How Mosquito Immune System Attacks Specific Infections, Including Malaria Parasite

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Researchers have determined a new mechanism by which the mosquitoes’ immune system can respond with specificity to infections with various pathogens, including the parasite that causes malaria in humans, using one single gene.

Unlike humans and other animals, insects do not make antibodies to target specific infections. According to researchers at the Johns Hopkins Bloomberg School of Public Health, mosquitoes use a mechanism known as alternative splicing to arrange different combinations of binding domains, encoded by the same AgDscam gene, into protein repertoires that are specific for different invading pathogens. The researchers’ findings were published October 18 in the journal Cell Host & Microbe and could lead to new ways to prevent the spread of a variety of mosquito born illnesses.

Mosquitoes and other insects use their primitive innate immune systems to successfully fight infections with a broad spectrum of viruses, bacteria, fungi and parasites, despite the lack of antibodies that are part of the more sophisticated human immune system. The effectiveness of the human immune system is to a large degree based on the ability to produce an enormous variety of antibodies containing different immunoglobulin domains that can specifically tag and label a pathogen for destruction. This great variety of pathogen-binding antibodies is achieved by combining different immunoglobulin gene segments and further mutate them through mechanisms called somatic recombination and hypermutation. While mosquitoes also have genes encoding immunoglobulin domains, they lack these specific mechanisms to achieve pathogen recognition diversity.

The Johns Hopkins researchers discovered a different way by which mosquitoes can combine immunoglobulin domains of a single gene called AgDscam (Anopheles gambiae Down Syndrome Cell Adhesion Molecule) to produce a variety of pathogen-binding proteins. The AgDscam gene is subjected to a mechanism called alternative splicing that combines different immunoglobulin domains into mature AgDscam proteins, depending on which pathogen has infected the mosquito. The researchers showed that this alternative splicing is guided by the immune signal transducing pathways (analogous to electrical circuits) that they previously demonstrated to activate defenses against different malaria parasites and other pathogens. While alternative splicing of the AgDscam gene does not nearly achieve the degree of pathogen recognition diversity of human antibodies, it does nevertheless vastly increase the variety of pathogen binding molecules.

“Using antibodies to fight infection is like fishing with a harpoon—it’s very targeted. The mosquito’s innate immune system is more like fishing with a net—it catches a bit of everything,” explained George Dimopoulos, PhD, senior investigator of the study and professor with the Johns Hopkins Malaria Research Institute. “However, we discovered that immune pathway-guided alternative splicing of the AgDscam gene renders the mosquito’s immune net, so to speak, more specific than previously suspected. The mosquito’s immune system can come up with approximately 32,000 AgDscam protein combinations to target infections with greater specificity.”

Dimopoulos and his group are developing a malaria control strategy based on mosquitoes that have been genetically modified to possess an enhanced immune defense against the malaria parasite Plasmodium. One obstacle to this approach is the great variety of Plasmodium strains that may interact somewhat differently with the mosquito’s immune system.

“Some of these strains may not be detected by the engineered immune system proteins that mediate their killing. Our new discovery may provide the means to create genetically modified mosquitoes that can target a broader variety of parasite strains, like casting a net rather than shooting with a harpoon,” said Dimopoulos.

Malaria kills more than 800,000 people worldwide each year. Many are children.

“Anopheles NF-kB –Regulated Splicing Factors Direct Pathogen-Specific Repertoires of the Hypervariable Pattern Recognition Receptor AgDscam” was written by Yuemei Dong, Chris M. Cirimotich, Andrew Pike, Ramesh Chandra and George Dimopoulos.

The research was supported by grants from the National Institutes of Health/National Institute of Allergy and Infectious Disease, the Calvin A. and Helen H. Lang Fellowship, and the Johns Hopkins Malaria Research Institute.

Sources: Johns Hopkins Bloomberg School of Public Health; Cell Host & Microbe

Researchers Discover Proteins in Mosquitoes that Help Fight Malaria Infection

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Researchers have discovered the function of a series proteins within the mosquito that transduce a signal that enables the mosquito to fight off infection from the parasite that causes malaria in humans. Together, these proteins are known as immune deficiency (Imd) pathway signal transducing factors, are analogous to an electrical circuit. As each factor is switched on or off it triggers or inhibits the next, finally leading to the launch of an immune response against the malaria parasite.

The latest study, conducted at the Johns Hopkins Malaria Research Institute, builds upon earlier work of the research team, in which they found that silencing one gene of this circuit, Caspar, activated Rel2, an Imd pathway transcription factor of the Anopheles gambiae mosquito. The activation of Rel2 turns on the effectors TEP1, APL1 and FBN9 that kill malaria-causing parasites in the mosquito’s gut. More significantly, this study discovered the Imd pathway signal transducing factors and effectors that will mediate a successful reduction of parasite infection at their early ookinete stage, as well as in the later oocyst stage when the levels of infection were similar to those found in nature.

“Identifying and understanding how all of the players work is crucial for manipulating the Imd pathway as an invention to control malaria. We now know which genes can be manipulated through genetic engineering to create a malaria resistant mosquito” said George Dimopoulos PhD, professor in the Department of Molecular Microbiology and Immunology at the Johns Hopkins Bloomberg School of Public Health.

To conduct the study, Dimopoulos’s team used a RNA interference method to “knock down” the genes of the Imd pathway. As the components were inactivated, the researchers could observe how the mosquito’s resistance to parasite infection would change.

“Imagine a string of Christmas lights or other circuit that will not work when parts aren’t aligned in the right sequence. That is how we are working with the mosquito’s immune system,” explained Dimopolous. “We manipulate the molecular components of the mosquito’s immune system to identify the parts necessary to kill the malaria parasites.”

Malaria kills more than 800,000 people worldwide each year. Many are children.

The authors of “Anopheles Imd pathway factors and effectors in infection intensity-dependent anti-Plasmodium action” are Lindsey S. Garver, Ana C. Bahia, Suchismita Das, Jayme A. Souza-Neo, Jessica Shiao, Yuemei Dong and George Dimopoulos.

The research was funded by the Johns Hopkins Malaria Research Institute and was published June 7, 2012 in the journal PLoS Pathogens.

Source: Johns Hopkins Bloomberg School of Public Health