Malaria is caused by infection with intraerythrocytic protozoa of the genus Plasmodium that are transmitted by Anopheles mosquitoes. Although a variety of anti-parasite effector genes have been identified in anopheline mosquitoes, little is known about the signaling pathways that regulate these responses during parasite development. Here we demonstrate that the MEK-ERK signaling pathway in Anopheles is controlled by ingested human TGF-β1 and finely tunes mosquito innate immunity to parasite infection. Specifically, MEK-ERK signaling was dose-dependently induced in response to TGF-β1 in immortalized cells in vitro and in the A. stephensi midgut epithelium in vivo. At the highest treatment dose of TGF-β1, inhibition of ERK phosphorylation increased TGF-β1-induced expression of the anti-parasite effector gene nitric oxide synthase (NOS), suggesting that increasing levels of ERK activation negatively feed back on induced NOS expression. At infection levels similar to those found in nature, inhibition of ERK activation reduced P. falciparum oocyst loads and infection prevalence in A. stephensi and enhanced TGF-β1-mediated control of P. falciparum development. Taken together, our data demonstrate that malaria parasite development in the mosquito is regulated by a conserved MAPK signaling pathway that mediates the effects of an ingested cytokine.

We showed previously that ingested human insulin activates the insulin/IGF-1 signaling pathway in Anopheles stephensi and increases the susceptibility of these mosquitoes to Plasmodium falciparum. In other organisms, insulin can alter immune responsiveness through regulation of NF-B transcription factors, critical elements for innate immunity that are also central to mosquito immunity. We show here that insulin signaling decreased expression of NF-B-regulated immune genes in mosquito cells stimulated with either bacterial or malarial soluble products. Further, human insulin suppressed mosquito immunity through sustained phosphatidylinositol 3-kinase activation, since inhibition of this pathway led to decreased parasite development in the mosquito. Together, these data demonstrate that activation of the insulin/IGF-1 signaling pathway by ingested human insulin can alter NF-B-dependent immunity, and ultimately the susceptibility, of mosquitoes to P. falciparum.

Reactive oxygen species (ROS) have been implicated in direct killing of pathogens, increased tissue damage, and regulation of immune signaling pathways in mammalian cells. Available research suggests that analogous phenomena affect the establishment of Plasmodium infection in Anopheles mosquitoes. We have previously shown that provision of human insulin in a blood meal leads to increased ROS levels in Anopheles stephensi. Here, we demonstrate that provision of human insulin significantly increased parasite development in the same mosquito host in a manner that was not consistent with ROS-induced parasite killing or parasite escape through damaged tissue. Rather, our studies demonstrate that ROS are important mediators of both the mitogen-activated protein kinase and phosphatidylinositol 3-kinase/Akt signaling branches of the mosquito insulin signaling cascade. Further, ROS alone can directly activate these signaling pathways and this activation is growth factor specific. Our data, therefore, highlight a novel role for ROS as signaling mediators in the mosquito innate immune response to Plasmodium parasites.

The overexpression of activated, myristoylated Akt in the midgut of female transgenic Anopheles stephensi results in resistance to infection with the human malaria parasite Plasmodium falciparum but also decreased lifespan. In the present study, the understanding of mitochondria-dependent midgut homeostasis has been expanded to explain this apparent paradox in an insect of major medical importance. Given that Akt signaling is essential for cell growth and survival, we hypothesized that sustained Akt activation in the mosquito midgut would alter the balance of critical pathways that control mitochondrial dynamics to enhance parasite killing at some cost to survivorship. Toxic reactive oxygen and nitrogen species (RNOS) rise to high levels in the midgut after blood feeding, due to a combination of high NO production and a decline in FOXO-dependent antioxidants. Despite an apparent increase in mitochondrial biogenesis in young females (3 d), energy deficiencies were apparent as decreased oxidative phosphorylation and increased [AMP]/[ATP] ratios. In addition, mitochondrial mass was lower and accompanied by the presence of stalled autophagosomes in the posterior midgut, a critical site for blood digestion and stem cell-mediated epithelial maintenance and repair, and by functional degradation of the epithelial barrier. By 18 d, the age at which An. stephensi would transmit P. falciparum to human hosts, mitochondrial dysfunction coupled to Akt-mediated repression of autophagy/mitophagy was more evident and midgut epithelial structure was markedly compromised. Inhibition of RNOS by co-feeding of the nitric-oxide synthase inhibitor L-NAME at infection abrogated Akt-dependent killing of P. falciparum that begins within 18 h of infection in 3–5 d old mosquitoes. Hence, Akt-induced changes in mitochondrial dynamics perturb midgut homeostasis to enhance parasite resistance and decrease mosquito infective lifespan. Further, quality control of mitochondrial function in the midgut is necessary for the maintenance of midgut health as reflected in energy homeostasis and tissue repair and renewal.

Severe pediatric malaria is an important risk factor for developing disseminated infections with nontyphoidal Salmonella serotypes (NTS). While recent animal studies on this subject are lacking, early work suggests that an increased risk for developing systemic NTS infection during malaria is caused by hemolytic anemia, which leads to reduced macrophage microbicidal activity. Here we established a model for oral Salmonella enterica serotype Typhimurium challenge in mice infected with Plasmodium yoelii nigeriensis. Initial characterization of this model showed that 5 days after coinoculation, P. yoelii nigeriensis infection increased the recovery of S. Typhimurium from liver and spleen by approximately 1,000-fold. The increased bacterial burden could be only partially recapitulated by antibody-mediated hemolysis, which increased the recovery of S. Typhimurium from liver and spleen by 10-fold. These data suggested that both hemolysis and P. yoelii nigeriensis-specific factors contributed to the increased susceptibility to S. Typhimurium. The mechanism by which hemolysis impaired resistance to S. Typhimurium was further investigated. In vitro, S. Typhimurium was recovered 24 h after infection of hemophagocytic macrophages in 2-fold-higher numbers than after infection of mock-treated macrophages, making it unlikely that reduced macrophage microbicidal activity was solely responsible for hemolysis-induced immunosuppression during malaria. Infection with P. yoelii nigeriensis, but not antibodymediated hemolysis, reduced serum levels of interleukin-12p70 (IL-12p70) in response to S. Typhimurium challenge. Collectively, studies establishing a mouse model for this coinfection suggest that multiple distinct malaria-induced immune defects contribute to increased susceptibility to S. Typhimurium.