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Innate immune system can kill HIV when a viral gene is deactivated


Human cells have an intrinsic capacity to destroy HIV. However, the virus has evolved to contain a gene that blocks this ability. When this gene is removed from the virus, the innate human immune system destroys HIV by mutating it to the point where it can no longer survive.

This phenomenon has been shown in test tube laboratory experiments, but now researchers at the University of North Carolina School of Medicine have demonstrated that the same phenomenon occurs in a humanized mouse model, suggesting a promising new target for tackling the virus, which has killed nearly 30 million people worldwide since it first appeared three decades ago.

A family of human proteins called APOBEC3 effectively restrict the growth of HIV and other viruses, but this action is fully counteracted by the viral infectivity factor gene (vif) in HIV. In the study, researchers intravenously infected humanized mice with HIV. They found that the most commonly transmitted strains of HIV are completely neutralized by APOBEC3 proteins when vif is removed from the virus.

“Without the vif gene, HIV can be completely destroyed by the body’s own immune system,” said J. Victor Garcia, PhD, professor of medicine at the UNC School of Medicine and senior author on the study. “These results suggest a new target for developing drugs fully capable of killing the virus.”

Garcia and his colleagues pioneered the humanized mouse model used for these studies. The aptly named “BLT” mouse is created by introducing human bone marrow, liver and thymus tissues into animals without an immune system of their own. The mice have a fully functioning human immune system and can be infected with HIV in the same manner as humans. In previous research, Garcia and his team have effectively prevented intravenous, rectal, vaginal and oral transmission of HIV in the mice with pre-exposure prophylaxis (PrEP).

For the current study, Garcia and his colleagues also infected BLT mice with another, highly harmful strain of the virus. The results show that this strain of HIV does continue to replicate, even without vif, but at a much slower rate and without harming the human immune system. Further, the researchers found that virus replication in this case was limited to one tissue—the thymus—in the entire body.

“These findings demonstrate a fundamental weakness in HIV,” said John F. Krisko, PhD, lead author on the study. “If this weakness can be exploited, it might eventually lead to a cure for HIV/AIDS,” Krisko said.

 

Journal reference:

John F. Krisko, Francisco Martinez-Torres, John L. Foster, J. Victor Garcia. HIV Restriction by APOBEC3 in Humanized MicePLoS Pathogens, 2013; 9 (3): e1003242 DOI: 10.1371/journal.ppat.1003242

Provided by University of North Carolina Health Care

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New Vaccine-Design Approach Targets Viruses Such as HIV


A team led by scientists from The Scripps Research Institute (TSRI) and the International AIDS Vaccine Initiative (IAVI) has unveiled a new technique for vaccine design that could be particularly useful against HIV and other fast-changing viruses.

New Vaccien Design HIV

The report, which appears March 28, 2013, in Science Express, the early online edition of the journal Science, offers a step toward solving what has been one of the central problems of modern vaccine design: how to stimulate the immune system to produce the right kind of antibody response to protect against a wide range of viral strains. The researchers demonstrated their new technique by engineering an immunogen (substance that induces immunity) that has promise to reliably initiate an otherwise rare response effective against many types of HIV.

“We’re hoping to test this immunogen soon in mice engineered to produce human antibodies, and eventually in humans,” said team leader William R. Schief, who is an associate professor of immunology and member of the IAVI Neutralizing Antibody Center at TSRI.

Seeking a Better Way

For highly variable viruses such as HIV and influenza, vaccine researchers want to elicit antibodies that protect against most or all viral strains — not just a few strains, as seasonal flu vaccines currently on the market. Vaccine researchers have identified several of these broadly neutralizing antibodies from long-term HIV-positive survivors, harvesting antibody-producing B cells from blood samples and then sifting through them to identify those that produce antibodies capable of neutralizing multiple strains of HIV. Such broadly neutralizing antibodies typically work by blocking crucial functional sites on a virus that are conserved among different strains despite high mutation elsewhere.

However, even with these powerful broadly neutralizing antibodies in hand, scientists need to find a way to elicit their production in the body through a vaccine. “For example, to elicit broadly neutralizing antibodies called VRC01-class antibodies that neutralize 90 percent of known HIV strains, you could try using the HIV envelope protein as your immunogen,” said Schief, “but you run into the problem that the envelope protein doesn’t bind with any detectable affinity to the B cells needed to launch a broadly neutralizing antibody response.”

To reliably initiate that VRC01-class antibody response, Schief and his colleagues therefore sought to develop a new method for designing vaccine immunogens.

From Weak to Strong

Joseph Jardine, a TSRI graduate student in the Schief laboratory, evaluated the genes of VRC01-producing B cells in order to deduce the identities of the less mature B cells — known as germline B cells — from which they originate. Germline B cells are major targets of modern viral vaccines, because it is the initial stimulation of these B cells and their antibodies that leads to a long-term antibody response.

In response to vaccination, germline B cells could, in principle, mature into the desired VRC01-producing B cells — but natural HIV proteins fail to bind or stimulate these germline B cells so they cannot get the process started. The team thus set out to design an artificial immunogen that would be successful at achieving this.

Jardine used a protein modeling software suite called Rosetta to improve the binding of VRC01 germline B cell antibodies to HIV’s envelope protein. “We asked Rosetta to look for mutations on the side of the HIV envelope protein that would help it bind tightly to our germline antibodies,” he said.

Rosetta identified dozens of mutations that could help improve binding to germline antibodies. Jardine then generated libraries that contained all possible combinations of beneficial mutations, resulting in millions of mutants, and screened them using techniques called yeast surface display and FACS. This combination of computational prediction and directed evolution successfully produced a few mutant envelope proteins with high affinity for germline VRC01-class antibodies.

Jardine then focused on making a minimal immunogen — much smaller than HIV envelope — and so continued development using the “engineered outer domain (eOD)” previously developed by Po-Ssu Huang in the Schief lab while Schief was at the University of Washington. Several iterative rounds of design and selection using a panel of germline antibodies produced a final, optimized immunogen — a construct they called eOD-GT6.

A Closer Look

To get a better look at eOD-GT6 and its interaction with germline antibodies, the team turned to the laboratory of Ian A. Wilson, chair of the Department of Integrative Structural and Computational Biology and a member of the IAVI Neutralizing Antibody Center at TSRI.

Jean-Philippe Julien, a senior research associate in the Wilson laboratory, determined the 3D atomic structure of the designed immunogen using X-ray crystallography — and, in an unusual feat, also determined the crystal structure of a germline VRC01 antibody, plus the structure of the immunogen and antibody bound together.

“We wanted to know whether eOD-GT6 looked the way we anticipated and whether it bound to the antibody in the way that we predicted — and in both cases the answer was ‘yes’,” said Julien. “We also were able to identify the key mutations that conferred its reactivity with germline VRC01 antibodies.”

Mimicking a Virus

Vaccine researchers know that such an immunogen typically does better at stimulating an antibody response when it is presented not as a single copy but in a closely spaced cluster of multiple copies, and with only its antibody-binding end exposed. “We wanted it to look like a virus,” said Sergey Menis, a visiting graduate student in the Schief laboratory.

Menis therefore devised a tiny virus-mimicking particle made from 60 copies of an obscure bacterial enzyme and coated it with 60 copies of eOD-GT6. The particle worked well at activating VRC01 germline B cells and even mature B cells in the lab dish, whereas single-copy eOD-GT6 did not.

“Essentially it’s a self-assembling nanoparticle that presents the immunogen in a properly oriented way,” Menis said. “We’re hoping that this approach can be used not just for an HIV vaccine but for many other vaccines, too.”

The next step for the eOD-GT6 immunogen project, said Schief, is to test its ability to stimulate an antibody response in lab animals that are themselves engineered to produce human germline antibodies. The difficulty with testing immunogens that target human germline antibodies is that animals typically used for vaccine testing cannot make those same antibodies. So the team is collaborating with other researchers who are engineering mice to produce human germline antibodies. After that, he hopes to learn how to drive the response, from the activation of the germline B cells all the way to the production of mature, broadly neutralizing VRC01-class antibodies, using a series of designed immunogens.

Schief also hopes they will be able to test their germline-targeting approach in humans sooner rather than later, noting “it will be really important to find out if this works in a human being.”

The first authors of the paper, “Rational HIV immunogen design to target specific germline B cell receptors,” were Jardine, Julien and Menis. Co-authors were Takayuki Ota and Devin Sok of the Nemazee and Burton laboratories at TSRI, respectively; Travis Nieusma of the Ward laboratory at TSRI; John Mathison of the Ulevitch laboratory at TSRI; Oleksandr Kalyuzhniy and Skye MacPherson, researchers in the Schief laboratory from IAVI and TSRI, respectively; Po-Ssu Huang and David Baker of the University of Washington, Seattle; Andrew McGuire and Leonidas Stamatatos of the Seattle Biomedical Research Institute; and TSRI principal investigators Andrew B. Ward, David Nemazee, Ian A. Wilson, and Dennis R. Burton, who is also head of the IAVI Neutralizing Center at TSRI.

Story Source:

The above story is reprinted from materials provided by Scripps Research Institute.

Journal Reference:

Joseph Jardine, Jean-Philippe Julien, Sergey Menis, Takayuki Ota, Oleksandr Kalyuzhniy, Andrew McGuire, Devin Sok, Po-Ssu Huang, Skye MacPherson, Meaghan Jones, Travis Nieusma, John Mathison, David Baker, Andrew B. Ward, Dennis R. Burton, Leonidas Stamatatos, David Nemazee, Ian A. Wilson, and William R. Schief. Rational HIV Immunogen Design to Target Specific Germline B Cell Receptors. Science, 28 March 2013 DOI: 10.1126/science.1234150

New Malaria Vaccine Depends on… Mosquito Bites?

April 25, 2011 Leave a comment

The same menace that spreads malaria — the mosquito bite — could help wipe out the deadly disease, according to researchers working on a new vaccine at Tulane University.

(left) Nirbhay Kumar, professor of tropical medicine at Tulane University, is working on a vaccine that aims to wipe out malaria using the same menace that spreads it — the mosquito bite. (Credit: Image courtesy of Tulane University)


The PATH Malaria Vaccine Initiative (MVI), established in 1999 through a grant from the Bill & Melinda Gates Foundation, announced February 15 a collaboration with Tulane University School of Public Health and Tropical Medicine and India’s Gennova Biopharmaceuticals Ltd. to produce and test a novel vaccine that aims to inoculate mosquitoes when they bite people.

The vaccine would work by triggering an immune response in people so they produce antibodies that target a protein the malaria parasite needs to reproduce within a mosquito.

Malaria, which kills nearly 800,000 people every year worldwide, is caused by a microscopic parasite that alternates between human and mosquito hosts at various stages of its lifecycle. Once a mosquito bites a vaccinated person, the antibodies would neutralize the protein essential for malaria parasite’s reproduction, effectively blocking the parasite’s — and the mosquito’s — ability to infect others.

The vaccine relies on a protein — known as Pfs48/45 — which is very difficult to synthetically produce, says Nirbhay Kumar, professor of tropical medicine at Tulane.

“With MVI’s support we can now work with Gennova to produce sufficient quantity of the protein and develop a variety of vaccine formulations that can be tested in animals to determine which one give us the strongest immune response,” Kumar says.

Such transmission blocking vaccines, though not yet widely tested in humans, are attracting widespread interest due to their potential to be used in conjunction with more traditional malaria vaccines and other interventions — such as malaria drugs and bed nets — to make gradual elimination and even eradication of the disease a reality.

“We’re investing in developing transmission blocking malaria vaccines to support two long-term goals: introducing an 80 percent efficacious malaria vaccine by the year 2025 and eventually eradicating malaria altogether,” says Dr. Christian Loucq, director of MVI. “A vaccine that breaks the cycle of malaria transmission will be important to our success.”

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Tulane University.