Schistosoma mansoni infection reduces the incidence of murine cerebral malaria

Abstract

Background: Plasmodium and Schistosoma are two of the most common parasites in tropical areas. Deregulation of the immune response to Plasmodium falciparum, characterized by a Th1 response, leads to cerebral malaria (CM), while a Th2 response accompanies chronic schistosomiasis.

Methods: The development of CM was examined in mice with concomitant Schistosoma mansoni and Plasmodium berghei ANKA infections. The effect of S. mansoni egg antigen injection on disease development and survival was also determined. Cytokine serum levels were estimated using ELISA. Statistical analysis was performed using t-test.

Results: The results demonstrate that concomitant S. mansoni and P. berghei ANKA infection leads to a reduction in CM. This effect is dependent on infection schedule and infecting cercariae number, and is correlated with a Th2 response. Schistosomal egg antigen injection delays the death of Plasmodium-infected mice, indicating immune involvement.

Conclusions: This research supports previous claims of a protective effect of helminth infection on CM development. The presence of multiple parasitic infections in patients from endemic areas should therefore be carefully noted in clinical trials, and in the development of standard treatment protocols for malaria. Defined helminth antigens may be considered for alleviation of immunopathological symptoms.

Figure 1

Brain histology of non-infected and PbA-infected ICR mice. Brains were removed from control non-infected mice (n = 10) or PbA-infected moribund mice (n = 10) on day 8 post- PbA inoculation. Representative sections are shown for each group. (A) Representative section of the cortex of a control mouse, demonstrating lack of vessel distention or pathology (magnification 20×). (B) Representative section of the submeningeal cortex of a control mouse, demonstrating the lack of haemorrhages or cellular infiltrations (magnification 10×). (C) Mononuclear cell aggregation in a meningeal vessel, increased cellularity, and piknotic nuclei in a prostrate mouse (magnification 20×). (D) Distended thallamic blood vessel; mononuclear cell interacting with activated endothelial cells (boxed) in a prostrate mouse (magnification 40×). (E) "Spongy" tissue - evidence of edema in a PbA-infected mouse suffering from coma (magnification 40×). (F) Meningeal haemorrahges in a PbA-infected mouse suffering from coma (magnification 10×).

Figure 2

Changes in temperature and body weight, and survival of mice infected with PbA 4 weeks (A), or 7 weeks (B) after S. mansoni infection, and in corresponding PbA- and S. mansoni infected control mice. Weight and temperature were monitored until days 9-11, when most of the deaths in the PbA-infected groups occurred. Error bars represent SD. *p < 0.001 (t-test), co-infected vs. S. mansoni and PbA-infected groups; ^§p < 0.0001 (t-test), PbA- vs. co-infected mice; **p < 0.002 (t-test), co-infected vs. PbA-infected mice.

Figure 3

The effect of S. mansoni infection on the development of cerebral malaria in ICR mice. Mice infected with 5 × 10⁴ PbA-infected erythrocytes (A), 5 × 10⁴ PbA-infected erythrocytes, seven weeks after injection of 50 S. mansoni cercariae (B), or 5 × 10⁴ PbA-infected erythrocytes, seven weeks after injection of 100 S. mansoni cercariae (C). Each line represents one mouse; D notes mouse death. Co-infection of mice with 100 cercariae caused a significant reduction in cerebral malaria (p < 0.0001, t-test). Shaded areas denote the period of CM susceptibility and death.

Figure 4

Average serum cytokine levels (pg/ml) ± SD in infected and non-infected ICR mice (n = 5 at each time point). Mice were infected with PbA 7 weeks after S. mansoni infection. Cytokine levels were measured on day 1 (A), day 7 (B), and day 10 post-PbA infection (C).

Figure 5

Average Th2-Th1 ratio. The balance between Th1 and Th2 responses was determined using the cytokine levels of each mouse, as follows: Th2/Th1 ratio = (sum of IL-4 and IL-10 levels) - (sum of IFNγ and TNF levels). Average values ± SD are presented per day. p < 0.05 (t-test) when comparing *non-infected and S. mansoni-infected mice; **non-infected and PbA-infected mice; ^†S. mansoni- and PbA-infected mice; ^‡S. mansoni- and co-infected mice; ^§PbA- and co-infected mice.

Figure 6

Parasitaemia (A) and survival (B) of mice administered IPSE/alpha-1 by i.v. injection from day 4 before PbA infection to day 7 post-infection. Control mice were injected with PBS. *Significant difference in delay to death (p = 0.005, t-test). Shaded areas denote the period of CM susceptibility and death.

Figure 7

Parasitaemia (A) and survival (B) of control mice (n = 10) and mice administered complete S. mansoni egg extract (SmEA; n = 10) or egg extract from which IPSE/alpha-1 was removed (SmEA ΔIPSE/alpha-1; n = 10), by i.v. injection, on days 7 and 0 before PbA infection and days 3 and 6 post-infection. Control mice were injected with PBS. *Parasitaemia was slightly lower in mice treated with SmEA or SmEA ΔIPSE/alpha-1 compared to control mice, on days 7-10 post-infection (p < 0.05, t-test). **A significant delay to death was seen between control and treated mice (p < 0.05, t-test), but not between the treated groups (p = 0.2, t-test). Shaded areas denote the period of CM susceptibility and death.