Target antigen, age, and duration of antigen exposure independently regulate immunoglobulin G subclass switching in malaria

Abstract

The isotype/subclass of immunoglobulin determines antibody function, but rather little is known about factors that direct class switching in vivo. To evaluate factors that might influence the maturation of the antibody response during infection, we conducted a seroepidemiological study of the immunoglobulin G (IgG) subclass response to four merozoite-associated antigens of Plasmodium falciparum in a mountainous region of northeastern Tanzania, where malaria endemicity declines with increasing altitudes. We found that IgG1/IgG3 class switching is independently affected by the nature of the antigen, cumulative exposure to the antigen, and the maturity of the immune system (i.e., the age of the individual). These observations provide insights into the effects of immune system maturity, the duration and intensity of antigen exposure, and inherent characteristics of individual antigens on the process of class switching in human B cells. Our data also throw light on the consequences of class switch decisions on the gradual acquisition of antimalarial immunity.

FIG. 1.

Single-point IgG1 and IgG3 OD values are valid proxies for serum IgG1 and IgG3 midpoint titers. The graph shows the relationship between OD values, measured at a dilution of 1:1,000, and titers, determined as the midpoints of the fitted sigmoids for each of 30 malaria-immune sera measuring anti-MSP-2 IgG3 (circles) and anti-AMA-1 IgG1 (squares). The unweighted least-square best fit sigmoid (dotted line) is given by the following equation: titer = 905 × [3.90/(OD + 0.074) − 1]. The r² value shown is for the linear least-square fit of the logit-transformed data, omitting values with titers of <10.

FIG. 2.

Age- and exposure-dependent variation in prevalence of IgG1 and IgG3 antibodies to malarial merozoite-associated antigens. The prevalence (%) of sera giving an OD of more than the mean + 3 standard deviations of the ODs for nonexposed European sera for IgG1 (▪) or IgG3 (□) antibodies to MSP-1₁₉ (a), MSP-2 (b), AMA-1 (c), and GPI (d) is shown. Data are arranged by increasing age (grouped as follows: A, 0 to 4 years; B, 5 to 14 years; and C, 15 to 45 years) and by increasing malaria prevalence (low, 1%; Mod1, 22%; Mod2, 32%; and high, 49%); residents of only two villages (moderate [18%] and high [49%] prevalence) were screened for anti-GPI antibodies. Differences in antibody prevalence with age were assessed using nonparametric (Wilcoxon rank-sum) tests and are indicated for IgG1 (* and **) and IgG3 (# and ##). * or #, P ≤ 0.05; ** or ##, P ≤ 0.005.

FIG. 3.

Age- and exposure-dependent variation in titers of IgG1 and IgG3 antibodies to malarial merozoite-associated antigens. Median log titers for IgG1 (a and c) and IgG3 (b and d) antibody binding to MSP-1₁₉, AMA-1, MSP-2, and GPI by increasing age (a and b) and by increasing malaria intensity (low, 1%; moderate, 18 to 32%; high, 49%) (c and d) are shown. Differences in titers were assessed using nonparametric (Wilcoxon rank-sum) tests and are indicated as follows: *, P ≤ 0.05; **, P ≤ 0.005.

FIG. 4.

Increasing polarization of IgG subclass responses to malaria antigens with increasing age. Data from all villages were combined, and the median ODs for IgG1 (▪) and IgG3 (□) were plotted by age group. (a) Recombinant MSP-1₁₉ (rMSP-1₁₉); (b) rMSP-2; (c) rAMA-1; (d) GPI. Differences in median ODs were assessed using nonparametric (Wilcoxon rank-sum) tests. *, P ≤ 0.05; **, P ≤ 0.005; ***, P ≤ 0.0005.

FIG. 5.

Increasing polarization of IgG subclass responses with increasing malaria transmission (parasite prevalence). Data from all age groups were combined, and the median ODs for IgG1 (▪) and IgG3 (□) were plotted by transmission intensity (parasite prevalence). (a) rMSP-1₁₉; (b) rMSP-2; (c) rAMA-1; (d) GPI. Differences in median ODs were assessed using nonparametric (Wilcoxon rank-sum) tests. *, P ≤ 0.05; **, P ≤ 0.005; ***, P ≤ 0.0005.