Human malaria in immunocompromised mice: an in vivo model to study defense mechanisms against Plasmodium falciparum

Abstract

We have recently described that sustained Plasmodium falciparum growth could be obtained in immunodeficient mice. We now report the potential of this new mouse model by assaying the effect of the passive transfer of antibodies (Abs) which in humans have had a well-established effect.Our results show that the total African adult hyperimmune immunoglobulin Gs (HI-IgGs) strongly reduce P. falciparum parasitemia similarly to that reported in humans, but only when mice are concomitantly reconstituted with human monocytes (HuMNs). In contrast, neither HI-IgGs nor HuMNs alone had any direct effect upon parasitemia. We assessed the in vivo effect of epitope-specific human Abs affinity-purified on peptides derived either from the ring erythrocyte surface antigen (RESA) or the merozoite surface protein 3 (MSP3). The inoculation of low concentrations of anti-synthetic peptide from MSP3, but not of anti-RESA Abs, consistently suppressed P. falciparum in the presence of HuMNs. Parasitemia decrease was stronger and faster than that observed using HI-IgGs and as fast as that induced by chloroquine. Our observations demonstrate that this mouse model is of great value to evaluate the protective effect of different Abs with distinct specificity in the same animal, a step hardly accessible and therefore never performed before in humans.

Figure 1

In vivo transfer of either HI-IgGs or HuPBMCs in P.f.-HuRBC-BXN mice. Course of P. falciparum parasitemia in P.f.-HuRBC-BXN mice undergoing the complementary immunomodulation protocol, injected intraperitoneally on day 0 with P. falciparum–infected HuRBCs. Noninfected HuRBCs were injected subsequently at 3–4-d intervals. (A) Shown is the mean ± SD from the parasitemia in seven representative BXN mice. (B) Mean parasitemia in three P.f.-HuRBC-BXN mice after intraperitoneal injection of HI-IgGs at a dose of 200 mg/kg. (C) Mean parasitemia in five P.f.-HuRBC-BXN mice after intraperitoneal injection of 30 × 10⁶ HuPBMCs/mouse. Arrows, day of injection.

Figure 2

In vivo transfer of HI-IgGs together with HuPBMCs (A and B) or MNs (C and D) in P.f.-HuRBC-BXN mice. Individual parasitemia in two out of four mice injected sequentially with either 200 mg/kg of HI-IgGs at day 9, followed 6 d later by HuPBMCs (3 × 10⁷) (A) or the same in the reverse order on days 6 and 9, respectively (B). Arrows show treatment days. In mouse A (but not in mouse B) an additional dose of 100 mg/kg of HI-IgGs was added because of the longer delay between the two treatments and the fast catabolism of human IgGs. Two other mice undergoing the same protocol showed the same effect on the course of P. falciparum parasitemia. C and D show the individual parasitemia of two out of four mice undergoing the same protocol except that purified HuMNs were used at a rate of 3 × 10⁶ ntraperitoneally instead of total PBMCs, together with 200 mg/kg of HI-IgGs. In mouse D, only the initial IgG injection was of 200mg/kg and the further two were of 100 mg/kg. In two other mice, not shown in the figure, receiving IgG and/or MNs on days 10 and 15, or 16 and 22, respectively, the decrease of parasitemia occurred over 4 d after the first combined injection. Only mouse D showed a partial response to treatment. +, one or two parasites observed in thin blood smears; −, parasites no longer detectable in thin blood smears.

Figure 3

In vivo transfer of anti-MSP3b peptide Abs with HuMNs in P.f.-HuRBC-BXN mice. (A) Course of parasitemia in a mouse receiving a single injection of HuMNs (3 × 10⁶). (B) Parasitemia in a mouse receiving a single injection of anti-MSP3b peptide–specific human Abs (at a rate of 250 μl of a solution titrating at 1:200 by IFA). (C and D) Course of parasitemia in two mice receiving first HuMNs (3 × 10⁶) and thereafter the anti-MSP3b Abs (250 μl) together with an additional dose of HuMNs (3 × 10⁶). + and −, presence or absence of few residual parasites in thin blood smears. Arrows, day of injection.

Figure 4

Sequential effect of anti-RESA and anti-MSP3b Abs. Course of parasitemia in two P.f.-HuRBC-BXN mice receiving sequential treatment first by HuMNs, then by HuMNs together with anti-RESA peptide Abs (250 μl of a solution with a titer of 1:2,000 by IFA), and followed by HuMNs together with anti-MSP3b peptide Abs (250 μl, IFA 1:200). The arrows show the days at which treatment was administered. −, absence of residual parasites in thin blood smears.

Figure 5

Decrease of parasitemia in HI-IgG– and anti-MSP3b–treated mice. Comparison between the mean decrease in parasitemias (± SD) in two groups of six mice, one treated by total African HI-IgGs (□), the other by affinity-purified anti-MSP3 Abs (▪). In both groups, HuMNs and Abs were injected on day 0. Note that the percentage of reduction of parasitemia is shown as a logarithmic scale.