Serum-derived immune factors as correlates of Artemisinin-based combination therapy efficacy in treatment of uncomplicated Plasmodium falciparum malaria in Kombewa, Western Kenya

Abstract

Artemisinin-based combination therapy (ACT) is recommended first-line treatment for malaria in a number of Sub-Saharan African (SSA) countries. Recent reports of decline in efficacy of ACT and emergence of ACT resistant Plasmodium falciparum isolates has raised global health concern. The underlying mechanisms for the development of resistance to ACT is however, not fully understood. Naturally acquired immunity to P. falciparum is associated with clinical protection against malaria and has been shown to influence efficacy of antimalarial drugs. To what extent pre-existing naturally acquired immunity to malaria affects efficacy of ACT remains to be established. The present study hypothesize that soluble immune factors present in sera of malaria-exposed (immune) individuals enhance the efficacy of ACT for treatment of uncomplicated P. falciparum malaria. Therefore, the study aims to determine the effect of serum-derived immune factors on in vitro growth of P. falciparum by serum from malaria immune and non-immune participants then correlate the effect of serum-derived immune factors on in vitro growth of P. falciparum in immune participants and ACT efficacy for uncomplicated malaria. To test the hypothesis, sera from participants (n=118) (i.e. immune sera) previously enrolled in a two-arm (i.e. artesunate-mefloquine or artemether-lumefantrine), randomized open-label trial conducted in Kisumu Country, western Kenya, were assessed for in vitro growth inhibitory activity (GIA) of 3D7 P. falciparum strain, then compared with pooled sera from malaria naïve volunteers (n=6) (i.e. non-immune sera). Each sample was divided into two portions from which one was heat inactivated, and GIA was performed at 10% and 1% serum concentration. Continuous variables were compared using Mann Whitney test and One-way analysis of variance with Tukey’s post hoc. Spearman correlation coefficient test was used to correlate GIA and parasite clearance rate. Median parasite clearance rate was used as cut-off to assess treatment outcome, where fast clearers (n=80) had parasite clearance slope half-life (PCt1/2) above the median parasite clearance rate while faster clearers (n=25) had PCt1/2 below the median parasite clearance rate. Serum from immune participants significantly inhibited P. falciparum growth compared to non-immune (p < 0.0001). Heat inactivation further diminished growth inhibitory activity of immune sera (p = 0.009). There was age-independent inhibitory activity (p > 0.05). In addition, GIA correlated with parasite clearance rate after adjusting with age (< 5years vs > 5years (p < 0.0001). Further analysis showed significant positive correlation between GIA and faster parasite clearance in participants aged > 5years (p = 0.02). The results of this study suggest that serum-derived immune factors affect the efficacy of ACT for treatment of uncomplicated P. falciparum malaria. These findings will provide insight into improving on effective use of ACT drugs (dosage) in area where malaria is endemic.