Publications

Understanding the impact of forces generated by blood flow on biological processes in the circulatory system, such as the invasion of human red blood cells by malaria parasites, is currently limited by the lack of experimental systems that integrate them. Recent systematic quantification of the growth of Plasmodium falciparum, the species that causes the majority of malaria mortality, under a range of shaking conditions has shown that parasite invasion of erythrocytes is affected by the shear stress to which the interacting P. falciparum merozoites and their target red blood cells are exposed. Blood flow could similarly impact shear stress and therefore invasion in vivo, but there is currently no method to test the impact of flow-induced forces on parasite invasion. We have developed a microfluidic device with four channels, each with dimensions similar to those of a post-capillary venule, but with different flow velocities. Highly synchronised P. falciparum parasites are injected into the device, and parasite egress and invasion rates are quantified using newly developed custom video analysis, which fully automates cell type identification and trajectory tracking. The device was tested with both wild-type P. falciparum lines and lines in which genes encoding proteins involved in parasite invasion had been deleted. Deletion of erythrocyte binding antigen 175 (PfEBA175) has a significant impact on invasion under flow, but not in static culture. These findings establish for the first time that flow conditions can critically affect parasite invasion in a genotype-dependent manner. The method can be applied to other biological processes affected by fluid motion, such as cell adhesion, migration, and mechanotransduction.

Duffy antigen receptor for chemokines (DARC) is the primary red blood cell (RBC) receptor for invasion of human RBCs by Plasmodium vivax and Plasmodium knowlesi parasites. By contrast, Plasmodium falciparum parasites use multiple RBC receptors for invasion. Whether DARC is one of these receptors has never been systematically explored. We used flow cytometry and microscopy-based approaches to investigate whether P. falciparum parasites preferentially invade specific Duffy RBC phenotypes and explored 2 potential explanations for invasion preference—differences in RBC biophysical properties and surface protein composition. P. falciparum parasites showed a consistent preference for Duffy-positive RBCs, and some biophysical properties and surface protein expression varied between Duffy-positive and Duffy-negative RBCs. We then used our in vitro invasion data to parametrize an evolutionary-epidemiological model of the relationship between P. falciparum and the FYBES allele. Our model accounts for immunity against P. falciparum virulence, gained through exposure, and thus mutations that impede infection are not always advantageous. The inhibition of P. falciparum invasion that we observed in vitro leads to FYBES frequencies increasing at low levels of P. falciparum transmission but decreasing at high levels of transmission. The impact of P. falciparum on the prevalence of Duffy negativity may therefore be most apparent in lower transmission settings. Our findings show a link between Duffy negativity and P. falciparum and suggest that DARC may directly or indirectly be involved in P. falciparum invasion of human RBCs which could, together with P. vivax, explain the distribution of Duffy negativity in sub-Saharan Africa.