Safety and comparability of controlled human Plasmodium falciparum infection by mosquito bite in malaria-naïve subjects at a new facility for sporozoite challenge

Abstract

Background: Controlled human malaria infection (CHMI) studies which recapitulate mosquito-borne infection are a critical tool to identify protective vaccine and drug candidates for advancement to field trials. In partnership with the Walter Reed Army Institute of Research, the CHMI model was established at the Seattle Biomedical Research Institute's Malaria Clinical Trials Center (MCTC). Activities and reagents at both centers were aligned to ensure comparability and continued safety of the model. To demonstrate successful implementation, CHMI was performed in six healthy malaria-naïve volunteers.

Methods: All volunteers received NF54 strain Plasmodium falciparum by the bite of five infected Anopheles stephensi mosquitoes under controlled conditions and were monitored for signs and symptoms of malaria and for parasitemia by peripheral blood smear. Subjects were treated upon diagnosis with chloroquine by directly observed therapy. Immunological (T cell and antibody) and molecular diagnostic (real-time quantitative reverse transcriptase polymerase chain reaction [qRT-PCR]) assessments were also performed.

Results: All six volunteers developed patent parasitemia and clinical malaria. No serious adverse events occurred during the study period or for six months post-infection. The mean prepatent period was 11.2 days (range 9-14 days), and geometric mean parasitemia upon diagnosis was 10.8 parasites/µL (range 2-69) by microscopy. qRT-PCR detected parasites an average of 3.7 days (range 2-4 days) earlier than blood smears. All volunteers developed antibodies to the blood-stage antigen merozoite surface protein 1 (MSP-1), which persisted up to six months. Humoral and cellular responses to pre-erythrocytic antigens circumsporozoite protein (CSP) and liver-stage antigen 1 (LSA-1) were limited.

Conclusion: The CHMI model was safe, well tolerated and characterized by consistent prepatent periods, pre-symptomatic diagnosis in 3/6 subjects and adverse event profiles as reported at established centers. The MCTC can now evaluate candidates in the increasingly diverse vaccine and drug pipeline using the CHMI model.

Trial registration: ClinicalTrials.gov NCT01058226.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Study flow diagram.

Eighteen subjects were screened for eligibility to participate in the trial and 7 healthy volunteers were considered eligible and willing to participate. On the day of enrollment, 6 subjects were enrolled and one backup subject was discharged from the study. The six subjects underwent CHMI and completed the 56 day study. Five subjects returned for optional long term safety and immunology follow up assessments at 3 and 6 months post-challenge.

Figure 2. Comparison of prepatent and incubation periods.

Kaplan-Meier survival curve showing the percentage of subjects without patent parasitemia by blood smear (blue line) or without symptoms (dashed red line) following challenge.

Figure 3. qRT-PCR-based course of parasitemia.

Parasite density based on qRT-PCR measurements are presented individually for each participant.

Figure 4. Humoral immune responses to P. falciparum antigens.

ELISAs were performed on the indicated days post-CHMI to test for responses against the indicated P. falciparum antigens. The positivity cut-off (dotted line) was calculated per ELISA plate as three standard deviations above the mean of the two negative control wells. All samples with an OD higher than the calculated cut-off were deemed positive. Data are presented for the five subjects completing the follow up at 3 and 6 months. Data to Day 56 for the sixth subject did not differ considerably from the five subjects in the graph.

Figure 5. Cellular immune response to CSP and LSA-1.

IFNγ ELISpot assays were performed using the indicated CSP and LSA-1 peptide pools on the indicated days post-CHMI. Spot forming units (SFU) per million PBMC are shown per individual subject for all six subjects.