An Aptamer-siRNA Chimera Suppresses HIV-1 Viral Loads and Protects from Helper CD4+ T Cell Decline in Humanized Mice

A dual-function aptamer that targets both a HIV-1 surface protein and a critical messenger RNA can inhibit HIV infection in humanized mice. A Small but Deadly Therapy for HIV Simultaneously precise and fragile, small interfering RNAs (siRNAs) have not as yet shown much success as therapeutic agents, despite their high specificity for selected targets. When injected into an organism, siRNAs tend to be destroyed in the blood, sometimes provoking the body’s own innate immune defenses against this foreign object. But there are ways around these problems, and Neff et al. have developed one by harnessing an RNA aptamer in the pursuit of a successful approach to vanquishing the HIV. Their aptamer, artificially evolved to bind tightly to the gp120 molecule on the surface of HIV, interrupts cell infection by the virus by itself. But by attaching an inhibitory siRNA directed against the essential tat/rev gene to one end of the aptamer, the authors enhanced its deadliness. HIV-infected mice treated with this chimeric agent showed reduced viral loads and improved CD4+ T cell status, indicating that such an approach may be beneficial to HIV-infected patients. Mice do not generally become infected with HIV, and so they have not been as informative as nonhuman primates in the fight against AIDS. But the authors Neff et al. got around this problem using an improved version of the BALB/c mouse, in which the animals have been engrafted with human hematopoietic stem cells and so carry a human instead of a mouse immune system. When infected with HIV, these animals showed viremia and CD4+ T cell loss, mimicking key aspects of AIDS. Treatment with the chimeric aptamer-siRNA against gp120 and tat/rev decreased HIV concentrations in the blood, with many mice showing a sharp drop, and restored the CD4+ T cell level—a measure of immune function that decreases in HIV-infected patients. siRNA was detected in the immune cells of the mice, showing that the chimera delivered its cargo to the intended target, and the amounts of tat-rev RNA were markedly reduced in these same cells, showing that the siRNA was doing its job to thwart viral gene expression. The dual-function aptamer avoided another problem often found with therapeutic RNAs: No interferon responses were generated, indicating that the chimera was not causing an immune reaction. These results are good news for two reasons: They show that the inhibition of two key molecules crucial for the HIV life cycle can in turn inhibit viral spread, pointing to an alternative therapy for this still burdensome infection. They also lay out a general approach for delivering siRNAs to particular target cells, shown here to be useful for HIV but potentially applicable to other diseases that require exact delivery of a deadly agent to a tiny target. Therapeutic strategies designed to treat HIV infection with combinations of antiviral drugs have proven to be the best approach for slowing the progression to AIDS. Despite this progress, there are problems with viral drug resistance and toxicity, necessitating new approaches to combating HIV-1 infection. We have therefore developed a different combination approach for the treatment of HIV infection in which an RNA aptamer, with high binding affinity to the HIV-1 envelope (gp120) protein and virus neutralization properties, is attached to and delivers a small interfering RNA (siRNA) that triggers sequence-specific degradation of HIV RNAs. We have tested the antiviral activities of these chimeric RNAs in a humanized Rag2−/−γc−/− (RAG-hu) mouse model with multilineage human hematopoiesis. In this animal model, HIV-1 replication and CD4+ T cell depletion mimic the situation seen in human HIV-infected patients. Our results show that treatment with either the anti-gp120 aptamer or the aptamer-siRNA chimera suppressed HIV-1 replication by several orders of magnitude and prevented the viral-induced helper CD4+ T cell decline. In comparison to the aptamer alone, the aptamer-siRNA combination provided more extensive inhibition, resulting in a significantly longer antiviral effect that extended several weeks beyond the last injected dose. The aptamer thus acts as a broad-spectrum HIV-neutralizing agent and an siRNA delivery vehicle. The combined aptamer-siRNA agent provides an attractive, nontoxic therapeutic approach for treatment of HIV infection.