| January 6, 2012 |
Full-Length, Natural, Human Antibodies Thwart HIV in Mice
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The war on the HIV pandemic is being won slowly but surely, one battle at a time. An important battle in that war may have been won by a California Institute of Technology (Caltech) research team led by Drs. Alejandro Balazs and David Baltimore. Although numerous antibodies that can neutralize most HIV strains have been identified, a conventional immunogen-based vaccine that elicits stable antibody production has not been developed. Taking an alternative approach, the Caltech team recently demonstrated that an optimized vector engineered to encode full-length, natural, human HIV-neutralizing antibodies prevents HIV infection in mice (Balazs et al. 2011). The novel approach, called vectored immunoprophylaxis (VIP), works very differently than conventional vaccines and may be just the weapon needed to significantly roll back the HIV pandemic.
A Better antibody-encoding vector is produced
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| The immunological responses of C57BL/6J (above) and BALB/cJ (below) are extremely well characterized in the Mouse Phenome Database. | |
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Previously constructed adeno-associated virus (AAV)-based transgene vectors contained heparin-binding capsids, which elicit an immune response from cytotoxic T cells. This immune response can destroy the cells transduced with the virus and eliminate therapeutic gene expression. To circumvent this response, the Caltech team constructed a vector with a capsid from a non-heparin-binding serotype 8 AAV. To reduce immune responses against the transgene, the researchers used a transgene that encodes full-length heavy- and light-chain variable regions of the naturally occurring human HIV-neutralizing b12 antibody. To ensure good muscle tissue expression, the researchers engineered the transgene to be driven by a novel synthetic CASI promoter. To test the antibody-generating ability of this construct, the researchers injected it into the gastrocnemius muscle of four mouse strains. Two of them, C57BL/6J (000664) and BALB/cJ (000651), are immunocompetent; the other two, NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (005557, NSG) and a Rag2-deficient mouse, are immunodeficient. The Caltech team tested the construct in these four strains to see if the absence or presence of the endogenous mouse immune system would confound its effects. The vector and its encoding transgene lived up to expectations: they induce exceptional and long-lasting amounts of encoded antibody in all four mouse strains.
New vector prevents HIV infection in mice
To determine if their optimized VIP protects mice from an HIV infection, the researchers injected it into humanized NSG mice. These are mice that develop a near-human immune system after being populated with human peripheral blood mononuclear cells (PBMCs). They then infected these humanized mice with HIV. As a control, they infected some NSG mice that had been injected with a luciferase-expressing construct instead of a b12-expressing construct. The Caltech team found that, following HIV challenge, the luciferase-expressing controls exhibit a dramatic loss of CD4 cells. In contrast, the b12-expressing mice exhibit no CD4 cell loss.
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| Humanized NSG mice are one of the best model organisms for studying the human immune system in vivo. They are being widely used to research multiple human infectious diseases as well as human hematopoiesis. |
The researchers then compared the efficacy of the b12-expressing construct harboring one of three historically known HIV-neutralizing antibodies: 2G12, 4E10, and 2F5. They found that humanized NSG mice injected with constructs that express any one of those three antibodies exhibit a range of CD4 cell loss. They are only partly protected from an intravenous HIV challenge. Additionally, eight weeks after an HIV challenge, the b12-expressing mice show no detectable signs of the HIV-expressed p24 antigen. On the other hand, mice that express any one of the other three antibodies exhibit varying amounts of the antigen.
The Caltech researchers found that the HIV protection conferred by the b12-expressing construct is quite robust. The b12-expressing mice produce high levels of human IgG specific for gp120 (an HIV envelope glycoprotein). None of the b12-expressing mice exhibit CD4 cell loss, even when challenged with very high HIV doses. The controls, however, exhibit CD4 cell loss in proportion to the HIV dose administered.
Finally, the Caltech team compared the efficacy of the b12-expressing construct to one expressing VRC01, one of several very effective HIV-neutralizing antibodies recently identified. The team found that both constructs induce dose-dependent antibody expression in NSG mice. However, the maximally effective dose required for the VRC01-expressing construct is higher than that for the b12-expressing construct. Suboptimal concentrations of either antibody are only partly protective, with several mice exhibiting no detectable CD4 cell loss and others exhibiting delayed CD4 cell loss.
In summary, the Balazs-Baltimore team developed a novel vectored immunoprophylactic (VIP) that protects mice from HIV infection. This VIP consists of a non-heparin-binding capsid and genes that encode naturally occurring, full-length, human HIV-neutralizing antibodies. When injected intramuscularly in mice, it confers long-lasting protection against even very high-dose HIV infections. The team believes that a similar VIP approach would confer broad, powerful HIV protection to humans. The approach may also be very effective in protecting against other infectious diseases and in therapies that require continuous production of monoclonal antibodies.
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