TCR clonotypes modulate the protective effect of HLA class I molecules in HIV-1 infection

Increased sequence coverage through combined targeting of variant and conserved epitopes correlates with control of HIV replication

A major challenge in the development of an HIV vaccine is that of contending with the extensive sequence variability found in circulating viruses. Induction of HIV-specific T-cell responses targeting conserved regions and induction of HIV-specific T-cell responses recognizing a high number of epitope variants have both been proposed as strategies to overcome this challenge. We addressed the ability of cytotoxic T lymphocytes from 30 untreated HIV-infected subjects with and without control of virus replication to recognize all clade B Gag sequence variants encoded by at least 5% of the sequences in the Los Alamos National Laboratory HIV database (1,300 peptides) using gamma interferon and interleukin-2 (IFN-γ/IL-2) FluoroSpot analysis. While targeting of conserved regions was similar in the two groups (P = 0.47), we found that subjects with control of virus replication demonstrated marginally lower recognition of Gag epitope variants than subjects with normal progression (P = 0.05). In viremic controllers and progressors, we found variant recognition to be associated with viral load (r = 0.62, P = 0.001). Interestingly, we show that increased overall sequence coverage, defined as the overall proportion of HIV database sequences targeted through the Gag-specific repertoire, is inversely associated with viral load (r = -0.38, P = 0.03). Furthermore, we found that sequence coverage, but not variant recognition, correlated with increased recognition of a panel of clade B HIV founder viruses (r = 0.50, P = 0.004). We propose sequence coverage by HIV Gag-specific immune responses as a possible correlate of protection that may contribute to control of virus replication. Additionally, sequence coverage serves as a valuable measure by which to evaluate the protective potential of future vaccination strategies.

Developing Combined HIV Vaccine Strategies for a Functional Cure

Increasing numbers of HIV-infected individuals have access to potent antiretroviral drugs that control viral replication and decrease the risk of transmission. However, there is no cure for HIV and new strategies have to be developed to reach an eradication of the virus or a natural control of viral replication in the absence of drugs (functional cure). Therapeutic vaccines against HIV have been evaluated in many trials over the last 20 years and important knowledge has been gained from these trials. However, the major obstacle to HIV eradication is the persistence of latent proviral reservoirs. Different molecules are currently tested in ART-treated subjects to reactivate these latent reservoirs. Such anti-latency agents should be combined with a vaccination regimen in order to control or eradicate reactivated latently-infected cells. New in vitro assays should also be developed to assess the success of tested therapeutic vaccines by measuring the immune-mediated killing of replication-competent HIV reservoir cells. This review provides an overview of the current strategies to combine HIV vaccines with anti-latency agents that could act as adjuvant on the vaccine-induced immune response as well as new tools to assess the efficacy of these approaches.

Both HLA-B*57 and plasma HIV RNA levels contribute to the HIV-specific CD8+ T cell response in HIV controllers

CD8(+) T cell responses are thought to play an important role during HIV infection, particularly in HIV controllers (HIC) in whom viral replication is spontaneously controlled without any treatment. We have demonstrated that CD8(+) T cells from these subjects are able to suppress viral replication in vitro. In parallel, HIV-specific CD8(+) responses were shown to be strong and of high quality, with proliferative abilities and cytotoxic capacities, in HIC. The HLA-B*57 allele, which is associated with a better clinical outcome in HIV infection, is overrepresented in HIC. However, we showed that these patients constitute a heterogeneous group that includes subjects who present weak suppression of viral replication in vitro and HIV-specific responses. We performed an extensive study of 101 HIC (49 HLA-B*57(+) and 52 HLA-B*57(-)) to determine the impact of HLA-B*57 on the HIV-specific CD8(+) response. The HLA-B*57-restricted response displayed better qualitative features, such as higher functional avidity, higher proliferation capacity, and a higher level of cytokine production, than responses not restricted by HLA-B*57. However, the highest frequencies of HIV-specific CD8(+) T cells were observed only in a subset of HLA-B*57(+) subjects. They were tightly associated with the ability to suppress viral replication in vitro. In contrast, the subset of HLA-B*57(+) subjects with a weak ability to suppress viral replication had significantly lower ultrasensitive viral loads than all the other groups of controllers. In conclusion, both HLA-B*57 and the amount of ultrasensitive viral load seem to play a role in HIV-specific CD8(+) T cell responses in HIC.

The same major histocompatibility complex polymorphism involved in control of HIV influences peptide binding in the mouse H-2Ld system

Single-site polymorphisms in human class I major histocompatibility complex (MHC) products (HLA-B) have recently been shown to correlate with HIV disease progression or control. An identical single-site polymorphism (at residue 97) in the mouse class I product H-2L(d) influences stability of the complex. To gain insight into the human polymorphisms, here we examined peptide binding, stability, and structures of the corresponding L(d) polymorphisms, Trp(97) and Arg(97). Expression of L(d)W97 and L(d)R97 genes in a cell line that is antigen-processing competent showed that L(d)R97 was expressed at higher levels than L(d)W97, consistent with enhanced stability of self-peptide·L(d)R97 complexes. To further examine peptide-binding capacities of these two allelic variants, we used a high affinity pep-L(d) specific probe to quantitatively examine a collection of self- and foreign peptides that bind to L(d). L(d)R97 bound more effectively than L(d)W97 to most peptides, although L(d)W97 bound more effectively to two peptides. The results support the view that many self-peptides in the L(d) system (or the HLA-B system) would exhibit enhanced binding to Arg(97) alleles compared with Trp(97) alleles. Accordingly, the self-peptide·MHC-Arg(97) complexes would influence T-cell selection behavior, impacting the T-cell repertoire of these individuals, and could also impact peripheral T cell activity through effects of self-peptide·L(d) interacting with TCR and/or CD8. The structures of several peptide·L(d)R97 and peptide·L(d)W97 complexes provided a framework of how this single polymorphism could impact peptide binding.

T-cell memory differentiation: insights from transcriptional signatures and epigenetics

A critical component of vaccine design is to generate and maintain antigen-specific memory lymphocytes of sufficient quantity and quality to give the host life-long protection against re-infection. Therefore, it is important to understand how memory T cells acquire the ability for self-renewal while retaining a potential for heightened recall of effector functions. During acute viral infection or following vaccination, antigen-specific T cells undergo extensive phenotypic and functional changes during differentiation to the effector and memory phases of the immune response. The changes in cell phenotype that accompany memory T-cell differentiation are predominantly mediated through acquired transcriptional regulatory mechanisms, in part achieved through epigenetic modifications of DNA and histones. Here we review our current understanding of epigenetic mechanisms regulating the off-on-off expression of CD8 and CD4 T-cell effector molecules at naive, effector and memory stages of differentiation, respectively, and how covalent modifications to the genome may serve as a mechanism to preserve 'poised' transcriptional states in homeostatically dividing memory cells. We discuss the potential of such mechanisms to control genes that undergo on-off-on patterns of expression including homing and pro-survival genes, and the implications on the development of effector-memory and central-memory T-cell differentiation. Lastly, we review recent studies demonstrating epigenetic modifications as a mechanism for the progressive loss of transcriptional adaptation in antigen-specific T cells that undergo sustained high levels of T-cell receptor signalling.

Identification of cross-clade CTL epitopes in HIV-1 clade A/E-infected individuals by using the clade B overlapping peptides

Identification of cross-clade T cell epitopes is one of key factors for the development of a widely applicable AIDS vaccine. We here investigated cross-clade CD8(+) T cell responses between clade B and A/E viruses in chronically HIV-1 clade A/E-infected Japanese individuals. CD8(+) T cell responses to 11-mer overlapping peptides derived from Nef, Gag, and Pol clade B consensus sequences were at a similar level to those to the same peptides found in clade B-infected individuals. Fifteen cross-clade CTL epitopes were identified from 13 regions where the frequency of responders was high in the clade A/E-infected individuals. The sequences of 6 epitopes were conserved between the clade B and clade A/E viruses whereas 9 epitopes had different amino acid sequences between the 2 viruses. CD8(+) T cells specific for the 6 conserved epitopes recognized cells infected with the clade A/E virus, whereas those for 8 diverse epitopes recognized both the clade A/E virus-infected and clade B-infected cells. All of the cross-clade CD8(+) T cells specific for conserved and diverse epitopes were detected in chronically HIV-1 clade A/E-infected individuals. These results show that in addition to conserved regions polymorphic ones across the clades can be targets for cross-clade CTLs.

HIV vaccine to induce cytotoxic T cells recognizing conserved HIV-1/2-epitopes derived from the most frequent HLA types of the human population

Evaluation of: Pleguezuelos O, Stoloff GA, Caparros-Wanderley W. Synthetic immunotherapy induces HIV virus specific Th1 cytotoxic response and death of an HIV-1 infected human cell line through classic complement activation. Virol. J. 10(1), 107 (2013). AIDS vaccine development represents an unprecedented challenge in both immunogen design and delivery to induce potent and long-lasting HIV-specific immune responses, including neutralizing antibodies and cytotoxic T lymphocytes (CTL). Pleguezuelos and coworkers recognized that immunogen design must address both HIV and HLA diversity to make a global vaccine. The HIV-v synthetic polypeptide vaccine described here sets a new standard in antigen design by selecting conserved regions of global HIV-1 and HIV-2 isolates and epitopes from most frequent HLA types of the human population. The new vaccine induced both antibody and CTL responses. Importantly, the authors demonstrated vaccine-specific HLA restricted CD8(+) CTL responses for one HLA allele that was involved in the antigen design. HIV-v vaccine is a new candidate to be tested in human subjects carrying frequent HLA types.

HLA-B27 and antigen presentation: at the crossroads between immune defense and autoimmunity

The HLA-B27 is historically studied as a susceptibility factor in spondyloarthropathies and, primarily, in ankylosing spondylitis (AS). Over the recent years however, it has been rediscovered as protective factor against some severe viral infections. This is due to the high capacity of virus-specific, HLA-B27-restricted CD8+ T cells for both intrinsic (i.e. polyfunctionality, high avidity, low sensitivity to Treg cell-mediated suppression) and extrinsic (i.e. rapid and efficient antigen processing and presentation) factors. It is tempting to speculate that these two aspects are not independent and that the association of B27 molecules to autoimmunity is the downside of this superior functional efficacy which, in given genetic backgrounds and environmental conditions, can support a chronic inflammation leading to spondyloarthropathies. Still, the pathogenic role of HLA-B27 molecules in AS is elusive. Here, we focus on the biology of HLA-B27 from the genetics to the biochemistry and to the structural/dynamical properties of B27:peptide complexes as obtained from atomistic molecular dynamics simulation. Overall, the results point at the antigen presentation as the key event in the disease pathogenesis. In particular, an extensive comparison of HLA-B*2705 and B*2709 molecules, that differ in a single amino acid (Asp116 to His116) and are differentially associated with AS, indicates that position 116 is crucial for shaping the entire peptide-presenting groove.

Individual MHCI-Restricted T-Cell Receptors are Characterized by a Unique Peptide Recognition Signature

Effective immunity requires that a limited TCR repertoire is able to recognize a vast number of foreign peptide-MHCI (peptide-major histocompatibility complex class I) molecules. This challenge is overcome by the ability of individual TCRs to recognize large numbers of peptides. Recently, it was demonstrated that MHCI-restricted TCRs can recognize up to 10⁶ peptides of a defined length. Astonishingly, this remarkable level of promiscuity does not extend to peptides of different lengths, a fundamental observation that has broad implications for CD8⁺ T-cell immunity. In particular, the findings suggest that effective immunity can only be achieved by mobilization of “length-matched” CD8⁺ T-cell clonotypes. Overall, recent findings suggest that every TCR is specific for a unique set of peptides, which can be described as a unique “peptide recognition signature” (PRS) and consists of three components: (1) peptide length preference, (2) number of peptides recognized; and, (3) sequence identity (e.g., self versus pathogen derived). In future, the ability to de-convolute peptide recognition signatures across the normal and pathogenic repertoire will be essential for understanding the system requirements for effective CD8⁺ T-cell immunity and elucidating mechanisms which underlie CD8⁺ T-cell mediated disease.

Pressure from TRIM5α contributes to control of HIV-1 replication by individuals expressing protective HLA-B alleles

The expression of certain HLA class I alleles, including HLA-B*27 and HLA-B*57, is associated with better control of human immunodeficiency virus type 1 (HIV-1) infection, but the mechanisms responsible are not fully understood. We sought evidence that pressure from the human restriction factor TRIM5α (hTRIM5α) could contribute to viral control. The hTRIM5α sensitivity of viruses from both HLA-B*57-positive (HLA-B*57(+)) and HLA-B*27(+) patients who spontaneously controlled viral replication, but not viruses from viremic patients expressing these alleles, was significantly greater than that of viruses from patients not expressing these protective HLA-B alleles. Overall, a significant negative correlation between hTRIM5α sensitivity and viral load was observed. In HLA-B*57(+) patients, the T242N mutation in the HLA-B*57-restricted TW10 CD8(+) T lymphocyte (CTL) epitope was strongly associated with hTRIM5α sensitivity. In HLA-B*27(+) controllers, hTRIM5α sensitivity was associated with a significant reduction in emergence of key CTL mutations. In several patients, viral evolution to avoid hTRIM5α sensitivity was observed but could be associated with reduced viral replicative capacity. Thus, in individuals expressing protective HLA-B alleles, the combined pressures exerted by CTL, hTRIM5α, and capsid structural constraints can prevent viral escape both by impeding the selection of necessary resistance/compensatory mutations and forcing the selection of escape mutations that increase hTRIM5α sensitivity or impair viral replicative capacity.

Single naive CD4+ T cells from a diverse repertoire produce different effector cell types during infection

A naive CD4(+) T cell population specific for a microbial peptide:major histocompatibility complex II ligand (p:MHCII) typically consists of about 100 cells, each with a different T cell receptor (TCR). Following infection, this population produces a consistent ratio of effector cells that activate microbicidal functions of macrophages or help B cells make antibodies. We studied the mechanism that underlies this division of labor by tracking the progeny of single naive T cells. Different naive cells produced distinct ratios of macrophage and B cell helpers but yielded the characteristic ratio when averaged together. The effector cell pattern produced by a given naive cell correlated with the TCR-p:MHCII dwell time or the amount of p:MHCII. Thus, the consistent production of effector cell subsets by a polyclonal population of naive cells results from averaging the diverse behaviors of individual clones, which are instructed in part by the strength of TCR signaling.

Interrogating the relationship between naïve and immune antiviral T cell repertoires

Understanding how naïve virus-specific CD8+ T cells influence the type of immune response generated after virus infection is critical for the development of enhanced therapeutic and vaccination strategies to exploit CD8+ T cell-mediated immunity. Recent technological advances in T cell isolation and T receptor sequencing have allowed for greater understanding of the basic structure of immune T cell repertoires, the diversity of responses within and between individuals, and changes in repertoires over time and in response to infection conditions. In this review, we discuss the current understanding of how T cell repertoires contribute to potent antiviral responses. Additionally we compare the state of the art in receptor sequencing, highlighting the advantages and disadvantages of the three most common approaches: next-generation sequencing, template-switch anchored RT-PCR, and multiplex single cell PCR. Finally, we describe how TCR sequencing has delineated the relationship between naïve and immune T cell repertoires.

Rapid perturbation in viremia levels drives increases in functional avidity of HIV-specific CD8 T cells

The factors determining the functional avidity and its relationship with the broad heterogeneity of antiviral T cell responses remain partially understood. We investigated HIV-specific CD8 T cell responses in 85 patients with primary HIV infection (PHI) or chronic (progressive and non-progressive) infection. The functional avidity of HIV-specific CD8 T cells was not different between patients with progressive and non-progressive chronic infection. However, it was significantly lower in PHI patients at the time of diagnosis of acute infection and after control of virus replication following one year of successful antiretroviral therapy. High-avidity HIV-specific CD8 T cells expressed lower levels of CD27 and CD28 and were enriched in cells with an exhausted phenotype, i.e. co-expressing PD-1/2B4/CD160. Of note, a significant increase in the functional avidity of HIV-specific CD8 T cells occurred in early-treated PHI patients experiencing a virus rebound after spontaneous treatment interruption. This increase in functional avidity was associated with the accumulation of PD-1/2B4/CD160 positive cells, loss of polyfunctionality and increased TCR renewal. The increased TCR renewal may provide the mechanistic basis for the generation of high-avidity HIV-specific CD8 T cells. These results provide insights on the relationships between functional avidity, viremia, T-cell exhaustion and TCR renewal of antiviral CD8 T cell responses.

CD8 T cells directed against virus are complex and functionally heterogeneous. One relevant component of CD8 T cells is their functional avidity which reflects their sensitivity to cognate antigens, i.e. how prone T cells are to respond when they encounter low doses of antigens. In patients with chronic and established HIV infection, we observed that the sensitivity of HIV-specific CD8 T cells was not different between patients with progressive or non-progressive disease. In contrast, the sensitivity of HIV-specific CD8 T cells was significantly lower in patients with early and recent HIV infection. Furthermore, CD8 T cells of high avidity were preferentially associated with a state of functional impairment known as exhaustion. Of interest, some patients treated with antiretroviral therapy during acute infection spontaneously interrupted their treatment and experienced a rebound of virus. In these patients, the avidity of HIV-specific CD8 T cells increased and this increase was associated to stronger cell exhaustion and greater renewal of the population of antiviral CD8 T cells, thus potentially providing the mechanistic basis for the generation of high-avidity CD8 T cells. Overall, our data suggest that rapid perturbation in viremia levels drove increases in the functional avidity of HIV-specific CD8 T cells.

Unravelling the mechanisms of durable control of HIV-1

Untreated HIV-1 infection typically progresses to AIDS within 10 years, but less than 1% of infected individuals remain healthy and have normal CD4(+) T cell counts and undetectable viral loads; some individuals have remained this way for 35 years and counting. Through a combination of large population studies of cohorts of these 'HIV-1 controllers' and detailed studies of individual patients, a heterogeneous picture has emerged regarding the basis for this remarkable resistance to AIDS progression. In this Review, we highlight the host genetic factors, the viral genetic factors and the immunological factors that are associated with the controller phenotype, we discuss emerging methodological approaches that could facilitate a better understanding of spontaneous HIV-1 immune control in the future, and we delineate implications for a 'functional cure' of HIV-1 infection.

Lessons to be Learned from Natural Control of HIV - Future Directions, Therapeutic, and Preventive Implications

Accumulating data generated from persons who naturally control HIV without the need for antiretroviral treatment has led to significant insights into the possible mechanisms of durable control of AIDS virus infection. At the center of this control is the HIV-specific CD8 T cell response, and the basis for this CD8-mediated control is gradually being revealed. Genome wide association studies coupled with HLA sequence data implicate the nature of the HLA-viral peptide interaction as the major genetic factor modulating durable control of HIV, but host genetic factors account for only around 20% of the variability in control. Other factors including specific functional characteristics of the TCR clonotypes generated in vivo, targeting of vulnerable regions of the virus that lead to fitness impairing mutations, immune exhaustion, and host restriction factors that limit HIV replication all have been shown to additionally contribute to control. Moreover, emerging data indicate that the CD8⁺ T cell response may be critical for attempts to purge virus infected cells following activation of the latent reservoir, and thus lessons learned from elite controllers (ECs) are likely to impact the eradication agenda. On-going efforts are also needed to understand and address the role of immune activation in disease progression, as it becomes increasingly clear that durable immune control in ECs comes at a cost. Taken together, the research achievements in the attempt to unlock the mechanisms behind natural control of HIV will continue to be an important source of insights and ideas in the continuous search after an effective HIV vaccine, and for the attempts to achieve a sterilizing or functional cure in HIV positive patients with progressive infection.

CD8(+) T cell cross-reactivity profiles and HIV-1 immune escape towards an HLA-B35-restricted immunodominant Nef epitope

Antigen cross-reactivity is an inbuilt feature of the T cell compartment. However, little is known about the flexibility of T cell recognition in the context of genetically variable pathogens such as HIV-1. In this study, we used a combinatorial library containing 24 billion octamer peptides to characterize the cross-reactivity profiles of CD8⁺ T cells specific for the immunodominant HIV-1 subtype B Nef epitope VY8 (VPLRPMTY) presented by HLA-B^*35∶01. In conjunction, we examined naturally occurring antigenic variations within the VY8 epitope. Sequence analysis of plasma viral RNA isolated from 336 HIV-1-infected individuals revealed variability at position (P) 3 and P8 of VY8; Phe at P8, but not Val at P3, was identified as an HLA-B^*35∶01-associated polymorphism. VY8-specific T cells generated from several different HIV-1-infected patients showed unique and clonotype-dependent cross-reactivity footprints. Nonetheless, all T cells recognized both the index Leu and mutant Val at P3 equally well. In contrast, competitive titration assays revealed that the Tyr to Phe substitution at P8 reduced T cell recognition by 50–130 fold despite intact peptide binding to HLA-B^*35∶01. These findings explain the preferential selection of Phe at the C-terminus of VY8 in HLA-B^*35∶01⁺ individuals and demonstrate that HIV-1 can exploit the limitations of T cell recognition in vivo.

Immunogenetics: Genome-Wide Association of Non-Progressive HIV and Viral Load Control: HLA Genes and Beyond

Very early after the identification of the human immunodeficiency virus (HIV), host genetics factors were anticipated to play a role in viral control and disease progression. As early as the mid-1990s, candidate gene studies demonstrated a central role for the chemokine co-receptor/ligand (e.g., CCR5) and human leukocyte antigen (HLA) systems. In the last decade, the advent of genome-wide arrays opened a new era for unbiased genetic exploration of the genome and brought big expectations for the identification of new unexpected genes and pathways involved in HIV/AIDS. More than 15 genome-wide association studies targeting various HIV-linked phenotypes have been published since 2007. Surprisingly, only the two HIV-chemokine co-receptors and HLA loci have exhibited consistent and reproducible statistically significant genetic associations. In this chapter, we will review the findings from the genome-wide studies focusing especially on non-progressive and HIV control phenotypes, and discuss the current perspectives.

Peptide-binding motifs associated with MHC molecules common in Chinese rhesus macaques are analogous to those of human HLA supertypes and include HLA-B27-like alleles

Chinese rhesus macaques are of particular interest in simian immunodeficiency virus/human immunodeficiency virus (SIV/HIV) research as these animals have prolonged kinetics of disease progression to acquired immunodeficiency syndrome (AIDS), compared to their Indian counterparts, suggesting that they may be a better model for HIV. Nevertheless, the specific mechanism(s) accounting for these kinetics remains unclear. The study of major histocompatibility complex (MHC) molecules, including their MHC/peptide-binding motifs, provides valuable information for measuring cellular immune responses and deciphering outcomes of infection and vaccine efficacy. In this study, we have provided detailed characterization of six prevalent Chinese rhesus macaque MHC class I alleles, yielding a combined phenotypic frequency of 29 %. The peptide-binding specificity of two of these alleles, Mamu-A2*01:02 and Mamu-B*010:01, as well as the previously characterized allele Mamu-B*003:01 (and Indian rhesus Mamu-B*003:01), was found to be analogous to that of alleles in the HLA-B27 supertype family. Specific alleles in the HLA-B27 supertype family, including HLA-B*27:05, have been associated with long-term nonprogression to AIDS in humans. All six alleles characterized in the present study were found to have specificities analogous to HLA supertype alleles. These data contribute to the concept that Chinese rhesus macaque MHC immunogenetics is more similar to HLA than their Indian rhesus macaque counterparts and thereby warrants further studies to decipher the role of these alleles in the context of SIV infection.

Innate and Adaptive Immunity in Long-Term Non-Progression in HIV Disease

Long-term non-progressors (LTNP) were identified after 10-15 years of the epidemic, and have been the subject of intense investigation ever since. In a small minority of cases, infection with nef/3'LTR deleted attenuated viral strains allowed control over viral replication. A common feature of LTNP is the readily detected proliferation of CD4 T-cells in vitro, in response to p24. In some cases, the responding CD4 T-cells have cytotoxic effector function and may target conserved p24 epitopes, similar to the CD8 T-cells described below. LTNP may also carry much lower HIV DNA burden in key CD4 subsets, presumably resulting from lower viral replication during primary infection. Some studies, but not others, suggest that LTNP have CD4 T-cells that are relatively resistant to HIV infection in vitro. One possible mechanism may involve up-regulation of the cell cycle regulator p21/waf in CD4 T-cells from LTNP. Delayed progression in Caucasian LTNP is also partly associated with heterozygosity of the Δ32 CCR5 allele, probably through decreased expression of CCR5 co-receptor on CD4 T-cells. However, in approximately half of Caucasian LTNP, two host genotypes, namely HLA-B57 and HLA-B27, are associated with viral control. Immunodominant CD8 T-cells from these individuals target epitopes in p24 that are highly conserved, and escape mutations have significant fitness costs to the virus. Furthermore, recent studies have suggested that these CD8 T-cells from LTNP, but not from HLA-B27 or HLA-B57 progressors, can cross-react with intermediate escape mutations, preventing full escape via compensatory mutations. Humoral immunity appears to play little part in LTNP subjects, since broadly neutralizing antibodies are rare, even amongst slow progressors. Recent genome-wide comparisons between LTNP and progressors have confirmed the HLA-B57, HLA-B27, and delta32 CCR5 allelic associations, plus indicated a role for HLA-C/KIR interactions, but have not revealed any new genotypes so far. Nevertheless, it is hoped that studying the mechanisms of intracellular restriction factors, such as the recently identified SAMHD1, will lead to a better understanding of non-progression.

Cell-Mediated Immunity in Elite Controllers Naturally Controlling HIV Viral Load

The natural course of human immunodeficiency virus (HIV) infection is characterized by high viral load, depletion of immune cells, and immunodeficiency, ultimately leading to acquired immunodeficiency syndrome phase and the occurrence of opportunistic infections and diseases. Since the discovery of HIV in the early 1980s a naturally selected population of infected individuals has been emerged in the last years, characterized by being infected for many years, with viremia constantly below detectable level and poor depletion of immune cells. These individuals are classified as “elite controllers (EC) or suppressors” and do not develop disease in the absence of anti-retroviral therapy. Unveiling host factors and immune responses responsible for the elite status will likely provide clues for the design of therapeutic vaccines and functional cures. Scope of this review was to examine and discuss differences of the cell-mediated immune responses between HIV+ individuals with disease progression and EC.

Structural basis for the differential classification of HLA-A*6802 and HLA-A*6801 into the A2 and A3 supertypes

High polymorphism is one of the most important features of human leukocyte antigen (HLA) alleles, which were initially classified by serotyping but have recently been re-grouped into supertypes according to their peptide presentation properties. Two relatively prevalent HLA alleles HLA-A*6801 and HLA-A*6802, are classified into the same serotype HLA-A68. However, based on their distinct peptide-binding characteristics, HLA-A*6801 is grouped into A3 supertype, whereas HLA-A*6802 belongs to A2 supertype, similar to HLA-A*0201. Thusfar, the structural basis of the different supertype definitions of these serotyping-identical HLA alleles remains largely unknown. Herein, we determined the structures of HLA-A*6801 and HLA-A*6802 presenting three typical A3 and A2 supertype-restricted peptides, respectively. The binding capabilities of these peptides to HLA-A*6801, HLA-A*6802, and HLA-A*0201 were analyzed. These data indicate that the similar conformations of the residues within the F pocket contribute to close-related peptide binding features of HLA-A*6802 and HLA-A*0201. However, the overall structure and the peptide conformation of HLA-A*6802 are more similar to HLA-A*6801 rather than HLA-A*0201 which illuminates the similar serotype grouping of HLA-A*6802 and HLA-A*6801. Our findings are helpful for understanding the divergent peptide presentation and virus-specific CTL responses impacted by MHC micropolymorphisms and also elucidate the molecular basis of HLA supertype definitions.

Wrestling with the repertoire: the promise and perils of next generation sequencing for antigen receptors

Next generation sequencing technologies are revolutionizing the study of immune repertoires. These methods provide a previously unimaginable amount of sequence data, unfortunately accompanied by numerous challenges associated with error correction and interpretation that remain to be solved. For antigen receptors, these challenges will require dedicated solutions beyond those developed for genome sequencing, which may differ depending on the sequencing technology used and the purpose of the experiment. Many investigators are developing such methods, based on different sequencing platforms, but critical details of protocol and performance are proprietary. The field will move forward when these methods are shared and standardized, and when the accuracy, sensitivity and reproducibility of various sequencing, and analytic methods are evaluated using standardized samples in comparative experiments.

Long-Term Non-Progression and Broad HIV-1-Specific Proliferative T-Cell Responses

Complex mechanisms underlying the maintenance of fully functional, proliferative, HIV-1-specific T-cell responses involve processes from early T-cell development through to the final stages of T-cell differentiation and antigen recognition. Virus-specific proliferative CD4 and CD8 T-cell responses, important for the control of infection, are observed in some HIV-1(+) patients during early stages of disease, and are maintained in long-term non-progressing subjects. In the vast majority of HIV-1(+) patients, full immune functionality is lost when proliferative HIV-1-specific T-cell responses undergo a variable progressive decline throughout the course of chronic infection. This appears irreparable despite administration of potent combination antiretroviral therapy, which to date is non-curative, necessitating life-long administration and the development of effective, novel, therapeutic interventions. While a sterilizing cure, involving clearance of virus from the host, remains a primary aim, a "functional cure" may be a more feasible goal with considerable impact on worldwide HIV-1 infection. Such an approach would enable long-term co-existence of host and virus in the absence of toxic and costly drugs. Effective immune homeostasis coupled with a balanced response appropriately targeting conserved viral antigens, in a manner that avoids hyperactivation and exhaustion, may prove to be the strongest correlate of durable viral control. This review describes novel concepts underlying full immune functionality in the context of HIV-1 infection, which may be utilized in future strategies designed to improve upon existing therapy. The aim will be to induce long-term non-progressor or elite controller status in every infected host, through immune-mediated control of viremia and reduction of viral reservoirs, leading to lower HIV-1 transmission rates.

Ecological analysis of antigen-specific CTL repertoires defines the relationship between naive and immune T-cell populations

Ecology is typically thought of as the study of interactions organisms have with each other and their environment and is focused on the distribution and abundance of organisms both within and between environments. On a molecular level, the capacity to probe analogous questions in the field of T-cell immunology is imperative as we acquire substantial datasets both on epitope-specific T-cell populations through high-resolution analyses of T-cell receptor (TCR) use and on global T-cell populations analyzed via high-throughput DNA sequencing. Here, we present the innovative application of existing statistical measures (used typically in the field of ecology), together with unique statistical analyses, to comprehensively assess how the naïve epitope-specific CD8(+) cytotoxic T lymphocyte (CTL) repertoire translates to that found following an influenza-virus-specific immune response. Such interrogation of our extensive, cumulated TCR CDR3β sequence datasets, derived from both naïve and immune CD8(+) T-cell populations specific for four different influenza-derived epitopes (D(b)NP(366), influenza nucleoprotein amino acid residues 366-374; D(b)PA(224), influenza acid polymerase amino acid residues 224-233; D(b)PB1-F2(62), influenza polymerase B 1 reading frame 2 amino acid residues 62-70; K(b)NS2(114), and influenza nonstructural protein 2 amino acid residues 114-121), demonstrates that epitope-specific TCR use in an antiviral immune response is the consequence of a complex interplay between the intrinsic characteristics of the naïve cytotoxic T lymphocyte precursor pool and extrinsic (likely antigen driven) influences, the contribution of which varies in an epitope-specific fashion.

Attenuation of multiple Nef functions in HIV-1 elite controllers

Background

Impaired HIV-1 Gag, Pol, and Env function has been described in elite controllers (EC) who spontaneously suppress plasma viremia to < 50 RNA copies/mL; however, activity of the accessory protein Nef remains incompletely characterized. We examined the ability of 91 Nef clones, isolated from plasma of 45 EC and 46 chronic progressors (CP), to down-regulate HLA class I and CD4, up-regulate HLA class II invariant chain (CD74), enhance viral infectivity, and stimulate viral replication in PBMC.

Results

In general, EC Nef clones were functional; however, all five activities were significantly lower in EC compared to CP. Nef clones from HLA-B*57-expressing EC exhibited poorer CD4 down-regulation function compared to those from non-B*57 EC, and the number of EC-specific B*57-associated Nef polymorphisms correlated inversely with 4 of 5 Nef functions in these individuals.

Conclusion

Results indicate that decreased HIV-1 Nef function, due in part to host immune selection pressures, may be a hallmark of the EC phenotype.

Attenuation of multiple Nef functions in HIV-1 elite controllers

BACKGROUND

Impaired HIV-1 Gag, Pol, and Env function has been described in elite controllers (EC) who spontaneously suppress plasma viremia to < 50 RNA copies/mL; however, activity of the accessory protein Nef remains incompletely characterized. We examined the ability of 91 Nef clones, isolated from plasma of 45 EC and 46 chronic progressors (CP), to down-regulate HLA class I and CD4, up-regulate HLA class II invariant chain (CD74), enhance viral infectivity, and stimulate viral replication in PBMC.

RESULTS

In general, EC Nef clones were functional; however, all five activities were significantly lower in EC compared to CP. Nef clones from HLA-B*57-expressing EC exhibited poorer CD4 down-regulation function compared to those from non-B*57 EC, and the number of EC-specific B*57-associated Nef polymorphisms correlated inversely with 4 of 5 Nef functions in these individuals.

CONCLUSION

Results indicate that decreased HIV-1 Nef function, due in part to host immune selection pressures, may be a hallmark of the EC phenotype.

Poor HIV control in HLA-B*27 and B*57/58 noncontrollers is associated with limited number of polyfunctional Gag p24-specific CD8+ T cells

OBJECTIVES

Analysis of immune response in HIV controllers, a unique group of infected individuals who are able to control HIV naturally, has provided us a chance to investigate the roles of host immune responses in HIV control.

DESIGN

In this study, the functional quality of HIV Gag p24-specific CD8 T-cell responses was assessed in two groups of clinically distinct, HLA-B*27, HLA-B*57/58-matched individuals, viremic controllers [plasma HIV load (pVL) ≤ 2000 copies/ml) and noncontrollers (pVL >2000 copies/ml) to determine its impacts on natural HIV clinical outcome.

METHODS

An ex-vivo interferon (IFN)-γ ELISpot assay was used to screen for each individual's HIV Gag p24-specific T-cell responses. Intracellular cytokine staining assay was used to determine their functional quality (as number of cytokine being produced).

RESULTS

We found that, in contrast to previous studies, all Thai volunteers with HLA-B*5801 were uniformly noncontrollers. Viremic controllers were observed with a significantly larger number of high functional quality p24-specific CD8 T cells than noncontrollers (P < 0.05). This superior quality of responses was observed at both total p24 and epitope-specific level. Moreover, the absolute number of high functional quality Gag p24-specific CD8 T cells was significantly in a negative correlation with pVL (r = -0.6984, P = 0.0006) and also in a positive correlation with CD4 T-cell count (r = 0.5648, P = 0.0095).

CONCLUSION

We concluded that an adequate number of high functional quality Gag p24-specific CD8 T cells is strongly associated with a natural HIV controller status.

[Control of HIV infection and selection/ accumulation of HIV escape mutants by HIV-specific cytotoxic T lymphocytes (CTLs)]

HIV-specific cytotoxic T lymphocytes (CTLs) play an important role in control of HIV infection. HLA class I molecules, which mediate recognition of HIV-infected cells by the CTLs, have great diversity. Some HLA class I molecules are well known to associate with HIV control. HIV escapes from the CTLs through amino acid mutations within CTL epitopes. The rapid and extensive spread of HIV escape mutants provides accumulation of the mutants at a population level, demonstrating strong evidence of HIV adaptation to HLA class I. In this review, we discuss the role of the CTLs in controlling HIV, the relationship between HLA alleles and HIV control, and the accumulation of HIV mutants selected by the CTLs at a population level.

Depletion and dysfunction of Vγ2Vδ2 T cells in HIV disease: mechanisms, impacts and therapeutic implications

Infection with human immunodeficiency virus (HIV) disrupts the balance among γδ T cell subsets, with increasing Vδ1+ cells and substantial depletion of circulating Vδ2+ cells. Depletion is an indirect effect of HIV in CD4-negative Vδ2 cells, but is specific for phosphoantigen-responsive subpopulations identified by the Vγ2-Jγ1.2 (also called Vγ9-JγP) T cell receptor rearrangement. The extent of cell loss and recovery is related closely to clinical status, with highest levels of functional Vδ2 cells present in virus controllers (undetectable viremia in the absence of antiretroviral therapy). We review the mechanisms and clinical consequences for Vδ2 cell depletion in HIV disease. We address the question of whether HIV-mediated Vδ2 cell depletion, despite being an indirect effect of infection, is an important part of the immune evasion strategy for this virus. The important roles for Vδ2 cells, as effectors and immune regulators, identify key mechanisms affected by HIV and show the strong relationships between Vδ2 cell loss and immunodeficiency disease. This field is moving toward immune therapies based on targeting Vδ2 cells and we now have clear goals and expectations to guide interventional clinical trials.

Vertical T cell immunodominance and epitope entropy determine HIV-1 escape

HIV-1 accumulates mutations in and around reactive epitopes to escape recognition and killing by CD8+ T cells. Measurements of HIV-1 time to escape should therefore provide information on which parameters are most important for T cell-mediated in vivo control of HIV-1. Primary HIV-1-specific T cell responses were fully mapped in 17 individuals, and the time to virus escape, which ranged from days to years, was measured for each epitope. While higher magnitude of an individual T cell response was associated with more rapid escape, the most significant T cell measure was its relative immunodominance measured in acute infection. This identified subject-level or "vertical" immunodominance as the primary determinant of in vivo CD8+ T cell pressure in HIV-1 infection. Conversely, escape was slowed significantly by lower population variability, or entropy, of the epitope targeted. Immunodominance and epitope entropy combined to explain half of all the variability in time to escape. These data explain how CD8+ T cells can exert significant and sustained HIV-1 pressure even when escape is very slow and that within an individual, the impacts of other T cell factors on HIV-1 escape should be considered in the context of immunodominance.

Correlates of protective cellular immunity revealed by analysis of population-level immune escape pathways in HIV-1

HLA class I-associated polymorphisms identified at the population level mark viral sites under immune pressure by individual HLA alleles. As such, analysis of their distribution, frequency, location, statistical strength, sequence conservation, and other properties offers a unique perspective from which to identify correlates of protective cellular immunity. We analyzed HLA-associated HIV-1 subtype B polymorphisms in 1,888 treatment-naïve, chronically infected individuals using phylogenetically informed methods and identified characteristics of HLA-associated immune pressures that differentiate protective and nonprotective alleles. Over 2,100 HLA-associated HIV-1 polymorphisms were identified, approximately one-third of which occurred inside or within 3 residues of an optimally defined cytotoxic T-lymphocyte (CTL) epitope. Differential CTL escape patterns between closely related HLA alleles were common and increased with greater evolutionary distance between allele group members. Among 9-mer epitopes, mutations at HLA-specific anchor residues represented the most frequently detected escape type: these occurred nearly 2-fold more frequently than expected by chance and were computationally predicted to reduce peptide-HLA binding nearly 10-fold on average. Characteristics associated with protective HLA alleles (defined using hazard ratios for progression to AIDS from natural history cohorts) included the potential to mount broad immune selection pressures across all HIV-1 proteins except Nef, the tendency to drive multisite and/or anchor residue escape mutations within known CTL epitopes, and the ability to strongly select mutations in conserved regions within HIV's structural and functional proteins. Thus, the factors defining protective cellular immune responses may be more complex than simply targeting conserved viral regions. The results provide new information to guide vaccine design and immunogenicity studies.

Functional avidity: a measure to predict the efficacy of effector T cells?

The functional avidity is determined by exposing T-cell populations in vitro to different amounts of cognate antigen. T-cells with high functional avidity respond to low antigen doses. This in vitro measure is thought to correlate well with the in vivo effector capacity of T-cells. We here present the multifaceted factors determining and influencing the functional avidity of T-cells. We outline how changes in the functional avidity can occur over the course of an infection. This process, known as avidity maturation, can occur despite the fact that T-cells express a fixed TCR. Furthermore, examples are provided illustrating the importance of generating T-cell populations that exhibit a high functional avidity when responding to an infection or tumors. Furthermore, we discuss whether criteria based on which we evaluate an effective T-cell response to acute infections can also be applied to chronic infections such as HIV. Finally, we also focus on observations that high-avidity T-cells show higher signs of exhaustion and facilitate the emergence of virus escape variants. The review summarizes our current understanding of how this may occur as well as how T-cells of different functional avidity contribute to antiviral and anti-tumor immunity. Enhancing our knowledge in this field is relevant for tumor immunotherapy and vaccines design.

Stochastic effects are important in intrahost HIV evolution even when viral loads are high

Blood plasma viral loads and the time to progress to AIDS differ widely among untreated HIV-infected humans. Although people with certain HLA (HLA-I) alleles are more likely to control HIV infections without therapy, the majority of such untreated individuals exhibit high viral loads and progress to AIDS. Stochastic effects are considered unimportant for evolutionary dynamics in HIV-infected people when viral load is high or when selective forces strongly drive mutation. We describe a computational study of host-pathogen interaction demonstrating that stochastic effects can have a profound influence on disease dynamics, even in cases of high viral load and strong selective pressure. These stochastic effects are pronounced when the virus must traverse a fitness "barrier" in sequence space to escape the host's cytotoxic T-lymphocyte (CTL) response, as often occurs when a fitness defect imposed by a CTL-driven mutation must be compensated for by other mutations. These "barrier-crossing" events are infrequent and stochastic, resulting in divergent disease outcomes in genetically identical individuals infected by the same viral strain. Our results reveal how genetic determinants of the CTL response control the probability with which an individual is able to control HIV infection indefinitely, and thus provide clues for vaccine design.

HIV and HLA class I: an evolving relationship

Successful vaccine development for infectious diseases has largely been achieved in settings where natural immunity to the pathogen results in clearance in at least some individuals. HIV presents an additional challenge in that natural clearance of infection does not occur, and the correlates of immune protection are still uncertain. However, partial control of viremia and markedly different outcomes of disease are observed in HIV-infected persons. Here, we examine the antiviral mechanisms implicated by one variable that has been consistently associated with extremes of outcome, namely HLA class I alleles, and in particular HLA-B, and examine the mechanisms by which this modulation is likely to occur and the impact of these interactions on evolution of the virus and the host. Studies to date provide evidence for both HLA-dependent and epitope-dependent influences on viral control and viral evolution and have important implications for the continued quest for an effective HIV vaccine.

Modulation of HIV reservoirs by host HLA: bridging the gap between vaccine and cure

Latent HIV reservoirs are the greatest challenge facing an HIV cure. Here, we review recent evidence supporting an important role for the host immune response, in particular HLA class I-restricted CD8+ T lymphocytes, in modulating HIV reservoirs during natural infection. These observations indicate that factors governing immune-mediated control of HIV may also contribute to the clearance of viral reservoirs. As such, critical gaps in our understanding of HIV immunology hinder efforts to develop both an effective HIV vaccine as well as novel therapies that may lead to a cure. The importance of elucidating correlates of protective cellular immunity should be recognized during research to develop and test potential HIV elimination strategies.