Update on Transplacental Transfer of IgG Subclasses: Impact of Maternal and Fetal Factors

Abstract

Transplacental antibody transfer from mother to fetus provides protection from infection in the first weeks of life, and the four different subclasses of IgG (IgG1, IgG2, IgG3, and IgG4) have diverse roles in protection against infection. In this study, we evaluated concentrations and transplacental transfer ratios of the IgG subclasses in a healthy UK-based cohort of mother-cord pairs, and investigated associations with maternal, obstetric, and fetal factors. In agreement with previous studies, we found a strong association between maternal and cord IgG for all subclasses. We report a transfer efficiency hierarchy of IgG1>IgG3>IgG4=IgG2 in our study population, and our review of the literature demonstrates that there is no consensus in the hierarchy of subclass transfer, despite the commonly made statement that the order is IgG1>IgG4>IgG3>IgG2. We report additional data regarding negative associations between elevated maternal IgG concentrations and maternal/cord transfer ratios, finding an effect on IgG1, IgG2, and IgG3 subclasses. Levels of IgG subclasses were the same between venous and arterial blood samples from the umbilical cord, but there was a significantly higher level of total IgG in arterial blood. We found no correlation between placental FcRn protein levels and IgG transfer in our cohort, suggesting that IgG is the main determinant of observed differences in transplacental transfer ratios at term. Neonatal IgG1 and IgG4 levels were increased with later gestation at delivery, independent of any increase in transplacental transfer, indicating that the benefit of later gestation is through accumulation of these subclasses in the fetus. Neonatal IgG2 levels and transfer ratios were reduced in rhesus-negative pregnancies, suggesting that administered anti-D antibodies may compete for transplacental transfer of this subclass. Maternal influenza vaccination resulted in elevated maternal and neonatal levels of IgG4, whereas maternal Tdap vaccination had no impact on neonatal levels of the subclasses, nor transfer. However, within Tdap vaccinated pregnancies, later gestation at Tdap vaccination was associated with higher transplacental transfer. Our study provides information regarding levels and transfer of IgG subclasses in healthy term pregnancies and demonstrates the importance of recording detailed clinical information in studies of antibody transfer, including parity, ethnicity, and timing of maternal vaccine delivery.

Keywords: IgG; antibody; immunology; infection; maternal vaccination; neonatal; placenta; pregnancy.

Figure 1

Overview of studies investigating maternal to fetal transfer ratios of IgG subclasses. We identified 17 papers that measured paired maternal and cord levels of all four IgG subclasses (–, –29). Studies had either employed radioimmunoassay, immunodiffusion, laser nephelometry or ELISA. The line graph represents the mean to fetal transfer ratio on a log(2) axis. Levels >1 (above the dotted line) indicates higher levels in cord blood compared to maternal blood. The dotted black line indicates the mean of the 16 studies. For Malek et al. (29) we only used the values from term pregnancies (37–41weeks). For two studies, we performed ratio calculations based on reported maternal and cord concentrations (21, 29). For two other studies, values were calculated by reading from the published graphs (13, 15). For Costa-Carvalho et al. (13) this required us to read the ratios from the summary graph produced in the review by Palmerio et al. (12). In Hay et al. (28), the maternal levels of IgG1 were the lowest of all the other subclasses and maternal IgG2 was the highest (double the level of IgG1).

Figure 2

Maternal and cord blood IgG subclass profiles and transfer ratios. (A) IgG1-4 concentrations (μg/ml) in maternal (M) and cord (C) blood on a log(2) axis. (B) Maternal to fetal transfer ratios of IgG1-4 represented on log(2) scale. Values >1 indicates higher levels in cord blood, indicated by dashed line. Analysis performed on log(2) transformed transfer ratios, using Repeated Measures One Way ANOVA and Holm Sidak's multiple comparisons test. (C) Maternal to fetal transfer ratios of IgG1–4 normalized to levels of total IgG in maternal blood as an indicator of transfer efficiency. Analysis by Repeated Measures One Way ANOVA and Holm Sidak's multiple comparisons test. (D) Maternal to fetal transfer ratios of IgG1–4 represented on log(2) scale broken down by gestational age at delivery. Values >1 indicates higher levels in cord blood, indicated by dashed line. Analysis performed on log(2) transformed transfer ratios, using Repeated Measures One Way ANOVA and Holm Sidak's multiple comparisons test. Hierarchy of IgG1>IgG3>IgG2>IgG4 for 37–38 weeks and 40+ weeks gestation. Hierarchy of IgG1>IgG3>IgG4>IgG2 for 39 weeks gestation. (E) Dot-line plots showing the relationship between IgG concentrations in maternal and cord pairs. Groups compared by paired t test on nLog-transformed data. (F) Correlations between maternal and cord IgG concentrations. Lines indicate simple linear regression. nLog values analyzed by Pearson r-test. N = 114 maternal and cord pairs. ****p < 0.0001, ***p < 0.001, **p < 0.01, *p < 0.05.

Figure 3

Impact of maternal IgG concentrations on the rate of maternal to fetal IgG transfer. Heatmap with overlaid scatterplots showing the effect of increasing concentrations of maternal IgG (X axis) on the transfer rates of total IgG (TIgG) and IgG1–4 from mother to fetus (Y axis, Log2 scale). Horizontal dashed line represents equal levels between mother and fetal circulations. Correlations between nlog-transformed maternal IgG concentrations and log(2)-transformed transfer ratios were analyzed by Spearman's rank correlation coefficient. The scale on the right indicates the color associated with the Spearman r-values. Inverse correlations are depicted in the darker shades, and weaker and no associations are depicted in lighter shades toward yellow. The lines on the scatterplots indicate simple linear regressions. ****p < 0.0001, ***p < 0.001, **p < 0.01, and *p < 0.05.

Figure 4

Comparison of IgG subclasses in umbilical cord vein and arterial blood. (A) Illustration of the placental circulation, showing the umbilical arteries bringing deoxygenated blood from the placenta to the fetus, and oxygenated blood returning to the fetus from the placenta via the umbilical vein. (B) Dot-line plots showing the relationship between IgG concentrations in paired umbilical cord vein and artery serum samples (n = 30). Groups compared by paired t-test on nLog-transformed data. **p < 0.01 (C) Correlations between maternal and cord IgG concentrations. Lines indicate simple linear regression. nLog values analyzed by Pearson r-test. N = 30 matched umbilical vein/umbilical artery serum pairs and 20 matched maternal/umbilical vein/umbilical artery serum trios.

Figure 5

Correlation of placental FcRn levels with transfer of IgG subclasses. (A) Western blot for FcRn in lysates of placental villous tissue. Proteins were prepared under reducing denaturing conditions. Top labels indicate gestational age of the sample in weeks+days. (B) Total protein stain (left panel) and densitometry analysis (right panel). (C) Correlation between placenta FcRn protein levels (normalized to total protein stain) and maternal to fetal IgG transfer ratios (log(2) transformed) analyzed by Spearman's rank correlation coefficient. All correlations non-significant at p > 0.05 level (Total IgG p = 0.51, Spearman r 0.156; IgG1 p = 0.80, r = −0.062; IgG2 p = 0.54, r = 0.146; IgG3 p = 0.67, r = −0.101; IgG4 p = 0.26, r = −0.266). N = 20.

Figure 6

Correlation between gestation at Tdap vaccination and placental transfer of total IgG and IgG subclasses. (A) Correlation between the gestation that pregnant women received the Tdap vaccine and the maternal to fetal IgG transfer ratio for total IgG and IgG subclasses [log(2) Y-axis]. Analysis was performed on log(2) transformed ratios by Spearman's rank correlation coefficient, and resultant p values are stated in the bottom right corner of each plot. Coefficients: Total IgG: 0.242, IgG1: 0.316, IgG2: 0.171, IgG3: 0.353, and IgG4: 0.316. (B) Correlation between the gestation at vaccination and the antibody concentration in maternal blood and cord blood at birth. Analysis performed on log-transformed data by Spearman's rank correlation coefficient. All correlations non-significant at p > 0.05 level. Lines indicate simple linear regression. Coefficients for maternal IgG: Total IgG: −0.044; p = 0.752, IgG1: −0.145; p = 0.262, IgG2: −0.180; p = 0.161, IgG3: 0.013; p = 0.919, IgG4: 0.029; p = 0.823. Coefficients for cord IgG: Total IgG: 0.235; p = 0.079, IgG1: 0.197; p = 0.125, IgG2: 0.134; p = 0.296, IgG3: 0.164; p = 0.199, and IgG4: 0.191; p = 0.133. N = 63.