Immunotherapeutic Approaches to HIV Cure and Remission

Ming J. Lee; S. Fidler; John Frater

Disclosures

Curr Opin Infect Dis. 2022;35(1):31-41. 

In This Article

Clinical Strategies for HIV Cure

Broadly Neutralising Antibodies

Neutralising antibodies prevent HIV from infecting host cells - blocking virus entry through disrupting virus-receptor interactions - and facilitate antibody-dependent cell mediated cytotoxicity. Due to genetic diversity and error-prone viral replication allowing viral escape from antibody neutralisation, high levels of somatic hypermutation are required to generate broadly neutralising antibodies (bNAbs).[18] Development of high-throughput neutralisation assays and single-cell antibody cloning techniques has led to the possibility of generating potent bNAbs with greater neutralisation breadth to viral strains.[19]

The target of all bNAbs is the HIV envelope glycoprotein (Env), which is heavily glycosylated, forming a 'glycan shield' that protects the virus against humoral responses that recognise glycans as 'self'.[20] Recent studies have confirmed the concept of 'glycan holes or the absence of particular proteins in env, which may offer exposed proteins as a relatively easier target.[21] For example, the use of an immunogen with a filled glycan hole at residue 241, and removal of surrounding glycans was found to redirect neutralising antibody responses to the newly unmasked epitopes.[22] The C3/465 glycan hole cluster has also been shown to be a major neutralising target in prevention of SHIV mucosal infection.[23]

bNAbs with different HIV Env targets, including long-acting antibody variants ('LS' variants) have been reported to be well tolerated with infrequent adverse events.[24–27] However, studies of bNAb monotherapy, using 3BNC117,[28] VRC01,[29] and 10–1074,[26] have shown viral rebound, and development of resistance through the selection of preexisting resistant viral populations after bNAb levels wane. A combination of 3BNC117 and 10–1074 was administered with an ATI in nine individuals with latent reservoirs sensitive to both bNAbs maintained viral suppression for a median of 21 weeks.[30] Resistance to both bNAbs did not arise in any participants, supporting the use of combination bNAbs for greater effectiveness against HIV compared to monotherapy. Bispecific bNAbs, able to target V2, and V3 glycan regions using the same antibody, were more potent compared to the individual parent antibodies in vitro[31] and these multispecific antibodies may further improve the effectiveness of bNAbs. Planned or ongoing trials of combination bNAbs registered on clinicaltrials.gov are summarised in Table 1.[32]

A subset of human[30] and macaques[33] receiving combination bNAbs have demonstrated unexpectedly long-term viral remission, greater than 30 months and 4 years, respectively, raising the possibility of antibody-mediated CD8 T-cell responses contributing to prolonged control. This effect may be due to increased HIV gag-directed T-cell immunity, and MIP1-β-expressing CD8 T cells.[34] BNAbs may also enhance antibody-directed cellular cytotoxicity (ADCC) in a synergistic mechanism – binding to different viral Env trimer epitopes exposes CD4-induced epitopes for further host antibody-ADCC responses.[35] There is still debate around the existence of this bNAb-induced 'vaccinal' response, and results from ongoing randomised placebo-controlled trials of combination bNAbs may provide further insight to determine if these immunological changes are indeed bNAb driven.

When investigating the effect of bNAbs on the HIV reservoir in the gut, where the largest HIV reservoir is present, antibodies may not achieve concentrations as high as in serum. In HIV-negative individuals, VRC01 antibodies in rectal and vaginal tissue were measured to be 10-fold lower compared to serum.[36] It needs to be determined if this will be the case in HIV-positive individuals; the tissue concentrations of bNAbs in PWH may achieve even lower concentrations due to viral antigen present in tissue providing an antigen sink effect. However for VRC01, the lower tissue antibody concentration compared to plasma, still maintained HIV neutralisation when tissue from VRC01 recipients were challenged with HIV ex vivo compared to controls not receiving VRC01.

For immune-privileged anatomical sites such as the central nervous system (CNS), intravenously infused antibodies may penetrate the blood brain barrier poorly, as seen in nonhuman primate models.[37] However, data from human trials are lacking. The use of modified Fc bNAbs containing 2 amino acids substitution M428L/N434S (commonly shortened to LS), increases bNAb half-life in vivo by increasing affinity for the neonatal Fc receptor,[38] leading to higher serum levels, and hypothetically may also allow accumulation to higher concentrations in tissue. The use of nanocapsules may improve CNS delivery of bNAbs in nonhuman primates[39] and may provide better CNS penetration options.

The development of an accurate, simplified assay with rapid turnaround to predict viral resistance to bNAb would be essential to determine the utility of bNAbs in clinical practice. The use of quantitative viral outgrowth assays and TZM-bl neutralisation assays are time consuming and expensive and may miss replication-competent provirus that remains latent during stimulation.[40] Novel machine learning algorithms used to predict bNAb susceptibility based on Env sequences have achieved high overall prediction accuracies of bNAb resistance[41–44] and may allow prediction of bNAb therapy efficacy similar to resistance testing for ART.

Recent advances in the delivery of bNAbs may provide promising ways to elicit high long-term bNAb concentrations in vivo, such as the use of CRISPR/Cas9 to edit mice or human B cells to express mature bNAbs.[45,46] Another approach is the use of viral vectors such as adeno-associated virus (AAV) delivery of anti-HIV monoclonal antibodies, which generated sustained viral control in 4 monkeys, however antidrug antibodies (ADA) limited the delivery of these bNAbs.[47] In human trials, 5 out of 8 HIV-positive individuals produced functional VRC07 4–6 weeks after AAV administration which remained at, or above, the 4–6 weeks peak at 1 year in 3 out of 5 individuals.[48]

Therapeutic Vaccination

Rapid rates of error-prone HIV replication can result in immune escape mutations to both T cells and antibody responses, through escape mutations resulting in avoidance of antigen recognition[49] and processing[50] potentially challenging bNAb efficacy and the therapeutic vaccines. Various therapeutic vaccine classes aim to boost HIV-specific T-cell responses in terms of magnitude or breadth of antigen specificity, with the aim of targeting and killing cells containing HIV, including potentially those in the reservoir. These include inactivated virus, subunit vaccines with recombinant envelope glycoprotein rgp160, DNA vaccines, viral vectors (modified vaccinia Ankara [MVA], adenovirus, vesicular stomatitis virus, canary pox virus), RNA vaccines, lentiviral vectors and dendritic cell vehicles.[51] These vaccine efforts, whereas safe, have predominantly proven to induce only limited effect on viral control. For example, the use of a therapeutic vaccine regimen comprising a multiantigen plasmid DNA vaccine, followed by an attenuated live viral vector containing HIV gag gene had no effect on the kinetics of viral rebound or HIV reservoir size after ATI.[52] However, recent promising results from the AELIX-002 trial using HIVACAT T cell immunogen vaccines (HTI) reported safe and highly immunogenic responses following administration of a combination of DNA.HTI, MVA.HTI, and ChAdOx1.HTI vaccines in early treated PWH, with 40% of vaccine-recipients compared to 8% of placebo-recipients remaining off ART for 22 weeks.[53] The novel HTI design approach uses T cell response data from more than 1000 individuals with HIV, to direct the T cell response to the most vulnerable sites of HIV.[54]

Progress in mRNA technology for therapeutic HIV vaccination may re-energise the therapeutic vaccine field. The improved safety, efficacy, and ease of manufacturing compared with previous vaccine technology make them attractive vaccine candidates, as reviewed by Mu et al.[55] One advantage may be that the constant production of mRNA-encoded immunogens leads to a slow but prolonged antigen delivery in vivo, with the potential to improve germinal centre and neutralising antibody responses in vivo.[56]

Latency Reversal Agents and 'Kick and Kill' Approaches

One of the challenges to immune elimination of the HIV reservoir is the lack of expression of target antigens by latently infected cells. To address this, the 'kick and kill' or 'shock and kill' strategy aims to activate latent cells to express HIV antigen using latency reversal agent (LRAs), allowing CTL or NK cells to identify and eradicate HIV-containing reservoir cells. Early LRAs included global T-cell activators such as OKT3 and interleukin 2,[57] but were limited by severe toxicities.[58] Subsequent classes of LRA include epigenetic LRAs such as histone deacetylase inhibitors (HDACi) (vorinostat, panobinostat and romidepsin); signal agonist LRAs including activators of protein kinase C (PKCa), Bryostatin-1; IL-15 superagonists (e.g. N803); toll-like receptor agonists; molecules which mimic second mitochondrial activator of caspases (SMAC mimetics or SMACm) and recently, stimulator of interferon genes (STING) agonists.

The RIVER study, a randomised trial of vorinostat, a histone deacetylase inhibitor LRA, and a therapeutic HIV vaccine (ChAdV63. HIVconsv and MVA.HIVconsv) did not show any difference in measures of the HIV reservoir despite evidence of augmented HIV-specific CD4 and CD8 T cell responses.[59] Participants did not interrupt ART, and although the lack of any impact on reservoir size using different assays is circumstantial evidence, an impact on time to viral rebound cannot be proven. Similarly, trials of other LRAs including romidepsin, panobinostat and valproic acid have not shown significant or meaningful changes in the size of the viral reservoir.[60]

Potential explanations for the lack of success with LRAs include an insufficiently robust 'kick', as even the most potent LRAs are not able to activate more than a fraction of the latent reservoir containing cells ex vivo,[61] and LRAs used in clinical trials are often used at lower doses to avoid the systemic inflammation and off-target toxicities associated with these compounds.[62] Some LRAs also impair NK[63] and CTL function,[64] and there is also evidence that latent reservoir cells exhibit inherent resistance to CD8-mediated killing even in the presence of functional cellular immunity.[65]

The search for safer but adequately potent LRA candidates is ongoing. TLR-7 agonists,[66–69] TLR-9 agonists,[70] TLR-3 agonists (Poly-ICLC),[71] Protein Kinase C agonists (bryostatin-1)[72] have all been unable to consistently and significantly reverse latency. STING agonists were shown to activate latently infected cells, increasing SIV RNA and decreasing SIV DNA levels ex vivo in macaque peripheral blood mononuclear cells.[73]

The SMACm AZD5582 was shown to induce latency reversal in two animal models with minimal side effects and better HIV-specificity.[74] Depletion of CD8+ lymphocytes to remove CD8-mediated viral suppression has been investigated in combination with N-803, leading to a synergistically greater effect leading to more robust viral reactivation in macaques compared to CD8 depletion or N-803 administration alone.[75] Similarly, CD8 lymphocyte depletion augmented the efficacy of the SMACm AZD5582 in SIV-infected rhesus macaques,[76] suggesting that antagonising the CD8-mediated mechanisms of viral suppression may boost the effects of LRAs. Should SMAC mimetics be combined with a potent cytotoxic reservoir targeting agent, one might envisage the increasingly ineffective 'kick and kill' approach being replaced with a new 'SMAC and whack' strategy.

Immune Modulation for Enhanced T-cell Function

CD8 T cell exhaustion, characterised by cellular markers such as PD-1, CTLA-4, LAG-3, TIGIT or Tim-3, likely leads to the immune dysregulation associated with HIV.[77] These inhibitory markers contribute to latency and may be amenable to therapeutic targeting to achieve latency reversal. Treatment of rhesus macaques with anti-PD-1 and anti-CTLA-4 antibodies appeared to induce reactivation and subsequent reduction of the reservoir including decreased intact proviral measurements.[78] However, autoimmune adverse effects may limit use of ICBs in future trials,[79] and there is increasing evidence for some irreversibility of immune exhaustion based on epigenetic approaches.[80,81]

Engineering Enhanced T Responses

Two individuals have achieved sustained HIV remission and likely 'cure' following haematopoietic stem cell transplant (HSCT) from donors with homozygous CCR5Δ32 mutations.[82,83] However, the high one year mortality rate (41–44%) and complication rate[84] suggest HSCT is not a viable treatment option for most PWH, without another clinical indication for HSCT. But these cases lend support to engineered T-cell approaches through gene therapy approaches, including removal of CCR5 receptors to prevent HIV cell entry or modifying chimeric antigen receptors (CAR) in CAR-T cells for augmented HIV-specific CTL responses.

Developments in CAR-T cell technology have been directed to increased potency and overcoming viral antigen escape. BNAb-based CARs recognise a breadth of HIV strains (over 95%) through targeting conserved sites in the Env protein,[85] and bispecific CARs improve targeting of conserved epitopes exposed through simultaneous binding of CD4 and gp120.[86] The use of a universal CAR-T cell platform (convertibleCAR-T cells), able to bind to a multitude of bNAbs, allows a single CAR-T cell infusion to be paired with different antibodies for greater breadth of activity. They may be introduced in an inert state with the ability to be turned 'on' with the appropriate activation and proliferation signals.[87] Another development is the use of stem cell-derived CAR T-cells, which can produce CAR T-cells that physiologically expand without the need for ex vivo expansion. They have been shown to persist in tissue-associated viral reservoirs for nearly 2 years in macaque models and may provide an effective long-lasting scalable solution for CAR-T cell delivery.[88]

Defining the Ideal Cure

A cure or remission strategy should be safe and portable. It should achieve long-term viral suppression (at least 6 months after stopping ART), negate any risk of onward viral transmission, and ideally protect against re-infection. Finally, the cure must be affordable to allow access to those with the highest need, often living with HIV in low resource settings with multiple challenges to sustain ART drug stocks.[89]

Limitations and Challenges for Future Therapies

Although the goal is to achieve a safe, effective, and durable cure for HIV, ART is safe, relatively cheap and with fewer barriers for implementation compared to current cure strategies. Therefore, it is imperative that a cure strategy meets a high threshold of safety and efficacy to provide a viable alternative to currently available ART, which may provide a barrier for future research. In the example of CAR-T cells, these thresholds may differ from their use in oncology treatment where alternatives have a greater toxicity profile, and the presence of moderate or severe adverse events may not be tolerable for PWH in the presence of safer alternatives. Other challenges towards HIV cure remain, such as the search for improved markers or assays of latency to reduce the need for treatment interruptions as well as a robust marker for effective HIV-specific immune responses.

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