Acquisition of growth-inhibitory antibodies against blood-stage Plasmodium falciparum

Abstract

Background: Antibodies that inhibit the growth of blood-stage Plasmodium falciparum may play an important role in acquired and vaccine-induced immunity in humans. However, the acquisition and activity of these antibodies is not well understood.

Methods: We tested dialysed serum and purified immunoglobulins from Kenyan children and adults for inhibition of P. falciparum blood-stage growth in vitro using different parasite lines. Serum antibodies were measured by ELISA to blood-stage parasite antigens, extracted from P. falciparum schizonts, and to recombinant merozoite surface protein 1 (42 kDa C-terminal fragment, MSP1-42).

Results: Antibodies to blood-stage antigens present in schizont protein extract and to recombinant MSP1-42 significantly increased with age and were highly correlated. In contrast, growth-inhibitory activity was not strongly associated with age and tended to decline marginally with increasing age and exposure, with young children demonstrating the highest inhibitory activity. Comparison of growth-inhibitory activity among samples collected from the same population at different time points suggested that malaria transmission intensity influenced the level of growth-inhibitory antibodies. Antibodies to recombinant MSP1-42 were not associated with growth inhibition and high immunoglobulin G levels were poorly predictive of inhibitory activity. The level of inhibitory activity against different isolates varied.

Conclusions: Children can acquire growth-inhibitory antibodies at a young age, but once they are acquired they do not appear to be boosted by on-going exposure. Inhibitory antibodies may play a role in protection from early childhood malaria.

Figure 1. Levels of IgG to schizont protein extract and recombinant MSP1-42, according to age and parasitemic status, among samples from the Ngerenya 1998 cohort.

A, B: Median absorbance (Optical Density, OD) to schizont protein extract and MSP1-42, respectively, for aparasitemic participants (n = 91). C, D: Median absorbance to schizont protein extract and MSP1-42, respectively, for parasitemic participants (n = 59). Sera were used at a dilution of 1∶1000. Error bars represent the inter-quartile range. P values were calculated using a Kruskal-Wallis test. All samples were tested in duplicate. The difference in median IgG levels for aparasitemic compared to parasitemic participants was not significant for schizont extract or MSP1-42 (P = 0.082 and P = 0.14, respectively).

Figure 2. In-vitro growth inhibition of parasite lines 3D7 and W2mef, according to parasitemic status, among samples from the Ngerenya 1998 cohort.

A, B: Mean growth of 3D7 and W2mef, respectively, for aparasitemic participants. C, D: Mean growth of 3D7 and W2mef, respectively, for parasitemic participants. Growth is expressed relative to control (PBS). Error bars represent standard deviation. P values were calculated using a one-way ANOVA. All samples were tested in duplicate in two separate assays. For all samples, including parasitemic and aparasitemic individuals together, mean growth (%, ±SD) was: W2mef (n = 138) 31.8±6.5, 2–5 years; 35.1±9.0, 6–14 years; 38.1±10.3, 18–81 years; 3D7 (n = 140) 42.4±8.3, 2–5 years; 44.2±12.4, 6–14 years; 45.4±7.5, 18–81 years.

Figure 3. The association between in vitro growth of parasite lines 3D7 or W2mef and IgG measured by ELISA, according to parasitemic status, for the Ngerenya 1998 cohort.

A, B: Correlation between ELISA response to schizont extract and 3D7 or W2mef growth, respectively. C, D: Correlation between ELISA response to MSP1-42 and 3D7 or W2mef growth, respectively. r_s values represent Spearman's rank correlation coefficients. Results for aparasitemic and parasitemic individuals are represented by black or grey diamonds, respectively. Samples used in growth-inhibition assays were dialysed serum.

Figure 4. Correlation between IgG to schizont protein extract measured by ELISA and in vitro growth inhibition using purified immunoglobulins.

A 3D7 parasite line. B W2mef parasite line. Samples used in the assays were immunoglobulins purified using ammonium sulphate precipitation from Ngerenya 1998 serum samples (n = 52). r_s represents Spearman's rank correlation coefficients.