Ineffectual targeting of HIV-1 Nef by cytotoxic T lymphocytes in acute infection results in no functional impairment or viremia reduction

HIV T-cell immunogen design and delivery

PURPOSE OF THE REVIEW

Not all T-cell responses against HIV are created equally and responses of certain epitope specificities have been associated with superior control of infection. These insights have spurred the development of a wide range of immunogen sequences, each with particular advantages and limitations.

RECENT FINDINGS

We review some of the most advanced designs that have reached or are close to reaching human clinical trials, with a special focus on T-cell immunogen developed for therapeutic use. We also touch upon the importance of how immunogens are delivered and point out the lamentable fact that there is essentially no alignment between different designs and vaccine regimens, which is a major hindrance to accelerated advances in the field.

SUMMARY

The design of an immunogen able to induce T-cell responses of adequate specificity and functionality is subject of a wide range of preclinical and clinical studies. Few designs have shown promise to date, but emerging data highlight the critical contribution of specificity to effective antiviral activity in vivo .

Consequences of HLA-associated mutations in HIV-1 subtype C Nef on HLA-I downregulation ability

Identification of CD8+ T lymphocyte (CTL) escape mutations that compromise the pathogenic functions of the Nef protein may be relevant for an HIV-1 attenuation-based vaccine. Previously, HLA-associated mutations 102H, 105R, 108D, and 199Y were individually statistically associated with decreased Nef-mediated HLA-I downregulation ability in a cohort of 298 HIV-1 subtype C infected individuals. In the present study, these mutations were introduced by site-directed mutagenesis into different patient-derived Nef sequence backgrounds of high similarity to the consensus C Nef sequence, and their ability to downregulate HLA-I was measured by flow cytometry in a CEM-derived T cell line. A substantial negative effect of 199Y on HLA-I downregulation and Nef expression was observed, while 102H and 105R displayed negative effects on HLA-I downregulation ability and Nef expression to a lesser extent. The total magnitude of CTL responses in individuals harboring the 199Y mutation was lower than those without the mutation, although this was not statistically significant. Overall, a modest positive relationship between Nef-mediated HLA-I downregulation ability and total magnitude of CTL responses was observed, suggesting that there is a higher requirement for HLA-I downregulation with increased CTL pressure. These results highlight a region of Nef that could be targeted by vaccine-induced CTL to reduce HLA-I downregulation and maximize CTL efficacy.

CD8+ Cytotoxic T Lymphocyte Responses and Viral Epitope Escape in Acute HIV-1 Infection

Epitope escape from HIV-1-targeted CD8⁺ cytotoxic T lymphocyte (CTL) responses occurs rapidly after acute infection and contributes to the eventual failure of effective immune control of HIV-1 infection. Because the early CTL response is key in determining HIV-1 disease outcome, studying the process of epitope escape is crucial for understanding what leads to failure of immune control in acute HIV-1 infection and will provide important implications for HIV-1 vaccine design. HIV-1-specific CD8⁺ T lymphocyte responses against viral epitopes were mapped in six acutely infected individuals, and the magnitudes of these responses were measured longitudinally during acute infection. The evolution of autologous circulating viral epitopes was determined in four of these subjects. In-depth testing of CD8⁺ T lymphocyte responses against index and all autologous-detected variant epitopes was performed in one subject. Among the four individuals examined, 10 of a total of 35 CD8⁺ T cell responses within Gag, Pol, and Nef showed evidence of epitope escape. CTL responses with viral epitope variant evolution were shown in one subject, and this evolution occurred with and without measurable CTL responses against epitope variants. These results demonstrate a dynamic period of viral epitope evolution in early HIV-1 infection due to CD8⁺ CTL response pressure.

Challenges in the design of a T cell vaccine in the context of HIV-1 diversity

The extraordinary variability of HIV-1 poses a major obstacle to vaccine development. The effectiveness of a vaccine is likely to vary dramatically in different populations infected with different HIV-1 subtypes, unless innovative vaccine immunogens are developed to protect against the range of HIV-1 diversity. Immunogen design for stimulating neutralizing antibody responses focuses on “breadth” – the targeting of a handful of highly conserved neutralizing determinants on the HIV-1 Envelope protein that can recognize the majority of viruses across all HIV-1 subtypes. An effective vaccine will likely require the generation of both broadly cross-neutralizing antibodies and non-neutralizing antibodies, as well as broadly cross-reactive T cells. Several approaches have been taken to design such broadly-reactive and cross-protective T cell immunogens. Artificial sequences have been designed that reduce the genetic distance between a vaccine strain and contemporary circulating viruses; “mosaic” immunogens extend this concept to contain multiple potential T cell epitope (PTE) variants; and further efforts attempt to focus T cell immunity on highly conserved regions of the HIV-1 genome. Thus far, a number of pre-clinical and early clinical studies have been performed assessing these new immunogens. In this review, the potential use of these new immunogens is explored.