Hematopoietic-stem-cell-based gene therapy for HIV disease

Abstract

Although combination antiretroviral therapy can dramatically reduce the circulating viral load in those infected with HIV, replication-competent virus persists. To eliminate the need for indefinite treatment, there is growing interest in creating a functional HIV-resistant immune system through the use of gene-modified hematopoietic stem cells (HSCs). Proof of concept for this approach has been provided in the instance of an HIV-infected adult transplanted with allogeneic stem cells from a donor lacking the HIV coreceptor, CCR5. Here, we review this and other strategies for HSC-based gene therapy for HIV disease.

Figure 1. Intracellular immunization with gene-modified hematopoietic stem cells

Long-lived, self-renewing, multilineage hematopoietic stem cells (HSCs) could be modified such that they and their progeny resist HIV infection. The host could thereafter be repopulated with a hematopoietic system (including CD4+ T and myeloid targets for HIV) that is resistant to the replication and spread of HIV.

Figure 2. Basic design of a SIN configured lentivirus vector

A. A traditional retroviral vector with two functional long terminal repeats (LTRs) that contain strong enhancer and promoter elements. Integration of this vector can lead to activation of nearby proto-oncogenes and thus leukemia. B. A self-inactivating (SIN) lentivirus vector design. The 3′ LTR is modified (delta) so it does not contain any transcriptional control elements. Thus, upon transduction and integration into the genome, both 5′ and 3′ LTRs are defective and will not contain any functional promoter or enhancer elements. A weaker internal promoter (e.g., PGK) can then be inserted to drive expression of the transgene and thus decrease the risk of activating any nearby proto-oncogenes.

Figure 3. Approaches to modifying hematopoietic stem cells for HIV resistance

Both transient and permanent CCR5 suppression can be achieved using non-integrating delivery vectors. By contrast, durable expression of HIV resistance elements will likely require integrating delivery vectors. While concerns remain about the possibility of insertional mutagenesis (as observed previously with gamma-retrovirus vectors), the safety of integrating approaches has improved greatly in recent years with the development of lentivirus and foamy virus vectors. Heavier arrows depict desired outcomes; lighter arrows depict less desirable outcomes.

Figure 4. The replicative cycle of HIV

Two major strains of HIV-1 (R5 and X4) bind to target cells by concerted interactions between the envelope protein (Gp120) and CD4, and the chemokine co-receptors, CCR5 and CXCR4, respectively, leading to a fusion event with the plasma membrane that allows for entry of the virion capsid into the cytoplasm. Reverse transcription of viral genomic RNA forms a series of replicative intermediates that may ultimately integrate into the host cell genome. Transcription and generation of spliced and un-spliced forms of the viral RNA allows for movement and packaging of the diploid viral genome in the cytoplasm, a step enabled in part by HIV-1 protease. Budding and release of new viral progeny for repeated rounds of infection is then facilitated by virally-encoded release and infectivity factors. Each of these steps can be (or might be) disabled by specific drug and/or gene therapy.