Chantelle Ahlenstiel and Tony Kelleher
Block and Lock: A Pathway to Remission
‘Block and lock’ is an emerging option in the pursuit of an HIV cure. Kirby Institute researchers have developed an animated infographic on ‘block and lock’. This clearly describes the groundbreaking work to develop the approach.
Watch the short video here:
Professor Tony Kelleher spoke about the ‘block and lock’ strategy at the Australasian HIV&AIDS Conference in September 2019. Read on for a summary of the talk, and an exploration of the work on the ‘block and lock’ approach to HIV cure.
Overview
Researchers are investigating a number of HIV cure strategies to permanently control HIV-1 infection. These include the ‘shock and kill’ and ‘block and lock’ approaches. The ‘shock and kill’ strategy attempts to reactivate, or ‘shock’, latently infected cells awake using latency reversing agents. It then relies on an immune response or other targeted method to ‘kill’ the cell. This strategy has (so far) shown limited success in clinical trials. ‘Block and lock’ attempts to mimic naturally occurring latency via latency inducing agents. These agents ‘block’ the production of new virus and ‘lock’ in a permanent state of latency.
Getting technical – how ‘block and lock’ works
The virus in the HIV reservoir is in a latent state. This means it is integrated into the host genome, but not actively transcribing into virus RNA. The universal controller of gene expression is a gene’s promoter. The HIV promoter controlling expression of all HIV genes is the 5`LTR. The HIV-1 5`LTR therefore controls proviral transcription and is a major determinant of post-integration latency. The HIV-1 5’LTR has binding sites for HIV Tat and other host transcription factors that activate the HIV 5`LTR promoter.
The “block and lock” strategy could help achieve drug-free HIV remission. This could be done by switching off, or silencing, the HIV 5`LTR via gene therapy. This process uses short interfering RNA (siRNA) targets called si143 and siPromA. These short sequences can be found in the HIV-1 5`LTR promoter region, but not in the human genome. The sequences therefore specifically target cells of the HIV reservoir.
Experiments show the potential for ‘block and lock’
Professor Kelleher presented data from in vitro experiments. This showed that cells carrying si143 and siPromA can block HIV-1 transcription for a long time (>21 days from a single dose). This has been shown in a range of cell types. Data from a lab model of latent HIV showed the protective siRNAs made cells resistant to reactivation with commonly used latency reversing agents. This demonstrates that the siRNAs have a potent ‘lock’ effect.
The next part of the presentation focussed on work to identify new targets for siRNA. Professor Kelleher’s research group has identified a number of conserved siRNA targets in the HIV-1 5`LTR. This work used the MultipLExed tARget sequeNce aNalysis (LEARNN) program developed by the Kirby Institute together with Professor Miles Davenport. The sequence analysis suggests that combining four siRNA targets will silence 99.89% of global HIV-1 subtypes. This means the cure strategy would be applicable globally, including in regions with a high HIV burden.
The final part of the presentation included exciting data from a humanised mouse model of HIV-1. In these experiments, researchers infused mice with human CD34+ stem cells. These cells carried the protective siRNA PromA and a siRNA targeting CCR5 (the HIV-1 co-receptor). Control group mice were susceptible to HIV. Mice carrying the PromA and CCR5 siRNAs, however, were protected against HIV. These mice had normal CD4 T cell numbers and reduced viral loads. Work is now underway to replicate the conditions needed for human clinical trials. This includes ART treatment and analytical treatment interruption (ATI) to assess the potential for rebound of viral load.
Not there yet, but watch this space
While optimistic about the approach, Professor Kelleher acknowledged limitations of the in vivo experiments. These include that modified cells represent a minority of cells in the periphery, and that the extent of protection from HIV infection is directly related to the level of siRNA expression. To address these limitations, further experiments are underway. This includes work to understand the distribution of the modified cells and to improve delivery systems for getting the siRNAs into cells.
The ‘block and lock’ strategy is gaining momentum and will be an intriguing area to watch as the work moves closer to human trials.
Read more from the Kirby Institute about this work here.
We’ve previously written about the block and lock strategy here.
Biographies
Tony Kelleher is the Director of the Kirby Institute, UNSW Sydney. He is a clinician scientist, trained in internal medicine and pathology. Tony has played a role in multiple phase I- to phase IV clinical trials and cohort studies exploring the treatment and prevention of HIV infection. He has established novel assays for the measurement of CD4+ T cell responses, and the use of fine needle lymph node biopsies to study immune responses and the HIV reservoir. Tony and his team are currently exploring the “block and lock” strategy for control of the viral reservoir.
Chantelle Ahlenstiel is a Senior Research Fellow in the Immunovirology and Pathogenesis Program at the Kirby Institute. She did her postdoctoral training at the Vaccine Research Center, NIH. In 2009, she moved to the Kirby Institute, where she studies RNA-directed epigenetic silencing of HIV-1 and SIV.
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