Whether vaccine-elicited CD8+ T cells can control HIV-1 rebound when antiretroviral therapy (ART) is interrupted remains a central open question for the HIV cure field. In the source trial (Thompson et al., PLOS ONE 2016), a DNA prime / modified vaccinia Ankara (MVA) boost therapeutic vaccine in ART-suppressed adults boosted Gag-specific CD8+ T cells yet failed to prevent or control viral rebound during analytical treatment interruption: virus re-emerged in every participant, rebound virus carried escape mutations archived in the latent reservoir, and in most participants the elicited CD8+ T cells never measurably engaged the re-emergent virus. Subsequent clinical trials of T-cell vaccines (MVA-B, DNA/LIPO-5, and conserved-region HTI vaccines — HTI being the HIVACAT T-cell immunogen, which targets conserved regions of the viral proteome), including deployment in early-treated infection and combined with a Toll-like receptor 7 (TLR7) agonist, still show universal rebound, with only statistical associations between vaccine-induced T-cell magnitude and delayed rebound; the strongest human signal to date is a 10-person single-arm combination immunotherapy study (therapeutic vaccine plus broadly neutralizing antibodies plus a TLR9 agonist) in which 7 of 10 participants exhibited post-intervention control of at least limited durability. The mechanistic barriers identified in 2016 — archived immune-escape variants in the reservoir, insufficient CD8+ T-cell breadth and quality, and exclusion or functional disarming of CD8+ T cells at the follicular sites where re-emergence begins — are each being mapped but none has a demonstrated clinical solution. The problem asks what vaccination strategy, timing, and combination can elicit CD8+ T cells that achieve durable ART-free virologic control of HIV-1 in humans, and which of these barriers is rate-limiting.

Background

Antiretroviral therapy (ART) suppresses HIV-1 replication but cannot cure infection because a latent reservoir of proviruses persists in long-lived CD4+ T cells; stopping ART almost invariably leads to viral rebound within weeks. An analytical treatment interruption (ATI) is a monitored, protocolized pause of ART used in cure-research trials to test whether an experimental intervention can maintain virologic control without drugs. CD8+ T cells are the cytotoxic arm of cellular immunity: they kill infected cells that display short viral peptides (epitopes) on human leukocyte antigen (HLA) class I molecules. HIV-1 mutates rapidly, so variants that carry mutations escaping recognition by a given CD8+ T-cell clone are selected during untreated infection; crucially, such escape variants become archived in the latent reservoir and can re-emerge unchanged when ART stops, pre-empting T-cell responses that target the wild-type sequence. Therapeutic vaccination — immunizing an already-infected person on suppressive ART — aims to elicit T-cell responses strong and broad enough to contain this rebound, which would reduce or eliminate lifelong dependence on ART.

The source study (Thompson et al., PLOS ONE 2016; trial GV-TH-01 of the GOVX-B11 vaccine, a DNA prime and MVA boost regimen expressing the clade B HIV-1 Gag, Pol, and Env proteins — the viral core, enzyme, and envelope antigens) vaccinated nine ART-suppressed adults and then performed an ATI. The vaccine was safe and boosted Gag-specific CD8+ T cells in 8 of 9 participants, without signs of T-cell exhaustion, yet virus re-emerged in all participants (median 4 weeks). In the two participants whose Gag-specific CD8+ T cells expanded in response to rebound virus, viremia transiently fell more than 10-fold before mutations in known CD8+ epitopes were selected, and in the others the re-emergent virus appeared to have escaped the boosted responses already at re-emergence. The authors concluded that therapeutic vaccination faced three requirements: (i) ART must be initiated early enough to limit archiving of escape variants in reservoirs; (ii) elicited CD8+ T cells must target a breadth of epitopes not easily overcome by escape; and (iii) the elicited CD8+ T cells must traffic to, and act at, the anatomic sites of re-emergence before replicating virus can diversify — sites that include the B-cell follicles of lymphoid tissue, which infected CD4+ T cells occupy and from which cytotoxic CD8+ T cells are largely excluded.

Problem Statement

Determine whether therapeutic vaccination can elicit CD8+ T-cell responses that control re-emergent HIV-1 and sustain ART-free virologic control after treatment interruption in humans, and identify which of the barriers demonstrated in the source trial is rate-limiting. Concretely, the problem comprises four interlinked questions, all of which a complete answer must address: (a) Early deployment — does initiating vaccination on early ART, before immune-escape variants are archived in the latent reservoir, change reservoir escape-variant composition and rebound dynamics enough to make vaccine-elicited control achievable? (b) Breadth and design — what immunogen content, epitope breadth, conserved-versus-autologous targeting, and vector characteristics (including induction of tissue-homing CD8+ T-cell phenotypes) elicit responses that recognize a sufficient fraction of latent and rebounding variants to prevent escape? (c) Site access — can vaccine-elicited CD8+ T cells be induced or otherwise manipulated to traffic into and exert antiviral effector function at the lymphoid-follicle and other tissue sites where viral re-emergence initiates, before replication allows escape? (d) Combination — does combining therapeutic vaccination with interventions that reduce the inducible reservoir or stimulate innate immunity (for example TLR agonists or broadly neutralizing antibodies) convert vaccine-elicited responses into durable post-ART control? A complete answer provides human interventional evidence of durable virologic control off ART, together with evidence attributing that control to vaccine-elicited CD8+ T cells, or determines that no current vaccination strategy yet achieves such control; in either case it identifies which of the barriers is rate-limiting. Here 'durable virologic control' means sustained suppression of plasma HIV-1 RNA to a low or undetectable level for at least 24 weeks off ART, the operational threshold used to define post-treatment control.

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