Multiple Isomers of Photolumazine V Bind MR1 and Differentially Activate MAIT Cells

In response to microbial infection, the nonclassical Ag-presenting molecule MHC class I-related protein 1 (MR1) presents secondary microbial metabolites to mucosal-associated invariant T (MAIT) cells. In this study, we further characterize the repertoire of ligands captured by MR1 produced in Hi5 (Trichoplusia ni) cells from Mycobacterium smegmatis via mass spectrometry. We describe the (to our knowledge) novel MR1 ligand photolumazine (PL)V, a hydroxyindolyl-ribityllumazine with four isomers differing in the positioning of a hydroxyl group. We show that all four isomers are produced by M. smegmatis in culture and that at least three can induce MR1 surface translocation. Furthermore, human MAIT cell clones expressing distinct TCR β-chains differentially responded to the PLV isomers, demonstrating that the subtle positioning of a single hydroxyl group modulates TCR recognition. This study emphasizes structural microheterogeneity within the MR1 Ag repertoire and the remarkable selectivity of MAIT cell TCRs.

Deaza-modification of MR1 ligands modulates recognition by MR1-restricted T cells

MR1-restricted T (MR1T) cells recognize microbial small molecule metabolites presented on the MHC Class I-like molecule MR1 and have been implicated in early effector responses to microbial infection. As a result, there is considerable interest in identifying chemical properties of metabolite ligands that permit recognition by MR1T cells, for consideration in therapeutic or vaccine applications. Here, we made chemical modifications to known MR1 ligands to evaluate the effect on MR1T cell activation. Specifically, we modified 6,7-dimethyl-8-D-ribityllumazine (DMRL) to generate 6,7-dimethyl-8-D-ribityldeazalumazine (DZ), and then further derivatized DZ to determine the requirements for retaining MR1 surface stabilization and agonistic properties. Interestingly, the IFN-γ response toward DZ varied widely across a panel of T cell receptor (TCR)-diverse MR1T cell clones; while one clone was agnostic toward the modification, most displayed either an enhancement or depletion of IFN-γ production when compared with its response to DMRL. To gain insight into a putative mechanism behind this phenomenon, we used in silico molecular docking techniques for DMRL and its derivatives and performed molecular dynamics simulations of the complexes. In assessing the dynamics of each ligand in the MR1 pocket, we found that DMRL and DZ exhibit differential dynamics of both the ribityl moiety and the aromatic backbone, which may contribute to ligand recognition. Together, our results support an emerging hypothesis for flexibility in MR1:ligand-MR1T TCR interactions and enable further exploration of the relationship between MR1:ligand structures and MR1T cell recognition for downstream applications targeting MR1T cells.

Riboflavin Metabolism Variation among Clinical Isolates of Streptococcus pneumoniae Results in Differential Activation of Mucosal-associated Invariant T Cells

Streptococcus pneumoniae is an important bacterial pathogen that causes a range of noninvasive and invasive diseases. The mechanisms underlying variability in the ability of S. pneumoniae to transition from nasopharyngeal colonization to disease-causing pathogen are not well defined. Mucosal-associated invariant T (MAIT) cells are prevalent in mucosal tissues such as the airways and are believed to play an important role in the early response to infection with bacterial pathogens. The ability of MAIT cells to recognize and contain infection with S. pneumoniae is not known. In the present study, we analyzed MAIT-cell responses to infection with clinical isolates of S. pneumoniae serotype 19A, a serotype linked to invasive pneumococcal disease. We found that although MAIT cells were capable of responding to human dendritic and airway epithelial cells infected with S. pneumoniae, the magnitude of response to different serotype 19A isolates was determined by genetic differences in the expression of the riboflavin biosynthesis pathway. MAIT-cell release of cytokines correlated with differences in the ability of MAIT cells to respond to and control S. pneumoniae in vitro and in vivo in a mouse challenge model. Together, these results demonstrate first that there are genetic differences in riboflavin metabolism among clinical isolates of the same serotype and second that these likely determine MAIT-cell function in response to infection with S. pneumoniae. These differences are critical when considering the role that MAIT cells play in early responses to pneumococcal infection and determining whether invasive disease will develop.

MR1 displays the microbial metabolome driving selective MR1-restricted T cell receptor usage

MR1-restricted T cells (MR1Ts) are a T cell subset that recognize and mediate host defense to a broad array of microbial pathogens, including respiratory pathogens (e.g., Mycobacterium tuberculosis, Streptococcus pyogenes, and Francisella tularensis) and enteric pathogens (e.g., Escherichia coli and Salmonella species). Mucosal-associated invariant T (MAIT) cells, a subset of MR1Ts, were historically defined by the use of a semi-invariant T cell receptor (TCR) and recognition of small molecules derived from the riboflavin biosynthesis pathway presented on MR1. We used mass spectrometry to identify the repertoire of ligands presented by MR1 from the microbes E. coli and Mycobacterium smegmatis. We found that the MR1 ligandome is unexpectedly broad, revealing functionally distinct ligands derived from E. coli and M. smegmatis. The identification, synthesis, and functional analysis of mycobacterial ligands reveal that MR1T ligands can be distinguished by MR1Ts with diverse TCR usage. These data demonstrate that MR1 can serve as an immune sensor of the microbial ligandome.

Variability in German Cockroach Extract Composition Greatly Impacts T Cell Potency in Cockroach-Allergic Donors

German cockroach extract is used clinically to evaluate allergen-specific sensitization and for subcutaneous allergen-specific immunotherapy, though there are no guidelines for standardization in its manufacture. We performed an immunological evaluation of 12 different cockroach extracts prepared from different sources and their potency to induce allergen-specific T cell reactivity. PBMC from 13 cockroach allergic donors were expanded in vitro with 12 different German cockroach extracts. After culture expansion, cells were re-stimulated with the different extracts and T cell responses were assessed by FluoroSpot (IL-5, IFNγ and IL-10 production). In parallel to the extracts, single allergen peptide pools for allergens from groups 1, 2, 4, 5, and 11 were tested to determine allergen immunodominance. Furthermore, to assess allergy specificity, PBMC from 13 non-allergic donors were also tested with the most potent extract and T cell responses were compared to the allergic cohort. Dramatic variations in T cell reactivity were observed to the different cockroach extract batches. Response magnitudes varied over 3 logs within a single donor. IL-5 production in the allergic cohort was significantly higher compared to the non-allergic cohort (p=0.004). Allergen content determination by ELISA detected much lower concentrations of Bla g 5 compared to Bla g 1 and 2. Mass spectrometric analysis revealed that Bla g 5 was present in similar amounts to Bla g 1 and 2 in extracts made from whole body, whereas it was not detected in extracts made from fecal matter, suggesting that Bla g 5 is not excreted into feces. Different donors exhibit different response patterns to different extracts, potentially dependent on the donor-specific T cell allergen immunodominance pattern and the allergen content of the extract tested. These findings have dramatic implications for the selection of potent extracts used for diagnostic purposes or allergen-specific immunotherapy.

Urinary Peptides As a Novel Source of T Cell Allergen Epitopes

Mouse allergy in both laboratory workers and in inner-city children is associated with allergic rhinitis and asthma, posing a serious public health concern. Urine is a major source of mouse allergens, as mice spray urine onto their surroundings, where the proteins dry up and become airborne on dust particles. Here, we tested whether oligopeptides that are abundant in mouse urine may contribute to mouse allergic T cell response. Over 1,300 distinct oligopeptides were detected by mass spectrometry analysis of the low molecular weight filtrate fraction of mouse urine (LoMo). Posttranslationally modified peptides were common, accounting for almost half of total peptides. A pool consisting of 225 unique oligopeptides of 13 residues or more in size identified within was tested for its capacity to elicit T cell reactivity in mouse allergic donors. Following 14-day in vitro stimulation of PBMCs, we detected responses in about 95% of donors tested, directed against 116 distinct peptides, predominantly associated with Th2 cytokines (IL-5). Peptides from non-urine related proteins such as epidermal growth factor, collagen, and Beta-globin accounted for the highest response (15.9, 9.1, and 8.1% of the total response, respectively). Peptides derived from major urinary proteins (MUPs), kidney androgen-regulated protein (KAP), and uromodulin were the main T cell targets from kidney or urine related sources. Further ex vivo analysis of enrichment of 4-1BB expressing cells demonstrated that LoMo pool-specific T cell reactivity can be detected directly ex vivo in mouse allergic but not in non-allergic donors. Further cytometric analysis of responding cells revealed a bone fide memory T cell phenotype and confirmed their Th2 polarization. Overall, these data suggest that mouse urine-derived oligopeptides are a novel target for mouse allergy-associated T cell responses, which may contribute to immunopathological mechanisms in mouse allergy.

T cell recognition of Mycobacterium tuberculosis peptides presented by HLA-E derived from infected human cells

HLA-E is a non-conventional MHC Class I molecule that has been recently demonstrated to present pathogen-derived ligands, resulting in the TCR-dependent activation of αβ CD8+ T cells. The goal of this study was to characterize the ligandome displayed by HLA-E following infection with Mycobacterium tuberculosis (Mtb) using an in-depth mass spectrometry approach. Here we identified 28 Mtb ligands derived from 13 different source proteins, including the Esx family of proteins. When tested for activity with CD8+ T cells isolated from sixteen donors, nine of the ligands elicited an IFN-γ response from at least one donor, with fourteen of 16 donors responding to the Rv0634A19-29 peptide. Further evaluation of this immunodominant peptide response confirmed HLA-E restriction and the presence of Rv0634A19-29-reactive CD8+ T cells in the peripheral blood of human donors. The identification of an Mtb HLA-E ligand that is commonly recognized may provide a target for a non-traditional vaccine strategy.

Immune response in Parkinson’s disease driven by HLA display of α-synuclein peptides

Abnormal processing of self-proteins can produce epitopes presented by major histocompatibility (MHC) proteins to be recognized by specific T cells that escaped central tolerance during thymic selection. Such actions by the acquired immune system are widely held to produce autoimmune disorders such as Type-1 diabetes. While not considered to possess autoimmune features, neurodegenerative diseases are characterized by the altered processing of specific proteins. One of the major pathological features of Parkinson’s disease are the presence of intraneuronal aggregrates known as Lewy bodies and neurites composed of α-synuclein. Genetic studies associate Parkinson’s disease with DRB5*01 and DRB1*15:01 MHC alleles. To address the hypothesis that Parkinson’s disease is associated with T cell recognition of epitopes derived from α-synuclein and presented by specific MHC alleles, we recruited 67 Parkinson’s disease patients and 36 age-matched non-Parkinson’s healthy controls. We found that a defined set of peptides derived from α-synuclein, act as antigenic epitopes displayed by these alleles and drive helper and cytotoxic T cell responses in Parkinson’s disease patients. We identified two antigenic regions in α-synuclein, the first near the N terminus (aa31-46), which was bound by DRB1*15:01 and DRB5*01:01, and the second near the C terminus, which required phosphorylation of an amino acid residue (S129). These α-synuclein epitopes were shown to arise from natural processing of both extracellular native α-synuclein and the fibrilized form associated with Parkinson’s disease pathogenesis. These responses may explain the association of Parkinson’s disease with alleles of the acquired immune system.

Unconventional Peptide Presentation by Major Histocompatibility Complex (MHC) Class I Allele HLA-A*02:01: BREAKING CONFINEMENT

Peptide antigen presentation by major histocompatibility complex (MHC) class I proteins initiates CD8⁺ T cell-mediated immunity against pathogens and cancers. MHC I molecules typically bind peptides with 9 amino acids in length with both ends tucked inside the major A and F binding pockets. It has been known for a while that longer peptides can also bind by either bulging out of the groove in the middle of the peptide or by binding in a zigzag fashion inside the groove. In a recent study, we identified an alternative binding conformation of naturally occurring peptides from Toxoplasma gondii bound by HLA-A*02:01. These peptides were extended at the C terminus (PΩ) and contained charged amino acids not more than 3 residues after the anchor amino acid at PΩ, which enabled them to open the F pocket and expose their C-terminal extension into the solvent. Here, we show that the mechanism of F pocket opening is dictated by the charge of the first charged amino acid found within the extension. Although positively charged amino acids result in the Tyr-84 swing, amino acids that are negatively charged induce a not previously described Lys-146 lift. Furthermore, we demonstrate that the peptides with alternative binding modes have properties that fit very poorly to the conventional MHC class I pathway and suggest they are presented via alternative means, potentially including cross-presentation via the MHC class II pathway.

Toxoplasma gondii peptide ligands open the gate of the HLA class I binding groove

HLA class I presentation of pathogen-derived peptide ligands is essential for CD8+ T-cell recognition of Toxoplasma gondii infected cells. Currently, little data exist pertaining to peptides that are presented after T. gondii infection. Herein we purify HLA-A*02:01 complexes from T. gondii infected cells and characterize the peptide ligands using LCMS. We identify 195 T. gondii encoded ligands originating from both secreted and cytoplasmic proteins. Surprisingly, T. gondii ligands are significantly longer than uninfected host ligands, and these longer pathogen-derived peptides maintain a canonical N-terminal binding core yet exhibit a C-terminal extension of 1–30 amino acids. Structural analysis demonstrates that binding of extended peptides opens the HLA class I F’ pocket, allowing the C-terminal extension to protrude through one end of the binding groove. In summary, we demonstrate that unrealized structural flexibility makes MHC class I receptive to parasite-derived ligands that exhibit unique C-terminal peptide extensions.

DOI:http://dx.doi.org/10.7554/eLife.12556.001

Toxoplasma gondii is a parasite that can infect most warm-blooded animals and cause a disease called toxoplasmosis. In humans, toxoplasmosis generally does not cause any noticeable symptoms, but it can cause serious problems in pregnant women and individuals with weakened immune systems.

T. gondii is one of many parasites that hide within human cells in an attempt to avoid detection by the immune system. However, proteins called Human Leukocyte Antigens, or HLAs, can reveal hidden parasites by carrying small sections of them from the inside the infected cell to the cell’s surface. The immune system can then recognize the fragments as foreign and attack the parasite.

HLAs typically pick up parasite fragments of a certain length, which enables the immune system to recognize that what is being displayed is a piece of parasite. By purifying HLAs from cells that have been infected by T. gondii, McMurtrey et al. have now learned more about which fragments of the parasite are displayed to the immune system. This analysis revealed that the parasite somehow manipulates the HLAs to carry parasite fragments that are considerably longer than can be explained with our current knowledge of how HLAs work. By using a technique called X-ray crystallography, McMurtrey et al. also show that the structure of the HLA assumes a previously unseen configuration when interacting with fragments of T. gondii.

In the future, it will be important to understand how infected cells give rise to unusual structural configurations of HLAs and to unravel how these structures affect the immune system’s ability to fight infections.

DOI:http://dx.doi.org/10.7554/eLife.12556.002

Direct interaction of recombinant TAPBPR with MHC-I molecules: stabilization of peptide-free MHC-I promotes high affinity peptide loading (APP5P.102)

The loading of MHC-I molecules with peptides for cell surface display is a crucial step in self-tolerance and activation of CD8 T cells. We studied TAPBPR (TAP binding protein-related) protein, a tapasin homolog, which is widely expressed and IFN-γ inducible, but is not part of the classical MHC-I peptide-loading complex. We produced recombinant soluble TAPBPR and evaluated its interactions with several recombinant MHC-I molecules in vitro, by gel-shift, size exclusion chromatography, ultracentrifugation, and surface plasmon resonance. We show that TAPBPR binds MHC-I after photolysis of a bound peptide, and that the TAPBPR/MHC-I complex is dissociated by exposure to peptides that bind the MHC-I molecule, indicating a role of TAPBPR in stabilizing a peptide-receptive form of the MHC-I/β2m complex. Peptide-dependent release of MHC-I from TAPBPR is directly proportional to the peptide’s affinity for MHC-I. Peptide binding experiments indicate a role for TAPBPR in selection of high affinity peptides. Mutagenesis of TAPBPR and MHC-I confirm the importance of amino acid residues conserved with the putative tapasin/MHC-I binding site and reveal additional residues important for the TAPBPR/MHC-I interaction. Molecular docking simulations suggest a detailed mechanism for the interaction of TAPBPR with peptide free MHC-I. These studies are consistent with the view that TAPBPR functions as a chaperone that stabilizes peptide-free MHC-I to permit binding of high affinity peptides.

HIV-1 NEF ligands predominate in the HLA class I of infected CD4+ T cells (APP2P.108)

Class I HLA reveal virus-derived ligands to cytotoxic T lymphocytes (CTL) whose function is to eliminate infected cells. The Los Alamos National Laboratory (LANL) database reports numerous HIV-1 CTL epitopes, none of which have elicited protective immunity in vaccine testing. The goal of this study was to determine the number and nature of HIV-1 ligands available for CTL targeting through presentation by the HLA class I of virus-infected cells. Class I presented peptides were recovered from immunoaffinity purified HLA-A*11:01 gathered from HIV-1 (NL4-3)-infected human CD4+ SUP-T1 T cells. These peptides were fractionated by RP-HPLC and mapped by tandem mass spectrometry (MS/MS). Seven HIV-derived peptides were confirmed by MS/MS as unique to infected cells. Twelve LANL HIV epitopes, including the well-studied Gag p24 epitope ACQGVGGPGHK (AK11), were clearly absent from the HLA-A*11:01 of infected cells. Of the 7 HIV-1 ligands found to be presented by A*11:01, 4 were derived from Nef, 2 from Gag, and 1 from Pol. When these 7 ligands were tested for T cell reactivity in a gamma interferon ELISPOT assay using PBMC from HIV-1 infected individuals (including elite controllers), the Nef-derived peptides were found to be highly reactive. These data demonstrate that the class I HLA of HIV-1 infected CD4+ T cells present a handful of viral peptide ligands for recognition by CTL and that reported HIV-1 CTL epitopes might not be presented by the HLA class I of infected cells.

Detection and validation of HLA Class I presented viral epitopes using T cell receptor mimics (APP2P.107)

Class I Human Leukocyte Antigens distinguish healthy cells from infected cells by presenting peptides at the cell surface. Viral epitopes confirmed as unique to infected cells can lead to successful development of vaccines and therapeutics, and a tool to validate viral epitope presentation on a variety of cell lineages is essential. Here, comparative mass spectrometry shows that the West Nile virus peptide epitopes SVGGVFTSV and ILRNPGYAL are presented by HLA-A*02:01 and HLA-B*07:02 of infected cells. To generate monoclonal antibodies against these HLA/WNV peptide epitopes, mice were immunized with peptide/HLA complexes, splenocytes were fused to myeloma cells, and single clones were picked and grown. Hybridoma supernatants were screened for recognition of the appropriate HLA/WNV peptide complex on peptide-pulsed cells with irrelevant HLA/peptides acting as a negative control. The T cell receptor mimic (TCRm) monoclonal antibodies RL15A and RL29A were found to be specific for A*02:01/SVG9 and B*07:02/ILR9, respectively. The RL15A and RL29A T cell receptor mimic mAb were then used to track viral epitope presentation on WNV infected cell lines and primary cells. In summary, we demonstrate the implementation of a mass spectrometry system for the direct discovery of class I HLA-presented viral epitopes from infected cells followed by the complementary use of TCRm mAb as a companion diagnostic for tracking epitope presentation during the course of a viral infection.

Peptide presentation by different HLA-Class I molecules during viral infection (APP2P.106)

Class I Human Leukocyte Antigens reveal intracellular viruses to the immune system by presenting viral epitopes at the cell surface. In this study we hypothesized that during infection particular HLA class I alleles consistently present more virus-derived peptide ligands to virus-specific CD8+ T cells than do other HLA class I. To test this hypothesis, we infected cells with West Nile virus (WNV) and directly identified and enumerated the virus specific peptides presented by various HLA class I molecules. Cells expressing a number of different HLA-A and HLA-B class I were cultured in bioreactors and infected with WNV. HLA/peptide complexes were harvested from infected and uninfected cells, peptides were eluted from affinity-purified HLA, comparatively mapped by mass spectroscopy, and sequenced. Twenty-four peptides, 20 eluted from HLA-A complexes (HLA-A*02:01, A*01:01, A*11:01 and A*24:02) and 4 from HLA-B complexes (HLA-B*27:05, B*07:02 and B*35:01), were identified as unique to infected cells. Ligands represented different parts of the viral polyprotein demonstrating that peptides sampled by class I HLA are distributed widely throughout the WNV proteome. These data indicate that, in WNV-infected cells, HLA-A present more virus-specific peptides than do HLA-B suggesting a potentially different role for HLA class I loci in developing immune responses that control WNV infection whereby HLA-A is responsible for diverse reactivity and HLA-B leads to more focused immunity.

Profiling class II HLA alloantibodies in transplant patient sera (P2222)

Allogeneic immunizing events including pregnancy, blood transfusion, and transplantation promote strong antibody responses to HLA. Such anti-HLA antibodies preclude organ transplantation, foster hyperacute rejection, and contribute to chronic transplant failure. Diagnostic antibody-screening assays can detect alloreactive antibodies and determine their HLA specificity, yet a number of key antibody attributes remain unexplored. The goal here was to provide the first detailed profile of antibodies directed to allogeneic HLA. Methodologically, sixteen sensitized patients were identified with alloantibodies to HLA-DR11, antibodies reactive with this class II HLA antigen were purified using a novel immunoaffinity column constructed with milligram quantities of native DR11, and these purified DR11 specific antibodies were categorized. Results show that allogeneic antibodies to DR11 were found in the serum at a median concentration of 2.3ug/ml and consisted of the IgM, IgG, IgA, and IgE isotypes. IgG2, IgM, and IgE were elevated while IgG1 was decreased in sensitized patients and, in most patients, these antibodies fixed complement. In conclusion, we show that HLA alloantibody responses are consistently comprised of multiple isotypes, that these alloantibodies reach an appreciable serum concentration, and that these alloantibodies fix complement.

Immunodominant and subdominant WNV epitopes are differentially presented (P5005)

There are many factors that are thought to contribute to T-cell immunodominance hierarchies including timing and levels of antigen presentation to T-cells. One of the most potent tools for studying antigen presentation for a specific determinant are monoclonal antibodies that are specific for not only MHC but a specific ligand, similar to a T cell receptor. In this study we utilize these antibodies to study levels and timing of antigen presentation of immunodominant (SVG9), sub-dominant (SLF9), and super sub-dominant (YTM9) T-cell epitopes during West Nile virus (WNV) infection. In this study, we demonstrate that these ligands are differentially expressed on the surface of a cell infected with WNV. Specifically, the super sub-dominant ligand (YTM9) was presented earlier and at 20 times the levels of the immunodominant (SVG9) and subdominant (SLF9) ligands. Transfection of ICP47 in the infected cells show that all three epitopes are TAP dependent. Using these highly specific reagents we show that there is differential ligand presentation within the restrictive allele. High levels of peptide HLA complexes on the surface of an infected do not correspond with immunodominance hierarchies as the super subdominant epitope YTM9 was presented at higher levels than the most dominant epitope SVG9. This differential in presentation was not due the TAP dependence of the ligands.

Classical antigen presentation by HLA-A2 of Mycobacterium tuberculosis derived ligands (106.40)

Mycobacterium tuberculosis (Mtb) remains a world health threat, with 8.8 million cases reported in 2010, 65% of which were new and recurrent cases of tuberculosis . Cellular immunity is critical for controlling Mtb and recent studies identify CD8+ T cells as important players in controlling infection. Cytotoxic T lymphocytes (CTLs) recognize peptide antigen presented by Major Histocompatibility Complex (MHC) Class I molecules and destroy infected cells, therefore discovery of CTL epitopes is of high importance. Using a method of soluble HLA production, we were able to generate soluble HLA-A2 from THP-1 monocyte cell lines transfected with a soluble form of HLA A*02:01. Transfected cells were seeded into a hollow fiber bioreactor and were infected with Mtb or left uninfected. Following production of 25 mgs of HLA-A2 from infected and uninfected cells, peptides were eluted from the supernatants of uninfected and infected cells. Peptides from each group were fractionated using HPLC and analyzed by mass spectrometry. We were able to identify six Mtb derived ligands presented by the MHC Class I molecule HLA-A*0201, two of which reacted well with donor PBMC from patients with both active and latent Mtb infection. Identifying new vaccine candidates is a top priority in the TB field and the reactive Mtb ligands identified here demonstrate that multiple Mtb proteins are processed for presentation by Class I HLA and a fraction of these ligands are immunologically reactive.

Toxoplasma gondii activates hypoxia-inducible factor (HIF) by stabilizing the HIF-1alpha subunit via type I activin-like receptor kinase receptor signaling

Toxoplasma gondii is an intracellular protozoan parasite that can cause devastating disease in fetuses and immune-compromised individuals. We previously reported that the alpha subunit of the host cell transcription factor, hypoxia-inducible factor-1 (HIF-1), is up-regulated by infection and necessary for Toxoplasma growth. Under basal conditions, HIF-1alpha is constitutively expressed but rapidly targeted for proteasomal degradation after two proline residues are hydroxylated by a family of prolyl hydroxylases (PHDs). The PHDs are alpha-ketoglutarate-dependent dioxygenases that have low K(m) values for oxygen, making them important cellular oxygen sensors. Thus, when oxygen levels decrease, HIF-1alpha is not hydroxylated, and HIF-1 is activated. How Toxoplasma activates HIF-1 under normoxic conditions remains unknown. Here, we report that Toxoplasma infection increases HIF-1alpha stability by preventing HIF-1alpha prolyl hydroxylation. Infection significantly decreases PHD2 abundance, which is the key prolyl hydroxylase for regulating HIF-1alpha. The effects of Toxoplasma on HIF-1alpha abundance and prolyl hydroxylase activity require activin-like receptor kinase signaling. Finally, parasite growth is severely diminished when signaling from this family of receptors is inhibited. Together, these data indicate that PHD2 is a key host cell factor for T. gondii growth and represent a novel mechanism by which a microbial pathogen subverts host cell signaling and transcription to establish its replicative niche.

Immune cross recognition of a conserved Flavivirus epitope (130.28)

We previously characterized an immunodomiant, HLA-A*0201 restricted, West Nile virus (WNV) peptide determinant. This dominant WNV peptide epitope, termed SVG9, lies within a highly conserved region of the envelope glycoprotein and demonstrates 100% homology among contemporary WNV strains. This region of the virus envelope is also highly conserved among other Flaviviruses such as Japanese encephalitis virus and Dengue virus (DV). Since immune cross reactivity has been reported within the Dengue virus serogroup, we hypothesized that T cells specific for this A*0201 restricted WNV stretch of envelope would cross-recognize different flaviviruses due to epitope conservation. As an initial test of this hypothesis we synthesized the corresponding envelope peptide for WNV (SVG9) and DV type 1 (SIG9). We utilized a competitive peptide binding assay to demonstrate that SVG9 and SIG9 have comparable and high A*0201 binding affinities. We then utilized PBMC from WNV and DV1 seropositive donors to assess T cell cross recognition of SVG9 and SIG9. T cells specific for the WNV SVG9 were cross-reactive for their DV1 Flaviviral counterpart SIG 9 and vice versa. These data demonstrate that cellular immunity is cross-reactive among different Flaviviruses. This observation has implications for vaccine development and for unraveling the immunopathology associated with infection.

Origin and number of viral peptides sampled by class I MHC following West Nile Virus infection (130.27)

West Nile Virus (WNV) was introduced into the United States in 1999 and it has emerged as the most common cause of arboviral neuroinvasive disease in the U.S. An estimated 1.65 million infections occurred between 1999 and 2008 and there is no specific treatment or vaccine available. WNV-infected cells can be detected and destroyed by the CD8+ cytotoxic lymphocytes via the presentation of viral peptides by major histocompatability complex class I molecules. Currently, no one knows how many WNV derived peptides decorate the MHC of infected cells, nor do we know which viral proteins MHC molecules sample. Understanding the number and origin of viral peptides available for immune recognition is a key prerequisite in the development of antiviral immunotherapeutics. Using an HLA-A and an HLA-B molecule, we previously reported that a given class I MHC molecule samples 2-6 viral peptides. Here we test this observation with additional HLA-A and HLA-B by secreting A*0101 and B*2705 from uninfected cells and then from WNV infected cells. Peptides eluted from the infected and uninfected class I molecules were compared by mass spectrometry, and peptides unique to infected cells were sequenced by tandem mass spectrometry. The resulting data demonstrate that different class I MHC molecules consistently present a like number of WNV peptides for immune recognition and that the viral peptides originate from clusters or “hot spots” within the viral polyprotein.

The memory T cell response to West Nile virus in symptomatic humans following natural infection is not influenced by age and is dominated by a restricted set of CD8+ T cell epitopes

We examined the West Nile virus (WNV)-specific T cell response in a cohort of 52 patients with symptomatic WNV infections, including neuroinvasive and non-invasive disease. Although all virus proteins were shown to contain T cell epitopes, certain proteins, such as E, were more commonly targeted by the T cell response. Most patients exhibited reactivity toward 3-4 individual WNV peptides; however, several patients exhibited reactivity toward >10 individual peptides. The relative hierarchy of T cell reactivities in all patients showed a fixed pattern that was sustained throughout the 12-mo period of the current study. Surprisingly, we did not observe any relationship between age and either the breadth or magnitude of the T cell response following infection. We also did not observe a relationship between disease severity and either the breadth or magnitude of the T cell response. The T cell epitopes were distributed in a non-random fashion across the viral polyprotein and a limited number of epitopes appeared to dominate the CD8(+) T cell response within our cohort. These data provide important new insight into the T cell response against WNV in humans.