Targeting the Latent Reservoir for HIV-1

Abstract

Antiretroviral therapy can effectively block HIV-1 replication and prevent or reverse immunodeficiency in HIV-1-infected individuals. However, viral replication resumes within weeks of treatment interruption. The major barrier to a cure is a small pool of resting memory CD4⁺ T cells that harbor latent HIV-1 proviruses. This latent reservoir is now the focus of an intense international research effort. We describe how the reservoir is established, challenges involved in eliminating it, and pharmacologic and immunologic strategies for targeting this reservoir. The development of a successful cure strategy will most likely require understanding the mechanisms that maintain HIV-1 proviruses in a latent state and pathways that drive the proliferation of infected cells, which slows reservoir decay. In addition, a cure will require the development of effective immunologic approaches to eliminating infected cells. There is renewed optimism about the prospect of a cure, and the interventions discussed here could pave the way.

Figure 1.

Viral dynamics in the presence of cART and curative interventions. (Top) Plasma HIV-1 RNA levels in typical patient. Clinical assays detect HIV-1 virions in the plasma through measurement of genomic HIV-1 RNA by quantitative RT-PCR. Each virion has two copies of the viral genome. Viremia increases exponentially after exposure, peaking at around two weeks post infection. Levels of plasma virus then fall, coincident with the development of a CTL response, to a steady state level (the set point). Viral replication continues through the course of untreated HIV-1 infection, driving CD4⁺ T cell depletion. cART produces a rapid biphasic decay in viremia to below the limit of detection of clinical assays (50 copies of HIV-1 RNA/ml of plasma), reflecting the short half-lives of two major populations of productively infected cells. However, residual viremia of ~1 copy/ml persists. Treatment intensification by addition of a fourth antiretroviral drug does not reduce residual viremia indicating that it is not due to ongoing cycles of replication but rather release of virus from long-lived latently infected cells that become activated. Nevertheless, with good adherence, infected individuals can expect continued suppression of viral replication and a near normal life span. Upon treatment interruption, viremia rebound rapidly, becoming clinically detectable within weeks of interruption and rising exponentially to the previous set point. (Middle) Effects of a hypothetical cure strategy in involving LRAs. LRAs may induce transient increases in plasma HIV-1 RNA as a result of the induction of viral gene expression from latently infected cells. If the virus producing cells die from viral cytopathic effects or are eliminated by immune mechanisms, then the reservoir will be reduced. This will be reflected in a reduction in the level of residual viremia and a delay in rebound following cART interruption, the length of which is related to the degree of reservoir reduction (Hill et al. 2014). A sterilizing cure requires complete reservoir elimination. (Bottom) Hypothetical vaccine strategies that enhance HIV-1-specific immunity to a degree that allows long-term control of HIV-1 replication would convert HIV⁺ individuals with progressive disease into EC and produce a functional cure.

Figure 2.

Proposed mechanism for the establishment of latent infection in resting CD4⁺ T cells. (A) Fate of resting CD4⁺ T cells following antigen (Ag) driven activation (horizontal axis) and following HIV-1 infection at times after activation (vertical axis). (B) Permissiveness of CD4⁺ T cells to different steps in the HIV-1 life cycle as a function of time after T cell activation. Resting CD4⁺ T cells are relatively resistant to HIV-1 infection due to low expression of the CCR5 coreceptor and reduced levels of dNTPs required for DNA synthesis during reverse transcription. Following activation, upregulation of the CCR5 allows viral entry. Levels of dNTPs increase allowing reverse transcription, and key host transcription factors translocate to the nucleus allowing viral gene expression and productive infection. Activated CD4⁺ T in a state of productive infection die rapidly. At 6–9 days after activation, CCR5 levels are high and dNTP levels are adequate for reverse transcription. However, viral gene expression is minimal and transient due to falling levels of activation dependent host transcription factors. Thus infection of cells during the effector to memory transition can result in latent infection. (C) Yield of latently infected cells following infection at different times after T cell activation. Model is based on experiments described in Shan et al., 2017.

Figure 3.

Challenges for the “kill” phase of HIV-1 cure strategies. Although HIV-1 induces a strong CTL response, effective killing of latently infected cells following latency reversal requires overcoming several challenges. (A) Ideal scenario in which a resting CD4⁺ T cell with an integrated provirus (black rectangle) is induced by an LRA to produce viral RNA and protein, leading to presentation of HIV-1 epitopes (black dot) which can be recognized by HIV-1-specific CTL after they have been activated by HIV-1 antigen (Ag) or a vaccine presenting the relevant epitope. (B) Escape mutations in dominant CTL epitopes prevent targeting of induced cells. (C) Certain LRAs inhibit CTL function. (D) HIV-1-specific CTLs have an exhausted phenotype marked by expression of PC-1 (red). Exhaustion is not fully reversed by ART, and blockade of inhibitory receptors may be required to re-establish functionality. (E) Induction of cells carrying defective proviruses may lead to the generation of target cells that present epitopes, thereby interfering with the lysis of cells carrying intact proviruses. (F) Some infected cells may reside in tissue sites for which there is limited access by CTL. See text for details and references.