Protective Effect of Chronic Schistosomiasis in Baboons Coinfected with Schistosoma mansoni and Plasmodium knowlesi

Abstract

Malaria and schistosomiasis coinfections are common, and chronic schistosomiasis has been implicated in affecting the severity of acute malaria. However, whether it enhances or attenuates malaria has been controversial due the lack of appropriately controlled human studies and relevant animal models. To examine this interaction, we conducted a randomized controlled study using the baboon (Papio anubis) to analyze the effect of chronic schistosomiasis on severe malaria. Two groups of baboons (n = 8 each) and a schistosomiasis control group (n = 3) were infected with 500 Schistosoma mansoni cercariae. At 14 and 15 weeks postinfection, one group was given praziquantel to treat schistosomiasis infection. Four weeks later, the two groups plus a new malaria control group (n = 8) were intravenously inoculated with 10(5) Plasmodium knowlesi parasites and monitored daily for development of severe malaria. A total of 81% of baboons exposed to chronic S. mansoni infection with or without praziquantel treatment survived malaria, compared to only 25% of animals infected with P. knowlesi only (P = 0.01). Schistosome-infected animals also had significantly lower parasite burdens (P = 0.004) than the baboons in the P. knowlesi-only group and were protected from severe anemia. Coinfection was associated with increased spontaneous production of interleukin-6 (IL-6), suggesting an enhanced innate immune response, whereas animals infected with P. knowlesi alone failed to develop mitogen-driven tumor necrosis factor alpha and IL-10, indicating the inability to generate adequate protective and balancing immunoregulatory responses. These results indicate that chronic S. mansoni attenuates the severity of P. knowlesi coinfection in baboons by mechanisms that may enhance innate immunity to malaria.

FIG 1

Experimental protocol. Four groups of experimental animals were included in the coinfection study: (i) animals infected with S. mansoni cercariae only (Schisto only); (ii) animals infected with schistosomiasis, followed by treatment with praziquantel (PZQ) and later infected with P. knowlesi (Schisto/PZQ+Pk); (iii) animals infected with S. mansoni (without PZQ treatment), followed by P. knowlesi infection (Schisto+Pk); and (iv) animals infected with P. knowlesi only (Pk only). Animals with a schistosome infection were treated with iron supplementation (Fe) between 8 and 10 weeks after schistosome infection.

FIG 2

S. mansoni egg counts in baboons with schistosomiasis. Treatment with praziquantel (PZQ trt) was administered to the Schisto/PZQ+Pk group at weeks 14 and 15 postinfection. Infection with P. knowlesi (Pk inf) occurred at week 20 after schistosomiasis infection.

FIG 3

Proportion of baboons surviving P. knowlesi malaria with or without prior S. mansoni infection. See the legend of Fig. 1 for a description of the various experimental groups.

FIG 4

Daily P. knowlesi parasitemia after i.v. infection of 10⁵ infected erythrocytes. Parasitemia was determined daily and expressed as number of parasites in 10,000 RBCs and graphed on a log scale. Truncated parasitemia levels (prior to day 18), indicated by arrows, show the results for a baboon that had to be euthanized (e.g., all but two animals in the P. knowlesi-only group). Animals that did not succumb to severe malaria were euthanized at day 18 after malaria infection.

FIG 5

Impact of S. mansoni and P. knowlesi coinfection on Hb levels. Shown are the hemoglobin values in grams per deciliter for each animal, taken before S. mansoni infection (BSI), at week 20 after S. mansoni infection and before malaria infection (BMI) for the Schisto-only, Schisto+Pk, and Schisto/PZQ+Pk groups, at and time zero (naive) for the P. knowlesi-only group. The hemoglobin levels after P. knowlesi infection (AMI) were determined at endpoint (7 to 23 days after P. knowlesi infection) for the Schisto+Pk, Schisto/PZQ+Pk, and P. knowlesi-only (Pk only) groups. For the Schisto-only group, the final Hb levels were taken at week 23 after S. mansoni infection.

FIG 6

Increased spontaneous IL-6 production is associated with active schistosomiasis infection and correlates with reduced risk of fatal malaria. IL-6 production in unstimulated PBMC cultures before S. mansoni infection (BSI), before malaria infection (BMI), and after P. knowlesi infection (AMI) was measured in culture supernatants from individual animals. (A) Cytokine production was compared between the different groups of animals. (B) Animals were grouped according to whether they succumbed to severe malaria (dead; AMI, 7 to 13 days after P. knowlesi infection) or recovered (survived; AMI, 23 days after P. knowlesi infection). The two groups were compared using a Student t test of log-transformed values.

FIG 7

Schistosome-infected baboons demonstrate reduced ConA-induced IFN-γ production and sustained TNF-α and IL-10 release after P. knowlesi infection. Plotted are the average concentrations and standard errors of the mean for the IFN-γ (A), TNF-α (B), IL-6 (C), and IL-10 (D) levels for each group of animals, before schistosomiasis infection (BSI), before malaria infection (BMI), and after malaria infection (AMI). Cultures of PBMCs were stimulated with ConA for 3 days.

FIG 8

Prior schistosome infection attenuates LPS-induced TNF-α and IL-10 production after P. knowlesi infection. The average cytokine levels of IL-6, IL-10, and TNF-α were recorded before schistosome infection (BSI) and before (BMI) and after (AMI) malarial infection.

FIG 9

Growth inhibition of P. knowlesi parasites. Growth inhibition in the three groups of animals was determined at 20 weeks after infection with S. mansoni for the Schisto/PZQ+ and Schisto+ groups and before malaria infection in animals with malaria only (Schisto −). For the negative control (inducing growth), cultures were grown in commercially available HuAb serum. For the positive control (growth inhibition), chloroquine (CQ) was added to the culture medium.