A short sequence in the COOH-terminus makes an adenovirus membrane glycoprotein a resident of the endoplasmic reticulum

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.

Understanding the mammalian TRAP complex function(s)

In eukaryotic cells, about one-third of the synthesized proteins are translocated into the endoplasmic reticulum; they are membrane or lumen resident proteins and proteins direct to the Golgi apparatus. The co-translational translocation takes place through the heterotrimeric protein-conducting channel Sec61 which is associated with the ribosome and many accessory components, such as the heterotetrameric translocon-associated protein (TRAP) complex. Recently, microscopic techniques, such as cryo-electron microscopy and cryo-electron tomography, have enabled the determination of the translocation machinery structure. However, at present, there is a lack of understanding regarding the roles of some of its components; indeed, the TRAP complex function during co-translational translocation needs to be established. In addition, TRAP may play a role during unfolded protein response, endoplasmic-reticulum-associated protein degradation and congenital disorder of glycosylation (ssr4 CDG). In this article, I describe the current understanding of the TRAP complex in the light of its possible function(s).

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.

Investigation of the Endoplasmic Reticulum Localization of UDP-Glucuronosyltransferase 2B7 with Systematic Deletion Mutants

UDP-Glucuronosyltransferase (UGT) plays an important role in the metabolism of endogenous and exogenous compounds. UGT is a type I membrane protein, and has a dilysine motif (KKXX/KXKXX) in its C-terminal cytoplasmic domain. Although a dilysine motif is defined as an endoplasmic reticulum (ER) retrieval signal, it remains a matter of debate whether this motif functions in the ER localization of UGT. To address this issue, we generated systematic deletion mutants of UGT2B7, a major human isoform, and compared their subcellular localizations with that of an ER marker protein calnexin (CNX), using subcellular fractionation and immunofluorescent microscopy. We found that although the dilysine motif functioned as the ER retention signal in a chimera that replaced the cytoplasmic domain of CD4 with that of UGT2B7, UGT2B7 truncated mutants lacking this motif extensively colocalized with CNX, indicating dilysine motif-independent ER retention of UGT2B7. Moreover, deletion of the C-terminal transmembrane and cytoplasmic domains did not affect ER localization of UGT2B7, suggesting that the signal necessary for ER retention of UGT2B7 is present in its luminal domain. Serial deletions of the luminal domain, however, did not affect the ER retention of the mutants. Further, a cytoplasmic and transmembrane domain-deleted mutant of UGT2B7 was localized to the ER without being secreted. These results suggest that UGT2B7 could localize to the ER without any retention signal, and lead to the conclusion that the static localization of UGT results from lack of a signal for export from the ER.

Cytoplasmic tail of coronavirus spike protein has intracellular targeting signals

Intracellular trafficking and localization studies of spike protein from SARS and OC43 showed that SARS spike protein is localized in the ER or ERGIC compartment and OC43 spike protein is predominantly localized in the lysosome. Differential localization can be explained by signal sequence. The sequence alignment using Clustal W shows that the signal sequence present at the cytoplasmic tail plays an important role in spike protein localization. A unique GYQEL motif is identified at the cytoplasmic terminal of OC43 spike protein which helps in localization in the lysosome, and a novel KLHYT motif is identified in the cytoplasmic tail of SARS spike protein which helps in ER or ERGIC localization. This study sheds some light on the role of cytoplasmic tail of spike protein in cell-to-cell fusion, coronavirus host cell fusion and subsequent pathogenicity.

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.

Lineage-Specific Changes in Biomarkers in Great Apes and Humans

Although human biomedical and physiological information is readily available, such information for great apes is limited. We analyzed clinical chemical biomarkers in serum samples from 277 wild- and captive-born great apes and from 312 healthy human volunteers as well as from 20 rhesus macaques. For each individual, we determined a maximum of 33 markers of heart, liver, kidney, thyroid and pancreas function, hemoglobin and lipid metabolism and one marker of inflammation. We identified biomarkers that show differences between humans and the great apes in their average level or activity. Using the rhesus macaques as an outgroup, we identified human-specific differences in the levels of bilirubin, cholinesterase and lactate dehydrogenase, and bonobo-specific differences in the level of apolipoprotein A-I. For the remaining twenty-nine biomarkers there was no evidence for lineage-specific differences. In fact, we find that many biomarkers show differences between individuals of the same species in different environments. Of the four lineage-specific biomarkers, only bilirubin showed no differences between wild- and captive-born great apes. We show that the major factor explaining the human-specific difference in bilirubin levels may be genetic. There are human-specific changes in the sequence of the promoter and the protein-coding sequence of uridine diphosphoglucuronosyltransferase 1 (UGT1A1), the enzyme that transforms bilirubin and toxic plant compounds into water-soluble, excretable metabolites. Experimental evidence that UGT1A1 is down-regulated in the human liver suggests that changes in the promoter may be responsible for the human-specific increase in bilirubin. We speculate that since cooking reduces toxic plant compounds, consumption of cooked foods, which is specific to humans, may have resulted in relaxed constraint on UGT1A1 which has in turn led to higher serum levels of bilirubin in humans.

Structure and mechanism of COPI vesicle biogenesis

Distinct trafficking pathways within the secretory and endocytic systems ensure prompt and precise delivery of specific cargo molecules to different cellular compartments via small vesicular (50-150nm) and tubular carriers. The COPI vesicular coat is required for retrograde trafficking from the cis-Golgi back to the ER and within the Golgi stack. Recent structural data have been obtained from X-ray crystallographic studies on COPI coat components alone and on COPI subunits in complex with either cargo motifs or Arf1, and from reconstructions of COPI coated vesicles by electron tomography. These studies provide important molecular information and indicate key differences in COPI coat assembly as compared with clathrin-based and COPII-based coats.

The transmembrane domain of the adenovirus E3/19K protein acts as an endoplasmic reticulum retention signal and contributes to intracellular sequestration of major histocompatibility complex class I molecules

The human adenovirus E3/19K protein is a type I transmembrane glycoprotein of the endoplasmic reticulum (ER) that abrogates cell surface transport of major histocompatibility complex class I (MHC-I) and MHC-I-related chain A and B (MICA/B) molecules. Previous data suggested that E3/19K comprises two functional modules: a luminal domain for interaction with MHC-I and MICA/B molecules and a dilysine motif in the cytoplasmic tail that confers retrieval from the Golgi apparatus back to the ER. This study was prompted by the unexpected phenotype of an E3/19K molecule that was largely retained intracellularly despite having a mutated ER retrieval motif. To identify additional structural determinants responsible for ER localization, chimeric molecules were generated containing the luminal E3/19K domain and the cytoplasmic and/or transmembrane domain (TMD) of the cell surface protein MHC-I K(d). These chimeras were analyzed for transport, cell surface expression, and impact on MHC-I and MICA/B downregulation. As with the retrieval mutant, replacement of the cytoplasmic tail of E3/19K allowed only limited transport of the chimera to the cell surface. Efficient cell surface expression was achieved only by additionally replacing the TMD of E3/19K with that of MHC-I, suggesting that the E3/19K TMD may confer static ER retention. This was verified by ER retention of an MHC-I K(d) molecule with the TMD replaced by that of E3/19K. Thus, we have identified the E3/19K TMD as a novel functional element that mediates static ER retention, thereby increasing the concentration of E3/19K in the ER. Remarkably, the ER retrieval signal alone, without the E3/19K TMD, did not mediate efficient HLA downregulation, even in the context of infection. This suggests that the TMD is required together with the ER retrieval function to ensure efficient ER localization and transport inhibition of MHC-I and MICA/B molecules.

Structural mechanism of ER retrieval of MHC class I by cowpox

One of the hallmarks of viral immune evasion is the capacity to disrupt major histocompatibility complex class I (MHCI) antigen presentation to evade T-cell detection. Cowpox virus encoded protein CPXV203 blocks MHCI surface expression by exploiting the KDEL-receptor recycling pathway, and here we show that CPXV203 directly binds a wide array of fully assembled MHCI proteins, both classical and non-classical. Further, the stability of CPXV203/MHCI complexes is highly pH dependent, with dramatically increased affinities at the lower pH of the Golgi relative to the endoplasmic reticulum (ER). Crystallographic studies reveal that CPXV203 adopts a beta-sandwich fold similar to poxvirus chemokine binding proteins, and binds the same highly conserved MHCI determinants located under the peptide-binding platform that tapasin, CD8, and natural killer (NK)-receptors engage. Mutagenesis of the CPXV203/MHCI interface identified the importance of two CPXV203 His residues that confer low pH stabilization of the complex and are critical to ER retrieval of MHCI. These studies clarify mechanistically how CPXV203 coordinates with other cowpox proteins to thwart antigen presentation.

Molecular basis for recognition of dilysine trafficking motifs by COPI

COPI mediates retrograde trafficking from the Golgi to the endoplasmic reticulum (ER) and within the Golgi stack, sorting transmembrane proteins bearing C-terminal KKxx or KxKxx motifs. The structure of KxKxx motifs bound to the N-terminal WD-repeat domain of β'-COP identifies electrostatic contacts between the motif and complementary patches at the center of the β'-COP propeller. An absolute requirement of a two-residue spacing between the terminal carboxylate group and first lysine residue results from interactions of carbonyl groups in the motif backbone with basic side chains of β'-COP. Similar interactions are proposed to mediate binding of KKxx motifs by the homologous α-COP domain. Mutation of key interacting residues in either domain or in their cognate motifs abolishes in vitro binding and results in mistrafficking of dilysine-containing cargo in yeast without compromising cell viability. Flexibility between β'-COP WD-repeat domains and the location of cargo binding have implications for COPI coat assembly.

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.

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

The E3-19K immunomodulatory protein from adenoviruses (Ads) inhibits antigen presentation by major histocompatibility complex (MHC) class I molecules. As a result, the ability of Ad-specific cytotoxic T lymphocytes (CTLs) to lyse infected cells is suppressed. The ER-lumenal domain of E3-19K is subdivided into a variable (residues 1 to ∼78/81) and conserved (residues ∼79/82 to 98) region followed by a linker (residues 99-107). Using molecular and cellular approaches, we characterized in detail the properties of the ER-lumenal domain of E3-19K that enable it to target MHC class I molecules. Proteolysis of recombinant serotype 2 E3-19K (residues 1-100) (with six His residues) generated a large N-terminal (residues 1-88) and a small C-terminal fragment (residues 94-100) in solution. Neither of these fragments associates with HLA-A*1101 as shown by a native gel band-shift assay. In contrast, the N-terminal 1-93 residues of Ad2 E3-19K exhibited the same binding affinity to HLA-A*1101 as E3-19K. Using a site-directed mutational analysis and flow cytometry, we show that Tyr(93), but not Tyr(88), critically modulates the cell-surface expression of MHC class I molecules. Taken together, these results indicate that the sequence comprising residues 89-93 (M(89)SKQY(93)), and in particular Tyr(93), in the conserved region of E3-19K is critical for its immunomodulatory function. Residues 89-93 likely form a linker or loop in E3-19K. Overall, our data provide novel insights into the structure of E3-19K and identify key determinants for association with and ER-retention of its cellular target protein. This knowledge is important for our understanding of the molecular basis of Ad pathogenesis.

Conserved amino acids within the adenovirus 2 E3/19K protein differentially affect downregulation of MHC class I and MICA/B proteins

Successful establishment and persistence of adenovirus (Ad) infections are facilitated by immunosubversive functions encoded in the early transcription unit 3 (E3). The E3/19K protein has a dual role, preventing cell surface transport of MHC class I/HLA class I (MHC-I/HLA-I) Ags and the MHC-I-like molecules (MHC-I chain-related chain A and B [MICA/B]), thereby inhibiting both recognition by CD8 T cells and NK cells. Although some crucial functional elements in E3/19K have been identified, a systematic analysis of the functional importance of individual amino acids is missing. We now have substituted alanine for each of 21 aas in the luminal domain of Ad2 E3/19K conserved among Ads and investigated the effects on HLA-I downregulation by coimmunoprecipitation, pulse-chase analysis, and/or flow cytometry. Potential structural alterations were monitored using conformation-dependent E3/19K-specific mAbs. The results revealed that only a small number of mutations abrogated HLA-I complex formation (e.g., substitutions W52, M87, and W96). Mutants M87 and W96 were particularly interesting as they exhibited only minimal structural changes suggesting that these amino acids make direct contacts with HLA-I. The considerable number of substitutions with little functional defects implied that E3/19K may have additional cellular target molecules. Indeed, when assessing MICA/B cell-surface expression we found that mutation of T14 and M82 selectively compromised MICA/B downregulation with essentially no effect on HLA-I modulation. In general, downregulation of HLA-I was more severely affected than that of MICA/B; for example, substitutions W52, M87, and W96 essentially abrogated HLA-I modulation while largely retaining the ability to sequester MICA/B. Thus, distinct conserved amino acids seem preferentially important for a particular functional activity of E3/19K.

MHC class I antigen presentation: learning from viral evasion strategies

The cell surface display of peptides by MHC class I molecules to lymphocytes provides the host with an important surveillance mechanism to protect against invading pathogens. However, in turn, viruses have evolved elegant strategies to inhibit various stages of the MHC class I antigen presentation pathway and prevent the display of viral peptides. This Review highlights how the elucidation of mechanisms of viral immune evasion is important for advancing our understanding of virus-host interactions and can further our knowledge of the MHC class I presentation pathway as well as other cellular pathways.

Role of genetic susceptibility to latent adenoviral infection and decreased lung function

BACKGROUND

Latent adenoviral infection may amplify cigarette smoke-induced lung inflammation and therefore play an important role in the development of chronic obstructive pulmonary disease (COPD). Adenoviruses can evade the human immune response via their 19-kDa protein (19K) which delays the expression of class I human leukocyte antigen (HLA) proteins. The 19K protein shows higher affinity to HLA-B7 and A2 compared with HLA-A1 and A3. The receptor for adenovirus (CXADR) and integrin beta(5) (ITGB5) are host factors which might affect adenovirus infection. Therefore, we investigated the contribution of HLA, CXADR, and ITGB5 genetic variants to the presence of the E1A gene and to level of lung function.

METHODS

Study subjects were assayed for HLA-B7, A1, A2 and A3 by PCR-based assays using allele-specific primers. Polymorphisms of the CXADR and ITGB5 genes were genotyped by PCR-based restriction fragment length polymorphism assays. Detection of adenoviral E1A gene was performed by a real-time PCR TaqMan assay.

RESULTS

E1A positive individuals had a lower FEV(1) compared with E1A negative individuals. However, there was no significant difference in E1A positivity rate between the high (HLA-B7 and A2) and low (HLA-A1 and A3) 19K affinity groups. There was also no significant difference in FEV(1) level between each affinity group. There was no significant difference in E1A positivity rate or lung function among the CXADR and ITGB5 genotypes.

CONCLUSIONS

Genetic variants in HLA, CXADR and ITGB5 do not influence latent adenoviral infections and are not associated with COPD.

Allele- and Locus-Specific Recognition of Class I MHC Molecules by the Immunomodulatory E3-19K Protein from Adenovirus

The E3-19K protein from human adenoviruses (Ads) retains class I MHC molecules in the endoplasmic reticulum. As a consequence, the cell surface expression of class I molecules is suppressed, allowing Ads to evade immune surveillance. Using native gel electrophoresis, gel filtration chromatography, and surface plasmon resonance, we show that a soluble form of the Ad type 2 (Ad2) E3-19K protein associates with HLA-A and -B molecules; equilibrium dissociation constants were in the nanomolar range and approximately 2.5-fold higher affinity for HLA-A (-A*0201, -A*0301, -A*1101, -A*3301, and -Aw*6801) relative to HLA-B (-B*0702 and -B*0801) molecules. Among the alleles of the HLA-A locus examined, HLA-A*3101 associated approximately 15-fold less avidly with soluble E3-19K. Soluble E3-19K interacted only very weakly with HLA-Cw*0304, and no interaction with HLA-Cw*0401 could be detected under identical conditions. Site-directed mutagenesis and flow cytometry demonstrated that MHC residue 56 plays a critical role in the association and endoplasmic reticulum retention of HLA-A molecules by E3-19K. This delineates the spatial environment around residue 56 as a putative E3-19K interaction surface on class I molecules. Overall, our data imply that a link may exist between host genetic factors and the susceptibility of individuals to Ad infections.

Porcine reproductive and respiratory syndrome virus-infected alveolar macrophages contain no detectable levels of viral proteins in their plasma membrane and are protected against antibody-dependent, complement-mediated cell lysis

Porcine reproductive and respiratory syndrome virus (PRRSV) can evade the host immune system, which results in prolonged virus replication for several weeks to several months. To date, the mechanisms of PRRSV immune evasion have not been investigated in detail. One possible immune-evasion strategy is to avoid incorporation of viral proteins into the plasma membrane of infected cells, as this prevents recognition by virus-specific antibodies and consequent cell lysis either by the classical complement pathway or by antibody-dependent, cell-mediated cytotoxicity. In this study, viral proteins were not observed in the plasma membrane of in vitro-infected macrophages by using confocal microscopy or flow cytometry. Subsequently, the sensitivity of PRRSV-infected macrophages towards antibody-dependent, complement-mediated cell lysis (ADCML) was determined by using an ADCML assay. A non-significant percentage of PRRSV-infected cells were killed in the assay, showing that in vitro PRRSV-infected macrophages are protected against ADCML. PRRSV proteins were not detected in the plasma membrane of in vivo-infected alveolar macrophages and ADCML was also not observed. Together, these data indicate that viral proteins are not incorporated into the plasma membrane of PRRSV-infected macrophages, which makes infected cells invisible to PRRSV-specific antibodies. This absence of viral proteins on the cell surface could explain the protection against ADCML observed for in vitro and in vivo PRRSV-infected macrophages, and may play a role in virus persistence.

The endoplasmic reticulum lumenal domain of the adenovirus type 2 E3-19K protein binds to peptide-filled and peptide-deficient HLA-A*1101 molecules

E3-19K is a type I membrane glycoprotein expressed by adenoviruses (Ads) to modulate host antiviral immune responses. We have developed an expression system for the endoplasmic reticulum lumenal domain (residues 1 to 100) of Ad type 2 E3-19K tagged with a C-terminal His6 sequence in baculovirus-infected insect cells. In this system, recombinant E3-19K is secreted into the culture medium. A characterization of soluble E3-19K by analytical ultracentrifugation and circular dichroism showed that the protein is monomeric and adopts a stable and correctly folded tertiary structure. Using a gel mobility shift assay and analytical ultracentrifugation, we showed that soluble E3-19K associates with soluble peptide-filled and peptide-deficient HLA-A*1101 molecules. This is the first example of a viral immunomodulatory protein that interacts with conformationally distinct forms of class I major histocompatibility complex molecules. The E3-19K/HLA-A*1101 complexes formed in a 1:1 stoichiometry with equilibrium dissociation constants (Kd) of 50 +/- 10 nM for peptide-filled molecules and of about 10 microM for peptide-deficient molecules. A temperature-dependent proteolysis study revealed that the association of E3-19K with peptide-deficient HLA-A*1101 molecules stabilizes the binding groove. Importantly, our studies showed that peptide-deficient HLA-A*1101 molecules sequestered by E3-19K are capable of binding antigenic peptides and maturing into peptide-filled molecules. This firmly establishes that E3-19K does not block binding of antigenic peptides. Together, our results suggest that Ads have evolved to exploit the late and early stages of the class I antigen presentation pathway.

Viral interference with MHC class I antigen presentation pathway: the battle continues

CD8+ cytotoxic T lymphocytes (CTLs) play a critical role in the defense against viral infections. In general, CD8+ CTLs recognize antigenic peptides in the context of the major histocompatibility complex (MHC) class I molecule. The MHC class I molecules are expressed on almost all the nucleated cells in the body. The trimolecular complex consisting of the class I heavy chain, beta2-microglobulin and the peptide are generated by the MHC class I antigen presentation pathway. This pathway is designed to sample the intracellular milieu and present the information to the CTLs trafficking the area. This rigorous sampling of intracellular environment enables the CTLs to quickly identify and eliminate the cells that synthesize non-self proteins as a result of a viral infection. Many viruses, including several viruses of veterinary importance, have evolved astounding strategies to interfere with the MHC class I antigen presentation pathway, as a means of evading the CTL response of the host. This review focuses on the diverse mechanisms of viral evasion of the MHC class I antigen presentation pathway with particular emphasis on viruses of veterinary importance.

ADP-overexpressing adenovirus elicits enhanced cytopathic effect by induction of apoptosis

Replication-competent adenoviruses (Ad's) are emerging as a promising new modality for treatment of cancer. Selective replication of viral agents in tumor may lead to improved efficacy over nonreplicating Ad's due to their inherent ability to multiply, lyse, and spread to surrounding cells. We have previously shown that an E1B 55 kDa-deleted adenovirus (YKL-1) exhibits tumor-specific replication and cell lysis, but its cytolytic effects were reduced in comparison to the wild-type adenovirus. To increase the oncolytic potency of YKL-1, we have reintroduced the Ad death protein (ADP) gene under the control of either a CMV or an MLP promoter at the E3 region of YKL-1, generating YKL-cADP and YKL-mADP Ad's, respectively. ADP is an 11.6 kDa protein encoded by the E3 transcription unit, and is required to kill adenovirus-infected cells efficiently. However, to date, the mechanism by which ADP mediates cell death has not been clearly defined. In this study, we report that ADP-overexpressing Ad markedly enhanced cytolytic effect (up to 100-fold) against all tumor cell lines tested, but did not increase cytopathic effect in normal skin fibroblast, BJ. Moreover, plaque size formed by YKL-cADP was substantially larger than that of YKL-1, indicating an enhancement in cell lysis. TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling) assay and Annexin-V/PI double staining indicate that ADP-mediated cytotoxicity was largely driven by apoptosis. Finally, YKL-cADP adenovirus also showed superior antitumor effect than YKL-1 and YKL-mADP in C33A cervical and Hep3B hepatoma xenograft tumor models. Taken together, these lines of evidence demonstrate that the newly generated adenovirus expressing ADP under the CMV promoter induces efficient but tumor-selective cell lysis, which is critical for adding therapeutic value to replicating adenovirus for its use in cancer gene therapy.

Functions and mechanisms of action of the adenovirus E3 proteins

In the evolutionary battle between viruses and their hosts, viruses have armed themselves with weapons to defeat the host's attacks on infected cells. Various proteins encoded in the adenovirus (Ad) E3 transcription unit protect cells from killing mediated by cytotoxic T cells and death-inducing cytokines such as tumor necrosis factor (TNF), Fas ligand, and TNF-related apoptosis-inducing ligand (TRAIL). The viral protein E3-gp19 K blocks MHC class-I-restricted antigen presentation, which diminishes killing by cytotoxic T cells. The receptor internalization and degradation (RID) complex (formerly E3-10.4 K/14.5 K) stimulates the clearance from the cell surface and subsequent degradation of the receptors for Fas ligand and TRAIL, thereby preventing the action of these important immune mediators. RID also downmodulates the epidermal growth factor receptor (EGFR), although what role, if any, this function has in immune regulation is uncertain. In addition, RID antagonizes TNF-mediated apoptosis and inflammation through a mechanism that does not primarily involve receptor downregulation. E3-6.7 K functions together with RID in downregulating some TRAIL receptors and may block apoptosis independently of other E3 proteins. Furthermore, E3-14.7 K functions as a general inhibitor of TNF-mediated apoptosis and blocks TRAIL-induced apoptosis. Finally, after expending great effort to maintain cell viability during the early part of the virus replication cycle, Ads lyse the cell to allow efficient virus release and dissemination. To perform this task subgroup C Ads synthesize a protein late in infection named ADP (formerly E3-11.6 K) that is required for efficient virus release. This review focuses on recent experiments aimed at discovering the mechanism of action of these critically important viral proteins.

A cytoplasmic region of the NSP4 enterotoxin of rotavirus is involved in retention in the endoplasmic reticulum

The rotavirus genome encodes two glycoproteins, one structural (VP7) and one non-structural (NSP4), both of which mature and remain in the endoplasmic reticulum (ER). While three amino acids in the N terminus have been proposed to function as a retention signal for VP7, no information is yet available on how NSP4 remains associated with the ER. In this study, we have investigated the ER retention motif of NSP4 by producing various C-terminal truncations. Deleting the C terminus by 52 amino acids did not change the intracellular distribution of NSP4, but an additional deletion of 38 amino acids diminished the ER retention and resulted in the expression of NSP4 on the cell surface. Brefeldin A treatment prevented NSP4 from reaching the cell surface, suggesting that C-terminal truncated plasma membrane NSP4 is transported through the normal secretory pathway. On the basis of these results, we propose that the region between amino acids 85 and 123 in the cytoplasmic region of NSP4 are involved in ER retention.

Overexpression of the ADP (E3-11.6K) protein increases cell lysis and spread of adenovirus

Adenoviruses replicate in the nucleus and induce lytic cell death. We have shown previously that efficient cell lysis and release of adenovirus from infected cells requires an 11.6-kDa protein named Adenovirus Death Protein (ADP). The adp gene is located in the early E3 transcription unit, but the gene is expressed primarily at very late stages of infection. The putative function of ADP was discerned previously from the use of virus mutants that lack functional ADP. Here we describe two adenovirus mutants, named VRX-006 and VRX-007, that overexpress ADP. VRX-006 lacks all other genes in the E3 region, and VRX-007 lacks all other E3 genes except 12.5K. VRX-006 and VRX-007 display the phenotype predicted by the proposed function for ADP: they produce early cytopathic effect, early cell lysis, large plaques, and increased cell-to-cell spread. They grow as well in cultured cells as does adenovirus type 5. These results are consistent with the conclusion that ADP functions in adenovirus infections to promote virus release from cells at the culmination of infection.

Viral interference with antigen presentation

CD8+ T cells play an important role in immunity to viruses. Just how important these cells are is demonstrated by the evolution of viral strategies for blocking the generation or display of peptide-major histocompatibility complex class I complexes on the surfaces of virus-infected cells. Here, we focus on viral interference with antigen presentation; in particular we consider the importance (and difficulty) of establishing the evolutionary significance (that is, the ability to enhance viral transmission) of viral gene products that interfere with antigen presentation in vitro.

Adenovirus RIDbeta subunit contains a tyrosine residue that is critical for RID-mediated receptor internalization and inhibition of Fas- and TRAIL-induced apoptosis

The adenovirus-encoded receptor internalization and degradation (RID) protein (previously named E3-10.4K/14.5K), which is composed of RIDalpha and RIDbeta subunits, down-regulates a number of cell surface receptors in the tumor necrosis factor (TNF) receptor superfamily, namely Fas, TRAIL receptor 1, and TRAIL receptor 2. Down-regulation of these "death" receptors protects adenovirus-infected cells from apoptosis induced by the death receptor ligands Fas ligand and TRAIL. RID also down-regulates certain tyrosine kinase cell surface receptors, especially the epidermal growth factor receptor (EGFR). RID-mediated Fas and EGFR down-regulation occurs via endocytosis of the receptors into endosomes followed by transport to and degradation within lysosomes. However, the molecular interactions underlying this function of RID are unknown. To investigate the molecular determinants of RIDbeta that are involved in receptor down-regulation, mutations within the cytoplasmic tail of RIDbeta were constructed and the mutant proteins were analyzed for their capacity to internalize and degrade Fas and EGFR and to protect cells from death receptor ligand-induced apoptosis. The results demonstrated the critical nature of a tyrosine residue near the RIDbeta C terminus; mutation of this residue to alanine abolished RID function. Mutating the tyrosine to phenylalanine did not abolish the function of RID, arguing that phosphorylation of the tyrosine is not required for function. These data suggest that this tyrosine residue forms part of a tyrosine-based sorting signal (Yxxphi). Additional mutations that target another potential sorting motif and several possible protein-protein interaction motifs had no discernible effect on RID function. It was also demonstrated that mutation of serine 116 to alanine eliminated phosphorylation of RIDbeta but did not affect any of the functions of RID that were examined. These results suggest a model in which the tyrosine-based sorting signal in RID plays a role in RID's ability to down-regulate receptors.

Construction and characterization of E1-minus replication-defective adenovirus vectors that express E3 proteins from the E1 region

Previous research has indicated that the adenovirus protein complex named RID, derived from the E3 transcription unit, functions to remove the receptors named Fas/Apo1/CD95 (Fas) and epidermal growth factor receptor (EGFR) from the surface of cells. (The RID complex is composed of the RIDalpha and RIDbeta polypeptides, previously named 10.4K and 14.5K, respectively.) In response to RID, Fas and EGFR appear to be internalized into endosomes and degraded in lysosomes. Fas is a death receptor in the tumor necrosis factor (TNF) receptor superfamily. RID inhibits apoptosis via the Fas pathway, presumably because RID gets rid of Fas. Earlier work further showed that another adenovirus E3-coded protein, E3-14.7K, inhibits apoptosis induced by TNF. Most of the above studies have been conducted using viable virus mutants that lack one or more of the genes for RID, E3-14.7K, or E1B-19K (this protein, coded by the E1B transcription unit, also inhibits apoptosis via the TNF and Fas pathways). Some studies have also been conducted with the genes for RID or E3-14.7K transiently or stably transfected into cells. We now report a new approach to studying the E3 genes. We have constructed four E1-minus replication-defective vectors that have all the E3 genes deleted from their natural position and then reinserted, in different permutations, into the deleted E1 region under control of the cytomegalovirus immediate early promoter. Vector Ad/RID only has the genes for RIDalpha and RIDbeta. Vector Ad/14.7K only has the gene for E3-14.7K. Vector Ad/RID/14.7K only has the genes for RIDalpha, RIDbeta, and E3-14.7K. Vector Ad/E3 has all E3 genes, but there are two missense mutations in the gene for Adenovirus Death Protein. These vectors expressed RID and/or E3-14.7K, as expected. The RID-expressing vectors forced the internalization and degradation of Fas and EGFR, and they inhibited apoptosis induced through the Fas pathway. These vectors should be useful reagents to study the E3 proteins.

Multiple determinants in voltage-dependent P/Q calcium channels control their retention in the endoplasmic reticulum

Surface expression level of voltage-dependent calcium channels is tightly controlled in neurons to avoid the resulting cell toxicity generally associated with excessive calcium entry. Cell surface expression of high voltage-activated calcium channels requires the association of the pore-forming subunit, Cavalpha, with the auxiliary subunit, Cavbeta. In the absence of this auxiliary subunit, Cavalpha is retained in the endoplasmic reticulum (ER) through mechanisms that are still poorly understood. Here, we have investigated, by a quantitative method based on the use of CD8 alpha chimeras, the molecular determinants of Cavalpha2.1 that are responsible for the retention, in the absence of auxiliary subunits, of P/Q calcium channels in the ER (referred to here as 'ER retention'). This study demonstrates that the I-II loop of Cavalpha2.1 contains multiple ER-retention determinants beside the beta subunit association domain. In addition, the I-II loop is not the sole domain of calcium channel retention as two regions identified for their ability to interact with the I-II loop, the N- and C-termini of Cavalpha2.1, also produce ER retention. It is also not an obligatory determinant as, similarly to low-threshold calcium channels, the I-II loop of Cavalpha1.1 does not produce ER retention in COS7 cells. The data presented here suggests that ER retention is suppressed by sequential molecular events that include: (i). a correct folding of Cavalpha in order to mask several internal ER-retention determinants and (ii). the association of other proteins, including the Cavbeta subunit, to suppress the remaining ER-retention determinants.

Novel role of Janus kinase 1 in the regulation of oncostatin M receptor surface expression

The oncostatin M receptor (OSMR) is part of a heterodimeric receptor complex that mediates signal transduction of the pleiotropic cytokine OSM via a signaling pathway involving Janus kinases (Jaks) and transcription factors of the signal transducers and activators of transcription (STAT) family. Upon heterologous expression of the OSMR in several cell lines, we observed that its surface expression was significantly enhanced by coexpression of the Janus kinases Jak1, Jak2, and Tyk2 but not Jak3. Chimeric receptors consisting of the extracellular region of the interleukin-5 receptor beta chain and the transmembrane and intracellular part of the OSMR were similarly up-regulated on the plasma membrane when Jak1 was coexpressed. The overall expression level of these constructs did not change significantly, but Jak1 coexpression increased the amount of endoglycosidase H-resistant, fully processed OSMR chimeras. Using mutated receptor and Jak1 constructs, we were able to demonstrate that association of Jak1 with the membrane proximal region of the receptor, but not its kinase activity, is necessary for this effect. Moreover, deletion of the OSMR box1/2 region also resulted in an improved surface expression indicating that this region may contain a signal preventing efficient receptor surface expression in the absence of associated Jaks. Finally we demonstrate that in Jak1-deficient cells, the endogenous OSMR is significantly down-regulated, an effect that can be reversed by transient expression of Jak1 in these cells.

Structural and functional dissection of human cytomegalovirus US3 in binding major histocompatibility complex class I molecules

The human cytomegalovirus US3, an endoplasmic reticulum (ER)-resident transmembrane glycoprotein, forms a complex with major histocompatibility complex (MHC) class I molecules and retains them in the ER, thereby preventing cytolysis by cytotoxic T lymphocytes. To identify which parts of US3 confine the protein to the ER and which parts are responsible for the association with MHC class I molecules, we constructed truncated mutant and chimeric forms in which US3 domains were exchanged with corresponding domains of CD4 and analyzed them for their intracellular localization and the ability to associate with MHC class I molecules. All of the truncated mutant and chimeric proteins containing the luminal domain of US3 were retained in the ER, while replacement of the US3 luminal domain with that of CD4 led to cell surface expression of the chimera. Thus, the luminal domain of US3 was sufficient for ER retention. Immunolocalization of the US3 glycoprotein after nocodazole treatment and the observation that the carbohydrate moiety of the US3 glycoprotein was not modified by Golgi enzymes indicated that the ER localization of US3 involved true retention, without recycling through the Golgi. Unlike the ER retention signal, the ability to associate with MHC class I molecules required the transmembrane domain in addition to the luminal domain of US3. Direct interaction between US3 and MHC class I molecules could be demonstrated after in vitro translation by coimmunoprecipitation. Together, the present data indicate that the properties that allow US3 to be localized in the ER and bind MHC class I molecules are located in different parts of the molecule.

Viral subversion of the immune system

This review describes the diverse array of pathways and molecular targets that are used by viruses to elude immune detection and destruction. These include targeting of pathways for major histocompatibility complex-restricted antigen presentation, apoptosis, cytokine-mediated signaling, and humoral immune responses. The continuous interactions between host and pathogens during their coevolution have shaped the immune system, but also the counter measures used by pathogens. Further study of their interactions should improve our ability to manipulate and exploit the various pathogens.

Transcription mapping and characterization of 284R and 121R proteins produced from early region 3 of bovine adenovirus type 3

We established the transcription map of early region (E) 3 of bovine adenovirus 3 (BAV-3) by Northern blot, S1 nuclease protection assays, cDNA sequencing, and RT-PCR analysis. Five major classes of mRNAs were identified, which shared the 3' ends. Four classes of mRNAs transcribed from the E3 promoter also shared the 5' end, while one major class of mRNA transcribed from the major late promoter contained a tripartite leader sequence at the 5' end. These five transcripts have the potential to encode four proteins, namely 284R, 121R, 86R, and 82R. To identify the proteins, rabbit antiserum was prepared using a bacterial fusion protein encoding 284R or 121R protein. Serum against 284R immunoprecipitated protein of 26-32 kDa in in vitro translated and transcribed mRNA and three proteins of 48, 67, and 125 kDa from BAV-3-infected cells. Western blots and enzymatic digestions confirmed that the 284R protein is a glycoprotein, which contains only N-linked oligosaccharides, both high mannose (48 kDa) and complex types (67 kDa). Serum against 121R immunoprecipitated a protein of 14.5 kDa from in vitro translated and transcribed mRNA and BAV-3-infected cells. Although 121R protein shows limited sequence similarity to a 14.7-kDa protein of human adenovirus 5, the 284R protein appears to be unique to BAV-3. Since proteins encoded by the E3 region appear to influence adenovirus pathogenesis, the 284R protein may contribute to the unique pathogenic properties of BAV-3.

Evaluation of an E1E4-deleted adenovirus expressing the herpes simplex thymidine kinase suicide gene in cancer gene therapy

Studies with first-generation adenoviral vectors have uncovered limitations that include finite transgene persistence, potential hepatotoxicity, and contamination with replication-competent adenovirus (RCA). To address these limitations within the context of cancer suicide gene therapy, a new adenoviral vector was developed containing the herpes simplex virus type 1 thymidine kinase (HSV tk) gene inserted in the E1 region of a recombinant vector containing deletions in the E1 and E4 regions of the Ad5 genome. The HSV tk minigene was placed under transcriptional control of a Rous sarcoma virus (RSV) promoter. This new E1E4-deleted vector was compared with the first-generation E1E3-deleted Ad.RSVtk vector. Generation of replication-competent adenovirus during production was eliminated. Using semiquantitative immunoblotting, the two vectors produced equivalent amounts of the expected 44-kDa tk-encoded protein in three different cell lines tested. The ability of the E1E4-deleted vector to sensitize tumor cells to ganciclovir (GCV) using in vitro assays and mixing studies was comparable to that of the E1E3-deleted vector. In vivo bystander effects were investigated using mixing studies in a syngeneic flank tumor model and demonstrated no difference between vectors in either immunocompetent or immunodeficient mice. To test the efficiency of these vectors in treating tumors in clinically relevant models, virus was injected intraperitoneally into tumor-bearing SCID mice and intrapleurally in a syngeneic rat mesothelioma model. After treatment of animals with ganciclovir, both vectors were roughly equivalent in their ability to increase mean survival (from approximately 40 to approximately 70 days) and markedly reduce tumor burden. Finally, formal toxicology studies were performed and showed similar amounts of local inflammation without systemic toxicity. In summary, this series of in vitro and in vivo experiments indicates that the performance of the recombinant E1E4-deleted adenoviral vector was virtually identical to that of the E1E3-deleted vector. Since the E1E4 vector has a much lower rate of recombination during production and has been shown to be less hepatotoxic in animal models, this new vector should prove superior to the first-generation Ad.HSVtk vectors in clinical cancer gene therapy trials.

Correction of murine galactosialidosis by bone marrow-derived macrophages overexpressing human protective protein/cathepsin A under control of the colony-stimulating factor-1 receptor promoter

Galactosialidosis (GS) is a human neurodegenerative disease caused by a deficiency of lysosomal protective protein/cathepsin A (PPCA). The GS mouse model resembles the severe human condition, resulting in nephropathy, ataxia, and premature death. To rescue the disease phenotype, GS mice were transplanted with bone marrow from transgenic mice overexpressing human PPCA specifically in monocytes/macrophages under the control of the colony stimulating factor-1 receptor promoter. Transgenic macrophages infiltrated and resided in all organs and expressed PPCA at high levels. Correction occurred in hematopoietic tissues and nonhematopoietic organs, including the central nervous system. PPCA-expressing perivascular and leptomeningeal macrophages were detected throughout the brain of recipient mice, although some neuronal cells, such as Purkinje cells, continued to show storage and died. GS mice crossed into the transgenic background reflected the outcome of bone marrow-transplanted mice, but the course of neuronal degeneration was delayed in this model. These studies present definite evidence that macrophages alone can provide a source of corrective enzyme for visceral organs and may be beneficial for neuronal correction if expression levels are sufficient.

Secretion and purification of HCV E1 protein forms as glutathione-S-transferase fusion in the baculovirus insect cell system

We have expressed the E1 protein of Hepatitis C Virus (HCV) in a new recombinant form by using a baculovirus transfer vector directing the expression of proteins fused to the carboxy-terminus of glutathione-S-transferase (GST). The E1 domain was expressed varying at its carboxy terminus in order to retain (GST-E1) or delete (GST-E1b) the C-terminal hydrophobic region that may be involved in membrane association. Following infection with the recombinant virus, GST-E1b was efficiently secreted into the culture media and could be purified in a single step with the minimum of denaturation by glutathione affinity chromatography. The purified product was specifically immunoprecipitated by HCV positive human sera suggesting the maintenance of an immuno-relevant tertiary structure despite removal of the hydrophobic anchor. By contrast, cells infected with a recombinant baculovirus expressing GST-E1 gave a fusion protein with an appropriate molecular weight but also a series of polypeptides of lower molecular weight consistent with cleavage at the C-terminus of E1. GST-E1 was not secreted into the medium and was associated predominantly with the membrane fraction following cell disruption; the lower molecular weight forms were soluble and secreted.

Role of tight junctions in establishing and maintaining cell polarity

The tight junction (TJ) is not randomly located on the cell membrane, but occupies a precise position at the outermost edge of the intercellular space and, therefore, is itself considered a polarized structure. This article reviews the most common experimental approaches for studying this relationship. We then discuss three main topics. (a) The mechanisms of polarization that operate regardless of the presence of TJs: We explore a variety of polarization mechanisms that operate at stages of the cell cycle in which TJs may be already established. (b) TJs and polarity as partners in highly dynamic processes: Polarity and TJs are steady state situations that may be drastically changed by a variety of signaling events. (c) Polarized distribution of membrane molecules that depend on TJs: This refers to molecules (mainly lipids) whose polarized distribution, although not the direct result of TJs, depends on these structures to maintain such distribution.

Function of the KKXX motif in endoplasmic reticulum retrieval of a transmembrane protein depends on the length and structure of the cytoplasmic domain

Transmembrane glycoproteins with type 1 topology can be retrieved to the endoplasmic reticulum (ER) by a retrieval signal containing a di-lysine (KK) motif near the C terminus. To investigate the structural requirements for ER retrieval, we have constructed mutants of the simian immunodeficiency virus (SIV) envelope (Env) protein with cytoplasmic tails of different lengths and containing a KK motif at the -3 and -4 positions. Such proteins were found to be retained intracellularly when the signal was located 18 amino acids or more away from the membrane spanning domain. The retrieval signal was found to be functional even when placed at the distal end of the wild-type SIV Env protein with 164 amino acids in the cytoplasmic tail, as shown by the lack of proteolytic processing and lack of cell surface expression of the mutant proteins. However, proteins with a cytoplasmic tail length of 13 amino acids or less having the di-lysine motif at the -3 and -4 positions were not retrieved to the ER since they were found to be processed and transported to the cell surface. The surface-expressed proteins were found to be functional in inducing cell fusion, whereas the proteins retained intracellularly were defective in fusion activity. We also found that the KK motif introduced near an amphipathic helical region in the cytoplasmic tail was not functional. These results demonstrate that the ability of the KK motif to cause protein retrieval and retention in the endoplasmic reticulum depends on the length and structure of the cytoplasmic domain. The ER retrieval of the mutant proteins was found to correlate with increased intracellular binding to beta COP proteins.

Expression of gp19K increases the persistence of transgene expression from an adenovirus vector in the mouse lung and liver

Activation of the cellular immune system and subsequent lysis of vector-transduced cells by adenovirus- or transgene-specific cytotoxic T lymphocytes have been shown to limit transgene expression in animal models. The adenovirus gp19K gene product associates with major histocompatibility complex class I proteins and prevents their maturation by sequestering them in the endoplasmic reticulum. gp19K has been shown to block the ability of adenovirus-specific cytotoxic T lymphocytes to recognize virus-infected cells in vitro. To determine if gp19K expression in an adenovirus vector would increase transgene persistence, a vector that replaces the E1 region of adenovirus with an expression cassette encoding both gp19K and beta-glucuronidase was constructed. This vector produced high levels of functional gp19K in infected cells. RNase protection analysis revealed efficient expression of the gp19K gene in the mouse lung. Enhanced persistence and increased beta-glucuronidase activity were observed in the lung and liver following delivery of the gp19K-expressing adenovirus vector in B10.HTG mice but not in BALB/c mice. Since gp19K binds to both class I alleles on B10.HTG mice but only one allele on BALB/c mice, these results suggest that the major histocompatibility complex class I haplotype of mice is important in determining the effectiveness of gp19K in vivo. Since gp19K has previously been shown to interact with every human major histocompatibility complex class I allele tested, the inclusion of gp19K in gene therapy vectors may increase vector persistence in human gene therapy trials.

Viruses use stealth technology to escape from the host immune system

In this review, we focus on recent investigations that reveal novel mechanisms by which viruses evade detection and elimination by the host immune system. In particular, we consider the evasion mechanisms of five persistent viruses: herpes simplex virus, human cytomegalovirus, mouse cytomegalovirus, Epstein-Barr virus and adenovirus. Unravelling the strategies used by viruses to survive within the host could identify new targets for antiviral drugs and for improved vaccines. Identification of the mechanisms that underlie these strategies might also reveal new, fundamental features of biology that occur in uninfected cells and are exploited by viruses.

A sorting motif localizes the foamy virus glycoprotein to the endoplasmic reticulum

We recently identified an endoplasmic reticulum (ER) retrieval signal-the dilysine motif-in the glycoproteins of all five foamy viruses (FVs) for which sequences were available (P. A. Goepfert, G. Wang, and M. J. Mulligan, Cell 82:543-544, 1995). In the present study, expression of recombinant human FV (HFV) glycoprotein and analyses of oligosaccharide modifications and precursor cleavage indicated that the protein was localized to the ER. HFV glycoproteins encoding seven different dilysine motif mutations were then expressed. The results indicated that disruptions of the dilysine motif resulted in higher levels of forward transport of the HFV glycoprotein from the ER through the Golgi apparatus to the plasma membrane. We conclude that the dilysine motif is responsible for ER sorting of the FV glycoprotein. Signal-mediated ER localization has not previously been described for a retroviral glycoprotein.

Molecular basis of immune evasion strategies by adenoviruses

Human adenoviruses have provided valuable insights into virus-host interactions at the clinical and experimental levels. In addition to the medical importance of adenoviruses in acute infections and the ability of the virus to persist in the host, adenovirus-based recombinants are being developed as potential vaccine vectors. It is now clear that adenoviruses employ various strategies to modulate the innate and the adaptive host immune defences. Adenovirus genome-coded products that interact with the immune response of the host have been identified, and to a large extent the molecular mechanisms of their functions have been revealed. Such knowledge will no doubt influence our approach to the areas of viral pathogenesis, vaccine development and immune modulation for disease management.

Human cytomegalovirus inhibits antigen presentation by a sequential multistep process

The human cytomegalovirus (HCMV) genomic unique short (US) region encodes a family of homologous genes essential for the inhibition of major histocompatibility complex (MHC) class I-mediated antigen presentation during viral infection. Here we show that US3, the only immediate early (IE) gene within the US region, encodes an endoplasmic reticulum-resident glycoprotein that prevents intracellular transport of MHC class I molecules. In contrast to the rapid degradation of newly synthesized MHC class I heavy chains mediated by the early gene product US11, we found that US3 retains stable MHC class I heterodimers in the endoplasmic reticulum that are loaded with peptides while retained in the ER. Consistent with the expression pattern of US3 and US11, MHC class I molecules are retained but not degraded during the IE period of infection. Our data identify the first nonregulatory role of an IE protein of HCMV and suggest that HCMV uses different T-cell escape strategies at different times during the infectious cycle.

Bimodal interaction of coatomer with the p24 family of putative cargo receptors

Cytoplasmic domains of members of the p24 family of putative cargo receptors were shown to bind to coatomer, the coat protein of COPI-coated transport vesicles. Domains that contained dilysine endoplasmic reticulum retrieval signals bound the alpha-, beta'-, and epsilon-COP subunits of coatomer, whereas other p24 domains bound the beta-, gamma-, and zeta-COP subunits and required a phenylalanine-containing motif. Transit of a CD8-p24 chimera from the endoplasmic reticulum through the Golgi complex was slowed when the phenylalanine motif was mutated, suggesting that this motif may function as an anterograde transport signal. The either-or bimodal binding of coatomer to p24 tails suggests models for how coatomer can potentially package retrograde-directed and anterograde-directed cargo into distinct COPI-coated vesicles.

A single gene encoding the fiber is responsible for variations in virulence in the fowl adenoviruses

Intertypic recombinant fowl adenoviruses (FAVs) were generated to determine regions of the viral genome involved in virulence. Recombinants were produced with two serotype 8 FAVs, mildly virulent CFA 3 and hypervirulent CFA 40. Restriction endonuclease fragments from the genomes of the two FAVs were used to transfect primary chicken kidney cells. Virulence testing of these recombinants located the region responsible for differences in virulence to an 8.4-kb fragment of the genome located between kb 26.6 and 35.0. According to data available for a serotype 10 FAV that had been partially characterized in the laboratory, this segment of the genome contained three genes of known identity (100K, 33K, and pVIII) and a region between kb 31 and 35 with unknown coding potential (although this information subsequently became available for a serotype 1 FAV, CELO). Therefore, the region between kb 30.5 and 34.5 was sequenced. The results revealed that the unknown region encoded a fiber gene on the right strand and several small open reading frames of unknown identity on the left strand. Further recombinant viruses containing defined exchanges within the 4-kb fragment were constructed, and virulence testing of these viruses indicated that the fiber was responsible for differences in virulence for CFA 40 and CFA 3.