Determinants of the endoplasmic reticulum (ER) lumenal-domain of the adenovirus serotype 2 E3-19K protein for association with and ER-retention of major histocompatibility complex class I molecules

Intracellular Sequestration of the NKG2D Ligand MIC B by Species F Adenovirus

The enteric human adenoviruses of species F (HAdVs-F), which comprise HAdV-F40 and HAdV-F41, are significant pathogens that cause acute gastroenteritis in children worldwide. The early transcription unit 3 (E3) of HAdVs-F is markedly different from that of all other HAdV species. To date, the E3 proteins unique to HAdVs-F have not been characterized and the mechanism by which HAdVs-F evade immune defenses in the gastrointestinal (GI) tract is poorly understood. Here, we show that HAdV-F41 infection of human intestinal HCT116 cells upregulated the expression of MHC class I-related chain A (MIC A) and MIC B relative to uninfected cells. Our results also showed that, for MIC B, this response did not however result in a significant increase of MIC B on the cell surface. Instead, MIC B was largely sequestered intracellularly. Thus, although HAdV-F41 infection of HCT116 cells upregulated MIC B expression, the ligand remained inside infected cells. A similar observation could not be made for MIC A in these cells. Our preliminary findings represent a novel function of HAdVs-F that may enable these viruses to evade immune surveillance by natural killer (NK) cells in the infected gut, thereby paving the way for the future investigation of their unique E3 proteins.

Immune evasion by adenoviruses: a window into host-virus adaptation

Human adenoviruses (HAdVs) are widespread pathogens that cause a number of partially overlapping, species-specific infections associated with respiratory, urinary, gastrointestinal, and ocular diseases. The early 3 (E3) region of adenoviruses is highly divergent between different species, and it encodes a multitude of proteins with immunomodulatory functions. The study of genetic diversity in the E3 region offers a unique opportunity to gain insight into how the various HAdVs have evolutionarily adapted in response to the selection pressures exerted by host immune defenses. The objective of this review was to discuss subversion of host antiviral immune responses by HAdVs, with a focus on suppression of MHC class I antigen presentation, as a window into host-HAdV adaptation.

A compendium of adenovirus genetic modifications for enhanced replication, oncolysis, and tumor immunosurveillance in cancer therapy

In this review, we specifically focus on genetic modifications of oncolytic adenovirus 5 (Ad5)-based vectors that enhance replication, oncolysis/spread, and virus-mediated tumor immunosurveillance. The finding of negative regulation of minor core protein V by SUMOylation led to the identification of amino acid residues, which when mutated increase adenovirus replication and progeny yield. Suppression of Dicer and/or RNAi pathway with shRNA or p19 tomato bushy stunt protein also results in significant enhancement of adenovirus replication and progeny yield. Truncation mutations in E3-19K or i-leader sequence or overexpression of adenovirus death protein (ADP) potently increase adenovirus progeny release and spread without affecting virus yield. Moreover, E3-19K protein, which was found to inhibit both major histocompatibility complex I (MHCI) and MHC-I chain-related A and B proteins (MICA/MICB) expression on the cell surface, protecting infected cells from T-lymphocyte and natural killer (NK) cell attack, may be tailored to selectively target only MHCI or MICA/MICB, or to lose the ability to downregulate both. At last, E3-19K protein may be exploited to deliver tumor-associated epitopes directly to the endoplasmic reticulum for loading MHCI in the transporter associated with antigen processing (TAP)-deregulated cells.

Structure of the Adenovirus Type 4 (Species E) E3-19K/HLA-A2 Complex Reveals Species-Specific Features in MHC Class I Recognition

Adenoviruses (Ads) subvert MHC class I Ag presentation and impair host anti-Ad cellular activities. Specifically, the Ad-encoded E3-19K immunomodulatory protein targets MHC class I molecules for retention within the endoplasmic reticulum of infected cells. We report the x-ray crystal structure of the Ad type 4 (Ad4) E3-19K of species E bound to HLA-A2 at 2.64-Å resolution. Structural analysis shows that Ad4 E3-19K adopts a tertiary fold that is shared only with Ad2 E3-19K of species C. A comparative analysis of the Ad4 E3-19K/HLA-A2 structure with our x-ray structure of Ad2 E3-19K/HLA-A2 identifies species-specific features in HLA-A2 recognition. Our analysis also reveals common binding characteristics that explain the promiscuous, and yet high-affinity, association of E3-19K proteins with HLA-A and HLA-B molecules. We also provide structural insights into why E3-19K proteins do not associate with HLA-C molecules. Overall, our study provides new information about how E3-19K proteins selectively engage with MHC class I to abrogate Ag presentation and counteract activation of CD8(+) T cells. The significance of MHC class I Ag presentation for controlling viral infections, as well as the threats of viral infections in immunocompromised patients, underline our efforts to characterize viral immunoevasins, such as E3-19K.

Adenovirus expressing β2-microglobulin recovers HLA class I expression and antitumor immunity by increasing T-cell recognition

Optimal tumor cell surface expression of human leukocyte antigen (HLA) class I molecules is essential for the presentation of tumor-associated peptides to T-lymphocytes. However, a hallmark of many types of tumor is the loss or downregulation of HLA class I expression associated with ineffective tumor antigen presentation to T cells. Frequently, HLA loss can be caused by structural alterations in genes coding for HLA class I complex, including the light chain of the complex, β2-microglobulin (β2m). Its best-characterized function is to interact with HLA heavy chain and stabilize the complex leading to a formation of antigen-binding cleft recognized by T-cell receptor on CD8+ T cells. Our previous study demonstrated that alterations in the β2m gene are frequently associated with cancer immune escape leading to metastatic progression and resistance to immunotherapy. These types of defects require genetic transfer strategies to recover normal expression of HLA genes. Here we characterize a replication-deficient adenoviral vector carrying human β2m gene, which is efficient in recovering proper tumor cell surface HLA class I expression in β2m-negative tumor cells without compromising the antigen presentation machinery. Tumor cells transduced with β2m induced strong activation of T cells in a peptide-specific HLA-restricted manner. Gene therapy using recombinant adenoviral vectors encoding HLA genes increases tumor antigen presentation and represents a powerful tool for modulation of tumor cell immunogenicity by restoration of missing or altered HLA genes. It should be considered as part of cancer treatment in combination with immunotherapy.

Crystal structure of adenovirus E3-19K bound to HLA-A2 reveals mechanism for immunomodulation

E3-19K binds to and retains MHC class I molecules in the endoplasmic reticulum, suppressing anti-adenovirus activities of T cells. We determined the structure of the adenovirus serotype 2 (Ad2, species C) E3-19K-HLA-A2 complex to 1.95-Å resolution. Ad2 E3-19K binds to the N terminus of the HLA-A2 groove, contacting the α1, α2 and α3 domains and β(2)m. Ad2 E3-19K has a unique structure comprising a large N-terminal domain, formed by two partially overlapping β-sheets arranged in a V shape, and a C-terminal α-helix and tail. The structure reveals determinants in E3-19K and HLA-A2 that are important for complex formation; conservation of some of these determinants in E3-19K proteins of different species and MHC I molecules of different loci suggests a universal binding mode for all E3-19K proteins. Our structure is important for understanding the immunomodulatory function of E3-19K.

Adenovirus E3-19K proteins of different serotypes and subgroups have similar, yet distinct, immunomodulatory functions toward major histocompatibility class I molecules

Our understanding of the mechanism by which the E3-19K protein from adenovirus (Ad) targets major histocompatibility complex (MHC) class I molecules for retention in the endoplasmic reticulum is derived largely from studies of Ad serotype 2 (subgroup C). It is not well understood to what extent observations on the Ad2 E3-19K/MHC I association can be generalized to E3-19K proteins of other serotypes and subgroups. The low levels of amino acid sequence homology between E3-19K proteins suggest that these proteins are likely to manifest distinct MHC I binding properties. This information is important as the E3-19K/MHC I interaction is thought to play a critical role in enabling Ads to cause persistent infections. Here, we characterized interaction between E3-19K proteins of serotypes 7 and 35 (subgroup B), 5 (subgroup C), 37 (subgroup D), and 4 (subgroup E) and a panel of HLA-A, -B, and -C molecules using native gel, surface plasmon resonance (SPR), and flow cytometry. Results show that all E3-19K proteins exhibited allele specificity toward HLA-A and -B molecules; this was less evident for Ad37 E3-19K. The allele specificity for HLA-A molecules was remarkably similar for different serotypes of subgroup B as well as subgroup C. Interestingly, all E3-19K proteins characterized also exhibited MHC I locus specificity. Importantly, we show that Lys(91) in the conserved region of Ad2 E3-19K targets the C terminus of the α2-helix (MHC residue 177) on MHC class I molecules. From our data, we propose a model of interaction between E3-19K and MHC class I molecules.