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 schizont-specific 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.

FIG 1

The parasitemia (a), total luminosity (measure of total parasites in an infected host) (b), and proportion of circulating parasites in the late stages of the life cycle (trophozoite or schizont stages) (c) were measured on days 2 to 6 postinfection (p.i.) for the WT (solid line) and rag1^−/− (dashed line) groups of mice (n = 6 mice per group). Error bars are reported as the standard errors of the means (±s/√n). The parasitemia and total luminosity increase consistently across all time points measured in WT mice. In contrast, a distinct slowing in growth of parasitemia is evident between days 4 and 5 in rag1^−/− mice. Further, the total luminosity reaches a peak in rag1^−/− mice on day 4. The proportion of late-stage parasites in WT mice is relatively consistent during the course of infection, unlike that in rag1^−/− mice that exhibit a distinct increase in the proportion of late-stage parasites to over 50% on day 4.

FIG 2

Simulation of parasite growth in an infected host with no parasite clearance and no parasite sequestration. This figure shows the concentration of parasites in the blood of the host (B) (solid line) and the percentage of total parasites older than 12 h (dashed line). When the parasite population is growing, it is evident that the proportion of parasites older than 12 h is low and that the proportion of these late-stage parasites increases once the growth in the total parasite burden begins to slow (after day 5), passing 50% after the total parasite burden has reached a peak and begins to decline (after day 5.5).

FIG 3

MCP-1 concentration measured in rag1^−/− mice. MCP-1 peaks on day 4 in rag1^−/− mice, the same point at which parasite growth stops in these mice. Groups contain 6 mice; error bars are reported as the standard errors of the means (±s/√n).

FIG 4

Targeting of late-stage parasites (older than 12 h) increases the proportion of late-stage parasites in the blood of the host. Four simulations of an infection were performed. In all simulations, there is no clearance until day 4. Just before day 4, parasite clearance is increased quickly (see Methods Part E in the supplemental material), targeting parasites of different ages. The simulations targeted parasites as follows: in the first simulation, parasites of all ages uniformly (blue); second, parasites older than 0.25 days/6 h (green); third, parasites older than 0.5 days/12 h (turquoise); and fourth, parasites older than 0.75 days/18 h (red) [clearance age, δ_age = 0, 0.25, 0.5, 0.75 days, respectively, with maximum clearance rates of 6/(1 − δ_age) for each δ_age]. The clearance rate applied to parasites of different ages for each simulation is shown in panel a. In each of these simulations the concentration of blood parasites increases across the first 3 days and peaks once clearance is increased in each simulation (b). The percentage of parasites older than 12 h is initially lower than 25% before a high clearance rate of parasites is introduced (c). The various clearance rates targeting different groups of parasites based on age have distinctly different impacts on the proportion of old parasites. A narrow clearance function that targets only parasites older than 0.75 days results in sharp increases in the percentage of parasites older than 12 h to above 50% after day 4.

FIG 5

The results of two models of parasite dynamics in a host with sequestration (solid lines) and without sequestration (dashed lines). Three quantities akin to those measured experimentally in Fig. 1 are presented as follows: parasitemia (a), total parasite burden (b), and the percentage of total parasites older than 12 h (c). Both models have the same parameters and initial conditions, with a host clearance of parasites beginning near day 4 of infection of the same magnitude (δ_rate = 24 day⁻¹) and targeting parasites older than 0.75 days. Parasites are able to sequester when they are 16 h (0.67 days) old, and these parasites sequester at s_rate = 2 day⁻¹.

FIG 6

The results of the mathematical model that includes sequestration. Four simulations are shown, with sequestration beginning at different parasite ages (s_age = 0.5, 0.6, 0.7, and 0.8 days, indicated in blue, green, teal, and red, respectively). The sequestration functions for each simulation are shown in panel a). The total parasite burden (b) and percentage of parasites older than 12 h (c) for each simulation are shown. In all simulations, host clearance of parasites begins just before day 4 of infection (see Methods Part E in the supplemental material) and targets parasites older than 0.75 days. It is evident that the earlier the age at which sequestration is possible, the higher the total parasite burden reached in infection. When the age that parasites can sequester is later than the age at which they become targeted for clearance (0.75 days), the parasite population stops growing.