HIV Persistence, Latency, and Cure Approaches: Where Are We Now?

Abstract

The latent reservoir remains a major roadblock to curing human immunodeficiency virus (HIV) infection. Currently available antiretroviral therapy (ART) can suppress active HIV replication, reduce viral loads to undetectable levels, and halt disease progression. However, antiretroviral drugs are unable to target cells that are latently infected with HIV, which can seed viral rebound if ART is stopped. Consequently, a major focus of the field is to study the latent viral reservoir and develop safe and effective methods to eliminate it. Here, we provide an overview of the major mechanisms governing the establishment and maintenance of HIV latency, the key challenges posed by latent reservoirs, small animal models utilized to study HIV latency, and contemporary cure approaches. We also discuss ongoing efforts to apply these approaches in combination, with the goal of achieving a safe, effective, and scalable cure for HIV that can be extended to the tens of millions of people with HIV worldwide.

Keywords: HIV; animal model; cure; latency; persistence; reservoir.

Figure 1

Steps of the HIV life cycle targeted by currently available antiretroviral drugs. HIV binds to the cellular receptor CD4 and a co-receptor (typically CCR5 or CXCR4) to enter a target cell through interaction with the viral gp120 envelope protein. Upon binding, the envelope transmembrane protein gp41 facilitates virus–host cell membrane fusion, triggering release of the viral core into the cytoplasm. A double-stranded DNA copy of the viral RNA is then reverse transcribed around the time it is shuttled into the nucleus, then the HIV integrase enzyme permanently inserts the HIV genome into the host-cell chromosomal DNA. Now stably integrated, the HIV provirus is transcribed into RNA, which is exported to the cytoplasm (in some cases after splicing). After the RNA is translated into proteins, immature virions are assembled and bud through the plasma membrane. The HIV protease enzyme catalyzes virion maturation by cleaving polypeptides within the new virion. Currently approved classes of antiretroviral drugs are shown in blue boxes. HIV latency occurs when the provirus pauses after integration, during which time it expresses little or no RNA and no viral proteins. As there are no currently approved antiretroviral drugs that target an integrated HIV provirus, latently infected cells can persist despite ART (red box).

Figure 2

HIV plasma viral loads at different phases of HIV infection. Upon acquisition of HIV, viral loads rapidly increase. After initial viremia is reduced by the adaptive immune response (including HIV-specific CD8⁺ T cells), infection proceeds to a long and typically asymptomatic period that often lasts 10 years. In this stage, very high levels of virus are produced each day leading to high plasma viral loads, but this is offset by the immune response and regeneration of immune cells by the hematopoietic system. If left untreated, infection will eventually progress to AIDS. This occurs when CD4⁺ T cell numbers are significantly reduced (<200 cells/microliter blood), and the virus has caused sufficient immunological damage so that efficient immune responses can no longer be mounted. The immunodeficient individual is then vulnerable to many opportunistic infections and diseases. Upon initiating ART treatment, HIV viral loads in plasma are ideally reduced to undetectable levels. For HIV to remain suppressed, ART must be continually taken to prevent viral rebound from latent reservoirs that are not cleared by currently available ART.

Figure 3

Schematic of common humanized mouse models for HIV infection and persistence. HIV does not infect non-modified mice, so common murine models for studying HIV replication, pathogenesis, and cure approaches rely on the use of immunodeficient mice transplanted with human cells and/or tissues (humanized mice). For production of mice with near-complete immune systems, this has historically involved highly immunodeficient recipient mouse strains including the NOD-SCID-common gamma (NSG) or NOD-rag-gamma (NRG), which are either irradiated or treated with busulfan to clear space in the bone marrow, followed by infusion with hematopoietic stem cells. This may be performed alone (hu-CD34) or, in the case of bone marrow/liver/thymus (BLT), performed in conjunction with the transplant of fetal liver and thymus tissue under the kidney capsule to generate a human thymus organoid where T cells can develop on human thymic stroma. More recent advances include the use of recipient mice bearing various important human immune genes that further improve the recapitulation of a working human immune system. For example, DRAGA mice (base strain NRG) expressing human HLAs (allowing optimized antigen presentation) or mice that augment human immune cell repopulation, such as NSG-Tg(IL-15) mice that produce human IL-15, allowing the improved development of NK cells.

Figure 4

Current HIV cure approaches. General approaches to developing a cure for HIV that are currently being explored within the field include block and lock, provirus editing/silencing, latency reversal and kill augmentation (kick and kill), and stem cell transplantation/gene therapy. Block and lock agents include didehydro-cortistatin A (dCA), JAK-STAT inhibitors, and BRD4 modulators. Common provirus editing/silencing approaches have utilized zinc nuclease fingers (ZNFs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR). Latency reversal cure approaches have included the use of protein kinase C (PKC) modulators, histone deacetylase inhibitors (HDACis), bromodomain extra-terminal motif (BET) bromodomain inhibitors, and second mitochondrial-derived activator of caspases (SMAC) mimetics. Kill augmentation has been explored with broadly neutralizing antibodies (bnAbs), programmed cell death protein (PD-1) boosted cytotoxic lymphocytes (CTLs), chimeric antigen receptor T and NK cells (CAR-T/CAR-NKs), and bispecific/trispecific antibodies, while modified stem cells with CCR5-∆32 bone marrow transplants have been used to apparently cure HIV in a select few individuals.