Acquired Clinical Immunity to Malaria in Nonhuman Primates Coinfected with Schistosoma and Plasmodium Parasites.

Naturally acquired immunity to malaria develops over several years and can be compromised by concomitant infections. This study explored the influence of chronic schistosomiasis on clinical outcome and immunity to repeated malaria infection. Two groups of baboons (n = 8 each), were infected with Schistosoma mansoni cercariae to establish chronic infections. One of the two groups was treated with praziquantel (PZQ) to eliminate schistosome infection. The two groups plus a new malaria control group (n = 8) were inoculated three times with Plasmodium knowlesi parasites at 1-month intervals. Clinical data and IgG, IgG1, memory T-cell, and monocyte levels were recorded. After three P. knowlesi infections, we observed (i) reduced clinical symptoms in all groups with each subsequent infection, (ii) increased IgG and IgG1 levels in the malaria control (Pk-only) group, (iii) increased IgG, IgG1, CD14+, and CD14- CD16+ levels in the Schistosoma-treated (Schisto/PZQ+Pk) group, and (iv) significantly lower IgG and IgG1 levels compared to those of the Pk-only group, reduced CD4+ CD45RO+ levels, and increased levels of CD14- CD16+ cells in the coinfected (Schisto+Pk) group. Chronic S. mansoni infection does not compromise establishment of clinical immunity after multiple malaria infections, with nonclassical monocytes seeming to play a role. Failure to develop robust antibody and memory T cells may have a long-term impact on acquired immunity to malaria infection.

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

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.

Parasite accumulation in placenta of non-immune baboons during Plasmodium knowlesi infection.

BACKGROUND: Placental malaria (PM) causes adverse pregnancy outcomes in the mother and her foetus. It is difficult to study PM directly in humans due to ethical challenges. This study set out to bridge this gap by determining the outcome of PM in non-immune baboons in order to develop a non-human primate model for the disease. METHODS: Ten pregnant baboons were acquired late in their third trimester (day 150) and randomly grouped as seven infected and three non-infected. Another group of four nulligravidae (non-pregnant) infected was also included in the analysis of clinical outcome. Malaria infection was intravenously initiated by Plasmodium knowlesi blood-stage parasites through the femoral vein on 160(th) day of gestation (for pregnant baboons). Peripheral smear, placental smear, haematological samples, and histological samples were collected during the study period. Median values of clinical and haematological changes were analysed using Kruskal-Wallis and Dunn's Multiple Comparison Test. Parasitaemia profiles were analysed using Mann Whitney U test. A Spearman's rank correlation was run to determine the relationship between the different variables of severity scores. Probability values of P <0.05 were considered significant. RESULTS: Levels of white blood cells increased significantly in pregnant infected (34%) than in nulligravidae infected baboons (8%). Placental parasitaemia levels was on average 19-fold higher than peripheral parasitaemia in the same animal. Infiltration of parasitized erythrocytes and inflammatory cells were also observed in baboon placenta. Malaria parasite score increased with increase in total placental damage score (rs = 0.7650, P <0.05) and inflammatory score (rs = 0.8590, P <0.05). Although the sample size was small, absence of parasitized erythrocytes in cord blood and foetal placental region suggested lack of congenital malaria in non-immune baboons. CONCLUSION: This study has demonstrated accumulation of parasitized red blood cells and infiltration of inflammatory cells in the placental intravillous space (IVS) of baboons that are non-immune to malaria. This is a key feature of placental falciparum malaria in humans. This presents the baboon as a new model for the characterization of malaria during pregnancy.

Functional evaluation of malaria Pfs25 DNA vaccine by in vivo electroporation in olive baboons.

Plasmodium falciparum Pfs25 antigen, expressed on the surface of zygotes and ookinetes, is one of the leading targets for the development of a malaria transmission-blocking vaccine (TBV). Our laboratory has been evaluating DNA plasmid based Pfs25 vaccine in mice and non-human primates. Previously, we established that in vivo electroporation (EP) delivery is an effective method to improve the immunogenicity of DNA vaccine encoding Pfs25 in mice. In order to optimize the in vivo EP procedure and test for its efficacy in more clinically relevant larger animal models, we employed in vivo EP to evaluate the immune response and protective efficacy of Pfs25 encoding DNA vaccine in nonhuman primates (olive baboons, Papio anubis). The results showed that at a dose of 2.5mg DNA vaccine, antibody responses were significantly enhanced with EP as compared to without EP resulting in effective transmission blocking efficiency. Similar immunogenicity enhancing effect of EP was also observed with lower doses (0.5mg and 1mg) of DNA plasmids. Further, final boosting with a single dose of recombinant Pfs25 protein resulted in dramatically enhanced antibody titers and significantly increased functional transmission blocking efficiency. Our study suggests priming with DNA vaccine via EP along with protein boost regimen as an effective method to elicit potent immunogenicity of malaria DNA vaccines in nonhuman primates and provides the basis for further evaluation in human volunteers.