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Of mice and HIV models
CAMBRIDGE, Mass.—The good news coming from a recent study published in the Aug. 22 issue of Cell is that small interfering RNA (siRNA) delivered directly to immune cells can suppress the human immunodeficiency virus in animal models of the disease. Perhaps the better news, using a long-view perspective, is researchers are finally zeroing in on making the animal models for HIV better—particularly in the challenging realm of making good HIV mouse models.
The authors of the paper, "T Cell-Specific siRNA Delivery Suppresses HIV-1 Infection in Humanized Mice," noted that along with the enduring problem of finding an effective strategy for delivering siRNA to the proper site in real-life systems, they have been stymied in the past by a lack of appropriate HIV animal models.
"Animal models for HIV-1 have suffered from either the lack of a system that precisely mirrors human HIV infection or, in the case of primate models, scarcity of species, high cost, and the need to use the related but distinct simian virus for infection," noted the authors.
Lead author Dr. Priti Kumar, an instructor and post-doctoral research fellow at Harvard University Medical School's Immune Disease Institute and Department of Pediatrics, adds that to overcome the animal model problem, several labs have been working on immuno-deficient mice that can be transfused with human T-cells and/or stem cells. For her study, the researchers used a new mouse strain, developed by co-authors Dr. Leonard Shultz, a researcher at the Jackson Laboratory, and University of Massachusetts researcher Dr. Dale Greiner, that can be transplanted with human immune cells or human stem cells that subsequently generate a human immune system in the animals.
"The key advance we achieved is that for the first time, these mice develop a complete functional human immune system including robust development of the cell target of HIV, CD4 T cells," Greiner explains. "Previous mouse models used for HIV failed to reproducibly develop CD4 T cells following hematopoietic stem cell engraftment."
"We built on a lot of earlier work that had helped produce mice with deficiencies that allowed us to more closely humanize them and overcome the innate immune system, a much more ancient kind of immune system that had tended to thwart our ability to make models that accurately predict human responses," Shultz says.
Earlier mouse models that lacked or had sharply deficient innate immune systems—though they were the gold standard for HIV and other research efforts for many years—still resisted engraftment, and the model that he and Greiner helped develop supports the higher level of engraftment that Kumar and her colleagues needed for the siRNA research.
More work is needed to get the mouse models perfected, Shultz and Greiner acknowledge, with Shultz calling humanized mice "a moving target that is always going to need improvement."
"The major advances needed are development of immunodeficient mouse models expressing human HLA molecules to permit appropriate T cell development," Greiner notes. "Expression of human species-specific soluble factors that are needed for robust human immune system development are also needed to enhance human immune system development in the immunodeficient mice further."
As for the original point of Kumar's research using the newer mouse model, her team demonstrated that they could thwart HIV in mice by delivering interfering RNAs to T-cells. By silencing three genes, the team protected these cells from much of the damage typically associated with HIV infection, a sign of hope that researchers might come up with effective siRNA-based HIV therapies for humans.
Because T-Cells are recalcitrant in the uptake of nucleic acids, Kumar and her team "coaxed" them to take up the molecules by attaching them to an antibody against a T-cell surface protein.
But even with a solution to that problem, they had to overcome rapid viral mutation. To reduce the problem, the researchers targeted three different sequences: the sequences coding for CCR5, a human T-cell surface protein used by HIV to enter the cells, and two well-conserved HIV proteins.
The antibody-siRNA cocktail they administered to treat immune-deficient mice containing HIV-negative human T-cells gave them dramatic results. The control mice and mice who were given other siRNAs lost T-cells as soon as ten days following HIV infection, while roughly 75 percent of the mice given the therapeutic siRNAs had T-cell profiles that were essentially like those of non-HIV mice. DDN