Characterization of an HLA-C-restricted CTL response in chronic HIV infection

High Resolution HLA-A, HLA-B, and HLA-C Allele Frequencies in Romanian Hematopoietic Stem Cell Donors

The HLA genes are associated with various autoimmune pathologies, with the control of the immune response also being significant in organs and cells transplantation. The aim of the study is to identify the HLA-A, HLA-B, and HLA-C alleles frequencies in the analyzed Romanian cohort. We performed HLA typing using next-generation sequencing (NGS) in a Romanian cohort to estimate class I HLA allele frequencies up to a six-digit resolution. A total of 420 voluntary donors from the National Registry of Voluntary Hematopoietic Stem Cell Donors (RNDVCSH) were included in the study for HLA genotyping. Peripheral blood samples were taken and brought to the Fundeni Clinical Institute during 2020-2021. HLA genotyping was performed using the Immucor Mia Fora NGS MFlex kit. A total of 109 different alleles were detected in 420 analyzed samples, out of which 31 were for HLA-A, 49 for HLA-B, and 29 for HLA-C. The most frequent HLA-A alleles were HLA-A*02:01:01 (26.11%), HLA-A*01:01:01 (12.5%), HLA-A*24:02:01 (11.67%), HLA-A*03:01:01 (9.72%), HLA-A*11:01:01, and HLA-A*32:01:01 (each with 8.6%). For the HLA-B locus, the most frequent allele was HLA-B*18:01:01 (11.25%), followed by HLA-B*51:01:01 (10.83%) and HLA-B*08:01:01 (7.78%). The most common HLA-C alleles were HLA-C*07:01:01 (17.36%), HLA-C*04:01:01 (13.47%), and HLA-C*12:03:01 (10.69%). Follow-up studies are ongoing for confirming the detected results.

The New Kid on the Block: HLA-C, a Key Regulator of Natural Killer Cells in Viral Immunity

The human leukocyte antigen system (HLA) is a cluster of highly polymorphic genes essential for the proper function of the immune system, and it has been associated with a wide range of diseases. HLA class I molecules present intracellular host- and pathogen-derived peptides to effector cells of the immune system, inducing immune tolerance in healthy conditions or triggering effective immune responses in pathological situations. HLA-C is the most recently evolved HLA class I molecule, only present in humans and great apes. Differentiating from its older siblings, HLA-A and HLA-B, HLA-C exhibits distinctive features in its expression and interaction partners. HLA-C serves as a natural ligand for multiple members of the killer-cell immunoglobulin-like receptor (KIR) family, which are predominately expressed by natural killer (NK) cells. NK cells are crucial for the early control of viral infections and accumulating evidence indicates that interactions between HLA-C and its respective KIR receptors determine the outcome and progression of viral infections. In this review, we focus on the unique role of HLA-C in regulating NK cell functions and its consequences in the setting of viral infections.

Convergent Evolution of HLA-C Downmodulation in HIV-1 and HIV-2

HLA-C-mediated antigen presentation induces the killing of human immunodeficiency virus (HIV)-infected CD4+ T cells by cytotoxic T lymphocytes (CTLs). To evade killing, many HIV-1 group M strains decrease HLA-C surface levels using their accessory protein Vpu. However, some HIV-1 group M isolates lack this activity, possibly to prevent the activation of natural killer (NK) cells. Analyzing diverse primate lentiviruses, we found that Vpu-mediated HLA-C downregulation is not limited to pandemic group M but is also found in HIV-1 groups O and P as well as several simian immunodeficiency viruses (SIVs). We show that Vpu targets HLA-C primarily at the protein level, independently of its ability to suppress NF-κB-driven gene expression, and that in some viral lineages, HLA-C downregulation may come at the cost of efficient counteraction of the restriction factor tetherin. Remarkably, HIV-2, which does not carry a vpu gene, uses its accessory protein Vif to decrease HLA-C surface expression. This Vif activity requires intact binding sites for the Cullin5/Elongin ubiquitin ligase complex but is separable from its ability to counteract APOBEC3G. Similar to HIV-1 Vpu, the degree of HIV-2 Vif-mediated HLA-C downregulation varies considerably among different virus isolates. In agreement with opposing selection pressures in vivo, we show that the reduction of HLA-C surface levels by HIV-2 Vif is accompanied by increased NK cell-mediated killing. In summary, our results highlight the complex role of HLA-C in lentiviral infections and demonstrate that HIV-1 and HIV-2 have evolved at least two independent mechanisms to decrease HLA-C levels on infected cells.IMPORTANCE Genome-wide association studies suggest that HLA-C expression is a major determinant of viral load set points and CD4+ T cell counts in HIV-infected individuals. On the one hand, efficient HLA-C expression enables the killing of infected cells by cytotoxic T lymphocytes (CTLs). On the other hand, HLA-C sends inhibitory signals to natural killer (NK) cells and enhances the infectivity of newly produced HIV particles. HIV-1 group M viruses modulate HLA-C expression using the accessory protein Vpu, possibly to balance CTL- and NK cell-mediated immune responses. Here, we show that the second human immunodeficiency virus, HIV-2, can use its accessory protein Vif to evade HLA-C-mediated restriction. Furthermore, our mutational analyses provide insights into the underlying molecular mechanisms. In summary, our results reveal how the two human AIDS viruses modulate HLA-C, a key component of the antiviral immune response.

The Dual Role of HLA-C in Tolerance and Immunity at the Maternal-Fetal Interface

To establish a healthy pregnancy, maternal immune cells must tolerate fetal allo-antigens and remain competent to respond to infections both systemically and in placental tissues. Extravillous trophoblasts (EVT) are the most invasive cells of extra-embryonic origin to invade uterine tissues and express polymorphic Human Leucocyte Antigen-C (HLA-C) of both maternal and paternal origin. Thus, HLA-C is a key molecule that can elicit allogeneic immune responses by maternal T and NK cells and for which maternal-fetal immune tolerance needs to be established. HLA-C is also the only classical MHC molecule expressed by EVT that can present a wide variety of peptides to maternal memory T cells and establish protective immunity. The expression of paternal HLA-C by EVT provides a target for maternal NK and T cells, whereas HLA-C expression levels may influence how this response is shaped. This dual function of HLA-C requires tight transcriptional regulation of its expression to balance induction of tolerance and immunity. Here, we critically review new insights into: (i) the mechanisms controlling expression of HLA-C by EVT, (ii) the mechanisms by which decidual NK cells, effector T cells and regulatory T cells recognize HLA-C allo-antigens, and (iii) immune recognition of pathogen derived antigens in context of HLA-C.

HLA-C downregulation by HIV-1 adapts to host HLA genotype

HIV-1 can downregulate HLA-C on infected cells, using the viral protein Vpu, and the magnitude of this downregulation varies widely between primary HIV-1 variants. The selection pressures that result in viral downregulation of HLA-C in some individuals, but preservation of surface HLA-C in others are not clear. To better understand viral immune evasion targeting HLA-C, we have characterized HLA-C downregulation by a range of primary HIV-1 viruses. 128 replication competent viral isolates from 19 individuals with effective anti-retroviral therapy, show that a substantial minority of individuals harbor latent reservoir virus which strongly downregulates HLA-C. Untreated infections display no change in HLA-C downregulation during the first 6 months of infection, but variation between viral quasispecies can be detected in chronic infection. Vpu molecules cloned from plasma of 195 treatment naïve individuals in chronic infection demonstrate that downregulation of HLA-C adapts to host HLA genotype. HLA-C alleles differ in the pressure they exert for downregulation, and individuals with higher levels of HLA-C expression favor greater viral downregulation of HLA-C. Studies of primary and mutant molecules identify 5 residues in the transmembrane region of Vpu, and 4 residues in the transmembrane domain of HLA-C, which determine interactions between Vpu and HLA. The observed adaptation of Vpu-mediated downregulation to host genotype indicates that HLA-C alleles differ in likelihood of mediating a CTL response that is subverted by viral downregulation, and that preservation of HLA-C expression is favored in the absence of these responses. Finding that latent reservoir viruses can downregulate HLA-C could have implications for HIV-1 cure therapy approaches in some individuals.

Analysis of the CD8+ T cell anti-HIV activity in heterologous cell co-cultures reveals the benefit of multiple HLA class I matches

CD8⁺ T lymphocytes can reduce the production of human immunodeficiency virus 1 (HIV-1) by CD4⁺ T cells by cytotoxic and non-cytotoxic mechanisms. To investigate the involvement of human leukocyte antigen (HLA) class I compatibility in anti-HIV responses, we co-cultured primary CD8⁺ T cells, isolated from the peripheral blood of HIV-1-infected individuals, with panels of autologous and heterologous acutely HIV-1-infected primary CD4⁺ T cells. Altogether, CD8⁺ T cell anti-HIV activity was evaluated in more than 200 co-cultures. Marked heterogeneity in HIV-1 replication levels was observed among the co-cultures sharing a common CD8⁺ T cell source. The co-cultures that exhibited greater than 50% reduction in HIV production were found to have significantly increased numbers of matching HLA class I alleles (Yates chi-square = 54.21; p < 0.001). With CD8⁺ T cells from HIV controllers and asymptomatic viremic individuals, matching HLA-B and/or HLA-C alleles were more predictive of strong anti-HIV activity than matching HLA-A alleles. Overall, HLA class I genotype matches were more closely associated with CD8⁺ T cell anti-HIV activity than supertype pairings. Antibodies against HLA class I and CD3 reduced the CD8⁺ T cell anti-HIV activity. Stimulated CD8⁺ T cells exhibited increased anti-HIV activity and reduced dependency on HLA compatibility. These findings provide evidence that the maximal suppression of HIV replication by CD8⁺ T cells requires the recognition of multiple epitopes. These studies provide insight for HIV vaccine development, and the analytic approach can be useful for the functional characterization of HLA class I alleles and tentative HLA class I supertypes.

Cytotoxic potential of decidual NK cells and CD8+ T cells awakened by infections

To establish a healthy pregnancy the maternal immune system must tolerate fetal allo-antigens, yet remain competent to respond to infections. The ability of decidual NK cells (dNK) to promote migration of fetal extravillous trophoblasts (EVT) and placental growth as well as the capacity of EVT to promote immune tolerance are topics of high interest and extensive research. However, the problem of how dNK and decidual CD8+ T cells (CD8+ dT) provide immunity to infections of the placenta and the mechanisms that regulate their cytolytic function has thus far largely been ignored. Fetal EVT are the most invasive cells of the placenta and directly interact with maternal decidual immune cells at this maternal-fetal interface. Besides the expression of non-polymorphic HLA-E and HLA-G molecules that are associated with immune tolerance, EVT also express highly polymorphic HLA-C molecules that can serve as targets for maternal dNK and CD8+ dT responses. HLA-C expression by EVT has a dual role as the main molecule to which immune tolerance needs to be established and as the only molecule that can present pathogen-derived peptides and provide protective immunity when EVT are infected. The focus of this review is to address the regulation of cytotoxicity of dNK and CD8+ dT, which is essential for maternal-fetal immune tolerance as well as recent evidence that both cell types can provide immunity to infections at the maternal-fetal interface. A particular emphasis is given to the role of HLA-C expressed by EVT and its capacity to elicit dNK and CD8+ dT responses.

HIV-1 Control by NK Cells via Reduced Interaction between KIR2DL2 and HLA-C∗12:02/C∗14:03

Natural killer (NK) cells control viral infection in part through the interaction between killer cell immunoglobulin-like receptors (KIRs) and their human leukocyte antigen (HLA) ligands. We investigated 504 anti-retroviral (ART)-free Japanese patients chronically infected with HIV-1 and identified two KIR/HLA combinations, KIR2DL2/HLA-C∗12:02 and KIR2DL2/HLA-C∗14:03, that impact suppression of HIV-1 replication. KIR2DL2+ NK cells suppressed viral replication in HLA-C∗14:03+ or HLA-C∗12:02+ cells to a significantly greater extent than did KIR2DL2- NK cells in vitro. Functional analysis showed that the binding between HIV-1-derived peptide and HLA-C∗14:03 or HLA-C∗12:02 influenced KIR2DL2+ NK cell activity through reduced expression of the peptide-HLA (pHLA) complex on the cell surface (i.e., reduced KIR2DL2 ligand expression), rather than through reduced binding affinity of KIR2DL2 to the respective pHLA complexes. Thus, KIR2DL2/HLA-C∗12:02 and KIR2DL2/HLA-C∗14:03 compound genotypes have protective effects on control of HIV-1 through a mechanism involving KIR2DL2-mediated NK cell recognition of virus-infected cells, providing additional understanding of NK cells in HIV-1 infection.

HIV and host immunogenetics: unraveling the role of HLA-C

Host genetic factors play a major role in determining the outcome of many infections including human immunodeficiency virus (HIV). Multiple host factors have been studied till date showing their varied role in susceptibility or resistance to HIV infection. HLA-C, however, has been recently started gaining interest in researchers mind revealing its polymorphisms to have an important effect on viral load set-points, disease progression as well as transmission. In this review report, we have compiled these significant findings of HLA-C in HIV infection, in an attempt to highlight the need for further research in the area in different ethnic population to establish its role in the infection.

HIV-1 Vpu Mediates HLA-C Downregulation

Many pathogens evade cytotoxic T lymphocytes (CTLs) by downregulating HLA molecules on infected cells, but the loss of HLA can trigger NK cell-mediated lysis. HIV-1 is thought to subvert CTLs while preserving NK cell inhibition by Nef-mediated downregulation of HLA-A and -B but not HLA-C molecules. We find that HLA-C is downregulated by most primary HIV-1 clones, including transmitted founder viruses, in contrast to the laboratory-adapted NL4-3 virus. HLA-C reduction is mediated by viral Vpu and reduces the ability of HLA-C restricted CTLs to suppress viral replication in CD4+ cells in vitro. HLA-A/B are unaffected by Vpu, and primary HIV-1 clones vary in their ability to downregulate HLA-C, possibly in response to whether CTLs or NK cells dominate immune pressure through HLA-C. HIV-2 also suppresses HLA-C expression through distinct mechanisms, underscoring the immune pressure HLA-C exerts on HIV. This viral immune evasion casts new light on the roles of CTLs and NK cells in immune responses against HIV.

CD8+ effector T cells at the fetal-maternal interface, balancing fetal tolerance and antiviral immunity

During pregnancy CD8+ effector T cells need optimal immune regulation to prevent a detrimental response to allogeneic fetal cells while providing immune protection to infections. A significant proportion of (prospective) mothers carry naïve or memory CD8+ T cells with a TCR that can directly bind to paternal MHC molecules. In addition, a high percentage of pregnant women develop specific T cell responses to fetal minor histocompatibility antigens (mHags). Under normal conditions, fetal-maternal MHC and mHag mismatches lead to elevated lymphocyte activation but do not induce pregnancy failure. Furthermore, viral infections alter the maternal CD8+ T cell response by changing the CD8+ T cell repertoire and increasing the influx of CD8+ T cells to decidual tissue. The normally high T cell activation threshold at the fetal-maternal interface may prevent efficient clearance of viral infections. Conversely, the increased inflammatory response due to viral infections may break fetal-maternal tolerance and lead to pregnancy complications. The aim of this review is to discuss the recent studies of CD8+ T cells in pregnancy, identify potential mechanisms for antigen-specific immune recognition of fetal extravillous trophoblast (EVT) cells by CD8+ T cells, and discuss the impact of viral infections and virus-specific CD8+ T cells during pregnancy.

The effect of HLA polymorphisms on the recognition of Gag epitopes in HIV-1 CRF01_AE infection

Introduction

The design of a globally effective vaccine rests on the identification of epitopes capable of eliciting effective cytotoxic T lymphocyte (CTL) responses across multiple HIV clades in different populations. This study aims to discern the effect of HLA polymorphisms and the cross-clade reactivity or clade-specificity of epitopes in Thailand where HIV-1 CRF01_AE is circulating.

Materials and Methods

14 peptides based on consensus HIV-1 CRF01_AE amino acid sequences were designed for use in IFN-γ ELISpot assays and ⁵¹Cr release assays among 66 HIV-1 CRF01_AE-infected Thai patients. For ELISpot responders carrying HLA alleles currently unknown to restrict CRF01_AE epitopes, in silico epitope-HLA prediction was performed.

Results

29/66 (43.9%) patients recognized at least one peptide. In total 79 responses were seen against all 14 peptides. 28/79 (35.4%) of the responses were in patients with HLA alleles previously reported to restrict CRF01_AE epitopes, 24/79 (30.4%) responses were in individuals with HLA alleles previously reported to restrict epitopes of HIV clades other than CRF01_AE, and the remaining 27/79 (34.2%) responses were not associated with HLA alleles previously known to restrict HIV epitopes. In silico epitope prediction detected 19 novel, epitope-HLA combinations, and 11/19 (57.9%) were associated with HLA-C alleles. We further confirmed a novel HLA restriction of a previously identified HIV-1 Gag epitope [p24_122–130: PPIPVGDIY (PY9)] by HLA-B*40:01 with a standard ⁵¹Cr release assay.

Discussion

CTL recognition sites in HIV-1 Gag were similar among different clades but the HLA restriction differed in Thai patients. This disparity in HLA restriction along different populations illustrated the importance of clade- and population-specific HLA analysis prior to CTL vaccine design.

HLA-B may be more protective against HIV-1 than HLA-A because it resists negative regulatory factor (Nef) mediated down-regulation

Human leukocyte antigen HLA-B alleles have better protective activity against HIV-1 than HLA-A alleles, possibly due to differences in HLA-restricted HIV-1-specific CD8+ cytotoxic T lymphocyte (CTL) function, but the mechanism is unknown. HIV-1 negative regulatory factor (Nef) mediates down-regulation of surface expression of class I HLA (HLA-I) and may therefore impair immune recognition by CTL. Because of sequence differences in the cytoplasmic domains, HLA-A and -B are down-regulated by Nef but HLA-C and -E are not affected. However, the latter are expressed at low levels and are not of major importance in the CTL responses to HIV-1. Here, we compared the role of the cytoplasmic domains of HLA-A and -B in Nef-mediated escape from CTL. We found HLA-B cytoplasmic domains were more resistant to Nef-mediated down-regulation than HLA-A cytoplasmic domains and demonstrated that these differences affect CTL recognition of virus-infected cells in vitro. We propose that the relative resistance to Nef-mediated down-regulation by the cytoplasmic domains of HLA-B compared with HLA-A contributes to the better control of HIV-1 infection associated with HLA-B-restricted CTLs.

Long-term persistence of limited HTLV-I Tax-specific cytotoxic T cell clones in a patient with adult T cell leukemia/lymphoma after allogeneic stem cell transplantation

PURPOSE

Adult T cell leukemia/lymphoma (ATL) is a highly aggressive malignancy of T cells caused by human T cell lymphotropic virus type 1 (HTLV-1). Recent clinical studies have suggested that allogeneic stem cell transplantation (HSCT) improves the clinical course of ATL by harnessing a graft-versus-ATL effect, and that donor-derived HTLV-1 Tax-specific CD8(+) cytotoxic T cells (CTLs) contribute to the graft-versus-ATL effect after HSCT. However, little is known about the immunological characteristics of Tax-specific CTLs in ATL patients who underwent HSCT.

METHODS

We serially analyzed frequencies, differentiation, functions and clonal dynamics of Tax-specific CTLs in paired samples of peripheral blood (PB) and bone marrow (BM) from an ATL patient after HSCT at the single-cell level. We used flowcytometric and single-cell T cell receptor (TCR) repertoire analysis methods without culture steps.

RESULTS

Donor-derived Tax-specific CTLs effectively suppressed HTLV-1 replication in both PB and BM at least during chronic graft-versus-host disease after HSCT. Furthermore, Tax-specific CTLs had comparable properties between BM and PB, except for preferential accumulation in BM rather than PB. Tax-specific CTLs persistently existed as less-differentiated CD45RA(-)CCR7(-) effector memory CTLs based on predominant phenotypes of CD27(+), CD28(+/-) and CD57(+/-). Our approach using single-cell TCR repertoire analysis method showed highly restricted oligoclonal responses of Tax-specific CTLs, and TCR BV7- or BV30- expressing two predominant CTL clones persistently existed and maintained strong cytotoxic activities against HTLV-1 in both PB and BM over three years after HSCT.

CONCLUSIONS

These findings about Tax-specific CTLs provide insights into future directions for studies on immunotherapy against ATL.

Interplay between HIV-1 and Host Genetic Variation: A Snapshot into Its Impact on AIDS and Therapy Response

As of February 2012, 50 circulating recombinant forms (CRFs) have been reported for HIV-1 while one CRF for HIV-2. Also according to HIV sequence compendium 2011, the HIV sequence database is replete with 414,398 sequences. The fact that there are CRFs, which are an amalgamation of sequences derived from six or more subtypes (CRF27_cpx (cpx refers to complex) is a mosaic with sequences from 6 different subtypes besides an unclassified fragment), serves as a testimony to the continual divergent evolution of the virus with its approximate 1% per year rate of evolution, and this phenomena per se poses tremendous challenge for vaccine development against HIV/AIDS, a devastating disease that has killed 1.8 million patients in 2010. Here, we explore the interaction between HIV-1 and host genetic variation in the context of HIV/AIDS and antiretroviral therapy response.

HLA-C and HIV-1: friends or foes?

The major histocompatibility complex class I protein HLA-C plays a crucial role as a molecule capable of sending inhibitory signals to both natural killer (NK) cells and cytotoxic T lymphocytes (CTL) via binding to killer cell Ig-like receptors (KIR). Recently HLA-C has been recognized as a key molecule in the immune control of HIV-1. Expression of HLA-C is modulated by a microRNA binding site. HLA-C alleles that bear substitutions in the microRNA binding site are more expressed at the cell surface and associated with the control of HIV-1 viral load, suggesting a role of HLA-C in the presentation of antigenic peptides to CTLs. This review highlights the role of HLA-C in association with HIV-1 viral load, but also addresses the contradiction of the association between high cell surface expression of an inhibitory molecule and strong cell-mediated immunity. To explore additional mechanisms of control of HIV-1 replication by HLA-C, we address specific features of the molecule, like its tendency to be expressed as open conformer upon cell activation, which endows it with a unique capacity to associate with other cell surface molecules as well as with HIV-1 proteins.

High frequency of HIV mutations associated with HLA-C suggests enhanced HLA-C-restricted CTL selective pressure associated with an AIDS-protective polymorphism

Delayed HIV-1 disease progression is associated with a single nucleotide polymorphism upstream of the HLA-C gene that correlates with differential expression of the HLA-C Ag. This polymorphism was recently shown to be a marker for a protective variant in the 3'UTR of HLA-C that disrupts a microRNA binding site, resulting in enhanced HLA-C expression at the cell surface. Whether individuals with "high" HLA-C expression show a stronger HLA-C-restricted immune response exerting better viral control than that of their counterparts has not been established. We hypothesized that the magnitude of the HLA-C-restricted immune pressure on HIV would be greater in subjects with highly expressed HLA-C alleles. Using a cohort derived from a unique narrow source epidemic in China, we identified mutations in HIV proviral DNA exclusively associated with HLA-C, which were used as markers for the intensity of the immune pressure exerted on the virus. We found an increased frequency of mutations in individuals with highly expressed HLA-C alleles, which also correlated with IFN-γ production by HLA-C-restricted CD8(+) T cells. These findings show that immune pressure on HIV is stronger in subjects with the protective genotype and highlight the potential role of HLA-C-restricted responses in HIV control. This is, to our knowledge, the first in vivo evidence supporting the protective role of HLA-C-restricted responses in nonwhites during HIV infection.

The emerging role of HLA-C in HIV-1 infection

Recently, genome-wide association studies have identified the major histocompatibility complex class I protein HLA-C as an important molecule that affects HIV disease progression. The association between HLA-C and HIV disease outcome was originally determined through a single nucleotide polymorphism (SNP) 35 kb upstream of the HLA-C locus. More recent work has focused on elucidating the functional significance of the -35 SNP, and several groups now have demonstrated HLA-C surface expression to be a key element in control of HIV viral load, with higher surface expression associating with slower disease progression. Most recently, control of HLA-C surface expression has been correlated with the presence of microRNA binding sites that affect HLA-C expression and control of HIV disease. This review highlights these results and explores the ways in which HLA-C surface expression could affect immune system function in the setting of HIV disease.

Maternal KIR and fetal HLA-C: a fine balance

NK cell effector function is regulated by a range of activating and inhibitory receptors, and many of their known ligands are MHC class I molecules. Human NK receptors encoded by the Killer immunoglobulin-like receptor (KIR) gene family recognize polymorphic HLA-C as well as some HLA-A and HLA-B molecules. KIRs are expressed by uterine NK (uNK) cells, which are distinctive NK cells directly in contact with the invading fetal placental cells that transform the uterine arteries during the first trimester. Trophoblast cells express both maternal and paternal HLA-C allotypes and can therefore potentially interact with KIRs expressed by uNK. Therefore, allorecognition of paternal HLA-C by maternal KIR might influence trophoblast invasion and vascular remodeling, with subsequent effects on placental development and the outcome of pregnancy. We discuss here the studies relating to KIR/HLA-C interactions with an emphasis on how these function during pregnancy to regulate placentation.

KIR/HLA interactions and pathogen immunity

The innate immune system is the first line of defence in response to pathogen infection. Natural killer (NK) cells perform a vital role in this response with the ability to directly kill infected cells, produce cytokines, and cross-talk with the adaptive immune system. These effector functions are dependent on activation of NK cells which is determined by surface receptor interactions with ligands on target cells. Of these receptors, the polymorphic killer immunoglobulin-like receptors (KIRs), which interact with MHC class 1 (also highly polymorphic), are largely inhibitory, and exhibit substantial genetic diversity. The result is a significant variation of NK cell repertoire between individuals and also between populations, with a multitude of possible KIR:HLA combinations. As each KIR:ligand interaction may have differential effects on NK cell activation and inhibition, this diversity has important potential influences on the host response to infections. Genetic studies have demonstrated associations between specific KIR:ligand combinations and the outcome of viral (and other) infections, in particular hepatitis C and HIV infection. Detailed functional studies are not required to define the mechanisms underpinning these disease associations.

HLA-C as a mediator of natural killer and T-cell activation: spectator or key player?

The biochemical properties of the HLA-C antigen differ substantially from those of HLA-A and -B molecules. For this reason, HLA-C diversity and expression at the cell surface are much lower than its counterparts and in consequence HLA-C-restricted responses have been infrequently detected and described. In this review we summarise the key differences between HLA-C and other class I molecules and provide an update on natural killer and T-cell responses restricted by HLA-C. We also discuss the different clinical settings associated with HLA-C alleles which mainly consist of autoimmune disorders, cancers and chronic infections.

HLA/KIR restraint of HIV: surviving the fittest

Multiple epidemiological studies have demonstrated associations between the human leukocyte antigen (HLA) loci and human immunodeficiency virus (HIV) disease, and more recently the killer cell immunoglobulin-like (KIR) locus has been implicated in differential responses to the virus. Genome-wide association studies have convincingly shown that the HLA class I locus is the most significant host genetic contributor to the variation in HIV control, underscoring a central role for CD8 T cells in resistance to the virus. However, both genetic and functional data indicate that part of the HLA effect on HIV is due to interactions between KIR and HLA genes, also implicating natural killer cells in defense against viral infection and viral expansion prior to initiation of an adaptive response. We review the HLA and KIR associations with HIV disease and the progress that has been made in understanding the mechanisms that explain these associations.

Antibodies and lentiviruses that specifically recognize a T cell epitope derived from HIV-1 Nef protein and presented by HLA-C

HIV selectively downregulates HLA-A and -B from the surfaces of infected cells to avoid detection by the immune system. In contrast, the HLA-C molecules are highly resistant to this downregulation. High expression level of HLA-C on the cell surface, which correlates with a single nucleotide polymorphism, is also associated with lower viral loads and slower progression to AIDS. These findings strongly suggest that HIV-1-derived peptides are efficiently presented by HLA-C and trigger the elimination of infected cells. Accordingly, the ability to detect these HLA-C-peptide complexes may be used for therapeutic targeting of HIV-1-infected cells and for measuring effective presentation of vaccine candidates after immunization with HIV-1-related proteins or genes. However, low level of HLA-C expression on the cell surface has impeded the development of such complex-recognizing reagents. In this study, we describe the development of a high-affinity human Ab that specifically interacts, at low pM concentrations, with a conserved viral T cell epitope derived from HIV-1 Nef protein and presented by HLA-C. The human Ab selectively detects this complex on different cells and does not interact with a control complex that differed only in the presented peptide. Engineering lentiviruses to display this Ab endowed them with the same specificity as the Ab, whereas coexpressing the Ab and Fas ligand enables the lentiviruses to kill specifically Nef-presenting cells. Abs and pseudoviruses with such specificity are likely to be highly valuable as building blocks for specific targeting and killing of HIV-1-infected cells.