Silencing genes in the vaginal mucosa by topical application of a cholesterol-modified siRNA to inhibit HIV transmission.

Human immunodeficiency virus (HIV) is one of the several viruses that infects the genital tract of women. Although anti-retroviral therapy has significantly improved HIV-associated morbidity and mortality, resistance to anti-retroviral therapy is a global problem. More than 10% of HIV in low and middle income countries is already resistant to this treatment.1 Additionally, unlike another genital tract-infecting virus, human papillomavirus (HPV), a successful vaccine is yet to be developed against HIV. There is an ongoing need for new approaches to combat HIV. Basar et al.2 propose that HIV infection can be inhibited using RNAi to target the infected cells, making the otherwise permissive cells (including T cells, dendritic cells and Langerhans cells) non-permissive for HIV infection. RNAi was chosen by the authors on the strength of the FDA approval of an RNAi therapy for hereditary transthyretin-mediated amyloidosis associated peripheral nerve disease. The principle of RNAi therapy is that double-stranded complementary sequences (small interfering RNA, or siRNA) for the gene target are introduced into the cell. This is bound by the endogenous enzymatic machinery, eventually resulting in degradation of the target mRNA. While this approach may be relatively straight forward in vitro, challenges to be addressed for successful gene silencing in vivo include targeting the cells of interest and delivery of the siRNA payload to the RNAi machinery in those cells. In their proof of principle study, Basar et al., took the approach of conjugating siRNA with cholesterol or lipid. They show that the cholesterol-conjugated siRNA penetrates into the cells of the vaginal mucosa in the mouse. It is interesting to note that the conjugated siRNA did not have access to the cellular RNAi machinery in vitro but did in vivo, reminding us to not draw too heavily on in vitro data and its translatability to in vivo efficacy. The authors delivered the modified siRNA using simple topical application into the genital tract of the mouse on two consecutive days. The targeted protein was CD45, a molecule that is widely expressed on immune cells. CD45 expression was reduced in CD11c+ dendritic cells in the epithelium and lamina propria, whereas CD11b+, F4/80+ macrophages took up the conjugated siRNA however CD45 was not silenced. Therefore, while Basar et al. demonstrated that delivery of siRNA can be achieved in the vaginal mucosal epithelium with relative ease, the appetite of cells for the conjugated siRNA and the efficiency of silencing in those cells are both highly variable. Finding ways to control these factors will be critical to a successful therapy. Furthermore, the siRNA approach is yet to be tested in an HIV infection model using an HIV-relevant target, a critical next step that brings this research a step closer to the clinic. Bearing those caveats in mind, is also important to consider how this approach might be applied in humans. How will the gene silencing treatment be co-ordinated to prevent HIV infection? This is not a gene editing approach, and gene silencing would have only transient effects. Is this therapy going to need to be frequently applied to be effective? If that is the case, then the therapy must be cost effective and accessible to those who are most at need. This raises the question of how this might be funded, especially when the greatest need is currently in low- and middle-income countries with limited resources. It is exciting to see the efficacy of this RNAi approach in the genital tract and to consider it in a global context where delivery of synthetic RNA into the human body is now routine and generally accepted (i.e. the Moderna and Pfizer-BioNTech SARS-CoV2 vaccines). In particular, as a simple topical therapeutic, there may be other applications beyond HIV inhibition. For example, this approach may be useful for other pathogens such as chlamydia or for the treatment of HPV infection. Immune modulation in the genital tract, such as the regulation of inflammation, also comes to mind. It will be interesting to watch RNAi approaches for the genital tract progress into clinical application in the future. No conflicts of interest to declare. Open access publishing facilitated by University of Otago, as part of the Wiley - University of Otago agreement via the Council of Australian University Librarians. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.