Placenta
Volume 28, Supplement , Pages S51-S56, April 2007

How Does the Maternal Immune System Contribute to the Development of Pre-eclampsia?

Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QP, UK

Accepted 27 November 2006. published online 10 February 2007.

Article Outline

Abstract 

An immunological aura has hovered over the study of pre-eclampsia for many years but there has still been little progress in explaining the various ‘immune’ phenomena associated with this elusive disease. When considering the primary defect of placentation that leads to pre-eclampsia the focus should be on the intermingling of the invasive placental trophoblast cells with maternal leukocytes in the uterine wall. The MHC status of trophoblast cells is a crucial factor to be considered, as these molecules can act as ligands for uterine immune cells, including T cells, NK cells and myelomonocytic cells. Extravillous trophoblast cells express an unusual combination of HLA-C, HLA-G and HLA-E molecules and only one of these HLA molecules, HLA-C, shows any appreciable polymorphism. In humans, uNK cells express an array of receptors, some of which are known to bind to the HLA class I molecules expressed by extravillous trophoblast cells. HLA-C is the dominant ligand for killer immunoglobulin-like receptors (KIR) expressed by uterine NK cells that may deliver an inhibitory or activating signal. KIR haplotypes comprise two groups, A and B; these differ principally by having additional activating receptors in the B haplotype. In any pregnancy, the maternal KIR genotype could be AA (no activating KIR) or AB/BB (presence of between one and five activating KIRs). The HLA-C ligands for KIR on trophoblast cells may belong to two groups, C1 and C2 that are defined by a dimorphism at position 80 of the α1 domain. This maternal–fetal immunological interaction, occurring at the site of placentation, therefore involves two polymorphic gene systems, maternal KIRs and fetal HLA-C molecules. Uterine NK-cell function is thus likely to vary in each pregnancy. In pre-eclamptic pregnancies we have found that some KIR/HLA-C combinations appear unfavourable to trophoblast-cell invasion due to the overall signals that the NK cell receives. The academic excitement of this work is the realisation that this is a novel form of allorecognition based on NK cells that operates entirely differently from self/non-self discrimination used by T cells.

Keywords: Pre-eclampsia, NK cell, Trophoblast, Killer immunoglobulin-like receptor, HLA-C

 

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1. Immune phenomena in pre-eclampsia 

Although pre-eclampsia remains a ‘disease of theories’, two observations about this disease have stood the test of time. Firstly, it is generally agreed that the primary pathogenesis of the condition in most cases is inadequate transformation of the spiral arteries by extravillous trophoblast cells (EVT) [1]. Secondly, the view that some sort of ‘immune maladaptation’ is responsible for the development of the disease is widely held [2], [3]. How can these two observations be reconciled?

The initial reports from Robertson, Pijnenborg and colleagues that trophoblast invasion was defective in pre-eclampsia have been confirmed in numerous studies and are now supported by Doppler ultrasound studies of maternal uterine blood flow [4], [5]. Many mechanisms have been proposed to explain what controls the degree of infiltration of trophoblast. The immune cells of the mother are good candidates because these are the cells in an individual that are capable of discriminating between foreign fetal trophoblast cells and ‘self’ maternal cells. When thinking about the regulation of placentation this means that it is the immune cells in the uterus that are important to consider. As the trophoblast cells migrate through the decidua they contact leukocytes that are predominantly components of the innate immune system, natural killer (NK) cells and macrophages [6]. How can the immune phenomena associated with pre-eclampsia be connected with the local uterine immune response?

Traditionally, immune responses have been characterised by the features of memory and specificity. Pre-eclampsia is mainly a disease of primiparous women and subsequent pregnancies seem to be at low risk [7], [8], [9]. This has been considered akin to a ‘vaccination’ by the placenta so that after a first exposure the mother is protected. The protective effect of the first pregnancy has been interpreted as similar to immunological memory but, importantly, this is only seen if the first pregnancy is normal. In contrast, if the first pregnancy is complicated by pre-eclampsia then subsequent pregnancies are still at considerable risk [10], [11]. Therefore, the pre-eclamptic placenta seems not to exert the same protective effect, an observation that is more difficult to explain in terms of immunological memory.

The concept of immunological specificity in pre-eclampsia refers to the disease being partner specific. Whether this occurs or not is a subject of continual debate because of additional confounding factors such as a long birth interval [8], [12]. Recent reports suggest that as the years accumulate after a first pregnancy then the risk does fall in women who have not had pre-eclampsia irrespective of the partner. However, there are many other studies that have shown that if the first pregnancy is normal then the risk does increase in subsequent pregnancies if there is a change of partner. Conversely, if a primipara has pre-eclampsia, then the risk falls with a change of partner [13], [14]. A further observation that might have an immunological explanation is that in pregnancies resulting after oocyte donation the risk of pre-eclampsia is increased up to 30% [15]. In this situation, in immunological terms, the fetus is entirely ‘non-self’ and completely lacks the recipient mother's ‘self’.

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2. Immunology of placentation: NK cell receptors and trophoblast ligands 

Both parents have a genetic contribution to pre-eclampsia but it is clear that the mother's contribution is greater than the father's [16], [17]. No particular genes have been identified as yet and the possibility that some could be immune system genes is still open. Certain ethnic groups have a particularly high incidence of the disease, notably Afro-Caribbeans and African Americans. Furthermore, there is an increased incidence when there is racial dissimilarity between the parents [18], [19]. The obvious gene system that shows considerable ethnic variation is the major histocompatibility complex (MHC), but there have not been any convincing associations of any particular MHC genes or of phenomena such as MHC sharing between parents to indicate that the MHC is of importance in the development of the syndrome.

How can all these observations be viewed in the context of the maternal immune response to the invading trophoblast? Previous attempts to explain how the immune system functions in normal and abnormal pregnancies have generally centred on the necessity for maternal T-cell immunomodulation as set out by Medawar [20]. The problems with using this old paradigm are two: Firstly, T cells are a minor component of lymphocytes at the placental bed; it is NK cells that make up to 70% of decidual leukocytes. Secondly, EVT does not display any major T-cell ligands, specifically HLA class II and HLA-A and HLA-B class I molecules [6]. Instead, the invading trophoblast cells express an unusual and unique combination of three class I molecules, HLA-G, HLA-E and HLA-C. Of these only HLA-C shows any appreciable polymorphism and will therefore vary depending on the father's contribution. Furthermore, HLA-C is the dominant ligand for NK cells, making it an attractive candidate for a trophoblast molecule that is recognised by uterine NK cells. The receptors that bind to different groups of HLA-C allotypes are known as killer immunoglobulin-like receptors (KIR) [21], [22]. Like the MHC, this family of genes also shows extreme genomic variation both in the number of individual KIR genes an individual possesses together with allelic polymorphism at each KIR locus. Thus, there is a receptor-ligand system, KIR in the mother and HLA-C in the fetal trophoblast, where both elements show polymorphism. It is possible that certain combinations of particular KIR with HLA-C ligands may not allow optimal trophoblast invasion and these would be associated with pre-eclampsia.

We therefore compared the KIR and HLA-C genotypes in women with pre-eclampsia and those who had normal pregnancies [23]. HLA-C allotypes can be discriminated by KIR as two groups that differ at position 80 of the α1 domain, HLA-C1asn80 and HLA-C2lys80 (Fig. 1). Both groups can be recognised by KIR that are either inhibitory or activating, so that the balance of signals the NK cells receives will vary depending on the combination of KIR available for a particular HLA-C ligand. KIR genotypes can be simply divided into two haplotypes, A and B. The A haplotype has only inhibitory KIR for HLA-C whereas the B haplotype has a variable number of activating KIR (Fig. 2). Almost all individuals have inhibitory KIR for both HLA-C groups [24]. Women were identified as having KIR AA, AB or BB genotypes on the basis of the multiple KIR genes they possessed. Similarly, the fetal HLA-C groups in each pregnancy were categorised as C1+1, C1+2, or C2+2. We then looked to see which particular combinations occurred in pre-eclampsia (Fig. 3). As can be seen, the poor combinations are when there is an AA KIR genotype in the mother combined with the presence of an HLA-C2 group in the fetus (Fig. 4). Notably, the maternal KIR genotype appears unimportant when a homozygous C1+1 is present in the fetus. An explanation for this may be that comparison of the strength of the interactions between the two HLA-C groups and their respective inhibitory KIR has indicated these are not equivalent molecular bonds. The interaction between the KIR2DL2/3 and HLA-C1 allotypes is weakly inhibitory compared to the strong KIR2DL1/HLA-C2 interaction [24]. This means that decidual NK cells will be more strongly inhibited in pregnancies where the mother has a HLA-C2 gene in her fetus.

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  • Fig. 1 

    KIR receptors and their HLA-C ligands. Both HLA-C1 and HLA-C2 groups have corresponding inhibitory and activating KIR receptors. Inhibitory receptors signal through intra-cellular ITIM motifs and activating receptors on association with the adaptor molecule DAP-12 which has ITAM motifs.

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  • Fig. 2 

    The A and B KIR haplotypes differ essentially by the number of activating receptors they possess. Both haplotypes may have inhibitory receptors for HLA-C1 and C2 but only B haplotypes have the corresponding activating receptors.

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  • Fig. 3 

    Certain combinations of maternal KIR and fetal HLA-C genotypes increase susceptibility to pre-eclampsia. A cross (×) indicates the increased risk of a poor clinical outcome.

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  • Fig. 4 

    The frequency of the maternal KIR AA genotype is significantly increased in pre-eclamptic pregnancies as compared to non-affected but only when the fetus has a HLA-C2 allele in homozygous or heterozygous state. There is no associated risk of pre-eclampsia when the fetus is a HLA-C1 homozygote.

The obvious question that arises is: what is the protective KIR gene or genes on the B haplotype that protect against pre-eclampsia when there is an HLA-C2 in the fetus? To answer this we have looked at the frequency of individual KIR genes on the B haplotype compared with controls. This B haplotype can have anything between one and five additional activating receptors (Fig. 2). Preliminary analysis shows that it is the KIR genes at the telomeric end of the haplotype that are significantly reduced in pre-eclampsia (Fig. 5) (unpublished). The presence of the activating receptor for HLA-C2 (KIR2DS1) amongst these protective KIR genes is intriguing and makes biological sense. The presence of this gene could override the very strong inhibitory signal given by HLA-C2 binding to the inhibitory KIR2DL1. We are now extending our analysis to different groups of women with other types of reproductive failure. In recurrent miscarriage and fetal growth restriction, preliminary results are similar (unpublished). This reinforces the view that failure of adequate trophoblast invasion is the primary problem in many of these pathological pregnancies.

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  • Fig. 5 

    KIR frequencies in control and affected pregnancies when HLA-C2 is present in the fetus. There is a significant lack of the closely-linked KIR genes at the telomeric end of the KIR B haplotype.

A speculative interpretation of the results is that the presence of HLA-C2 in the fetal trophoblast cells provides a strong inhibitory signal that needs to be overcome by the presence of activating KIR (Fig. 6). Recent research has highlighted that uterine NK cells make an unusual array of chemokines and angiogenic cytokines [25], [26], [27]. Furthermore, the levels made can be altered as a result of KIR ligation meaning that the cytokine soup that the invading trophoblast cells are exposed to could be different in pregnancies with different KIR/HLA-C combinations [28].

Our findings can provide an explanation for how an interaction between maternal NK cells and trophoblast using KIR/HLA-C interactions could modify the depth of trophoblast invasion in different pregnancies. Importantly, this interaction occurs temporally and spatially at the site of placentation when trophoblast cells invade in the early part of pregnancy. A molecular cross-talk between maternal lymphocytes and fetal trophoblast has never been defined in terms of T-cell reactions and indeed no convincing mechanism for T cells to recognise trophoblast has ever been described.

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3. Interpretation of immune phenomena in relation to maternal KIR and fetal HLA-C interaction 

Is it possible to explain the immune phenomena that are so frequently invoked as one of the causative mechanisms in the disease? Partner specificity could be explained by our findings. If the mother has a KIR AA KIR genotype and lacks the protective B haplotype genes she may have a successful first pregnancy with her partner if the fetus is HLA-C1+1. However, if a subsequent partner donates a HLA-C2 gene to their fetus this second pregnancy will then be at risk. Conversely, women with an AA KIR genotype who are lacking the protective B haplotype genes may have pre-eclampsia in the first pregnancy if her partner donates HLA-C2 genes to the fetus. If her second partner is HLA-C1+1 then this subsequent pregnancy should be normal. However, if she herself has a HLA-C2 gene she will always have a risk of the poor KIR/HLA-C2 combination with her fetus. These might be the unfortunate women who have recurrent pre-eclampsia. We can also speculate that in an oocyte donation pregnancy, any woman who is lacking the protective B haplotype genes will be at risk if there is a HLA-C2 gene in either the egg or sperm donor.

The idea that there is any kind of immunological ‘memory’ in pre-eclampsia is more difficult to explain. Any woman who lacks B haplotype genes will be at risk if she encounters C2 in the fetus either from herself or a partner. In contrast, women who have the protective B haplotype genes will always be protected in the first and any subsequent pregnancies whatever their partner's HLA-C genotype. Theoretically, women most at risk from reproductive failure are those who are both lacking the protective B haplotype genes and also have one or two HLA-C2 genes. Analysis of more pregnancies that are well characterised clinically will be needed to see if these speculative predictions are true.

Our results can provide an explanation for why there is more influence of the maternal rather than the paternal genetic contribution in pre-eclampsia (Fig. 7). The paternal contribution is only to provide one of the HLA-C genes to the fetus. Males who are homozygous for HLA-C2 will obviously carry an increased risk in any pregnancy they father and could perhaps be the ‘dangerous males’ [16], [29]. In contrast, the mother's KIR genotype will shape the response of uterine NK cells and, in addition, she will also donate an HLA-C2 gene to the fetus. It appears that KIR genes may be much more important for women than for men; they could even be considered as ‘feminist’ immune system genes.

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  • Fig. 7 

    Genetic predisposition to pre-eclampsia involves both maternal and paternal genes. However, the maternal genotype has a greater contribution to the risk. Odds ratios as published by Skjaerven et al. [17].

The KIR and HLA loci are on separate chromosomes and segregate independently. Given the association of HLA-C2 and KIR AA genotypes with reproductive failure it would seem logical to predict that these gene combinations would be selected against. We therefore looked at the published data on gene frequencies of these in different populations. There is a striking inverse association between KIR AA and HLA-C2 frequencies in a wide range of ethnic groups, so that these two genotypes are not usually found at high frequency together (Fig. 8). Intriguingly, the population that seems to have the highest frequency of both are Afro-Caribbeans, a group known to be at high risk of pre-eclampsia. These observations suggest that there is selective pressure on these two gene systems from reproductive success. In addition, our findings are in keeping with reports showing that Asian males are low-risk fathers because they have a low frequency of HLA-C2 [19]. This provides a fascinating link between the immune and reproductive systems. An obvious question to ask is: why are these genes retained in a population? There must be balancing selective pressures to retain the KIR AA and HLA-C2 genotypes and whether these are to provide defence against other diseases such as particular infections or whether they involve reproductive performance in some other way remains to be seen.

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4. Conclusions 

Overall, our findings do provide new ideas about how the maternal immune system may be operating in pregnancy to regulate placentation and the depth of intrusion of trophoblast cells into the mother. Importantly, this system does not follow any other immunological paradigm. What is operating in the uterus is a system where maternal NK cell receptors can variably bind to HLA-C ligands on the fetal trophoblast cells whose function is to transform the spiral arteries. This provides a potential explanation for why trophoblast modification of arteries does not occur properly in pathological pregnancies such as those complicated by pre-eclampsia and growth restriction.

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PII: S0143-4004(06)00278-5

doi:10.1016/j.placenta.2006.11.008

Placenta
Volume 28, Supplement , Pages S51-S56, April 2007

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