immune tolerance10. Lower numbers and altered function of Treg cells have been associated
with adverse pregnancy outcomes in both human and animal studies, and it has been shown that adequate numbers and function of Treg cells are essential for pregnancy success10. During pregnancy the balance of T cell subsets is essential for preventing
fetal rejection, for example increased levels of Th1 and Th17 related cells and cyto-kines are associated with adverse pregnancy outcomes. Depending on the cytokine environment, naïve T cells will differentiate to a T cell subset. Other immune cells which are important during pregnancy are NK cells and macrophages3. NK cells are
regarded as important in the regulation of vascularisation and invasion processes during pregnancy11. All immune cells work together to ensure fetal tolerance. A good
example of this is the recent finding that NK cells are able to inhibit the Th17 cells, and with that skew the maternal immune system towards tolerance12. The mechanisms
causing the shift of the maternal immune system towards fetal tolerance are not fully known, but most likely endogenous and exogenous factors, including exposure to semen, hormones, and lifestyle, modulate the maternal immune system towards tolerance10. The precise mechanisms of maternal fetal tolerance are complex and
numerous, however they are out of the scope of this review.
Maternal immunology in recurrent miscarriage
Several immunological mechanisms have been associated with recurrent miscarri-age1,13. Most studies indicate higher levels of activated immune cells and associated
cytokines, and lower levels of regulatory cells and associated cytokines in women with recurrent miscarriage. Differences have been found both locally as well as in the peripheral blood. Tissue recovered after both spontaneous as well as induced abortions showed lower levels of Treg cells in spontaneous miscarriages14,15. Other
studies showed that women with recurrent miscarriage have lower levels of Treg cells in their peripheral blood and also reduced suppressive function compared to a control
group14,16. These Treg cells could also be less stable, as gene expression of certain
genes related to Treg cell stability was less in women with recurrent miscarriage17.
Other evidence implying Treg cells in the pathophysiology of recurrent miscarriage are altered Treg / Th17 balances found in recurrent miscarriage. Higher levels of Th17 cells and Th17 associated cytokines have been found in decidual tissue and peripheral blood of women with recurrent miscarriage18,19, indicating an imbalance
between Treg and Th17 cells in these women20. Most likely the regulation of this
balance is dysfunctional, as it has been shown that peripheral blood mononuclear cells from women with recurrent miscarriage show greater proliferation of Th17 and fewer Treg cells once cultured in the presence of sperm antigens from their male partner17.
An interesting question would be whether this inadequate skewing of the maternal immune system is also influenced by paternal factors, and is caused by differences in semen constituents. It is known that seminal fluid induces Treg cell generation, and that the duration of sexual relationship is related to pregnancy outcome21,22.
Inadequate immune modulating factors in seminal fluid could lead to inadequate adaptation of the maternal immune system, and make a couple prone for recurrent miscarriage. More studies analyzing the role of semen constituents in couples with recurrent miscarriage are needed.
Another mechanism which is also implied in the immune regulation in recurrent miscarriage is altered Interleukin 6 (IL6) signaling. IL6 is known to influence the dif-ferentiation of naïve T cells away from Treg cells towards Th17 cells23. Studies have
found lower levels of IL6 in endometrial tissue in the mid-secretory phase of the cycle, together with increased serum levels of IL6 in women with recurrent miscarriage com-pared to controls24-27. The higher levels of IL6 could explain the finding that sera from
women with recurrent miscarriage inhibit the expansion of the Treg cell populations24.
Other immune cells associated with recurrent miscarriage are NK cells and their cytokines28-30. Although studies differ in their methods, most studies found higher levels
of NK cells in the peripheral blood of women with recurrent miscarriage30-32, whereas
also an altered balance in inhibitory and activating killer immunoglobulin-like recep-tors (KIR) was found24,33-35. KIRs are cell proteins on the surface of NK cells. Whereas
inhibitory KIRs prevent activation, activating KIRs leads to cytotoxic activity36. Altered
balances in these KIRs could lead to an improper activation of the NK cells. More evidence for NK cell dysfunction in recurrent miscarriage comes from a finding that NK cells were partially impaired in women with recurrent miscarriage, and with that were less successful in inhibiting Th17 cells12.
Immune modulators
Since the maternal immune system is implied in the etiology of part of the unex-plained recurrent miscarriages, several immune modulating therapies have been investigated, including aspirin, paternal leukocyte immunization, progesterone, and steroids37. As the causes of recurrent miscarriage are multi factorial, selection of
couples by immunological analysis could improve therapeutic outcomes and therefore influence the results of the different studies. However, in only a few studies pre-im-mune screening of the subjects was performed. Most studies used either paternal white cell immunization (12 trials, 641 women, Peto OR 1.23, 95% confidence interval (CI) 0.89 to 1.70) or intravenous administration of immunoglobulins (eight trials,303 women, Peto OR 0.98, 95% CI 0.61 to 1.58) as therapy37. Although, a
Cochrane review of 2006 concluded that overall these therapies do not improve live birth rate in women with recurrent miscarriage37, some studies did find normalization
of immune parameters in women with recurrent miscarriage. Methods between the studies vary and it can be argued that immunizing the women by injecting paternal cells intravenously is immunologically not an optimal method as most of the immune reaction will be cytotoxic driven. Immunizing subcutaneously could improve results, as this shows similarities to the ‘natural’ immunizing method (coitus), and dendrictic cells and macrophages will present the antigens22. Interestingly, a few studies have
shown that once immunizing recurrent miscarriage patients with mononuclear cells from their husbands, levels of IL6 decrease to normal levels24,25,38. More interestingly,
immunization of women with recurrent miscarriage with paternal cells led to an incre-ase in the Treg cell frequency comparable to levels of fertile women24. In mice models,
it has been shown that after increasing the levels of Treg cells abortion prone mice are protected against fetal loss28,39. Although these latter results hold potential, there is no
evidence yet that immunizing with paternal cells does improve pregnancy outcome. Futures studies should consider using the subcutaneously route to immunize women.
Because of the role of Treg cells in recurrent miscarriage, several other strategies have been used to improve their levels. The use of granulocyte colony stimulating factor (G-CSF) has been evaluated for the treatment of recurrent miscarriage. G-CSF and GM-CSF are cytokines known to recruit and activate tolerogeneic dendritic cells which are implied in the generation of Treg cells10,39,40. A small study showed a
reduction in the incidence of miscarriage in a following pregnancy in women treated with G-CSF (odds ratio 5.1; 95% CI 1.5–18.4), together with an increase in levels of HCG41. As these results were only obtained in small studies, these results have to
The use of progesterone is of recent interest. Progesterone suppresses Th17 cell responses and enhances development and possible function of Treg cells42,43. A recent
Cochrane review stated that there is no evidence that progesterone could prevent miscarriage44. Interestingly, a subgroup analysis, only analyzing women with
recur-rent miscarriage, within this Cochrane review of four trials analyzing the effect of progesterone in women with recurrent miscarriage showed a significant decrease in miscarriage (total of 225 women, Peto OR 0.39; 95% CI 0.21 to 0.72)7,44-47.
Howe-ver, most of this data is over 50 years old and these results should be confirmed in larger randomized clinical trials.
Other strategies include therapies to modulate NK cell numbers. In patients with recurrent miscarriage several abnormalities were demonstrated in cellular immunity and increased numbers of NK cells were found in peripheral blood. Since pred-nisolone decreases numbers of NK cells in women with recurrent miscarriage48, a
recent pilot feasibility study administered prednisolone or a placebo to women with recurrent miscarriage, in this study no difference was found in live birth rate (95% CI 0.79-2.86)49.
There is also no evidence for a beneficial treatment effect of intravenous immu-noglobulins in recurrent miscarriage patients37. One study analyzed the effects of
intravenous immunoglobulin on levels of Treg and Th17 cells in women with recur-rent miscarriage with prescreened immune abnormalities50. Levels of Treg cells and
Th17 cells did not change in the whole patient group, however, when subjects were divided in groups based on their immune cell levels, intravenous immunoglobulins led to higher levels of Treg cells in the lowest Treg cell number group and lower levels of Th17 cells in the higher Th17 cell number groups50. Although this study shows that
administration of immunoglobulins could restore part of an immune balance in part of recurrent miscarriage there was no effect seen on pregnancy rate50. However,
this was not a double-blind placebo controlled trial, therefore further evaluation is needed. Being able to influence both immune and haemostatic characteristics, the effect of aspirin on recurrent miscarriage has been evaluated. No improvement in live birth rate was however observed after administration of aspirin with or without a low molecular weight heparin51.
To summarize the above, there is no clear evidence yet that immune modulation benefit couples with recurrent miscarriage. Since the causes of recurrent miscarriage are multi factorial, inclusion and treatment of patients who are likely to benefit from immune modulating therapy is essential. Adequate screening of both maternal immune
cell subsets and their function and possibly also immune skewing factors in semen could define couples in which the immune system might be the cause of recurrent miscarriage. In these cases, specific immunotherapy might benefit pregnancy outcome. However, similar to the therapeutic options, no guidelines towards specific immune screening have been made yet.
Possible immune therapies have to be proven safe for both mother and child. During pregnancy the fetal immune system is shaped, and most likely the fetal immune system is influenced by maternal immune factors. There is evidence that prenatal maternal immune factors could influence the fetal immune system, and with that leading to fetal immune programming [5]. As modulating the fetal immune system via maternal modulation could lead to for example allergies, cardiovascular diseases in later life, and could also have an influence on neurodevelopment, great care is needed before administering modulators of the maternal immune system to women suffering from recurrent miscarriage. Although modulating the maternal immune system could very well benefit the couples seeking to reproduce, the risk for the fetus and its developing immune system has to be considered thoroughly.
CONCLUSION
Although there is a scientific rationale to modulate the maternal immune system as a therapy for recurrent miscarriage, caution is very much needed. Since the causes of recurrent miscarriage are multi factorial, a strategy towards selection of patients, who might benefit from maternal immune modulation, has to be developed. For most of the above mentioned modulators, the exact working mechanisms are not known, and the possible effects of such a (immune) modulator on fetal and neonatal immune function are also unknown. Although some immune modulators do normalize immune abnormalities found in women with recurrent miscarriage, there is no proven effect on pregnancy outcome yet. Therefore, more specific and demonstrably safe maternal immune modulators have to be developed. Possible promising immune modulators should first be tested in randomized clinical trials before being used in daily clinical practice. Although it can be tempting to give a promising therapy to these sometimes desperate couples, only therapies with a scientifically proven beneficial and safe effect should be administered to prevent providing false hope to couples experiencing recurrent miscarriage.
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