Evaluating controlled human malaria infection in Kenyan adults with varying degrees of prior exposure to Plasmodium falciparum using sporozoites administered by intramuscular injection

Abstract

Background: Controlled human malaria infection (CHMI) studies are a vital tool to accelerate vaccine and drug development. As CHMI trials are performed in a controlled environment, they allow unprecedented, detailed evaluation of parasite growth dynamics (PGD) and immunological responses. However, CHMI studies have not been routinely performed in malaria-endemic countries or used to investigate mechanisms of naturally-acquired immunity (NAI) to Plasmodium falciparum.

Methods: We conducted an open-label, randomized CHMI pilot-study using aseptic, cryopreserved P. falciparum sporozoites (PfSPZ Challenge) to evaluate safety, infectivity and PGD in Kenyan adults with low to moderate prior exposure to P. falciparum (Pan African Clinical Trial Registry: PACTR20121100033272).

Results: All participants developed blood-stage infection confirmed by quantitative polymerase chain reaction (qPCR). However one volunteer (110) remained asymptomatic and blood-film negative until day 21 post-injection of PfSPZ Challenge. This volunteer had a reduced parasite multiplication rate (PMR) (1.3) in comparison to the other 27 volunteers (median 11.1). A significant correlation was seen between PMR and screening anti-schizont Enzyme Linked Immunosorbent Assays (ELISA) OD (p = 0.044, R = -0.384) but not when volunteer 110 was excluded from the analysis (p = 0.112, R = -0.313).

Conclusions: PfSPZ Challenge is safe and infectious in malaria-endemic populations and could be used to assess the efficacy of malaria vaccines and drugs in African populations. Whilst our findings are limited by sample size, our pilot study has demonstrated for the first time that NAI may impact on PMR post-CHMI in a detectable fashion, an important finding that should be evaluated in further CHMI studies.

Keywords: CHMI; challenge; falciparum; immunity; malaria.

Figure 1

Study design and volunteer recruitment. 118 participants were excluded following screening (refer to Figure S1 for details). In each group, the total dose of sporozoites was split between two injection sites and administered as two 50 μL injections, one in each deltoid.

Figure 2

Antibody ELISA absolute OD readings measured at screening. (A) Anti-schizont. (B) Anti-MSP2. Serum diluted 1:1000. Negative controls = OD readings from UK malaria naïve adults (n = 30). Positive controls = OD readings from hyperimmune Kenyan adults living in malaria endemic regions (n = 6). Minimal exposure = subjects enrolled in groups 1, 3, and 5 (n = 14). Definite exposure = subjects enrolled in groups 2, 4, and 6 (n = 14). Screened subjects = all volunteers that had blood drawn at screening (n = 145). A significant difference was seen between minimally and definitely exposed volunteers for both antigens (Anti-schizont = p ≤ 0.0001; Anti-MSP2 = p = 0.006; Mann–Whitney test). Median values represented by lines through each dataset.

Figure 3

Analysis of adverse events associated with clinical malaria. (A) Comparison of the total number of AEs (excluding laboratory AEs) deemed possibly, probably or definitely related to clinical malaria infection in individuals diagnosed with malaria in Groups 1, 3, and 5 (Minimally exposed; mean = 7.1, median = 6.0) and Groups 2, 4, and 6 (Definitely exposed; mean = 6.3, median = 7.0) (Mann–Whitney test; p = 0.746). The median value is represented by a straight line through each plot. (B) Comparison of the duration of symptoms deemed possibly, probably or definitely related to clinical malaria infection in individuals diagnosed with malaria in Groups 1, 3, and 5 (Minimally exposed; mean = 11.8, median = 6.5) and Groups 2, 4, and 6 (Definitely exposed; mean = 6.3, median = 3.0) (Mann–Whitney test; p = 0.142). The median value is represented by a straight line through each plot. (C) Comparison of maximum severity of any symptom of clinical malaria infection between individuals diagnosed with malaria in in Groups 1, 3, and 5 (Minimally exposed) and Groups 2, 4, and 6 (Definitely exposed). (D) Laboratory AEs after CHMI deemed possibly, probably or definitely related to clinical P. falciparum infection. For “any laboratory abnormality” only the highest intensity AE per subject is counted.

Figure 4

qPCR-measured parasite density for each individual subject. Y-axis = parasites/mL. X-axis = days post challenge (calculated from hours post challenge). Red line = mean parasite density over time for all subjects.

Figure 5

Modeling of qPCR-measured parasite density to calculate parasite multiplication rates and liver-to-blood inocula. (A) Dot plot of PMR according to Group (p = 0.251; Kruskal–Wallis test). (B) Dot plot of LBI according to Group (p = 0.547; Kruskal–Wallis test). (C) PMR according to prior exposure to malaria (p = 0.206; Mann–Whitney test). The median value is represented by a straight line through each plot (D) LBI according to prior exposure to malaria (p = 0.700; Mann–Whitney test). Median value is represented by a straight line through each plot. (E) Relationship between PMR and anti-schizont ELISA OD measured at screening (Spearman rank = −0.384; p = 0.044). (F) Relationship between LBI and anti-schizont ELISA OD measured at screening (Spearman rank = −0.123; p = 0.534). In all graphs Volunteer 110′s data points are represented as open points. PMR = parasite multiplication rate (fold change in parasites/mL over 48 h). LBI = Liver-to-blood Inocula (total number of parasites released from liver).

Figure 6

qPCR results post-challenge for volunteer 110, group 2. Long dashed line = lower limit of detection (i.e., a probability of >50% of ≥1 positive result among three replicate PCR reactions) for qPCR assay (5 parasites/mL). Short dashed line = lower limit of quantification (defined as %CV < 20%) for qPCR assay (20 parasites/mL).