Engineering HIV-1-resistant T-cells from short-hairpin RNA-expressing hematopoietic stem/progenitor cells in humanized BLT mice

Studies of retroviral infection in humanized mice

Many important aspects of human retroviral infections cannot be fully evaluated using only in vitro systems or unmodified animal models. An alternative approach involves the use of humanized mice, which consist of immunodeficient mice that have been transplanted with human cells and/or tissues. Certain humanized mouse models can support robust infection with human retroviruses including different strains of human immunodeficiency virus (HIV) and human T cell leukemia virus (HTLV). These models have provided wide-ranging insights into retroviral biology, including detailed information on primary infection, in vivo replication and pathogenesis, latent/persistent reservoir formation, and novel therapeutic interventions. Here we describe the humanized mouse models that are most commonly utilized to study retroviral infections, and outline some of the important discoveries that these models have produced during several decades of intensive research.

CREB regulates memory allocation in the insular cortex

The molecular and cellular mechanisms of memory storage have attracted a great deal of attention. By comparison, little is known about memory allocation, the process that determines which specific neurons in a neural network will store a given memory. Previous studies demonstrated that memory allocation is not random in the amygdala; these studies showed that amygdala neurons with higher levels of the cyclic-AMP-response-element-binding protein (CREB) are more likely to be recruited into encoding and storing fear memory. To determine whether specific mechanisms also regulate memory allocation in other brain regions and whether CREB also has a role in this process, we studied insular cortical memory representations for conditioned taste aversion (CTA). In this task, an animal learns to associate a taste (conditioned stimulus [CS]) with the experience of malaise (such as that induced by LiCl; unconditioned stimulus [US]). The insular cortex is required for CTA memory formation and retrieval. CTA learning activates a subpopulation of neurons in this structure, and the insular cortex and the basolateral amygdala (BLA) interact during CTA formation. Here, we used a combination of approaches, including viral vector transfections of insular cortex, arc fluorescence in situ hybridization (FISH), and designer receptors exclusively activated by designer drugs (DREADD) system, to show that CREB levels determine which insular cortical neurons go on to encode a given conditioned taste memory.

Optimization of Critical Hairpin Features Allows miRNA-based Gene Knockdown Upon Single-copy Transduction

Gene knockdown using micro RNA (miRNA)-based vector constructs is likely to become a prominent gene therapy approach. It was the aim of this study to improve the efficiency of gene knockdown through optimizing the structure of miRNA mimics. Knockdown of two target genes was analyzed: CCR5 and green fluorescent protein. We describe here a novel and optimized miRNA mimic design called mirGE comprising a lower stem length of 13 base pairs (bp), positioning of the targeting strand on the 5' side of the miRNA, together with nucleotide mismatches in upper stem positions 1 and 12 placed on the passenger strand. Our mirGE proved superior to miR-30 in four aspects: yield of targeting strand incorporation into RNA-induced silencing complex (RISC); incorporation into RISC of correct targeting strand; precision of cleavage by Drosha; and ratio of targeting strand over passenger strand. A triple mirGE hairpin cassette targeting CCR5 was constructed. It allowed CCR5 knockdown with an efficiency of over 90% upon single-copy transduction. Importantly, single-copy expression of this construct rendered transduced target cells, including primary human macrophages, resistant to infection with a CCR5-tropic strain of HIV. Our results provide new insights for a better knockdown efficiency of constructs containing miRNA. Our results also provide the proof-of-principle that cells can be rendered HIV resistant through single-copy vector transduction, rendering this approach more compatible with clinical applications.

Pol III Promoters to Express Small RNAs: Delineation of Transcription Initiation

Pol III promoters such as U6 are commonly used to express small RNAs, including small interfering RNA, short hairpin RNA, and guide RNA, for the clustered regularly interspaced short palindromic repeats genome-editing system. However, whether the small RNAs were precisely expressed as desired has not been studied. Here, using deep sequencing to analyze small RNAs, we show that, for mouse U6 promoter, sequences immediately upstream of the putative initiation site, which is often modified to accommodate the restriction enzyme sites that enable easy cloning of small RNAs, are critical for precise transcription initiation. When the promoter is kept unmodified, transcription starts precisely from the first available A or G within the range of positions −1 to +2. In addition, we show that transcription from another commonly used pol III promoter, H1, starts at multiple sites, which results in variability at the 5′ end of the transcripts. Thus, inaccuracy of 5′ end of small RNA transcripts might be a common problem when using these promoters to express small RNAs based on currently believed concepts. Our study provides general guidelines for minimizing the variability of initiation, thereby enabling more accurate expression of small RNAs.

Optimized lentiviral vectors for HIV gene therapy: multiplexed expression of small RNAs and inclusion of MGMT(P140K) drug resistance gene

Gene therapy with hematopoietic stem and progenitor cells is a promising approach to engineering immunity to human immunodeficiency virus (HIV) that may lead to a functional cure for acquired immunodeficiency syndrome (AIDS). In support of this approach, we created lentiviral vectors with an engineered polycistronic platform derived from the endogenous MCM7 gene to express a diverse set of small antiviral RNAs and a drug resistance MGMT(P140K) marker. Multiple strategies for simultaneous expression of up to five RNA transgenes were tested. The placement and orientation of each transgene and its promoter were important determinants for optimal gene expression. Antiviral RNA expression from the MCM7 platform with a U1 promoter was sufficient to provide protection from R5-tropic HIV in macrophages and resulted in reduced hematopoietic toxicity compared with constructs expressing RNA from independent RNA polymerase III promoters. The addition of an HIV entry inhibitor and nucleolar TAR RNA decoy did not enhance antiviral potency over constructs that targeted only viral RNA transcripts. We also demonstrated selective enrichment of gene-modified cells in vivo using a humanized mouse model. The use of these less toxic, potent anti-HIV vectors expressing a drug selection marker is likely to enhance the in vivo efficacy of our stem cell gene therapy approach in treating HIV/AIDS.

Pre-clinical modeling of CCR5 knockout in human hematopoietic stem cells by zinc finger nucleases using humanized mice

Genetic strategies to block expression of CCR5, the major co-receptor of human immunodeficiency virus type 1 (HIV-1), are being developed as anti-HIV therapies. For example, human hematopoietic stem/precursor cells (HSPC) can be modified by the transient expression of CCR5-targeted zinc finger nucleases (ZFNs) to generate CCR5-negative cells, which could then give rise to HIV-resistant mature CD4(+) T cells following transplantation into patients. The safety and anti-HIV effects of such treatments can be evaluated by transplanting ZFN-treated HSPC into immunodeficient mice, where the extent of human cell engraftment, lineage differentiation and anti-HIV activity arising from the engineered HSPC can be examined. In this way, humanized mice are providing a powerful small animal model for pre-clinical studies of novel anti-HIV therapies.

Gene therapy strategies for HIV/AIDS: preclinical modeling in humanized mice

In the absence of an effective vaccine and lack of a complete cure, gene therapy approaches to control HIV infection offer feasible alternatives. Due to the chronic nature of infection, a wide window of opportunity exists to gene modify the HIV susceptible cells that continuously arise from the bone marrow source. To evaluate promising gene therapy approaches that employ various anti-HIV therapeutic molecules, an ideal animal model is necessary to generate important efficacy and preclinical data. In this regard, the humanized mouse models that harbor human hematopoietic cells susceptible to HIV infection provide a suitable in vivo system. This review summarizes the currently used humanized mouse models and different anti-HIV molecules utilized for conferring HIV resistance. Humanized mouse models are compared for their utility in this context and provide perspectives for new directions.

Development of hematopoietic stem cell based gene therapy for HIV-1 infection: considerations for proof of concept studies and translation to standard medical practice

Over the past 15 years we have been investigating an alternative approach to treating HIV-1/AIDS, based on the creation of a disease-resistant immune system through transplantation of autologous, gene-modified (HIV-1-resistant) hematopoietic stem and progenitor cells (GM-HSPC). We propose that the expression of selected RNA-based HIV-1 inhibitors in the CD4+ cells derived from GM-HSPC will protect them from HIV-1 infection and results in a sufficient immune repertoire to control HIV-1 viremia resulting in a functional cure for HIV-1/AIDS. Additionally, it is possible that the subset of protected T cells will also be able to facilitate the immune-based elimination of latently infected cells if they can be activated to express viral antigens. Thus, a single dose of disease resistant GM-HSPC could provide an effective treatment for HIV-1+ patients who require (or desire) an alternative to lifelong antiretroviral chemotherapy. We describe herein the results from several pilot clinical studies in HIV-1 patients and our strategies to develop second generation vectors and clinical strategies for HIV-1+ patients with malignancy who require ablative chemotherapy as part of treatment and others without malignancy. The important issues related to stem cell source, patient selection, conditioning regimen and post-infusion correlative studies become increasingly complex and are discussed herein.

Preclinical in vivo evaluation of the safety of a multi-shRNA-based gene therapy against HIV-1

Highly active antiretroviral therapy (HAART) has significantly improved the quality of life and the life expectancy of HIV-infected individuals. Still, drug-induced side effects and emergence of drug-resistant viral variants remain important issues that justify the exploration of alternative therapeutic options. One strategy consists of a gene therapy based on RNA interference to induce the sequence-specific degradation of the HIV-1 RNA genome. We have selected four potent short hairpin RNA (shRNA) candidates targeting the viral capside, integrase, protease and tat/rev open-reading frames and screened the safety of them during human hematopoietic cell development, both in vitro and in vivo. Although the four shRNA candidates appeared to be safe in vitro, one shRNA candidate impaired the in vivo development of the human immune system in Balb/c Rag2(-/-)IL-2Rγc(-/-) (BRG) mice. The three remaining shRNA candidates were combined into one single lentiviral vector (LV), and safety of the shRNA combination during human hematopoietic cell development was confirmed. Overall, we demonstrate here the preclinical in vivo safety of a LV expressing three shRNAs against HIV-1, which is proposed for a future Phase I clinical trial.Molecular Therapy-Nucleic Acids (2013) 2, e120; doi:10.1038/mtna.2013.48; published online 3 September 2013.