Development of high affinity antibodies to Plasmodium falciparum merozoite and sporozoite antigens during infancy and adulthood

Abstract

Antibodies are important for protection against malaria. For optimal protective activity, it is thought that antibodies need to have high affinity. A longitudinal study conducted in Uganda followed newborn infants and their mothers for nine months. The study found that antibody affinity (here measured as dissociation rate constant, k_d) against the merozoite antigens AMA1 and MSP2 decreased from birth to six months in the infants, then gradually increased to 9 months, but not reaching the level observed in the mothers. In contrast, affinity against the sporozoite antigen CSP, did not change throughout the study period. Among mothers, no significant changes in antibody affinity were observed for any antigen, which is consistent with expectations for adults in an endemic area. Comparing specific antibody affinities to total antibody levels revealed almost no correlations, indicating that antibody magnitude and affinity evolve differently during immune development. Significant correlations were observed between antibody affinities and some atypical memory B cells. In conclusion, our study shows that development of naturally acquired slowly dissociating (high affinity) antibodies against malaria can evolve separately across different antigens. This is important information for future vaccine development studies.

Keywords: AMA1; CSP; MSP2; affinity; antibodies; falciparum; malaria.

Figure 1

Overview of development of high affinity antibodies during infancy and in mothers. Graphs illustrate estimated means of antibody dissociation rate constants (k_d) for MSP2, CSP and AMA1 during (A) infancy (at birth, 2.5, 6 and 9 months) and (B) in mothers (at delivery and after 9 months). Lower k_d values indicate higher antibody affinity. *** indicates significant at P < 0.001, analyzed by linear mixed models.

Figure 2

Comparative analysis of antibody dissociation rate constant (k_d) for MSP2, AMA1 and CSP in infants and mothers. Box plots comparing the median k_d of antibodies targeting the antigens (A) MSP2, (B) AMA1 and (C) CSP between mothers at delivery and nine months postpartum, and their infants at birth and nine months. Box plots represent interquartile range (IQR), whiskers the range and horizontal lines the median; **, ***, **** indicate significant differences (p < 0.01, 0.001, < 0.0001, respectively), and ns (p ≥ 0.05) indicates not significant tested by the Mann-Whitney U test.

Figure 3

Comparison of antibody dissociation rate constants for AMA1, MSP2, and CSP in infants and mothers. Box plots illustrate the median antibody affinities (k_d) against AMA1, MSP2, and CSP in infants at (A) birth, (B) 2.5 months, (C) 6 months, (D) 9 months, and in mothers at (E) delivery and (F) 9 months postpartum. Box plots represent interquartile range (IQR), whiskers the range, and horizontal lines represent the median; *,**, ***, and **** indicate significant differences (p < 0.05, 0.01, 0.001, and 0.0001, respectively) tested by Kruskal-Wallis test.

Figure 4

Relationship between antibody dissociation rate constant and ELISA-determined antibody levels. In infants there was (A) a significant correlation between antibody level and dissociation rate constant only for IgM and CSP at 9 months, but not for any other tested correlations: (B) IgG and CSP, (C) IgM and AMA1, (D) IgG and AMA1, (E) IgM and MSP2 and (F) IgG and MSP2. Coefficient of correlation, rho, and P values were calculated using Spearman’s correlation test. **** significant at P < 0.0001.

Figure 5

Correlations between antibody affinity and B cell subsets. Figures representing (A) antibody dissociation rate constant for MSP2 and atypical memory B cells (MBCs) in infants at 9 months and (B) antibody dissociation rate constant for AMA1 and Pf+ atypical MBCs in mothers postpartum. Pearson’s coefficient was used to detect correlations between plasma antibody affinity and B cell subsets.