Comparison of leucocyte profiles between healthy children and those with asymptomatic and symptomatic Plasmodium falciparum infections

BACKGROUND: The immune mechanisms that determine whether a Plasmodium falciparum infection would be symptomatic or asymptomatic are not fully understood. Several studies have been carried out to characterize the associations between disease outcomes and leucocyte numbers. However, the majority of these studies have been conducted in adults with acute uncomplicated malaria, despite children being the most vulnerable group. METHODS: Peripheral blood leucocyte subpopulations were characterized in children with acute uncomplicated (symptomatic; n = 25) or asymptomatic (n = 67) P. falciparum malaria, as well as malaria-free (uninfected) children (n = 16) from Obom, a sub-district of Accra, Ghana. Leucocyte subpopulations were enumerated by flow cytometry and correlated with two measures of parasite load: (a) plasma levels of P. falciparum histidine-rich protein 2 (PfHRP2) as a proxy for parasite biomass and (b) peripheral blood parasite densities determined by microscopy. RESULTS: In children with symptomatic P. falciparum infections, the proportions and absolute cell counts of total (CD3 +) T cells, CD4 + T cells, CD8 + T cells, CD19 + B cells and CD11c + dendritic cells (DCs) were significantly lower as compared to asymptomatic P. falciparum-infected and uninfected children. Notably, CD15 + neutrophil proportions and cell counts were significantly increased in symptomatic children. There was no significant difference in the proportions and absolute counts of CD14 + monocytes amongst the three study groups. As expected, measures of parasite load were significantly higher in symptomatic cases. Remarkably, PfHRP2 levels and parasite densities negatively correlated with both the proportions and absolute numbers of peripheral leucocyte subsets: CD3 + T, CD4 + T, CD8 + T, CD19 + B, CD56 + NK, γδ + T and CD11c + cells. In contrast, both PfHRP2 levels and parasite densities positively correlated with the proportions and absolute numbers of CD15 + cells. CONCLUSIONS: Symptomatic P. falciparum infection is correlated with an increase in the levels of peripheral blood neutrophils, indicating a role for this cell type in disease pathogenesis. Parasite load is a key determinant of peripheral cell numbers during malaria infections.