Associations between an IgG3 polymorphism in the binding domain for FcRn, transplacental transfer of malaria-specific IgG3, and protection against Plasmodium falciparum malaria during infancy: A birth cohort study in Benin

Abstract

Background: Transplacental transfer of maternal immunoglobulin G (IgG) to the fetus helps to protect against malaria and other infections in infancy. Recent studies have emphasized the important role of malaria-specific IgG3 in malaria immunity, and its transfer may reduce the risk of malaria in infancy. Human IgGs are actively transferred across the placenta by binding the neonatal Fc receptor (FcRn) expressed within the endosomes of the syncytiotrophoblastic membrane. Histidine at position 435 (H435) provides for optimal Fc-IgG binding. In contrast to other IgG subclasses, IgG3 is highly polymorphic and usually contains an arginine at position 435, which reduces its binding affinity to FcRn in vitro. The reduced binding to FcRn is associated with reduced transplacental transfer and reduced half-life of IgG3 in vivo. Some haplotypes of IgG3 have histidine at position 435. This study examines the hypotheses that the IgG3-H435 variant promotes increased transplacental transfer of malaria-specific antibodies and a prolonged IgG3 half-life in infants and that its presence correlates with protection against clinical malaria during infancy.

Methods and findings: In Benin, 497 mother-infant pairs were included in a longitudinal birth cohort. Both maternal and cord serum samples were assayed for levels of IgG1 and IgG3 specific for MSP119, MSP2 (both allelic families, 3D7 and FC27), MSP3, GLURP (both regions, R0 and R2), and AMA1 antigens of Plasmodium falciparum. Cord:maternal ratios were calculated. The maternal IgG3 gene was sequenced to identify the IgG3-H435 polymorphism. A multivariate logistic regression was used to examine the association between maternal IgG3-H435 polymorphism and transplacental transfer of IgG3, adjusting for hypergammaglobulinemia, maternal malaria, and infant malaria exposure. Twenty-four percent of Beninese women living in an area highly endemic for malaria had the IgG3-H435 allele (377 women homozygous for the IgG3-R435 allele, 117 women heterozygous for the IgG3-R/H alleles, and 3 women homozygous for the IgG3-H435 allele). Women with the IgG3-H435 allele had a 78% (95% CI 17%, 170%, p = 0.007) increased transplacental transfer of GLURP-R2 IgG3 compared to those without the IgG3-H435 allele. Furthermore, in infants born to mothers with the IgG3-H435 variant, a 28% longer IgG3 half-life was noted (95% CI 4%, 59%, p = 0.02) compared to infants born to mothers homozygous for the IgG3-R435 allele. Similar findings were observed for AMA1, MSP2-3D7, MSP3, GLURP-R0, and GLURP-R2 but not for MSP119 and MSP2-FC27. Infants born to women with IgG3-H435 had a 32% lower risk of symptomatic malaria during infancy (incidence rate ratio [IRR] = 0.68 [95% CI 0.51, 0.91], p = 0.01) compared to infants born to mothers homozygous for IgG3-R435. We did not find a lower risk of asymptomatic malaria in infants born to women with or without IgG3-H435. Limitations of the study were the inability to determine (i) the actual amount of IgG3-H435 relative to IgG-R435 in serum samples and (ii) the proportion of malaria-specific IgG produced by infants versus acquired from their mothers.

Conclusions: An arginine-to-histidine replacement at residue 435 in the binding domain of IgG3 to FcRn increases the transplacental transfer and half-life of malaria-specific IgG3 in young infants and is associated with reduced risk of clinical malaria during infancy. The IgG3-H435 allele may be under positive selection, given its relatively high frequency in malaria endemic areas.

Fig 1. The cord-to-mother transfer ratio of malaria-specific IgG1, IgG3-R435, and IgG3-H435.

(A) The cord-to-mother transfer ratios (CMTRs) are represented: malaria-specific IgG1 (orange bars), IgG3 for women with the IgG3-H435 allele (dark blue bars, N = 120), and IgG3 for women homozygous for the IgG3-R435 allele (light blue bars, N = 377). Statistical comparison with IgG1 was performed for 377 women homozygous for IgG3-R435 and 120 women with the IgG3-H435 allele. Data are shown as box and whisker plots, and the p-values of the logistic regression univariate analysis are represented as follows: ** p ≤ 0.007, *** p ≤ 0.001. Only p-values ≤ 0.007 were considered significant after Bonferroni correction for multiple comparisons. (B) A multivariate logistic regression was performed to test whether individuals with IgG3-H435 had greater CMTR (over or below the median) relative to those with only IgG3-R435. The association is represented by the odds ratio (± 95% confidence interval) for each malaria antigen. Adjusted variables are shown in Table 1. There was no collinearity between all tested variables, and variance inflation factor (VIF) values were ≤ 1.08.

Fig 2. Increased persistence of maternally derived malaria-specific IgG3 among young infants born to women with the IgG3-H435 allele compared to those homozygous for IgG3-R435.

The odds ratio (± 95% confidence interval) of maternal malaria-specific IgG3-H435 persistence at 6 months relative to homozygous IgG3-R435 based on dichotomized cord-to-mother transfer ratio (over or below the median) using multivariate regression analysis adjusted for the variables shown in Table 1. To reduce the possibility that IgG3 was produced by malaria-infected infants, samples from any infant showing an increase in malaria-specific IgG3 between 0 and 3 months or between 3 and 6 months of age (any antigen) were removed from the analysis (N = 195). The odds ratio was obtained after adjustment for malaria-specific IgG3 in maternal peripheral blood (see Table 3). Of 302 infants, between 3 and 11 had missing data (according to the tested antigen) and were not included in the analysis (see details in Table 3). There was no collinearity between all tested variables, and variance inflation factor (VIF) values were all ≤2.17.

Fig 3. Increased transplacental transfer of GLURP-R2-specific IgG3 is associated with delayed time to first symptomatic malaria.

Dichotomized cord-to-mother transfer ratio (above or below the median; N = 493 infants). The log-rank analysis yielded p < 0.001, and the Cox proportional hazard analysis model adjusted for malaria exposure and placental malaria gave a hazard ratio of 0.59 (95% CI 0.45, 0.77, p < 0.001).

Fig 4. Maternal IgG3-H435 polymorphism is associated with a decreased risk of symptomatic malaria in infants from birth to 12 months of age.

The time to first symptomatic malaria episode during infancy is shown. The log-rank analysis yielded p = 0.102. The Cox proportional hazard analysis adjusted for malaria exposure and placental malaria gave a hazard ratio of 0.69 (95% CI 0.46, 1.05, p = 0.083).