1 Blood Transfusion Service, 2 Department of Gynaecology and Obstetrics and 3 Department of Medical Statistics, University Hospital Nijmegen, PO Box 9101, 6500 HB, The Netherlands
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Abstract |
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Key words: abortion/CD56/folic acid/KIR/natural killer cells
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Introduction |
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The notion that the immune system requires modulation in order to ensure successful pregnancy has in the case of RSA already led to preventive measures, e.g. i.v. injection of immunoglobulin (Ig)G (Rigal et al., 1994; Ruiz et al., 1996
) and paternal leukocyte immunization (Daya and Gunby, 1994
; Check et al., 1997
).
Of particular interest is the role of NK cells. During early pregnancy the so-called uterine NK cells form the major population of immune competent cells in the maternalfetal interface. These cells carry inhibitory receptors for HLA antigens, most notably CD94/NKG2a, CD158a and CD158b (King et al., 1997a; Ponte et al., 1999
). At the placental interface HLA-G is the major HLA antigen with prolonged and significant levels of protein expression, but (transient) expression of HLA-C has also been observed (Hutter et al., 1996
; King et al., 1996
). In-vitro studies with both uterine and peripheral NK cells have shown that the cytotoxic reactivity against HLA-G expressing targets of distinct origin was inhibited (Rouas Freiss et al., 1997
; Rajagopalan and Long, 1999
). This suggests that the interaction of HLA antigens and NK cells at the placental interface could be critical in determining the outcome of pregnancy. So far, few groups have studied peripheral NK cell characteristics in RSA women. Beer, Kwak and co-workers have shown that before and during pregnancy NK cell numbers in RSA women are elevated, whereby high numbers appeared predictive of subsequent miscarriage (Kwak et al., 1995
; Beer et al., 1996
). Also, evidence has been obtained that NK cell cytotoxic activity is increased in RSA women (Aoki et al., 1995
; Higuchi et al., 1995
; Ruiz et al., 1996
), particularly in those with subsequent pregnancy failure. Intravenous IgG treatment and paternal leukocyte immunization have already successfully been used to down-regulate NK cell activity (Higuchi et al., 1995
; Kwak et al., 1996
; Ruiz et al., 1996
).
We here report the results of a longitudinal study comprising pre- and post-conceptional data on both NK cell cytotoxicity and NK cell phenotypes in RSA women with either a successful or failed pregnancy. Thus, we were able to answer whether pre-conceptional NK cell parameters differ between RSA women and controls and whether these are indicative of subsequent miscarriage. We assessed the changes in NK parameters within groups from pre- to post-conceptional stages and how these values differed between RSA women and controls at several points during the course of pregnancy. In the course of this study we also assessed the effect of folic acid intake on NK cell characteristics.
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Materials and methods |
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NK cell cytotoxic reactivity
From all blood samples, peripheral blood mononuclear cells (PBMC) were isolated by density centrifugation (Lymphoprep®; Nycomed, Oslo, Norway) and stored in liquid nitrogen until use. NK cell reactivity was assessed using a 51Cr release bioassay. In short: effector cells were thawed and incubated overnight in medium containing RPMI 1640 Dutch modification (Life Technologies, Paisley, UK) supplemented with 10% fetal calf serum (FCS) (PAA, Linz, Austria), 0.02 mmol/l sodium pyruvate, 100U/ml penicillin and 100 µmol/l streptomycin (both Life Technologies); 1x106 K562 target cells were labelled with 3.7 MBq 51Cr (Amersham, UK) for 2 h. Then, 1x103 labelled target cells were added to 80x103, 40x103, 20x103 or 10x103 of effector cells resulting in four different effector/target (E/T) cell ratios (80:1, 40:1, 20:1, 10:1). Incubation was for 4 h in a humidified incubator at 37°C and 5% CO2. After incubation, 100 µl of supernatant was harvested and radioactivity was measured using a -counter (Wallac 1470
counter, Turku, Finland). The percentage specific NK cell cytotoxic reactivity was determined as follows:
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Flow cytometric labelling and analysis
Cells were phenotypically analysed by a two-step double labelling procedure. Briefly, cells were washed three times with fluorescence activated cell sorting (FACS) buffer [phosphate buffered saline (PBS) containing 0.5% bovine serum albumin (BSA)] and labelled first with (un)conjugated specific antibody, if necessary followed by conjugate binding (GAMFITC; Dako, Glostrup, Denmark). Thereafter the cells were labelled with a secondary conjugated specific antibody. All incubations were for 30 min and thereafter the cells were washed twice. The samples were run on an XL-Epics (Coulter Electronics) and 5000 or 10 000 events were collected based on live lymphocyte cell gating as indicated by propidium iodide (5 µg/ml) staining. Isotype matched antibodies were used to define marker settings, isotype matched controls were usually below background staining. Data were analysed by Coulter XL-2 and/or WINMDI software. For analyses of NK cell (subset) numbers conjugated CD16-FITC (clone DJ130c),
CD56-RPE (clone MOC-1),
CD3-FITC or
CD3-RPE (clone UCHT1) antibodies were used (all from Dako, Glostrup, Denmark). NK cell inhibiting/activating receptor expression was characterized by conjugated
CD158a-PE (clone EB6),
CD158b-PE (clone GL183) and unconjugated
NKG2a (clone Z199) antibodies (all from Immunotech, Marseille, France). Isotype matched control antibodies were directed against Aspergillus niger (Dako, Glostrup, Denmark).
Statistical methods
The t-test for independent samples was used to test for statistical significance of the differences in the mean values of the age and NK cell parameters between, first, the RSA group and the intake controls and, second, the RSA group and follow-up controls. The mean values with 95% confidence intervals (CI) are presented.
The t-test for paired samples was used to test for statistical significance of the differences in the mean values of age and NK cell parameters within the RSA group before and after folic acid supplementation. The mean values with 95% CI are presented.
Repeated measurement analysis adjusting for the pre-conceptional value was used to test for statistical significance of the differences between the RSA group and follow-up controls in the mean NK cell parameters at 8 and 12 weeks after conception. The estimated means with 95% CI are presented.
Differences between the groups may be influenced by the loss of information of those women who miscarried. Therefore the repeated measurement analysis was also performed using the method of last observation carried forward (LOCF). The LOCF method was performed at each assessment point, substituting missing values after miscarriage at all subsequent assessments by the last observed value after conception. The results of both analyses were similar and those using the LOCF method are presented here. It is obvious that these variables are not mutually independent. Therefore the P values are presented unadjusted for analysing multiple variables. Note that P values are adjusted for multiple comparisons of each variable (Table II: TukeyKramer).
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Results |
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During folic acid treatment NK cell levels in RSA women were compared with follow-up control women, successfully partaking in an IVF procedure and also receiving folic acid treatment. None of the parameters tested revealed a substantial difference between RSA women and controls. The estimated mean LU levels in the RSA and follow-up control group were 393.1 (95% CI: 362.2; 423.9) and 364.5 (95% CI: 338.1; 391.0) respectively (P = 0.165) and the mean total NK cell levels were 15.8% (95% CI: 12.8; 18.8) and 12.5% (95% CI: 8.8; 16.2) respectively (P = 0.169).
Effects of pre-conceptional folic acid supplementation in the RSA women
Since RSA women with a wish to conceive received folic acid treatment upon entering the clinic, we analysed whether the pre-conceptional intake of folic acid was associated with a subsequent change in NK cell parameters. A comparison of peripheral NK cell levels and activity before and during folic acid treatment within the RSA group (Table I) revealed no statistically significant change in cytotoxic activity. LU levels were 411.0 (95% CI: 378.1; 44.0) before and 393.1 (95% CI: 362.2; 423.9) during treatment (P = 0.185). Also, neither the total number of NK cells nor the number of T cells changed substantially. However, when looking at NK cell subsets, we did observe a significant rise in single CD56pos NK cells, from 1.4% (95% CI: 0.7; 2.2) up to 2.4 (95% CI: 1.5; 3.4) associated with a significant change in the ratio single CD56pos/double CD56posCD16pos cells from 0.25 (95% CI: 0.06; 0.44) up to 0.55 (95% CI: 0.28; 0.82)(P = 0.008).
Post-conceptional peripheral NK cell reactivity and cell numbers
We were also interested in finding out whether during the first trimester of pregnancy in RSA women (including both women who carried to term as well as those that aborted) NK and T cell parameters underwent distinct changes in comparison with controls.
Figure 1 shows the course of NK cell cytotoxicity levels and cell numbers starting from pre-conceptional values up to 12 weeks of menstrual age, in both RSA women and controls. From Figure 1
it is clear that differences between the groups are most prominent at week 8 of menstrual age. To be better able to evaluate the differences between the groups, the estimated values as derived from the repeated measurements model are depicted in Table II
. The differences were calculated for the overall period (column 2), and for each of the assessment points separately (columns 3 and 4). Adjusted for the pre-conceptional values, it appeared NK cell cytotoxicity (expressed as lytic units) at week 8 was significantly increased in the RSA group as compared with the controls [a difference of 55.2 (95% CI: 10.6; 99.8)]. But this difference tapered off and was almost negligible at week 12 [6.9 (95% CI: 37.1; 50.9)]. Similarly, NK cell levels in RSA women were higher during pregnancy [an overall difference of 4.0% (95% CI: 0.4; 7.6)], mainly due to a higher level of double CD56posCD16pos cells. This effect was again most prominent at week 8 of menstrual age [5.4% (95% CI: 0.2; 11.0)] but in this case was maintained at week 12. Levels of single CD56pos cells were lower in RSA women as compared with controls, both at weeks 8 and 12. In combination with the higher levels of double CD56posCD16pos cells this resulted in a notable reduction in the ratio single CD56pos/double CD56posCD16pos NK cells [0.65 (95% CI: 1.0; 0.3)]. Notably, the concomitant rise of NK cell cytotoxicity, increased numbers of CD16pos NK cells and decreased levels of single CD56pos cells fits the known characteristics of NK cell activation.
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Risk factors for miscarriage within the RSA group
Although all RSA women had a history of miscarriages, two-thirds of these women successfully carried their next pregnancy to term. Consequently, we looked for parameters that could predict the pregnancy outcome in the group of RSA women. The effect of the following parameters, measured before pregnancy, on the probability of a miscarriage was studied using univariate logistic regression analysis in the group of RSA women with two or more spontaneous abortions: age, LU, % NK cells, % double CD56posCD16pos cells, % single CD56pos cells, ratio single CD56pos/double CD56posCD16pos cells, % CD3pos cells, % double CD3pos CD56pos cells. Of these parameters only age proved to be a substantial risk factor with a 37% increased risk of miscarriage with each additional year of age (OR = 1.37; 95% CI: 1.07; 1.92; P = 0.03).
In addition, using the data from the regression analysis, we dichotomized the data for the above mentioned parameters and analysed both women with two or more abortions and those with three or more abortions. Table III shows the results for the pre-conceptional values (women were on folic acid).
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KIR/KAR and NKG2a expression on CD56pos cells
Both uterine and peripheral NK cells can be activated or inhibited via specific surface molecules. The killing inhibitory/activating receptors CD158a, CD158b and NKG2a are examples of such surface molecules and are thought to interact with their ligands, HLA-C and HLA-E (presenting processed HLA-G), respectively. These receptors are known to be expressed at the fetalmaternal interface. In order to ascertain whether there are differences in NK cell inhibitory receptor repertoire between RSA women and controls, we determined the expression of CD158a, CD158b and NKG2a receptors on peripheral NK cells by flow cytometry (as shown in Figure 2). Expression was similar for all groups studied. CD158a, CD158b and NKG2a were expressed on approximately 20, 30 and 45% of CD56pos cells respectively.
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Discussion |
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Notably, before pregnancy we could not detect a significant difference in either NK cell cytotoxicity or numbers of double CD56posCD16pos cells between RSA women (grouped irrespective of the outcome of their next pregnancy) and controls, but we did observe a marked difference between these groups in early pregnancy. Whereas both cytotoxicity and NK cell numbers increased in the RSA group with advancing pregnancy, peaking at week 8 menstrual age, the opposite effect was found in the controls. The controls were women partaking in an IVF procedure, but exactly the same trend was previously observed for other healthy pregnant women (Okamura et al., 1984), validating our choice of controls in this respect.
In the RSA women the ratio of single CD56pos/double CD56posCD16pos cells was altered in favour of the CD16-expressing cells. Since CD16, and not CD56, was recently characterized as a lysis receptor mediating direct cytotoxicity (Mandelboim et al., 1999), the concomitant rise in levels of cytotoxic reactivity found in the RSA group was not surprising.
The fact that in our study pre-conceptional cytotoxic reactivity or NK cell numbers in RSA women could not be linked to a higher risk of miscarriage as was previously shown (Aoki et al., 1995; Coulam et al., 1995
) cannot so easily be explained. As in our study, both groups used as definition two or more miscarriages; however, differences in protocols, for instance the influence of cryopreserved versus fresh PBMC (Kawai et al., 1988
), cannot be excluded. In any case directly after conception these parameters reached significantly higher levels in RSA women compared to controls. Together with the fact that within our group of RSA women pre-conceptional NK cell levels were found to be predictive of subsequent outcome, the observations suggest that these cells are either directly or indirectly involved in the process of spontaneous abortion.
As regards the intake of folic acid, we found a positive effect of daily supplementation on single CD56pos cells in RSA women. This was associated with a slight, but not significant drop in pre-conceptional cytotoxic reactivity. Folic acid is capable of lowering plasma homocysteine levels (Nelen et al., 1998), and a high plasma level of this protein is a known risk factor for reproductive disorders such as neural tube defects, placental pathology, pre-eclampsia and RSA (Obwegeser et al., 1999
). It is not yet clear whether the effects of folic acid on reproductive performance are limited to the homocysteine pathway or are in fact more extensive. Based on our data it would be of interest further to elucidate the role of folic acid on NK cell function.
In conclusion, we have found evidence for the hypothesis that in women with a history of recurrent spontaneous abortion low pre-conceptional peripheral NK cell levels are indicative for a subsequent successful pregnancy. During pregnancy, RSA women have markedly increased NK cell cytotoxicity, associated with a rise in double CD56posCD16pos cells and a concurrent drop in single CD56pos cells. Thus in RSA, diagnostic and therapeutic measures aimed at characterizing and modulating NK cell activity appear promising in the future.
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Acknowledgments |
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Notes |
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References |
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Submitted on September 24, 1999; accepted on January 6, 2000.