Exploratory analysis of the effect of helminth infection on the immunogenicity and efficacy of the asexual blood-stage malaria vaccine candidate GMZ2

Abstract

Background: Helminths can modulate the host immune response to Plasmodium falciparum and can therefore affect the risk of clinical malaria. We assessed here the effect of helminth infections on both the immunogenicity and efficacy of the GMZ2 malaria vaccine candidate, a recombinant protein consisting of conserved domains of GLURP and MSP3, two asexual blood-stage antigens of P. falciparum. Controlled human malaria infection (CHMI) was used to assess the efficacy of the vaccine.

Methodology: In a randomized, double-blind Phase I clinical trial, fifty, healthy, lifelong malaria-exposed adult volunteers received three doses of GMZ2 adjuvanted with either Cationic Adjuvant Formulation (CAF) 01 or Alhydrogel, or a control vaccine (Rabies) on days (D) 0, D28 and D56, followed by direct venous inoculation (DVI) of 3,200 P. falciparum sporozoites (PfSPZ Challenge) approximately 13 weeks after last vaccination to assess vaccine efficacy. Participants were followed-up on a daily basis with clinical examinations and thick blood smears to monitor P. falciparum parasitemia for 35 days. Malaria was defined as the presence of P. falciparum parasites in the blood associated with at least one symptom that can be associated to malaria over 35 days following DVI of PfSPZ Challenge. Soil-transmitted helminth (STH) infection was assessed by microscopy and by polymerase chain reaction (PCR) on stool, and Schistosoma infection was assessed by microscopy on urine. Participants were considered as infected if positive for any helminth either by PCR and/or microscopy at D0 and/or at D84 (Helm+) and were classified as mono-infection or co-infection. Total vaccine-specific IgG concentrations assessed on D84 were analysed as immunogenicity outcome.

Main findings: The helminth in mono-infection, particularly Schistosoma haematobium and STH were significantly associated with earlier malaria episodes following CHMI, while no association was found in case of coinfection. In further analyses, the anti-GMZ2 IgG concentration on D84 was significantly higher in the S. haematobium-infected and significantly lower in the Strongyloides stercoralis-infected groups, compared to helminth-negative volunteers. Interesting, in the absence of helminth infection, a high anti-GMZ2 IgG concentration on D84 was significantly associated with protection against malaria.

Conclusions: Our results suggest that helminth infection may reduce naturally acquired and vaccine-induced protection against malaria. Vaccine-specific antibody concentrations on D84 may be associated with protection in participants with no helminth infection. These results suggest that helminth infection affect malaria vaccine immunogenicity and efficacy in helminth endemic countries.

Fig 1. Flow chart of the study participants.

PCR: Polymerase chain reaction STH: Soil-transmitted helminth SH: Schistosoma haematobium N = number

Fig 2. Post immunization antibody concentration at D84 in helminth uninfected and infected groups (Helm- and Helm+).

Fig 2 illustrates the log of GMZ2-specific (A), MSP3-specific (B), GLURP-specific (C) IgG concentration at D84 in helminth negative (Helm-, n = 16) and helminth positive (Helm+, n = 23) groups. Comparisons were performed by univariate and multivariate linear regression adjusted for baseline data as independent covariable. The graphs are mean ± standard deviation. Statistical significance was set for p value below 0.05. *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001. NS = Non-significant.

Fig 3. Post immunization antibody concentration at D84 in Helm- and Helm+ subgroups.

Fig 3 illustrates the log of specific-GMZ2 (A), specific-MSP3 (B), specific GLURP (C) IgG concentration at D84 in Helm- and mono-infected by Schistosoma haematobium (Sh+, n = 4), those mono-infected with Strongyloides (Str+, n = 4), those mono-infected with Trichuris trichiura or hookworm (Tt+/Hw+, n = 3), and those coinfected by at least two different helminths (Coinf+, n = 12). The comparison was performed by multivariate linear regression adjusted for baseline vaccine-specific total IgG concentration as independent covariable. The graphs show mean ± standard deviation. Statistical significance was set for p value below 0.05 and is indicated when statistically significant. *p<0.05, **p<0.01, ***p<0.001. NS = Non-significant.

Fig 4. Time to occurrence of malaria episodes after CHMI.

Fig 5. Anti-GMZ2 IgG concentration and CHMI outcome groups according to helminth status Comparison of vaccine-specific total IgG concentration at D84 between those having clinical malaria (monotone increase of parasitemia with symptoms) and those without clinical malaria (low oscillating parasitemia and no symptoms (control) and those with no parasitemia and no symptoms (protected)) after the CHMI was performed in all participants (n = 12 vs 14, 5A), in helminth uninfected (n = 3 vs 8, 5B), and infected (n = 9 vs 7, 5C) participants.

The comparison was done by multivariate linear regression using baseline vaccine-specific total IgG concentration as variable of adjustment. Statistical significance was set for p value below 0.05. *p<0.05 NS = Non-significant.