Role of malaria exposure and off-target responses on RTS,S/AS02A vaccine immunogenicity and protection in Mozambican children

Abstract

RTS,S/AS01_E is the first malaria vaccine implemented for young African children. However, it provides partial protection against Plasmodium falciparum (Pf) malaria, and a better understanding of the mechanisms and determinants of vaccine immunity will help develop second-generation improved vaccines. We measured IgG to vaccine target and Pf blood-stage off-target proteins before and after vaccination in 874 children aged 1-4 years in a phase 2b trial of RTS,S/AS02_A in Mozambique. We found that naturally acquired PfCSP IgG levels pre-vaccination were positively associated with RTS,S immunogenicity. Increased levels of IgG to the C-terminus and NANP-repeat regions of PfCSP, and to PfMSP5 and PfMSP1 block 2, following vaccination, were significantly associated with a lower hazard of clinical malaria over 6 months. Thus, immune priming, anti-PfCSP C-terminus and off-target antibody responses contributed to malaria protection after adjusting for prior Pf exposure, and this could guide strategies for optimizing the immunogen and vaccine deployment.

Fig. 1. Study design.

Participants received either RTS,S/AS02_A or a comparator vaccine administered at month (M) 0, M1 and M2. Blood samples were collected at baseline (M0), 2 weeks after the third dose (M3), and 6.5 months after the third dose (M8). 743 children were enrolled in cohort 1 (Manhiça, low malaria transmission intensity [MTI]) and followed up by passive surveillance to detect clinical malaria episodes as endpoint, and 131 were enrolled in cohort 2 (Ilha Josina, high MTI), where participants received a combination of amodiaquine and sulfadoxine-pyrimethamine 14 days before the third vaccine dose to clear asymptomatic parasitemia, and were followed up using active detection of infection during 4.5 months starting 2 weeks post-vaccination to measure vaccine efficacy against infection.

Fig. 2. Kinetics of IgG levels to antigens included in the RTS,S vaccine from baseline to 6 months post-vaccination.

a anti-PfCSP IgG levels in Manhiça, b anti-PfCSP IgG levels in Ilha Josina. Boxplots display median IgG levels against PfCSP full length (fl), PfCSP NANP and PfCSP C-terminus (C-term) and the interquartile range (IQR). p-values correspond to the FDR-adjusted from a t-test comparing the IgG levels to each PfCSP construct between treatment groups at month (M) 3. c IgG levels to Hepatitis B surface antigen (HBsAg) in Ilha Josina. Boxplots display median and IQR of IgG levels to HBsAg at each timepoint (month) for each of the age groups (from 12 to 24 [12 < 24] and from 24 to 59 [24 < 59] months). Children in the older group received the Engerix vaccine as a comparator. p-values were obtained from paired t-tests between IgG levels at M0 and M3 and from two-sample t-tests between IgG levels at M3 in each age group in the RTS,S treatment group.

Fig. 3. Kinetics of IgG levels to non-vaccine Plasmodium falciparum (Pf) antigens in Manhiça.

Boxplots show median and interquartile range (IQR) of IgG levels against each of the Pf-blood-stage antigens at each time point (Month [M]). p-values are adjusted by false discovery rate (FDR) from t-tests comparing IgG levels at M3 between RTS,S and comparator groups, and from paired t-tests comparing IgG levels at M0 and M3 in RTS,S vaccinees.

Fig. 4. Correlations between pre-vaccination IgG levels to non-vaccine Plasmodium falciparum antigens and PfCSP and post-RTS,S-vaccination IgG levels to PfCSP in both sites together.

Correlations between IgG levels against PfCSP NANP repeat and PfCSP C-terminus (C-term) at month (M) 3 and IgG to PfEXP1, PfMSP2, PfCSP NANP and C-term at M0, assessed by the Pearson’s correlation coefficients. Displayed p-values are adjusted by FDR. a Outcome variable is M3 anti-PfCSP NANP IgG levels. b Outcome variable is M3 anti-PfCSP C-term IgG levels.

Fig. 5. Effect of baseline anti-PfCSP IgG levels on post-vaccination anti-PfCSP IgG levels adjusted by exposure and age in both sites.

Three multiple linear regression models are presented, each using one baseline anti-PfCSP IgG measurement (full-length [FL], NANP, or C-terminus [C-term]) as the predictor, with outcomes defined as the post-vaccination IgG levels at month (M) 3 against NANP (blue triangles) and C-term (red circles). These models were adjusted for baseline malaria exposure, measured as the sum of anti-PfEXP1 and anti-PfMSP2 IgG levels at M0, and age at first vaccination. Each forest plot displays the estimated effect sizes (multiplicative effect in the dependent variable per a 10-fold increase in the predictor) and their 95% confidence intervals.