The modulating effect of progesterone on endometrial structure and function are essential to the success of human reproduction. Due to the important role played by progesterone in human reproduction, exogenous supplementation of progesterone is the important treatment regimen in infertility treatment.
Natural micronized progesterone
Natural Micronized Progesterone (NMP) has the chemical structure identical to that of the hormone molecule produced in the human body. Natural micronized progesterone is manufactured in a laboratory from chemicals derived from plants (Mexican wild yams and soy). The term “micronized” refers to the small particle size of the progesterone itself, which greatly helps its absorption. Side effects associated with them are minimum, which may be attributed to their close resemblance to the natural hormone.1
Micronization of progesterone to particle sizes of <10μm increases the available surface area of the drug and enhances the aqueous dissolution rate and intestinal absorption.
The immune system
The primary function of the immune system is to eliminate infectious agents that have breached the natural barriers. This is brought about by the integrated actions of different cells and molecules that, directly or indirectly, lead to recovery from infection. It is the effector cells and molecules that actually bring about the elimination of infectious agents. Mast cells, granulocytes, natural killers cells are responsible for innate immunity whereas in adaptive immunity lymphocytes (T lymphocytes and B lymphocytes) play a major role. Dendritic cells are involved in the initiation and regulation of adaptive immune responses. They can be viewed as linking innate and adaptive immunity because they respond quite rapidly to the presence of infection and subsequently change their properties so they can activate lymphocytes.
The classical immune system recognizes the fetus as a foreign object and may initiate the immune system to expel or destroy the foreign object. But during pregnancy the maternal immune system is altered to tolerate the fetus which can be considered as allogenic graft. Series of immunomodulation takes place both at the local and systemic level in mother’s body to accept the fetus as a self and harmless object. The main protagonists of this immunomodulation are embryonic factors, uterine (endometrial) NK cells and, of course, the hormone progesterone.
Role of progesterone in early pregnancy
During the late luteal phase of menstrual cycle the endometrium undergoes decidual change, with the dominant cells being natural killer (NK) cells during the first trimester of pregnancy. NK cells are responsible for changes in the decidual arteries to increase the blood supply to the foeto-placental unit. Also these NK cells participate in immune modulation.2
The immunology in pregnancy is the result of combination of signals and responses originated from the maternal immune system and the fetal-placental system. When needed (in the presence of dangerous signals), these signals modulate the way the maternal immune system behaves. These modifications, which occur both at the foeto-maternal interface and in the systemic circulation, are driven by estrogens and progesterone whose blood concentrations increase during pregnancy. Progesterone plays an important role in modifying the maternal immune system. As soon as immune system recognizes the pregnancy, there will be activation of maternal immune system which results in upregulation of progesterone receptors on activated lymphocytes among placental cells and CD8+ cells.
In pregnancy, progesterone causes a decrease in peripheral NK cell numbers, activation, and cytotoxicity via direct action on NK cells and/or through promoting Th2 cytokine and progesterone induced blocking factor (PIBF) production by T cells. It also facilitates NK cell homing to the endometrium, via inducing expression of homing receptors and addressins on peripheral NK cells and the endometrium, respectively, and possibly through induction of vascular endothelial growth factor (VEGF) and macrophage inflammatory protein (MIP-1) expression by the endometrium. Endometrial stromal cells, under the influence of progesterone, produce IL-15, prolactin, and likely other unidentified factors, which may regulate uNK cell proliferation, differentiation, and production of cytokines and other molecules that support placental and trophoblast development and promote local immunomodulation.3
These T helper cells help immune cells by enhancing their actions in phagocytosis. These cells exert their effects by secretion of signaling molecules, cytokines. Th-2 type cytokines are known to play an important role in success of the pregnancy and at high concentration Th-1 type of cytokines, particularly IFN γ, are known to be deleterious for the pregnancy. Hence the presence of Th-2 cells indicate successful pregnancy. Under the influence of Th-1 cytokines, NK cells turn into classical NK cells expressing CD16 which can damage the trophoblast either directly through cytolytic activity or indirectly by producing inflammatory cytokines.3,4
Natural killer (NK) cells are bone marrow derived peripherally circulating cytolytic lymphocytes slightly more voluminous than B or T cells, which comprise approximately 10% of all peripheral blood lymphocytes. During normal human pregnancy, peripheral blood NK cells decrease in number. In addition, peripheral NK cells from pregnant women exhibit decreased lytic activity compared with NK cells from controls.3
Immunomodulation in successful pregnancy
Healthy fetal growth certainly requires a specific interaction between maternal cells and trophoblasts to form a placental unit. This happens when the maternal endometrium successfully accepts the foreign blastocyst during the timely implantation window. There is also an assortment of cells in various concentrations that are active during normal and pathologic pregnancy. These include NK cells, which are predominant immune cell population, and Th1, Th2 cytokines, which include tumor necrosis factor (TNF- α), interferon- γ and interleukins. It is the delicate interaction and balance between these various immune cells that results in successful pregnancy.
In successful pregnancy, the Th1/Th2 ratio reaches its peak during proliferative phase, significantly declines during secretary phase and is at lowest during early pregnancy decidua. During the early phase of pregnancy, a successful implantation occurs in a proinflammatory microenvironment, and a Th1-type response is followed by a shift to Th2, to control endocrine and immune interactions. Several cytokines such as TNF- α and IL-1 induce leukemia inhibitory factor (LIF) expression in the stroma and epithelial cells, and through their receptors provide paracrine signals to both embryonic tissues and uterine epithelium during implantation. Th1 response may be suppressed during pregnancy, via downregulation of nuclear factor. In addition, progesterone stimulates a Th2 type response, reduces proinflammatory cytokines, and represses allogenic responses, conferring fetal survival.
High levels of progesterone present at the interface of placenta and fetus induces the increased level of RANTES (regulator on activation, normal T cell expressed and secreted), which play an important role in Th1 response, contributing to a tolerogenic response at immune-privileged sites.5
In pregnancy, females respond in 3 positive ways. Her immunological system releases Th2 cells which are protective in nature. These in turn release interleukins 3, 4, 5, 6, 10 and 13, which safeguard the fetus. The second response is the production of asymmetric antibodies and these antibodies don’t bind to antigen and therefore there is no activation of complement cascade which is immunoprotective to fetus. Lastly NK cell production is decreased, thereby the fetus is protected.
Th1/Th2 ratio
Studies have demonstrated that during pregnancy Th1 responses are downregulated where as there is upregulation of Th2 response. Pregnancy seems to bring about a shift towards Th2 bias. These Th2 immune induce strong antibody production. IL-13, an anti-inflammatory Th2 cytokine has the capacity to limit inflammatory reactions that may arise locally at the site of implantation and within the placenta during pregnancy.
Asymmetric antibodies
During pregnancy, increased levels of asymmetric antibodies have been observed, which act as univalent (because of the presence of glycosyl group predominantly of the high mannose type in only one of two Fab fragments of the molecule) and as an antigen blocking agent. Due to their univalent behavior, they will be unable to form adequate antibody-antigen complexes, and also activate all the effector immune mechanisms. As no aggregation of the antibody takes place when it interacts with an antigen, effector immune mechanisms such as complement fixation, phagocytosis, opsonisation and clearance of antigen from the blood are not triggered. Nevertheless, since asymmetric antibodies can firmly combine with antigen, they act in a competitive way when they are remixed with precipitating symmetric antibodies of the same specificity and therefore, their action depends on their percentage in the mixture.6 During pregnancy there is preferential synthesis of asymmetric IgG molecules with anti-paternal specificity that are believed to exert a protective effect upon the fetus.
NK Cells
During normal human pregnancy, peripheral blood NK cells decrease in number, mainly as a result of a decrease in the CD16+ subset. In addition, peripheral NK cells from pregnant women exhibit decreased lytic activity compared with NK cells from controls. Furthermore, there is increased expression of inhibitory receptors among peripheral T cells and NK cells in the first weeks of pregnancy, reaching a maximum within the third month of gestation, with a subsequent decline to basal levels by the end of pregnancy (Figure 1).3
Immunomodulatory mechanism in abortogenic pregnancy
Immunomodulatory plays an important role in the abortogenic pregnancy. Increase in Th1 cytokines, with marked shift of Th1/Th2 bias, increased NK cell activity as well as production of symmetric antibodies predominate the abortogenic pregnancy. Direct effects of Th1 cytokines include the apoptosis of trophoblast cells by TNF α and IFN γ, inhibition of secretion of the growth-stimulating cytokines from the uterine epithelium and cytokine- induced activation of coagulation mechanisms, which may then lead to vasculitis affecting maternal blood supply to the implanted embryo. Also Th1 cells cause deleterious effects by activating other cytotoxic cells like NK cells, macrophages, and also by producing dangerous levels of nitrous oxide and TNF α, which together damage the conceptus leading to unsuccessful pregnancy.7
Peripheral blood NK cells are cytotoxic and an increase in number and activity has been associated with miscarriages and implantation failure.8
Increased levels of symmetric antibodies have been observed in abortogenic pregnancy. These antibodies form antigen-antibody complex and thereby activate the immune mechanism which results in the series of actions of phagocytosis, opsonization, etc. These reactions damage the conceptus leading to pregnancy loss.
The role of progesterone
Progesterone is critical for the establishment and the maintenance of pregnancy, both by its endocrine and immunological effects. It inhibits myometrial contractility and the onset of labor is thought to be the consequence of progesterone withdrawal. The effects of progesterone are mediated by a lymphocyte derived protein, progesterone-induced blocking factor (PIBF). This factor has pregnancy protective effects which may be mediated by inhibition of NK lymphocytes and induction of an anti-inflammatory cytokine response by increasing interleukin (IL)-3, IL-4, and IL-10 and decreasing IL-12 and interferon- γ.9
Immunosuppressant activity of progesterone prevents blastocyst rejection by increasing Th2 cells, facilitating implant by PIBF secretion.
Role of PIBF
PIBF is a protein, which can block NK cell-mediated lysis of cells. The expression of this protein by CD8 T lymphocytes (gamma/delta T cells) requires progesterone for expression, and it has therefore been called progesterone induced blocking factor. When there are sufficient levels of endogenous progesterone to form PIBF, this then safeguards the fetus by preventing inflammatory and thrombotic secondary reactions towards the trophoblast through three distinct actions:10
- By inducing asymmetric, protective ‘blocking’ antibodies
- By blocking NK cell degranulation
- By inducing T-helper 2 (Th2) cell dependent cytokines, thereby shifting the balance towards a Th2-dominated, cytoprotective immune response.
PIBF by acting on the phospholipase A2 enzyme interferes with arachidonic acid metabolism, induces a Th2 biased immune response, and by controlling NK activity exerts an anti-abortive effect (Figure 2).11
Role of progesterone supplementation in the management of RPL
Insufficient progesterone has been implicated as cause of sporadic and recurrent miscarriages. Progesterone has been used in different routes and doses to prevent miscarriage in early to mid-pregnancy. A study which evaluated the efficacy of progesterone supplementation in patients with recurrent pregnancy loss demonstrated increased live birth rate (63%) when compared to miscarriages (36%). This study provides support that progesterone supplementation reduces the subsequent miscarriage rate in patients with unexplained recurrent miscarriages.12
In assisted reproduction
Progesterone prepares the endometrium for pregnancy by stimulating proliferation in response to human chorionic gonadotropin (hCG), which is produced by the corpus luteum. This occurs in the luteal phase of the menstrual cycle. In assisted reproduction techniques (ART) the progesterone or hCG levels, or both, are low and the natural process is insufficient, so the luteal phase is supported with either progesterone, hCG or gonadotropin releasing hormone (GnRH) agonists. Luteal phase support improves implantation rate and thus pregnancy rates but the ideal method is still unclear. A review study analyzed the relative effectiveness and safety of various methods of luteal phase support in subfertile women undergoing assisted reproductive technology. This study showed a significant effect of progesterone for luteal phase support.13
Role of progesterone in preterm labor
In preterm labor, micronized progesterone supplementation helps in prolongation of labor. Prevention of preterm labor is a challenge to the attending healthcare specialist. One study evaluated the effect of micronized progesterone supplementation in the prevention of preterm labor. In this study women with singleton pregnancy with history of preterm labor, prophylactic circlage, uterine malformation or premature pain were chosen. Subjects were given 200 mg of micronized progesterone by vaginal route from gestational age of 28 weeks to 34 weeks 6 days and the rate of preterm labor in both, cases and controls as well as neonatal outcome with respect to morbidity and mortality was determined. Results of the study showed the incidence of preterm labor in subjects was 19.5% and in control group was 33.33%. This study showed that administration of micronized progesterone in pregnant women at high risk of developing preterm labor prolongs duration of pregnancy by an
average of two weeks and the difference between the subject and control groups is statistically significant (Figure 3).14
To Summarize…
Pregnancy is an altered physiological state, where the foetus which is a foreign organism to the mother, grows inside normally without any harm by the mother’s immune reactions. These immunologic changes which take place during pregnancy are unique. In this, number of factors influence their effect both at the systemic but mostly at the local uterine level to modulate the immune response which will then refrain from mounting an inflammatory response against the invading trophoblast. This immune modulation mainly involves NK cells, progesterone hormone along with other embryonic factors. The immune response is mainly controlled by PIBF under the influence of progesterone, to bias towards humoral response and suppresses cytotoxic response. Progesterone levels if sufficient at maternal-foetal interface stimulate the synthesis of PIBF which in turn result in asymmetric antibodies, Th2/Th1 bias, reduced NK cell activity which are necessary for successful pregnancy. In contrast, insufficient progesterone levels leads to decreased synthesis of PIBF which results in symmetric antibody production, Th1/Th2 bias and increased NK cell activity resulting in pregnancy loss.
Conclusion
Successful pregnancy has been considered a biologic example of semiallogeneic graft acceptance, in which the semiallogenic fetus is protected from immune attack from the mother. Studies have shown that Th2 cytokine profile favors successful pregnancy, while production of Th1inflammatory cytokines and complement activation result in higher rates of abortion.
Progesterone is essential in establishing and maintaining pregnancy. It inhibits myometrial contractility and the onset of labor is thought to be the consequence of progesterone withdrawal. Progesterone is critical for the establishment and the maintenance of pregnancy, both by its endocrine and immunological effects, as shown by a progesterone-dependent immunomodulation in maternal tolerance of the fetus, e.g. via PIBF and uterine homing of NK cells and up-regulation of HLA-Ggene expression in humans. Progesterone deficiency results in preterm birth, uneventful pregnancy, recurrent pregnancy loss. Various studies have supported the use of natural micronized progesterone in the prevention of miscarriages, preterm birth as well as recurrent pregnancy losses.
References
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