Mighty Mouse

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Mighty Mouse
A humanized BLT mouse model creates a vital platform for drug discovery
by Chael Needle

In the early stages of drug discovery, many candidates that show promise of effective anti-HIV activity may be in the running. But which ones deserve to move on?

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That question is answered in part by those researchers who can help make visible and measure the anti-HIV effects of these candidates before consideration of a human clinical trial is even broached. At this stage of research, in other words, it’s all about the launching pad. One of those launching pads is the humanized BLT (bone marrow-liver-thymus) mouse model, which uses human cells within the animal to simulate how human virus interacts with both the body and HIV-specific agents that seek to target it.

While the humanized BLT mouse model can be used to test many treatment and prevention approaches, it is especially important in a new wave of research into HIV eradication strategies, many of which seek to “kick” HIV out of its reservoir hiding spots, where it persists, despite antiretrovirals, in tissues and peripheral blood cells, in stable fashion, competent enough to replicate if it ever is given a chance.

Activating this latent HIV is necessary in order to eradicate the virus completely. Of course, there’s little reason to activate the virus if one doesn’t have a strategy to recognize it and then kill it.

Researchers know that stopping antiretrovirals often prompts latent HIV to start replicating, but, as this approach might create new rounds of infection, they want to see if latent HIV might be induced and attacked by an adjunctive agent even when an individual is on antiretroviral therapy.

Researcher J. Victor Garcia, PhD, Division of Infectious Diseases, Department of Medicine, UNC Center for AIDS Research, University of North Carolina School of Medicine, worked with a collaborative team across multiple sites on a study, whose results were recently published in the journal PloS Pathogens, that examined the effects of an adjunctive immunotoxin, 3B3-PE38, on latent HIV. 3B3 is an antibody that recognizes a specific HIV protein on infected cells, and then attaches to them, allowing PE38, a toxin, to enter and kill the virus-producing cells and spare, for the most part, others. The compound was developed by co-authors Edward Berger, PhD, and Ira Pastan, MD, from the National Institute of Allergy and Infectious Diseases (NIAID).

Both the distribution of, and the antiretrovirals’ and this targeted cytotoxic agent’s effects on, persistent HIV-infected cells were able to be measured because the mouse model is humanized; that is, it’s not an animal model that is merely similar to a human subject. It’s a step closer—an entire immune system made up of human cells, which can be infected by human virus and which can be treated with human antiretrovirals.

Notes Dr. Garcia, who pioneered the mouse model and is a senior author of this study, which he helped conceive and design: “The translation from our model into clinical application is facilitated by the fact that we’d be using exactly the same reagent.”

In the study, the mice were infected with HIV, the virus was then suppressed by antiretroviral therapy to the greatest degree possible, and then were given the immunotoxin. The goal was to see if an agent could target the virus-infected cells that remain ultimately out of reach of antiretrovirals. Notes Dr. Garcia, the first surprising result the team observed was viral RNA in the tissues of humanized mice undergoing the same antiretroviral therapies that would be administered to humans and remove the virus from the blood. “Viral RNA is an indication of the potential of the virus to replicate.…We found that there was this second reservoir of RNA-producing cells in virtually every tissue we examined—in the lungs, the liver, the thymus, the spleen. Everywhere we looked there was viral RNA. So we had an opportunity to test the new approaches to kill the cells, because we identified them as present.”

He explains: “We saw that, when we put the animal on antiretroviral therapy, there was an expected reduction in the viral RNA in those tissues, but then, when we added the adjunctive immunotoxin therapy, we saw further reduction in the amount of viral RNA produced in individual tissues. So we found an effect on top of what the drugs were doing.”

Garcia’s colleagues at the University of Minnesota, Stephen W. Wietgrefe and Ashley T. Haase, were able to “essentially go in and count how many cells in different tissues were actually producing viral RNA. And, remarkably, after the addition of the immunotoxin, the numbers of cells making RNA were dramatically reduced in virtually every tissue that we looked at. That was proof that the immunotoxin could go in and kill infected cells in the animal that was already well suppressed by antiretroviral therapy.” The research team noted a six-fold drop in the number of infected cells throughout the immune systems.

It’s not the agent’s effects but moreso the visibility of those effects that emboldens Dr. Garcia’s work toward a functional cure. “The biggest advance was not that we tested 3B3-PE38. It’s that we developed a platform in which we can evaluate different novel approaches to HIV eradication. So you can exchange the immunotoxin for another, or you can exchange the immunotoxin for a different type of antiviral or anti-HIV infected cell therapy, and then we can decide which one is the best. That could lead to prioritizing which approach to eradicating HIV could translate into clinical application.”

A mode of comparison is now possible. “There might be other approaches that investigators have that might be better or more well suited and we need to have a platform in order to compare them head-to-head,” he says. “If you think about it, if the goal is to eradicate, we might have Agent A, being an immunotoxin; Agent B, a bifunctional antibody; Agent C a cytotoxic T lymphocyte approach; or a gene therapy approach; and so on, then we don’t really know which one is going to work the best but we need to demonstrate which one actually works, which ones are effective, and this is what we can do with the BLT model.”

Chael Needle wrote about HPV-complication immunotherapy candidate Multikine in the January 2014 issue.