HIV Cure

HIV is so difficult to cure because the virus persists inside stable reservoirs that cannot be detected by the immune system.

This animation, created in collaboration with TED Ed, provides an introduction on HIV and AIDS and antiretroviral therapy, and provides a brief explanation of why HIV has been so difficult to cure.

Antiretroviral Therapy and the Search for a Cure

Management of HIV/AIDS is achieved using combinations of antiretroviral drugs. There are numerous classes of drugs that target different aspects of the HIV life cycle, and therapy always involves taking two or more classes of drugs in combination.

The most commonly prescribed drugs include those that prevent the viral genome from being copied and incorporated into the cell’s DNA. Other drugs prevent the virus from maturing, or block viral fusion, causing HIV to be unable to infect new cells in the body.

Antiretroviral therapy is highly effective at managing the levels of HIV. Continued use has been shown to keep HIV-infected individuals from ever progressing to AIDS, and can lower the viral count to nearly undetectable levels. With antiretroviral therapy, most people can expect to live long and healthy lives.

Unfortunately, antiretroviral therapy is not a cure for HIV. This is due to HIV’s ability to hide its instructions inside of cells where drugs cannot reach it.

During the HIV life cycle, HIV incorporates itself into its host cell’s DNA. Antiretroviral therapies can stop new viruses that might be produced from infecting new cells, but can’t eliminate the viral DNA from the host cell’s genome.

Most host cells will be killed by infection or will eventually die of old age, but a very small number of cells appear to live for a very long time in the body. Every so often, the viral DNA can get turned on, and the cell starts to produce new virus. This is why medication adherence is critical. Stopping medication, even for a short time, might result in new cells being infected with HIV.

Researchers are working hard to find a true cure for HIV that could completely eradicate the virus from an infected person. Current directions include finding a means to activate cells that are harboring viral DNA, forcing them “out into the open” where they can then be targeted by antiretroviral drugs. Researchers are also looking into ways of using genetic tools to delete viral DNA from the cell’s DNA.

Latently Infected T-Cells

A major challenge to curing HIV is the virus’ ability to “hide” undetected in cells — a stage referred to as latency. During the HIV life cycle, HIV integrates itself into its host cell’s DNA. There it persists even when it is not being actively transcribed to make new viruses. These latent viruses can stay dormant for many years.

Antiretroviral therapies can stop new viruses that might be produced from infecting new cells but can’t eliminate viral DNA from the host cell’s genome. Some of these HIV-infected cells are long-lived CD4 memory T cells and serve as the HIV reservoirs. During the homeostatic proliferation of these memory T cells, the pool of latent HIV also gets copied.

When HIV-positive individuals are on combination antiretroviral therapy (cART), they can live relatively normal and healthy lives without developing AIDS. cART also decreases the risk of HIV transmission. However, cART requires lifelong adherence to these medications.

Unfortunately, antiretroviral therapy is not a cure for HIV. This is due to HIV’s ability to hide its instructions inside of cells where drugs cannot reach it.

References

Abdel-Mohsen M, Richman D, Siliciano RF, et al. Recommendations for measuring HIV reservoir size in cure-directed clinical trials. Nat Med. 2020;26(9):1339-1350. doi:10.1038/s41591-020-1022-1

Deeks SG, Lewin SR, Ross AL, et al. International AIDS Society global scientific strategy: towards an HIV cure 2016. Nat Med. 2016;22(8):839-850. doi:10.1038/nm.4108

Latency-Reversing Agents (LRA)

Latency-reversing agents are used to try to eliminate HIV reservoirs. This strategy attempts to flush the virus out of the resting cells by reawakening dormant viral DNA in the latent reservoirs. This approach is usually accompanied by a second step which aims to effectively clear the infected cells.

The most common class of latency-reversing agents are HDAC (histone deacetylase) inhibitors, which can force latently infected cells to produce viruses. Histones, which are proteins that DNA wraps around, can regulate what genes are actively transcribed. In some regions of the genome, chromatin (that is, DNA and its associated proteins) is tightly condensed. As a result, DNA in these regions are not available for the cell’s transcription machinery (such as DNA polymerase II, or Pol II), to read and copy, and thus they are not active. This is thought to be a major mechanism by which HIV can lie dormant in cells. Histone deacetylase inhibitors act to relax the chromatin and can thus enable genes on that segment of DNA to be turned on.

Some latency-reversing agents are known to produce significant toxicity and must be administered at low doses. Scientists are currently researching new LRA drugs that are both safe and effective.

References

Deeks SG, Lewin SR, Ross AL, et al. International AIDS Society global scientific strategy: towards an HIV cure 2016. Nat Med. 2016;22(8):839-850. doi:10.1038/nm.4108

Verdone L, Agricola E, Caserta M, Di Mauro E. Histone acetylation in gene regulation. Brief Funct Genomic Proteomic. 2006;5(3):209-221. doi:10.1093/bfgp/ell028

Immune-Based Modulators

Therapeutic vaccines

There are two types of immune responses, referred to as innate and adaptive immunity. The innate response is the first line of defense against pathogens, and is considered to be more general and non-specific. The second line of defense, the adaptive immune response, recognizes specific pathogen fragments, called antigens. CD4 T cells, CD8 T cells and B cells are all part of the adaptive immune system. T cells and B cells are activated when presented with antigens, including by HIV antigens. HIV therapeutic vaccines expose an HIV-positive individual to HIV antigens that are designed to elicit a more effective adaptive immune response to the virus.

There are four strategies used to deliver non-infectious HIV antigens into the patient:

DNA and RNA vaccines: These genetic vaccines use DNA plasmids or mRNA that code for the antigen. They are then taken up by the patient’s cells, which then start to produce that specific antigen.
Viral vector vaccines: Genes encoding HIV antigens such as HIV envelope protein are inserted into a modified, non-pathogenic virus (for example, canarypox).
Protein or peptide vaccines: HIV proteins or protein fragments are delivered in this class of vaccine.
Dendritic cell vaccines: In this case, antigen-presenting dendritic cells are isolated from a patient and mixed with HIV antigens. These cells are then injected back into the patient.

What happens to antigens after they are introduced to an individual? Antigens are internalized and processed by immune system sentinels called antigen-presenting cells (specifically, dendritic cells and macrophages). These antigen fragments are then presented to helper T cells. Specific helper T cells can become activated after being presented with these antigens, causing them to activate and proliferate into clones. This army of helper T cells can release different signals that activate B cells to start producing antibodies. At the same time, cytotoxic T cells, which also recognize the same target antigen, are activated.

When cytotoxic T cells interact with infected cells that are displaying the specific target antigen on their surface (on proteins known as MHC1 receptors), the T cells produce granzymes and perforin which cause the infected cell to break down.

Broadly Neutralizing Antibodies - Passive Immunization

While many antibodies produced during an infection specifically recognize specific strains of a virus, other antibodies can recognize multiple virus strains. These types of antibodies are known as broadly neutralizing antibodies, or bnAbs. In the case of HIV, these antibodies can inhibit a broad array of different HIV isolates. Current vaccine research strategies focus on the induction of bnAb production.

Some bnAbs have been isolated from the B cells of HIV-positive individuals and sequenced. These bnAbs can then be manufactured and administered by subcutaneous injection or infusion to other infected individuals. The bnAbs can then recognize infected cells and target them for destruction by natural killer cells.

Gene and Cell Therapy

Researchers are also exploring other possible HIV cure approaches that focus on gene and cell therapies. The goal of gene therapy is to deliver therapeutic genes into a patient that will treat the disease. In cell therapy, living cells are transplanted into a patient to treat the disease.

One cell therapy approach involves engineering HIV target cells to render them resistant to HIV entry. Another approach is to modify cytotoxic T cells to selectively target and eliminate infected cells. Generally, these therapies rely on genetic modification to “edit” the blood cells of the patient so that they become resistant to HIV or become better at targeting and eliminating HIV and infected cells.

In gene therapy, anti-HIV genes are introduced into cells using a viral vector or an engineered nanoparticle. Gene therapies use different approaches to target HIV. In some cases, target genes are edited either by inserting a therapeutic sequence or by disrupting DNA sequences of proteins that are important in the HIV life cycle. Examples of gene editing methods include the use of TAL-effectors or zinc fingers with nuclease, and/or CRISPR-Cas9. With these techniques, a specific sequence of DNA is recognized and cut, and (in some cases) new DNA is introduced at the cut site.

A primary target for many gene therapy approaches is the CCR5 gene. CCR5 is a receptor found on the surface of white blood cells, including T cells, and is required by HIV to enter T cells. When this protein is absent (such as in individuals with a naturally occurring deletion, such as one called CCR5-Δ32), HIV cannot infect cells. An individual known as the “Berlin Patient” was cured of his leukemia and of HIV when he received a stem cell transplant from a donor who has a double CCR5-Δ32 deletion mutation. This mutation, however, is very rare. Many current cure approaches focus on introducing CCR5 deletions or mutations. One possible complication, however, is that a similar receptor to CCR5, called CXCR4, exists in white blood cells, and it is possible that HIV may be able to adapt in order to use CXCR4 to gain entry into cells in the absence of CCR5.

References

Kuhlmann AS, Peterson CW, Kiem HP. Chimeric antigen receptor T cell approaches to HIV cure. Curr Opin HIV AIDS. 2018;13(5):446-453.

Peterson CW, Kiem HP. Lessons from London and Berlin: Designing A Scalable Gene Therapy Approach for HIV Cure. Cell Stem Cell. 2019 May 2;24(5):685-687. doi: 10.1016/j.stem.2019.04.010. PMID: 31051132.

Haworth KG, Peterson CW, Kiem HP. CCR5-edited gene therapies for HIV cure: Closing the door to viral entry. Cytotherapy. 2017 Nov;19(11):1325-1338. doi: 10.1016/j.jcyt.2017.05.013. Epub 2017 Jul 24. PMID: 28751153.

Gene Therapy using engineered CD8+ cells

Chimeric antigen receptors (CARs) are engineered receptor proteins. They are called chimeric because they are a combination of two different proteins. In this case, an antigen-binding domain “glued” to the signaling domain of T cells. The signaling domain of T cells is what gives white blood cells the signal to release biochemical compounds that kill pathogens or infected/mutated cells. CAR-T therapies have been successfully used to treat some cancers.

In the case of HIV, researchers have engineered T cells or natural killer cells to express a chimeric receptor that can selectively bind and kill infected cells that express HIV envelope protein. An early example of these CAR-T cells expressed a receptor consisting of the extracellular domain of CD4 fused with the signaling domain of cytotoxic T cells. Since CD4 is the receptor that binds HIV envelope protein, any infected cells with Env on their surface should be recognized by this CAR-T cell. This would then trigger a signaling event that leads to the release of toxic particles, killing the HIV-infected cell. Disappointingly, however, researchers found that CAR-T cells expressing the chimeric CD-4 receptor can become infected with HIV, and these efforts failed to significantly cure patients of HIV in clinical trials.

In an effort to improve the design of CAR-T cell therapy for HIV patients, researchers engineered a receptor that fused the binding domain of a monoclonal antibody (called scFv) that can specifically recognize HIV Env protein with the T cell receptor’s signaling domain.

Unfortunately, this bNAb-based CAR was not found to be effective therapeutically, and researchers are continuing to engineer new CARs that have started to show promising results. Using multiple antigen-recognizing domains in a single CAR, for example, has been shown to improve protection in animal studies. Researchers have recently designed a receptor with two CAR molecules created out of multiple receptors that can bind HIV — called duoCAR. In animal models, duoCAR therapy was able to successfully eliminate HIV-infected cells and resist HIV infection. This may be a viable approach for controlling viral loads and eliminating latent cells in HIV patients.

References

Anthony-Gonda K, Bardhi A, Ray A, et al. Multispecific anti-HIV duoCAR-T cells display broad in vitro antiviral activity and potent in vivo elimination of HIV-infected cells in a humanized mouse model. Sci Transl Med. 2019;11(504):eaav5685. doi:10.1126/scitranslmed.aav5685

Kuhlmann AS, Peterson CW, Kiem HP. Chimeric antigen receptor T-cell approaches to HIV cure. Curr Opin HIV AIDS. 2018;13(5):446-453. doi:10.1097/COH.0000000000000485

Mu W, Carrillo MA, Kitchen SG. Engineering CAR T Cells to Target the HIV Reservoir. Front Cell Infect Microbiol. 2020;10:410. Published 2020 Aug 13. doi:10.3389/fcimb.2020.00410

Peterson CW, Kiem HP. Cell and Gene Therapy for HIV Cure. Curr Top Microbiol Immunol. 2018;417:211-248. doi: 10.1007/82_2017_71. PMID: 29256135.

Lenacapavir

Lenacapavir is an HIV capsid inhibitor developed by Gilead Sciences. Currently, it is being evaluated in phase II/III trials for people with multidrug-resistant HIV. Laboratory studies indicate the drug is unaffected by mutations associated with resistance to most approved antiretroviral therapies. Watch the animation below to see how researchers think that Lenacapavir blocks HIV entry into the nucleus.

References

Bester SM, Wei G, Zhao H, et al. Structural and mechanistic bases for a potent HIV-1 capsid inhibitor. Science. Oct 16 2020;370(6514):360-364. doi:10.1126/science.abb4808

Burdick RC, Li C, Munshi M, et al. HIV-1 uncoats in the nucleus near sites of integration. Proc Natl Acad Sci U S A. Mar 10 2020;117(10):5486-5493. doi:10.1073/pnas.1920631117

Jang S, Cook NJ, Pye VE, et al. Differential role for phosphorylation in alternative polyadenylation function versus nuclear import of SR-like protein CPSF6. Nucleic Acids Res. May 21 2019;47(9):4663-4683. doi:10.1093/nar/gkz206

Matreyek KA, Yücel SS, Li X, Engelman A. Nucleoporin NUP153 phenylalanine-glycine motifs engage a common binding pocket within the HIV-1 capsid protein to mediate lentiviral infectivity. PLoS Pathog. 2013;9(10):e1003693. doi:10.1371/journal.ppat.1003693

Yant SR, Mulato A, Hansen D, et al. A highly potent long-acting small-molecule HIV-1 capsid inhibitor with efficacy in a humanized mouse model. Nat Med. Sep 2019;25(9):1377-1384. doi:10.1038/s41591-019-0560-x

Zila V, Margiotta E, Turoňová B, et al. Cone-shaped HIV-1 capsids are transported through intact nuclear pores. Cell. Feb 18 2021;184(4):1032-1046.e18. doi:10.1016/j.cell.2021.01.025