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Generating and measuring effective vaccine-elicited HIV-specific CD8+ T cell responses.

There is growing consensus that eliciting CD8+ T cells in addition to antibodies may be required for an effective HIV vaccine for both prevention and cure. Here, we review key qualities of vaccine-elicited CD8+ T cells as well as major CD8+ T cell-based delivery platforms used in recent HIV vaccine clinical trials. Much progress has been made in improving HIV immunogen design and delivery platforms to optimize CD8+ T cell responses. With regards to viral vectors, recent trials have tested newer chimp and human adenovirus vectors as well as a CMV vector. DNA vaccine immunogenicity has been increased by delivering the vaccines by electroporation and together with adjuvants as well as administering them as part of a heterologous regimen. In preclinical models, self-amplifying RNA vaccines can generate durable tissue-based CD8+ T cells. While it may be beneficial for HIV vaccines to recapitulate the functional and phenotypic features of HIV-specific CD8+ T cells isolated from elite controllers, most of these features are not routinely measured in HIV vaccine clinical trials. Identifying a vaccine capable of generating durable T cell responses that target mutationally vulnerable epitopes and that can rapidly intercept infecting or rebounding virus remains a challenge for HIV. Comprehensive assessment of HIV vaccine-elicited CD8+ T cells, as well as comparisons between different vaccine platforms, will be critical to advance our understanding of how to design better CD8+ T cell-based vaccines for HIV.

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Guiding HIV-1 vaccine development with preclinical nonhuman primate research.

Nonhuman primates (NHPs) are seen as the closest animal model to humans in terms of anatomy and immune system makeup. Here, we review how preclinical studies in this model system are teaching the field of HIV vaccinology the basic immunology that is needed to induce broadly neutralizing antibodies (bnAbs) with vaccination and elicit protective T cell responses. These lessons are being translated into clinical trials to advance towards protective active vaccination against HIV-1 infection. Preclinical vaccination studies in NHPs have shown that highly engineered HIV-1 immunogens can initiate bnAb precursors providing proof of concept for Phase I clinical trials. Additionally, NHP models of HIV-1 infection are elucidating the pathways for bnAb development while serving as systems to evaluate vaccine protection. Innovative immunization strategies have increased affinity maturation of HIV-1 antibodies in long-lived germinal centers. Preclinical studies in macaques have defined the protective level of neutralizing antibodies and have shown that T cell responses can synergize with antibody-mediated immunity to provide protection in the presence of lower neutralizing antibody titers. The NHP model provides vaccine regimens and desired antibody and T cell responses that serve as benchmarks for clinical trials, accelerating HIV vaccine design.

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Anticipating HIV viral escape - resistance to active and passive immunization.

Active and passive immunization strategies are challenged by the extraordinary diversity of HIV, and the need for high titers of neutralizing antibodies to confer protective immunity. This review summarises recent studies and the barrier that these interventions will need to overcome to prevent viral resistance. Studies from the antibody mediated prevention trial identified a measure of protective titers, finding that higher titers than anticipated will be needed to prevent infection. This benchmark has advanced our ability to predict combinations of broadly neutralizing antibodies (bNAbs) that will provide optimal coverage. To limit escape, these combinations should ensure that the majority of viruses are bound by a minimum of two antibodies. The characterization of currently circulating viruses has revealed increased resistance to some bNAbs over time, highlighting the need for continued surveillance, especially in under-studied populations and subtypes. Active vaccination will face similar challenges in combating diversity, although despite successes in germline targeting, this approach is not yet able to elicit bNAbs. Cumulatively these studies highlight the need to target multiple antibody epitopes for maximum coverage, but also to restrict escape pathways. Successful immunization strategies should anticipate viral escape and devise strategies to counteract this.

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