A recent spate of studies suggest researchers are finding ways around the limitations of the model
By Regina McEnery
The first humanized mice were created more than two decades ago. There are now four major types of humanized mouse models being used to study an array of infectious diseases, not least HIV. Though primates are still considered the best model for such research, humanized mice have the advantage of being far less costly. They have thus been used to test new HIV drugs and the systemic delivery of neutralizing antibodies against the virus (see Mighty Mice, IAVI Report, Sep.-Oct. 2008). Scientists have also designed humanized mice that appear to recapitulate the persistence of HIV in reservoirs of latently infected CD4+ T cells. Such mice are likely to prove valuable to HIV cure research.
But they have so far proved to be less useful to HIV vaccine research, mainly due to limitations in their ability to generate functional T-cell responses against the virus that mimic those of humans. But four papers published recently suggest researchers have found a way around some of these barriers—most notably with the creation of the bone marrow-liver-thymus (BLT) humanized mouse. Those mice took a starring role at an all-day symposium at Harvard Medical School in Boston on Nov. 5 devoted to the application of humanized mouse models to AIDS vaccine development. “The immune responses in these models are very similar to what we see in human infection,” said Todd Allen, co-chair of the symposium and principal investigator at The Ragon Institute of Massachusetts General Hospital (MGH), Massachusetts Institute of Technology (MIT), and Harvard. “But we don’t know yet how well that will play out following vaccination of these mice. The biggest limitation is that this remains a model of a human immune system in a mouse environment.”
A flurry of findings
Allen led a recent study that caused a small stir in AIDS vaccine research circles. He and his colleagues found that BLT mice infected with HIV mounted cellular immune responses that closely mirrored those observed in HIV-infected humans. HIV also escaped from those responses in a manner that generally mimicked natural infection (Sci. Transl. Med. 4, 143ra98, 2012). Researchers detected HIV-specific cellular immune responses in the BLT mice, both through stimulation of epitope-specific T-cell lines and directly, ex vivo. Allen’s team also found that BLT mice expressing the protective HLA-B*57 allele suppressed the virus in a way that was almost identical to how humans who express the same gene control the virus. They mounted responses against conserved regions of HIV Gag that are associated with greater control of viral replication in humans. Allen’s lab is now looking at the potential to induce human HIV-specific immune responses in the humanized mice through vaccination.
A bone marrow-liver-thymus (BLT) mouse. Jen Torrance/The Jackson Laboratory
Though mice are much smaller than people, they can shed light on how HIV makes its way around the body. This was vividly illustrated by Allen’s Harvard colleague Thorsten Mempel at the mouse symposium. Mempel and his team recently tracked HIV-infected human T cells in the lymph node of a humanized mouse using a high-tech tool called intravital microscopy. This was the first time scientists have visualized the behavior of such cells in a live animal (Nature 490, 283, 2012). The study found that HIV-infected T cells migrate robustly in lymph nodes, suggesting that their mobility facilitated the local dissemination of HIV infection in lymph nodes.
The study also found that in humanized mice infected with an experimental strain of HIV that localizes to nuclei, the majority of elongated lymph node cells were multinucleated syncytia that likely developed through cell fusion. The uncoordinated motility of these syncytia and multiple adhesions to other CD4+ T cells in the lymph node resulted in the formation of continuous membrane surfaces that increased the effective length of infected cells some 10-fold. The researchers suggest that all this may facilitate cell-to-cell transmission of the virus and promote widespread HIV dissemination.
In yet another study, this one conducted by David Baltimore’s lab at California Institute of Technology, scientists injected humanized mouse muscle cells with a modified viral vector optimized for the production of various broadly neutralizing antibodies. The researchers used vectored immunoprophylaxis (VIP) which utilizes a specialized adeno-associated virus (AAV) vector optimized for the production of full-length antibody from muscle tissue. The study found that antibodies delivered by VIP resulted in the long-lived production of antibodies in the mice. Further, the mice receiving VIP appear to be fully protected from HIV even when challenged intravenously with high doses of HIV. Alex Balazs, a researcher in David Baltimore’s lab said at the mouse symposium that it remains to be seen whether the results seen in BLT mice can be replicated in humans. “History has shown us that humans don’t behave like mice,” said Balazs. “We have to be prepared for surprises.”
Humanized mice are contributing to research on novel therapies as well. Rockefeller University scientist Michel Nussenzweig has been testing cocktails of potent bNAbs as a therapy in humanized mice infected with HIV. He and his team have found that giving a single bNAb or even as many as three did not produce durable results; the virus rebounded weeks after the antibody treatment ceased. But when they increased the number of bNAbs to five, the virus had still not rebounded in seven of the eight mice after two months (Nature 492, 118, 2012). Researchers suspect that the expanding arsenal of more potent antibodies might improve the chances of passive antibody transfer working and, if so, might provide an alternative to the daily grind of antiretroviral therapy. Instead of a BLT mouse, researchers used one that was a cross between a severe combined immune deficiency mouse and a non-obese diabetic mouse.
The evolving humanized mouse
The BLT mouse was initially developed by virologist J. Victor Garcia-Martinez, who is now at the University of North Carolina, in conjunction with a team at the University of Minnesota. Scientists make the mice by surgically implanting them with human organoids, which are fetal liver and thymic tissue that mimic organs—in this case organs that are essential to the development of immune cells. The mice are then irradiated and given transplants of stem cells taken from human fetal livers. These cells take up residence in the bone marrow, establishing a source for the human immune system borne by BLT mice. Mice altered this way were found to have a wide range of human immune cells in their peripheral blood; the cells also infiltrated tissues and organs in the lungs, GI tract, and liver, just as they would in the human body.
But the transplanted BLT immune system is not identical to a human’s. For example, antibody-producing B lymphocytes don’t mature properly in these mice. Dale Greiner, a University of Massachusetts scientist who has authored two reviews on the impact of humanized mouse models on the study of human disease, said this may be because the lymphoid organs in such mice are disorganized. It is in these organs that the immune responses are amplified and refined, especially those involving the production of neutralizing antibodies—which are today a major focus of HIV vaccine research.
In humans, he said, all of the components are “where they need to be.” In humanized mice, “it is like walking into a warehouse, where everything is scattered.” Greiner said that the genetic engineering required to remove the immune system in these mice, so that it can be replaced by a human one, might inadvertently disrupt the genes required to “organize” their lymphatic system in an immunologically functional manner.
Still, researchers are optimistic about the future of humanized mice in AIDS vaccine research. “What I think would really catalyze the field,” said Andrew Tager, a Harvard Medical School scientist who collaborated with Allen on his recent study, “is if there could be funding for a consortium to focus on making this a better model with an eye toward answering more questions about HIV. How can we make the immune responses of the model even better? We have shown we are on track. The time is now.”