Human cell surface proteins selectively assembled into vesicular stomatitis virus virions

Identification of a Natural Viral RNA Motif That Optimizes Sensing of Viral RNA by RIG-I

Stimulation of the antiviral response depends on the sensing of viral pathogen-associated molecular patterns (PAMPs) by specialized cellular proteins. During infection with RNA viruses, 5'-di- or -triphosphates accompanying specific single or double-stranded RNA motifs trigger signaling of intracellular RIG-I-like receptors (RLRs) and initiate the antiviral response. Although these molecular signatures are present during the replication of many viruses, it is unknown whether they are sufficient for strong activation of RLRs during infection. Immunostimulatory defective viral genomes (iDVGs) from Sendai virus (SeV) are among the most potent natural viral triggers of antiviral immunity. Here we describe an RNA motif (DVG(70-114)) that is essential for the potent immunostimulatory activity of 5'-triphosphate-containing SeV iDVGs. DVG(70-114) enhances viral sensing by the host cell independently of the long stretches of complementary RNA flanking the iDVGs, and it retains its stimulatory potential when transferred to otherwise inert viral RNA. In vitro analysis showed that DVG(70-114) augments the binding of RIG-I to viral RNA and promotes enhanced RIG-I polymerization, thereby facilitating the onset of the antiviral response. Together, our results define a new natural viral PAMP enhancer motif that promotes viral recognition by RLRs and confers potent immunostimulatory activity to viral RNA.

IMPORTANCE

A discrete group of molecular motifs, including 5'-triphosphates associated with double-stranded RNA, have been identified as essential for the triggering of antiviral immunity. Most RNA viruses expose these motifs during their replication; however, successful viruses normally evade immune recognition and replicate to high levels before detection, indicating that unknown factors drive antiviral immunity. DVGs from SeV are among the most potent natural viral stimuli of the antiviral response known to date. These studies define a new natural viral motif present in DVGs that maximizes viral recognition by the intracellular sensor RIG-I, allowing fast and strong antiviral responses even in the presence of viral-encoded immune antagonists. This motif can be harnessed to increase the immunostimulatory potential of otherwise inert viral RNAs and represents a novel immunostimulatory enhancer that could be used in the development of vaccine adjuvants and antivirals.

A chimeric vesiculo/alphavirus is an effective alphavirus vaccine

While a large number of mosquito-transmitted alphaviruses are known to cause serious human diseases, there are no licensed vaccines that protect against alphavirus infections. The alphavirus chikungunya virus (CHIKV) has caused multiple recent outbreaks of chikungunya fever. This virus has the potential to cause a worldwide epidemic and has generated strong interest in development of a prophylactic CHIKV vaccine. We report here on the development of a potent experimental vaccine for CHIKV based on a chimeric vesicular stomatitis virus (VSV) expressing the entire CHIKV envelope polyprotein (E3-E2-6K-E1) in place of the VSV glycoprotein (G). These VSVΔG-CHIKV chimeras incorporated functional CHIKV glycoproteins into the viral envelope in place of VSV G. The chimeric viruses were attenuated for growth in tissue culture but could be propagated to high titers without VSV G complementation. They also generated robust neutralizing antibody and cellular immune responses to CHIKV in mice after a single dose and protected mice against CHIKV infection. VSVΔG-alphavirus chimeras could have general applicability as alphavirus vaccines.

Reduced sensitivity to human serum inactivation of enveloped viruses produced by pig cells transgenic for human CD55 or deficient for the galactosyl-alpha(1-3) galactosyl epitope

Complement activation mediated by the major xenogeneic epitope in the pig, galactosyl-alpha(1-3) galactosyl sugar structure (alpha-Gal), and human natural antibodies could cause hyperacute rejection (HAR) in pig-to-human xenotransplantation. The same reaction on viruses bearing alpha-Gal may serve as a barrier to zoonotic infection. Expressing human complement regulatory proteins or knocking out alpha-Gal epitopes in pig in order to overcome HAR may therefore pose an increased risk in xenotransplantation with regard to zoonosis. We investigated whether amphotropic murine leukemia virus, porcine endogenous retrovirus, and vesicular stomatitis virus (VSV) budding from primary transgenic pig aortic endothelial (TgPAE) cells expressing human CD55 (hCD55 or hDAF) was protected from human-complement-mediated inactivation. VSV propagated through the ST-IOWA pig cell line, in which alpha-galactosyl-transferase genes were disrupted (Gal null), was also tested for sensitivity to human complement. The TgPAE cells were positive for hCD55, and all pig cells except the Gal-null ST-IOWA expressed alpha-Gal epitopes. Through antibody binding, we were able to demonstrate the incorporation of hCD55 onto VSV particles. Viruses harvested from TgPAE cells were relatively resistant to complement-mediated inactivation by the three sources of human sera tested. Additionally, VSV from Gal-null pig cells was resistant to human complement inactivation. Such protection of enveloped viruses may increase the risk of zoonosis from pigs genetically modified for pig-to-human xenotransplantation.

Lipid rafts and pseudotyping

Specific interactions between envelope and core proteins govern the membrane assembly of most enveloped viruses. Despite this, mixed infections lead to pseudotyping, the association of the viral cores of one virus with the envelopes of another. How does this occur? We show here that the detergent-insoluble lipid rafts of the plasma membrane function as a natural meeting point for the transmembrane and core components of a phylogenetically diverse collection of enveloped viruses. As a result, viral particles preferentially incorporate both the envelope components of other viruses as well as the extra- and intracellular constituents of host cell lipid rafts, including gangliosides, glycosyl phosphatidylinositol-anchored surface proteins, and intracellular signal transduction molecules. Pharmacological disruption of lipid rafts interferes with virus production.

The rabies virion-associated 100-kDa polypeptide (VAP100) is a host-derived minor component of the viral envelope

We investigated a minor polypeptide component of 100-kDa detected in the rabies virion (referred to as VAP100) by using a monoclonal antibody (mAb), #16743, which was shown to recognize the SDS-denatured VAP100 antigen by immunoblot analyses. Although the VAP100 antigen was hardly detectable in the cell by usual immunoblot methods with this mAb, we could detect the antigen by a luminescent immunoblot method as well as by immunoprecipitation from the metabolically radiolabeled cell lysates and virions. Fluorescent antibody (FA) staining with mAb #16743 detected the uniformly distributed antigen on the formalin-fixed normal BHK-21 cells, while slight accumulation of the antigen was also seen in the Golgi area when the cells were permeabilized by treatment with Triton X-100 after fixation. Rabies virus infection induced alteration of the behavior of VAP100 to show a spotted distribution pattern in virus-infected cells. Double FA staining with mAb #16743 and rabbit antibody against the rabies virus envelope antigen demonstrated colocalized distribution of the viral envelope antigens and VAP100 in the cell. From these results, we think that VAP100 is a membrane-associated component of the cell, and its colocalized distribution with the viral envelope antigens in the cell implicates an intimate association of the VAP100 with viral envelope protein(s) and a reflection of possible involvement in the efficient incorporation of VAP100 into the virion.

Immunological studies of a 21 kDa cellular component efficiently incorporated into rabies virion grown in a BHK-21 cell culture

To investigate cellular components incorporated into the rabies virion, monoclonal antibodies (MAbs) were screened based on their reactivity with additional virion components. Two of the MAbs we prepared recognized a virion-associated 21 kDa polypeptide (referred to as VAP21) from a BHK-21 cell. Since the MAbs precipitated the rabies virion and trypsin digestion eliminated the VAP21 antigen from the virion but alkaline treatment (pH 11) did not, VAP21 seems to be anchored into the viral envelope and exposed on the virion surface. Although quantitative immunoblot analyses indicated an apparently increased concentration of VAP21 in the virion, the ratio of the content of VAP21 to that of viral glycoprotein (G) was several times decreased as compared to the ratio of those in the cell. These data suggest that sorting of VAP21 occurs during the viral budding process on the cell but that it might be inefficient, probably due to a more intimate association of VAP21 with the viral envelope proteins. This assumption seems to be consistent with the results of immunofluorescence studies; that is, VAP21 displayed colocalized distribution with viral envelope antigens in the cell. From these results, it is suggested that VAP21 closely associates with the viral envelope proteins in the cell, and this association might cause passive but relatively efficient incorporation of VAP21 into the virion.

Human cervical carcinoma cell lines contain an antigen identical to the tumor-specific 75 kDa antigen of HeLa cells: detection by viral acquisition

Purified vesicular stomatitis virus grown in the human cervical carcinoma HeLa cell line, VSV(HeLa), contains a 75 kDa tumor-specific antigen, detectable by immunoblotting of electrophoretically separated proteins with rabbit antiserum made against whole HeLa cells. Nearly identical results were obtained with VSV grown in the tumorigenic human hybrid ESH-5L cells, but not with the matched non-tumorigenic ESH-5E cells. Growth of VSV in 4 other independently isolated human cervical carcinoma cell lines led to the concentration of the same 75 kDa tumor-specific antigen by VSV. Infection of 2 other human cervical carcinoma cell lines did not lead to the detection of this antigen. The expression of the tumor-specific antigen correlated directly with the amount of RNA expression from human papillomavirus integrated in the DNA of these cells, irrespective of whether the papillomavirus was type 16 or 18.

The M protein of vesicular stomatitis virus associates specifically with the basolateral membranes of polarized epithelial cells independently of the G protein

Using monoclonal antibodies and indirect immunofluorescence microscopy, we investigated the distribution of the M protein in situ in vesicular stomatitis virus-(VSV) infected MDCK cells. M protein was observed free in the cytoplasm and associated with the plasma membrane. Using the ts045 mutant of VSV to uncouple the synthesis and transport of the VSV G protein we demonstrated that this distribution was not related to the presence of G protein on the cell surface. Sections of epon-embedded infected cells labeled with antibody to the M protein and processed for indirect horseradish peroxidase immunocytochemistry revealed that the M protein was associated specifically with the basolateral plasma membrane. The G and M proteins of VSV have therefore evolved features which bring them independently to the basolateral membrane of polarized epithelial cells and allow virus to bud specifically from that membrane.

An adenovirus 2-coded tumor antigen located on the endoplasmic reticulum and nuclear envelope is required for growth of transformed cells in Ca2+-deficient media

Rat embryo cell lines containing the adenovirus 2 E1a region together with normal or mutant forms of the N-terminal half of the E1b region (HindIII G fragment) were generated by using a dominant selection marker, neo. Biochemically transformed cells containing a nonmutated HindIII G fragment proliferated more rapidly in Ca2+-deficient media, whereas cells containing a specific deletion within the E1b-encoded, 175-amino-acid (175R) (19-kilodalton) T-antigen gene and nontransformed cells grew at a slower rate. Furthermore, transformed cells that did not express the 175R T antigen and untransformed cells could not replicate their DNA efficiently in low-Ca2+ medium. Our results suggest that Ca2+ ions may provide an important stimulus for cell proliferation in adenovirus-transformed cells through a mechanism that involves the functions of the 175R T antigen.

Production of acquired immunodeficiency syndrome-associated retrovirus in human and nonhuman cells transfected with an infectious molecular clone

We constructed an infectious molecular clone of acquired immunodeficiency syndrome-associated retrovirus. Upon transfection, this clone directed the production of infectious virus particles in a wide variety of cells in addition to human T4 cells. The progeny, infectious virions, were synthesized in mouse, mink, monkey, and several human non-T cell lines, indicating the absence of any intracellular obstacle to viral RNA or protein production or assembly. During the course of these studies, a human colon carcinoma cell line, exquisitely sensitive to DNA transfection, was identified.

An adenovirus 2-coded tumor antigen located on the endoplasmic reticulum and nuclear envelope is required for growth of transformed cells in Ca2+-deficient media

Rat embryo cell lines containing the adenovirus 2 E1a region together with normal or mutant forms of the N-terminal half of the E1b region (HindIII G fragment) were generated by using a dominant selection marker, neo. Biochemically transformed cells containing a nonmutated HindIII G fragment proliferated more rapidly in Ca2+-deficient media, whereas cells containing a specific deletion within the E1b-encoded, 175-amino-acid (175R) (19-kilodalton) T-antigen gene and nontransformed cells grew at a slower rate. Furthermore, transformed cells that did not express the 175R T antigen and untransformed cells could not replicate their DNA efficiently in low-Ca2+ medium. Our results suggest that Ca2+ ions may provide an important stimulus for cell proliferation in adenovirus-transformed cells through a mechanism that involves the functions of the 175R T antigen.

Further characterization of proteins assembled by vesicular stomatitis virus from human tumor cells

Vesicular stomatitis virus (VSV), when reproduced in human tumor cell lines, assembled a specific subset of cell-derived proteins. These were detected by 35S]methionine labeling of cells prior to infection and subsequent immunoprecipitation of VSV grown in these cells, as well as by direct immunoprecipitation of labeled cell extracts with antiserum directed against the VSV-assembled proteins. Their molecular weight (Mr) ranged between 15K and 180K; the larger proteins were glycosylated. Two of the major protein species (gp88 and gp130) were common to all four cell lines used (HeLa-cervical carcinoma, T47D-breast carcinoma, and HMB2 and SK1477-two melanoma cell lines). Proteins of other molecular weights were detected only in one or two of the cell lines. The melanoma cell lines (even in the absence of VSV) shed large particulate material which had contained the same spectrum of proteins that were assembled by VSV. The major protein component had an Mr of 30K. Some of the VSV-assembled proteins might possibly serve as specific tumor markers. It is also conceivable that the proteins assembled by VSV as well as the large particulate material might be products of defective endogenous human retroviruses.

Monoclonal antibody against an antigen selectively assembled into vesicular stomatitis virus virions from HeLa cells

A mouse hybridoma cell line IIB9, secreting IgG2b antibody specific for a HeLa cell antigen, was obtained by fusion of a mouse myeloma cell line with spleen cells from mice immunized with purified VSV tsO45 mutant (defective in assembly of G protein) which had been reproduced at a non-permissive temperature in HeLa cells. The monoclonal antibody IIB9 was strictly specific for HeLa cells in two tests: (1) reaction with VSV or Chandipura virus phenotypically mixed with host cell antigen, (2) complement-dependent cytotoxicity test (51Cr-release).

Immunoprecipitating human antigens associated with vesicular stomatitis virus grown in HeLa cells

Vesicular stomatitis virus (VSV) preparations made in HeLa cells, VSV(HeLa), appeared to contain non-viral structural proteins. This was suggested by neutralization of the virus with homologous and heterologous antisera made against VSV prepared in different cells. Antisera against uninfected HeLa cells failed to neutralize VSV(HeLa) but did immunoprecipitate the virus in the presence of Staphylococcus aureus. These immunoprecipitated VSV(HeLa) retained their infectivity, despite the presence of antibody and bacteria. The anti-HeLa cell serum did not react with VSV grown in rodent cells nor did anti-Vero cells serum immunoprecipitate VSV(HeLa). When the anti-HeLa cell serum was absorbed with whole HeLa cells, it no longer specifically precipitated VSV(HeLa). Because over 98% of infectious VSV(HeLa) was neutralizable by anti-VSV serum and immunoprecipitable by anti-HeLa serum, these virions were called mosaics. Physical identification of HeLa cell determinants on the mosaics was accomplished by further purification and radioiodination followed by selective immunoprecipitations with antisera. Two to three major bands with molecular weights around 75,000 Da were identified as HeLa cell determinants associated with the mosaic VSV(HeLa).

Identity of HeLa cell determinants acquired by vesicular stomatitis virus with a tumor antigen

Growth of vesicular stomatitis virus (VSV) in HeLa cells results in progeny containing non-VSV antigens with a molecular weight around 75,000. The non-VSV antigens were detected by antiserums to HeLa cell determinants. These antiserums precipitate whole virions but do not neutralize them. Because one of the antiserums is directed to a tumor-specific surface antigen of HeLa cells, it appears that VSV specifically acquires such antigens during its passage through human tumor cells.