Nancy B. Myers

Antigen-specific cytotoxic T lymphocytes protect against lethal West Nile virus encephalitis.

Infection with West Nile virus (WNV) causes fatal encephalitis in immunocompromised animals. Previous studies in mice have established that T cell protection is required for clearance of WNV infection from tissues and preventing viral persistence. The current study assessed whether specific WNV peptide epitopes could elicit a cytotoxic T lymphocyte (CTL) response capable of protecting against virus infection. Hidden Markov model analysis was used to identify WNV‐encoded peptides that bound the MHC class I proteins Kb or Db. Of the 35 peptides predicted to bind MHC class I molecules, one immunodominant CTL recognition peptide was identified in each of the envelope and non‐structural protein 4B genes. Addition of these but not control peptides to CD8+ T cells from WNV‐infected mice induced IFN‐γ production. CTL clones that were generated ex vivo lysed peptide‐pulsed or WNV‐infected target cells in an antigen‐specific manner. Finally, adoptive transfer of a mixture of envelope‐ and non‐structural protein 4B‐specific CTL to recipient mice protected against lethal WNV challenge. Based on this, we conclude that CTL responses against immundominant WNV epitopes confer protective immunity and thus should be targets for inclusion in new vaccines.

Kb, Kd, and Ld Molecules Share Common Tapasin Dependencies as Determined Using a Novel Epitope Tag

The endoplasmic reticulum protein tapasin is considered to be a class I-dedicated chaperone because it facilitates peptide loading by proposed mechanisms such as peptide editing, endoplasmic reticulum retention of nonpeptide-bound molecules, and/or localizing class I near the peptide source. Nonetheless, the primary functions of tapasin remain controversial as do the relative dependencies of different class I molecules on tapasin for optimal peptide loading and surface expression. Tapasin dependencies have been addressed in previous studies by transfecting different class I alleles into tapasin-deficient LCL721.220 cells and then monitoring surface expression and Ag presentation to T cells. Indeed, by these criteria, class I alleles have disparate tapasin-dependencies. In this study, we report a novel and more direct method of comparing tapasin dependency by monitoring the ratio of folded vs open forms of the different mouse class I heavy chains, Ld, Kd, and Kb. Furthermore, we determine the amount of de novo heavy chain synthesis required to attain comparable expression in the presence vs absence of tapasin. Our findings show that tapasin dramatically improves peptide loading of all three of these mouse molecules.

Endogenous MHC-Related Protein 1 Is Transiently Expressed on the Plasma Membrane in a Conformation That Activates Mucosal-Associated Invariant T Cells

The development of mucosal-associated invariant T (MAIT) cells is dependent upon the class Ib molecule MHC-related protein 1 (MR1), commensal bacteria, and a thymus. Furthermore, recent studies have implicated MR1 presentation to MAIT cells in bacteria recognition, although the mechanism remains undefined. Surprisingly, however, surface expression of MR1 has been difficult to detect serologically, despite ubiquitous detection of MR1 transcripts and intracellular protein. In this article, we define a unique mAb capable of stabilizing endogenous mouse MR1 at the cell surface, resulting in enhanced mouse MAIT cell activation. Our results demonstrated that under basal conditions, endogenous MR1 transiently visits the cell surface, thus reconciling the aforementioned serologic and functional studies. Furthermore, using this approach, double-positive thymocytes, macrophages, and dendritic cells were identified as potential APCs for MAIT cell development and activation. Based on this pattern of MR1 expression, it is intriguing to speculate that constitutive expression of MR1 may be detrimental for maintenance of immune homeostasis in the gut and/or detection of pathogenic bacteria in mucosal tissues.

Association of ERp57 with Mouse MHC Class I Molecules Is Tapasin Dependent and Mimics That of Calreticulin and not Calnexin

Before peptide binding in the endoplasmic reticulum, the class I heavy (H) chain-β2-microglobulin complexes are detected in association with TAP and two chaperones, TPN and CRT. Recent studies have shown that the thiol-dependent reductase, ERp57, is also present in this peptide-loading complex. However, it remains controversial whether the association of ERp57 with MHC class I molecules precedes their combined association with the peptide-loading complex or whether ERp57 only associates with class I molecules in the presence of TPN. Resolution of this controversy could help determine the role of ERp57 in class I folding and/or assembly. To define the mouse class I H chain structures involved in interaction with ERp57, we tested chaperone association of Ld mutations at residues 134 and 227/229 (previously implicated in TAP association), residues 86/88 (which ablate an N-linked glycan), and residue 101 (which disrupts a disulfide bond). The association of ERp57 with each of these mutant H chains showed a complete concordance with CRT, TAP, and TPN but not with calnexin. Furthermore, ERp57 failed to associate with H chain in TPN-deficient .220 cells. These combined data demonstrate that, during the assembly of the peptide-loading complex, the association of ERp57 with mouse class I is TPN dependent and parallels that of CRT and not calnexin.

A Single Peptide–MHC Complex Positively Selects a Diverse and Specific CD8 T Cell Repertoire

Goldilocks Immunology T cells are carefully calibrated in the thymus to react to invading pathogens and to ignore the self. This occurs through interactions between the T cell receptor and major histocompatibility complexes (MHCs) expressing self-peptides. A Goldilocks-like selection process is carried out whereby T cells that do not react or react too strongly to self-peptide MHCs are deleted, whereas those with interactions that are “just right” are allowed to survive. The result is T cells highly specific for a particular foreign peptide-MHC complex. Receipt of survival signals from “just-right” interactions (positive selection) and deletion of cells that are too reactive (negative selection) are spatially and temporally segregated in the thymus, and it is unclear at which stage T cells acquire their high degree of peptide-MHC specificity. By using mice expressing a single peptide-MHC complex, Wang et al. (p. 871) now show that this single complex is sufficient for selection of a CD8+ T cell repertoire with a broad range of specificity. Importantly, recognition of peptide MHC by these cells was highly specific, demonstrating that peptide-MHC specificity is acquired during positive selection in the thymus. Positive selection by a single peptide-MHC complex imparts exquisite specificity to developing T cells. Pathogen recognition by T cells is dependent on their exquisite specificity for self–major histocompatibility complex (MHC) molecules presenting a bound peptide. Although this specificity results from positive and negative selection of developing T cells in the thymus, the relative contribution of these two processes remains controversial. To address the relation between the selecting peptide-MHC complex and the specificity of mature T cells, we generated transgenic mice that express a single peptide–MHC class I complex. We demonstrate that positive selection of CD8 T cells in these mice results in an MHC-specific repertoire. Although selection on a single complex is peptide promiscuous, mature T cells are highly peptide specific. Thus, positive selection imparts MHC and peptide specificity on the peripheral CD8 T cell repertoire.

Physical Association of the K3 Protein of Gamma-2 Herpesvirus 68 with Major Histocompatibility Complex Class I Molecules with Impaired Peptide and β2-Microglobulin Assembly

ABSTRACT To persist in the presence of an active immune system, viruses encode proteins that decrease expression of major histocompatibility complex class I molecules by using a variety of mechanisms. For example, murine gamma-2 herpesvirus 68 expresses the K3 protein, which causes the rapid turnover of nascent class I molecules. In this report we show that certain mouse class I alleles are more susceptible than others to K3-mediated down regulation. Prior to their rapid degradation, class I molecules in K3-expressing cells exhibit impaired assembly with β2-microglobulin. Furthermore, K3 is detected predominantly in association with class I molecules lacking assembly with high-affinity peptides, including class I molecules associated with the peptide loading complex TAP/tapasin/calreticulin. The detection of K3 with class I assembly intermediates raises the possibility that molecular chaperones involved in class I assembly are involved in K3-mediated class I regulation.

The Peptide-Receptive Transition State of MHC Class I Molecules: Insight from Structure and Molecular Dynamics

MHC class I (MHC-I) proteins of the adaptive immune system require antigenic peptides for maintenance of mature conformation and immune function via specific recognition by MHC-I–restricted CD8+ T lymphocytes. New MHC-I molecules in the endoplasmic reticulum are held by chaperones in a peptide-receptive (PR) transition state pending release by tightly binding peptides. In this study, we show, by crystallographic, docking, and molecular dynamics methods, dramatic movement of a hinged unit containing a conserved 310 helix that flips from an exposed “open” position in the PR transition state to a “closed” position with buried hydrophobic side chains in the peptide-loaded mature molecule. Crystallography of hinged unit residues 46–53 of murine H-2Ld MHC-I H chain, complexed with mAb 64-3-7, demonstrates solvent exposure of these residues in the PR conformation. Docking and molecular dynamics predict how this segment moves to help form the A and B pockets crucial for the tight peptide binding needed for stability of the mature peptide-loaded conformation, chaperone dissociation, and Ag presentation.

Applications of major histocompatibility complex class I molecules expressed as single chains.

Generation of CD8 T-cell responses to pathogens and tumors requires optimal expression of class I major histocompatibility complex/peptide complexes, which, in turn, is dependent on host cellular processing events and subject to interference by pathogens. To create a stable structure that is more immunogenic and resistant to immune evasion pathways, we have engineered class I molecules as single-chain trimers (SCTs), with flexible linkers connecting peptide, β2m, and heavy chain. Herein we extend our earlier studies with SCTs to the Kb ligand derived from vesicular stomatitis virus (VSV) to characterize further SCTs as probes of immune function as well as their potential in immunotherapy. The VSVp-β2m-Kb SCTs were remarkably stable at the cell surface, and immunization with DNA encoding SCTs elicited complex-specific antibody. In addition, SCTs were detected by cytotoxic T-lymphocytes specific for the native molecule, and the covalently bound peptide was highly resistant to displacement by exogenous peptide. SCTs can also prime CD8 T-cells in vivo that recognize the native molecule. Furthermore, SCTs were resistant to downregulation by the immune evasion protein mK3 of γherpesvirus 68. Moreover, owing to their preassembled nature, SCTs should be resistant to other immune evasion proteins that restrict peptide supply. Thus, SCTs possess therapeutic potential both for prophylactic treatment and for the treatment of ongoing infection.

Polymorphism at Position 97 in MHC Class I Molecules Affects Peptide Specificity, Cell Surface Stability, and Affinity for β2-Microglobulin

The two mouse MHC class I alleles, Ld and Lq, share complete amino acid sequence identity except in the α2 domain, where they differ at six positions. Despite their similarity, Lq has a stronger association with β2-microglobulin (β2m), is expressed at higher levels on the cell surface, demonstrates an increased cell surface half-life, and has fewer open forms on the cell surface than Ld. To determine the basis for their phenotypic differences, Ld molecules containing chimeric Ld-Lq α2 domains were characterized, and these analyses implicated residue 97 (LdTrp and LqArg) as the polymorphic site responsible for the disparity in β2m association between the two alleles. Single substitution analysis at this site (LdW97R and LqR97W) confirmed this. Furthermore, the LdW97R mutant molecule has a longer cell surface half-life than either Lq or Ld, and fewer open forms of LdW97R are observed on the cell surface. In addition, both LdW97R and Lq possess decreased binding affinity for the Ld-restricted tum− P91A14–22 peptide compared with Ld. Collectively, these results and the known location of Trp97 in the peptide binding cleft of Ld strongly suggest that the substitution of Arg for Trp97 in Ld alters the peptide binding cleft, increasing its affinity for endogenous peptides, which results in greater cell surface stability and better retention of β2m. Furthermore, these results imply that Trp97 plays an important role in the ability of Ld to efficiently participate in alternative MHC class I Ag presentation pathways.

Peptide Length Variants p2Ca and QL9 Present Distinct Conformations to L d -Specific T Cells

Recent advances have provided insights into how the TCR interacts with MHC/peptide complexes and a rationale to predict optimal epitopes for MHC binding and T cell recognition. For example, peptides of nine residues are predicted to be optimal for binding to H2-Ld, although 8 mer epitopes have also been identified. It has been predicted that 8 mer and 9 mer length variant peptides bound to Ld present identical epitopes to T cells. However, in contrast to this prediction, we demonstrate here that the 8 mer peptide p2Ca and its 9 mer length variant QL9, extended by an N-terminal glutamine, assume distinct conformations when bound to Ld. We generated self-Ld-restricted CTL clones specific for p2Ca that recognize Ld/QL9 poorly if at all. This result is in sharp contrast to what has been observed with Ld-alloreactive T cells that possess a much higher affinity for Ld/QL9 than for Ld/p2Ca. Alanine substitutions of the N-terminal residues of the QL9 peptide rescue detection by these self-Ld/p2Ca-specific T cells, but decrease recognition by the Ld-alloreactive 2C T cell clone. In addition, 2C T cell recognition of the p2Ca peptide is affected by different alanine substitutions compared with 2C T cell recognition of the QL9 peptide. These data clearly demonstrate that the p2Ca and QL9 peptides assume distinct conformations when bound to Ld and, furthermore, demonstrate that there is flexibility in peptide binding within the MHC class I cleft.