Mechanisms of recognition of HIV-infected cells by CD8+ T cells that mediate effective antiviral responses

Abstract

CD8+ T cells are a major defense against viruses by killing infected cells, and unraveling the molecular mechanisms involved in their control of HIV-1 infection is thus a topic of paramount importance for developing therapeutic strategies. Conventional CD8+ T cells recognize infected cells by T cell receptor (TCR) engagement of cognate viral peptides presented on human leukocyte antigen (HLA) class I molecules. Control of HIV-1 infection is associated with “protective” HLA alleles but the causality behind this association is not completely understood. Protective HLA molecules often present conserved viral epitopes, suggesting that specificity is an important mechanism for this protection, but there is also evidence that CD8+ T cells, restricted by protective alleles, respond with higher functionality. To investigate peptide-independent effects of HLA, we examined recognition of the Gag peptide QASQEVKNW (QW9) that is presented by the protective HLA-B*57:01 and the non-protective HLA-B*53:01 molecules. We found that CD8+ T cell responses from HIV-infected persons recognizing QW9 differ functionally but not quantitatively in the context of these two disparate restricting HLA molecules. Using x-ray crystallography and molecular dynamics simulation, we structurally identified an unusual mode of distinctive QW9 peptide presentation by the two HLA molecules. The unique observation is that the central peptide residue K7 can assume either a buried conformation in the peptide-binding groove or an exposed one. These data give insight on the role that protective HLA alleles play in HIV control. The lack of effective HIV control by conventional CD8+ T cells in most infected persons led us to investigate whether there are alternative ways to mobilize CD8+ T cells to recognize and kill virus-infected cells, which could potentially be utilized in translational medical applications. Although CD8+ T cell recognition of virus-infected cells is characteristically restricted by HLA class I, we demonstrated the presence of HLA class II-restricted CD8+ T cell responses with antiviral properties in a small subset of HIV-infected individuals. In these cases, class II-restricted CD8+ T cells underwent clonal expansion and mediated killing of HIV-infected cells. In one instance, these cells comprised 12% of circulating CD8+ T cells, and exhibited two distinct co-expressed TCR chains, with only one contributing to binding of the class II HLA-peptide complex. These data indicate that class II-restricted CD8+ T cell responses can exist in a chronic human viral infection, and may contribute to immune control. Finally, we explored an HLA-independent mode of CD8+ T cell recognition using chimeric antigen receptors (CARs). CARs are synthetic proteins expressed on genetically-modified T cells (CAR T cells) to redirect their specificity to a desired antigen. Unlike TCRs that bind viral peptides presented on HLA, we designed HIV-specific CARs to detect viral Env antigens expressed on the surface of infected cells. Previous attempts to use CAR T cells against HIV relied on CD4 as the chimeric receptor, but this rendered cells expressing them susceptible to HIV infection. Basing CAR construct design on anti-HIV broadly neutralizing antibodies (bNAbs), we generated CAR T cells capable of binding to multiple strains of HIV Env and showing HIV-specific activation. We proved that, unlike other designs based on the CD4 molecule, bNAb-based CARs do not induce infectability of the CAR T cells. We conclude that using bNAbs as the basis for the design of anti-HIV CARs is a promising approach allowing for HIV-specific recognition of multiple strains without increasing susceptibility to infection. This approach, having an HLA-independent mode of recognition, makes a potential therapeutic application more broadly applicable. Together these studies provide additional perspectives on immune control of HIV infection, and have implications for preventive vaccines, immunotherapy and cure strategies.