Effect of Mature Blood-Stage Plasmodium Parasite Sequestration on Pathogen Biomass in Mathematical and In Vivo Models of Malaria

Abstract

Parasite biomass and microvasculature obstruction are strongly associated with disease severity and death in Plasmodium falciparum-infected humans. This is related to sequestration of mature, blood-stage parasites (schizonts) in peripheral tissue. The prevailing view is that schizont sequestration leads to an increase in pathogen biomass, yet direct experimental data to support this are lacking. Here, we first studied parasite population dynamics in inbred wild-type (WT) mice infected with the rodent species of malaria, Plasmodium berghei ANKA. As is commonly reported, these mice became moribund due to large numbers of parasites in multiple tissues. We then studied infection dynamics in a genetically targeted line of mice, which displayed minimal tissue accumulation of parasites. We constructed a mathematical model of parasite biomass dynamics, incorporating schizontspecific host clearance, both with and without schizont sequestration. Combined use of mathematical and in vivo modeling indicated, first, that the slowing of parasite growth in the genetically targeted mice can be attributed to specific clearance of schizonts from the circulation and, second, that persistent parasite growth in WT mice can be explained solely as a result of schizont sequestration. Our work provides evidence that schizont sequestration could be a major biological process driving rapid, early increases in parasite biomass during blood-stage Plasmodium infection. Malaria caused approximately 1,238,000 deaths worldwide in 2010 (1). Of the five different Plasmodium species that infect humans, Plasmodium falciparum is associated with greatest morbidity (2, 3). Parasite biomass and microvasculature obstruction are among the most reliable prognostic indicators in P. falciparum-infected humans, with high levels of either indicator being strongly associated with severe disease and negative outcome (4-6). High P. falciparum biomass could in theory result from poor clearance of freely circulating parasites by the host's mononuclear phagocytic system (MPS), principally macrophages in the spleen and liver. Clinical studies, including ex vivo perfusion of human spleens and analyses of splenectomized malaria patients, argue that humans can clear at least a proportion of circulating P. falciparum parasites (7-11). Furthermore, increased rigidity of red blood cells (RBCs) infected with P. falciparum parasites, particularly mature-stage trophozoites and schizonts, appears to render them less capable of passage through the spleen than either uninfected RBCs or ring-stage P. falciparum-infected RBCs (P. falciparum-iRBCs) (12). Thus, the human MPS can clear circulating mature P. falciparum-iRBCs, but its competency at doing so in vivo remains unclear. A mechanism that could facilitate increases in parasite biomass is evasion of the host MPS by sequestration of P. falciparum-iRBCs in peripheral tissues. A large body of literature has clearly identified P. falciparum-expressed ligands, which mediate adherence of schizont-and late-stage trophozoite-iRBCs to ...

Host-mediated impairment of parasite maturation during blood-stage Plasmodium infection

“…Consistent with previous findings (12)(13)(14), the growth of GFP pos donor parasites in acutely infected mice (PMF = 2.0 ± 0.1) was less than half that observed for naive controls (4.6 ± 0.3) over 24 h (P = 0.01, Mann-Whitney test) (Fig. 1C).…”

“…Given that the reduced parasite maturation rate was observed only a few days into the infection, we next investigated a possible role for host inflammatory responses in mediating this phenomenon (14). To begin exploring this, we first blocked type-I IFN cytokine signaling via the receptor IFNAR1, because our previous data had suggested this innate immune signaling pathway could influence circulating pRBC numbers and life stages in vivo (24) (Fig.…”

“…Infection of C57BL/6J mice with PbA elicits a rapid increase in parasitemia until day 4 postinfection (p.i. ), followed by a slowing in the growth rate of parasites (14). To reveal factors causing this slowed growth, we sought to examine an individual generation of pRBCs exposed to this in vivo environment.…”

“…We used rag1 −/− mice, which lack T and B cells and, most importantly, mount poor systemic inflammatory responses during infection, including weak production of TNF and IFNγ (Cytokine Responses in rag1 −/− and WT Mice and Fig. S5) (14). The maturation rate in naive and rag1 −/− mice was not significantly different from 1 (i.e., 24 h cycle time, P = 0.13, F test), nor was it significantly different from that in the naive WT mice (P = 0.10, F test).…”

“…Use of these experimental models has revealed that during acute infection, specifically before an effective parasitespecific antibody response has developed, the host is able to partially control parasite growth (or in other words, to limit the PMF) (12)(13)(14). However, the precise mechanisms by which the PMF is reduced in vivo remain unclear.…”