Octyl-beta-D-glucopyranoside extracts polyomavirus receptor moieties from the surfaces of mouse kidney cells

Virus receptors and tropism

Polyomaviruses are small, tumorigenic, nonenveloped viruses that infect several different species. Interaction of these viruses with cell surface receptors represents the initial step during infection of host cells. This interaction can be a major determinant of viral host and tissue tropism. This chapter reviews what is currently known about the cellular receptors for each of five polyomavirus family members: Mouse polyomavirus (PyV), JC virus (JCV), BK virus (BKV), Lymphotropic papovavirus (LPV) and Simian virus 40 (SV40). These polyomaviruses serve to illustrate the enormous diversity of virus-cell surface interactions and allow us to closely evaluate the role of receptors in their life cycles. The contribution of other factors such as transcriptional regulators and signaling pathways are also summarized.

Role of sialic acid-containing molecules and the α4β1 integrin receptor in the early steps of polyomavirus infection

Murine polyomavirus (MPyV) infection occurs through recognition of sialic acid (SA) residues present on the host cell membrane, but the nature of the molecules involved and the exact role of this interaction in virus cell entry still need to be clarified. In this work, mutations at residues R(77) or H(298) of the MPyV VP1 protein were shown to lead to a complete loss of virus infectivity, which, however, could be restored by lipofection of virus particles into the cytoplasm of the host cells. Using virus-like particles (VLPs), it was demonstrated that the non-infectivity of these mutants was due to impaired cell entry caused by total abrogation of SA-dependent cell binding. This indicates that SA residues are essential primary cell receptors for MPyV. As the alpha4beta1 integrin has been identified recently as a cell receptor for MPyV, the relationship, if any, was investigated between SA-containing and alpha4beta1 integrin receptors. The ability of mutants R(77)Q and H(298)Q and wt VLPs to bind to cells overexpressing the alpha4beta1 integrin was studied in SA-positive (BALB/c 3T3 cells and Pro-5 cells) and SA-deficient (Pro5-derived Lec-2 cells) backgrounds. Overexpression of alpha4beta1 integrin did not restore binding of mutant VLPs in any of these cell lines or, indeed, that of wt VLPs in a SA-deficient background. Moreover, evidence is provided that overexpression of the sialylated alpha4beta1 integrin enhances wt VLP cell binding, suggesting that, in addition to its function at a post-attachment level, alpha4beta1 integrin acts also as one of the SA-containing receptors for initial cell binding.

Bile acids enhance low density lipoprotein receptor gene expression via a MAPK cascade-mediated stabilization of mRNA

Recent studies have indicated that bile acids regulate the expression of several genes involved in bile acid and lipid metabolism as ligands for the farnesoid X receptor (FXR). We report here that bile acids are directly able to govern cholesterol metabolism by a novel mechanism. We show that chenodeoxycholic acid (CDCA) enhances low density lipoprotein (LDL) receptor gene expression in human cultured cell lines (HeLa, Hep G2, and Caco-2). The proteolytic activation of sterol regulatory element-binding protein-2 (SREBP-2), a major regulator for LDL receptor gene expression, is not affected by CDCA. Both deoxycholic acid and lithocholic acid as well as CDCA, but not ursodeoxycholic acid, increase the mRNA level for the LDL receptor, even when Hep G2 cells are cultured with 25-hydroxycholesterol, a potent suppressor of gene expression for the LDL receptor. Although it seems possible that FXR might be involved in genetic regulation, both reporter assays with a reporter gene containing the LDL receptor promoter as well as Northern blot analysis reveal that FXR is not involved in the process. On the other hand, inhibition of mitogen-activated protein (MAP) kinase activities, which are found to be induced by CDCA, abolishes the CDCA-mediated up-regulation of LDL receptor gene expression. We further demonstrate that CDCA stabilizes LDL receptor mRNA and that the MAP kinase inhibitors accelerate its turnover. Taken together, these results indicate that bile acids increase LDL uptake and the intracellular cholesterol levels through the activation of MAP kinase cascades in conjunction with a down-regulation of bile acid biosynthesis by FXR. This work opens up a new avenue for developing pharmaceutical interventions that lower plasma LDL by stabilizing LDL receptor mRNA.

A cell regulatory agent, CeReS-18, inhibits mouse 3T6 cell proliferation but not polyomavirus replication

We have purified a cell regulatory sialoglycopeptide, CeReS-18, from intact bovine cerebral cortex cells. This is an 18-kDa molecule that reversibly inhibits cellular DNA synthesis and the proliferation of a wide array of target cells. In the present study, the effect of CeReS-18 on mouse 3T6 host cell proliferation and polyomavirus replication was investigated. The results showed that CeReS-18 was able to inhibit 3T6 cell cycling in a concentration-dependent, calcium-sensitive, and reversible manner. Despite the inhibition of cell proliferation, CeReS-18 did not influence polyomavirus infection of 3T6 cells. Indirect immunofluorescent assays revealed that CeReS-18-treated, and cell cycle-arrested, 3T6 cells remained permissive to polyomavirus replication. Electron microscopy and immunogold labeling showed that new viral particles were assembled inside the nuclei of infected cells in the presence of CeReS-18 and during cell cycle arrest. The cellular requirements for the replication of polyomavirus DNA and the synthesis of viral proteins, as well as for the assembly of viral particles, therefore, remained available in CeReS-18-inhibited 3T6 cells. In addition, although polyomavirus infection can be mitogenic, infection of CeReS-18-treated 3T6 cells did not reverse the cell cycle arrest mediated by this cell cycle inhibitor.

Use of electron microscopic and immunogold labeling techniques to determine polyomavirus recombinant VP1 capsid-like particles entry into mouse 3T6 cell nucleus

Murine polyomavirus major structural protein VP1 could assemble into capsid-like particles when expressed in the baculovirus system. The recombinant capsid-like particles that were purified by CsCl density gradient centrifugation were capable of packaging host DNA. Electron microscopic and immunogold labeling techniques were used to study the entry of these VP1 recombinant capsid-like particles into mouse 3T6 cells. It was found that these VP1 recombinant capsid-like particles, which lack polyomavirus minor structural proteins (VP2 and VP3), use the same mechanism to enter mouse 3T6 cell cytoplasm and nucleus as that used by native polyomavirus virions.

Biochemical characterization of rotavirus receptors in MA104 cells

We have tested the effect of metabolic inhibitors, membrane cholesterol depletion, and detergent extraction of cell surface molecules on the susceptibility of MA104 cells to infection by rotaviruses. Treatment of cells with tunicamycin, an inhibitor of protein N glycosylation, blocked the infectivity of the SA-dependent rotavirus RRV and its SA-independent variant nar3 by about 50%, while the inhibition of O glycosylation had no effect. The inhibitor of glycolipid biosynthesis d, l-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) blocked the infectivity of RRV, nar3, and the human rotavirus strain Wa by about 70%. Sequestration of cholesterol from the cell membrane with beta-cyclodextrin reduced the infectivity of the three viruses by more than 90%. The involvement of N-glycoproteins, glycolipids, and cholesterol in rotavirus infection suggests that the virus receptor(s) might be forming part of lipid microdomains in the cell membrane. MA104 cells incubated with the nonionic detergent octyl-beta-glucoside (OG) showed a ca. 60% reduction in their ability to bind rotaviruses, the same degree to which they became refractory to infection, suggesting that OG extracts the potential virus receptor(s) from the cell surface. Accordingly, when preincubated with the viruses, the OG extract inhibited the virus infectivity by more than 95%. This inhibition was abolished when the extract was treated with either proteases or heat but not when it was treated with neuraminidase, indicating the protein nature of the inhibitor. Two protein fractions of around 57 and 75 kDa were isolated from the extract, and these fractions were shown to have rotavirus-blocking activity. Also, antibodies to these fractions efficiently inhibited the infectivity of the viruses in untreated as well as in neuraminidase-treated cells. Five individual protein bands of 30, 45, 57, 75, and 110 kDa, which exhibited virus-blocking activity, were finally isolated from the OG extract. These proteins are good candidates to function as rotavirus receptors.

Murine polyomavirus infection of 3T6 mouse cells shows evidence of predominant necrosis as well as limited apoptosis

The current study was developed to determine if polyomavirus infected 3T6 mouse cells evoked an apoptotic or a necrotic mechanism during infection. Infected cells were analyzed by flow cytometry, transmission electron microscopy (TEM), DNA electrophoresis and by measuring caspase-3 enzymatic activity. Infected cells that were analyzed at 72 h post-infection showed the following: flow cytometry analysis revealed a 5% increase in apoptotic cells and a 46% increase in necrotic cells when compared to uninfected cells; electron microscopy showed 10% cells with characteristic apoptotic morphology and 40% with necrotic appearance; caspase-3 activity was found to increase two fold when compared to uninfected cells and DNA fragmentation (laddering) was clearly evident late in infection. It was concluded that infected cells predominantly showed necrosis, although some cells showed apoptosis in late infection. Recombinant capsid-like particles composed of the polyomavirus structural proteins were not able to induce cell death.

Consequences of a subtle sialic acid modification on the murine polyomavirus receptor

Polyomaviruses are small, nonenveloped DNA tumor viruses with restricted host ranges. Virus binding to cell surface receptors is one determinant of viral tropism. Although murine polyomavirus is among the best characterized viruses, little is known about the sialic acid-containing receptor and its interaction with viral particles. By using nonradioactive virus binding assays as recently described for the B-lymphotropic papovavirus, murine polyomavirus particles were found to bind in a saturable and noncooperative manner to 25,000 receptors per 3T6 mouse fibroblast. The virus-receptor interaction at 4 degrees C was of high affinity (Kd = 1.8 x 10(-11) M), very fast (k1 = 1.7 x 10(7) M(-1) s(-1)), and stable (half-life = 38 min). Elongation of the N-acyl side chain of sialic acid by biosynthetic modulation with synthetic precursor analogs has been shown for other polyomaviruses to influence both sialic acid-dependent binding and infection (O. T. Keppler, P. Stehling, M. Herrmann, H. Kayser, D. Grunow, W. Reutter, and M. Pawlita, J. Biol. Chem. 270:1308-1314, 1995). In 3T6 cells in which about one-third of the sialic acids were modified, infection and binding of polyomavirus particles were significantly reduced. The number of receptors per cell was decreased to 18,000, with the remaining receptors displaying the same affinity as in untreated cells. Molecular modeling studies based on the three-dimensional structure of a mouse polyomavirus-sialyllactose complex recently solved by T. Stehle and coworkers (T. Stehle, Y. W. Yan, T. L. Benjamin, and S. C. Harrison, Nature 369:160-163, 1994) were performed. They suggest that the elongation of the N-acyl side chain by a single methylene group leads to steric hinderence, with the peptide backbone of a loop walling the tip of the shallow sialic acid binding groove. This collision appears to be incompatible with functional binding. The data are taken as a basis to discuss possible features of the organization and topology of the cellular receptor for mouse polyomavirus.

Protein cell receptors mediate the saturable interaction of African swine fever virus attachment protein p12 with the surface of permissive cells

Previous studies have demonstrated that the entry of African swine fever virus (ASFV) into Vero cells and swine macrophages is mediated by saturable binding sites located on the plasma membrane. The ASFV protein p12 has been implicated in virus attachment to the host cell, but the cellular component responsible for the interaction with the virus is largely unknown. We have studied the binding of recombinant p12 and ASFV to different cell lines. Permissive cells were able to bind p12 in saturable and nonsaturable interactions, as reported for ASFV. Experiments of binding recombinant p12 have been used for the initial characterization of the specific receptors on Vero cells. The treatment of cell surfaces with different enzymes and lectins resulted in the inhibition of the p12 binding activity by several proteases, but not by glycosidases or lipase, suggesting that the receptor is composed of protein, with no carbohydrates or lipids involved in the virus attachment to the cellular membrane. The recovery of receptor activity after pronase treatment was completed in 6 h in culture medium containing tunicamycin, and could not be restored in the presence of cycloheximide, confirming that synthesis of new proteins, but not glycosylation, was required for the recovery of the receptor activity. These data support the idea that membrane protein(s) on the surface of permissive cells act as receptors for ASFV and that this specific interaction is, at least, one necessary step in a productive virus infection.

Bile acid stimulation of early growth response gene and mitogen-activated protein kinase is protein kinase C-dependent

Hepatic stellate cells are exposed to elevated bile acid levels during hepatic injury and fibrogenesis. Upon activation, the stellate cell becomes a major effector cell during the development of hepatic fibrosis and cirrhosis. Bile acids may function as costimulatory signalling molecules. This hypothesis was tested in vitro using rat-derived hepatic stellate cells. Bile acids were studied at concentrations that occur during cirrhosis in vivo. Conjugated and unconjugated bile acids rapidly induced egr and fos gene expression as well as cytoplasmic mitogen-activated protein kinase (MAPK) activation. Protein kinase C was required for both egr induction and MAPK activation. These studies imply that bile acids could contribute to the perpetuation of hepatic fibrosis by helping to keep the stellate cell in an activated state.

Binding and internalization of human papillomavirus type 33 virus-like particles by eukaryotic cells

Infection of cells by human papillomaviruses (HPVs) associated with malignant genital lesions has not been studied because of the lack of an in vitro system and the unavailability of virions. We have now used virus-like particles (VLPs) of HPV type 33 to analyze the initial events in the interaction of the HPV capsid with cell lines. Binding of VLPs to HeLa cells was observed in biochemical assays and by immunofluorescence. VLP binding was inhibited by antisera raised against VLPs but not by monoclonal antibodies recognizing either L1 or L2 epitopes accessible on VLPs. Under saturating conditions, approximately 2 x 10(4) VLPs were bound per cell, with a dissociation constant of about 100 pM. VLPs composed of L1 alone bound as well as VLPs composed of both capsid proteins, indicating that L2 is not required for initial binding. VLPs dissociated into capsomers did not bind, demonstrating that intercapsomer contacts are required. Neither capsomers nor simian virus 40 virions competed with VLP binding. Uptake of VLPs by small and smooth endocytic vesicles was demonstrated by immunoelectron microscopy. Cellular binding of VLPs was sensitive to trypsin but not to sialidase, N-glycosidase, or octyl-beta-D-glycopyranoside treatment, suggesting that a cell surface protein is involved in the VLP binding. Cell lines originating from a variety of tissues and organisms as distantly related as insects and humans bound VLPs with similar efficiency and specificity. Therefore, the putative receptor mediating VLP attachment should be highly conserved and cannot be responsible for the species and tissue specificity of HPVs.

Characterization of protein involvement in rabies virus binding to BHK-21 cells

Prior studies established the specificity of rabies virus receptors on BHK-21 cells based on the saturability of the receptors and on competitive binding. In the present study, we used protease-treated cells to identify the involvement of protein in the specific binding of rabies virus to these cells. In addition, biochemical characterization of n-octylglucoside solubilized BHK-21 plasma membranes demonstrated the involvement of a protease sensitive, heat insensitive, integral membrane protein or protein complex in rabies virus binding to these cells. The membrane component that binds rabies virus is associated with a high molecular weight fraction of the n-octylglucoside-plasma membrane extract isolated by gel filtration. This high molecular weight fraction (approximately 450 KDa) is enriched with a cell surface integral membrane component that comigrates with denatured bovine serum fibronectin (220 KDa). This cellular component did not bind polyclonal antisera to fibronectin in Western blot (native or denatured) or immunoprecipitation experiments. Direct and specific virus binding to high molecular weight plasma membrane protein(s) separated on Western blots further confirmed the role of a protein receptor in rabies virus binding to these cells.

Comparison of bovine immune responses to affinity-purified bovine herpesvirus-1 antiidiotypes and glycoproteins

Bovine immune responses to rabbit antiidiotypic antibodies (anti-Id) against neutralizing monoclonal antibodies to bovine herpesvirus-1 (BHV-1) envelope glycoproteins and to BHV-1 glycoproteins were compared. Glycoprotein-immunized animals produced high titers of anti-BHV-1 antibodies and were protected against BHV-1 challenge. Recombinant bovine interleukin-2 (rIL-2)-treated, anti-Id-immunized animals showed a slight reduction in clinical disease, and one calf produced BHV-1-neutralizing antibodies. Treatment with rIL-2 augmented non-BHV-1-specific immune responses. However, even with rIL-2 as an adjuvant, the mixture of polyclonal anti-Id did not elicit a consistent, protective BHV-1-specific immune response in calves.

Class I major histocompatibility proteins are an essential component of the simian virus 40 receptor

The class I molecules encoded by the major histocompatibility complex (MHC) present endogenously synthesized antigenic peptide fragments to cytotoxic T lymphocytes. We show here that these proteins are an essential component of the cell surface receptor for simian virus 40 (SV40). First, SV40 binding to cells can be blocked by two monoclonal antibodies against class I human lymphocyte antigen (HLA) proteins but not by monoclonal antibodies specific for other cell surface proteins. Second, SV40 does not bind to cells of two different human lymphoblastoid cell lines which do not express surface class I MHC proteins because of genetic defects in the beta 2-microglobulin gene in one line and in the HLA complex in the other. Transfection of these cell lines with cloned genes for beta 2-microglobulin and HLA-B8, respectively, restored expression of their surface class I MHC proteins and resulted in concomitant SV40 binding. Finally, SV40 binds to purified HLA proteins in vitro and selectively binds to class I MHC proteins in a cell surface extract.

Reovirus binds to multiple plasma membrane proteins of mouse L fibroblasts

Plasma membranes from mouse L fibroblasts were isolated and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Resolved proteins were electroblotted to nitrocellulose paper and probed with 125I-labeled type 3 (T3) reovirus. Multiple protein bands with molecular weights ranging from 26 to 200 kDa were consistently recognized by the virus. Such binding was specific since it was blocked in the presence of unlabeled virus. That these proteins were exposed on the cell surface was confirmed by their susceptibility to sulfo-NHS-LC-biotin labeling of intact cells prior to membrane purification. Blots probed with wheat germ agglutinin (WGA)-gold showed a similar pattern of protein bands. These findings are consistent with the ability of WGA to block reovirus binding to L cells, and with our recent demonstration that the alpha-anomeric form of sialic acid is the minimal receptor determinant recognized by reovirus (R. W. Paul, A. H. C. Choi, and P. W. K. Lee, Virology 172, 382-385, 1989). Both type 1 and type 3 reoviruses were found to recognize the same set of multiple proteins on the blot, which is again consistent with the previous observation that the two serotypes compete with each other for binding to intact L cells.

Myristylated polyomavirus VP2: role in the life cycle of the virus

The double-stranded genome of the small DNA tumor virus, polyomavirus, is enclosed in a capsid composed of a major protein, VP1, which associates as pentameric capsomeres into an icosahedral structure, and two minor proteins, VP2 and VP3, whose functions and positions within the structure are unknown. The N-terminal glycine of the VP2 coat protein has been shown to be cotranslationally acylated with myristic acid. To study the function of this modification and the role of VP2 in the life cycle of polyomavirus, the N-terminal glycine, critical to the myristylation consensus sequence, has been altered to a glutamic acid or a valine residue by site-directed oligonucleotide mutagenesis. The glycine----glutamic acid mutant DNA has been further studied. When transfected into cells permissive for the polyomavirus full lytic life cycle, this mutant DNA replicated at levels comparable to those of wild-type viral DNA, and small amounts of nonrevertant (mutant) virus could be harvested from the cultures. The virus particles viewed by electron microscopy appeared slightly distorted, but the ratio of full to empty particles was similar to that produced in a wild-type viral infection. Mutant virus was capable of reinfecting permissive cells but with a considerably reduced efficiency.

Characterization of octyl glucoside-solubilized cell membrane receptors for binding measles virus

Optimum conditions were determined for solubilizing cell membrane receptors for binding measles virus (MV). Evidence for specific receptors was shown from the saturability of MV binding to intact Vero cells or African Green Monkey erythrocytes (MRBCs). Receptors, solubilized from Vero cells and MRBC with 1.5% octyl glucoside, inhibited MV attachment, infectivity, and hemagglutination activities. Extracts from chicken erythrocytes, which did not bind MV, were inactive in all assays. MV binding activity in Vero or MRBC extracts was stable to heating (100 degrees, 10 min) or neuraminidase treatment, but was inactivated by a range of proteases, including chymotrypsin, and bound to lentil and pea lectin agaroses, to indicate a glycoprotein component.

Synthesis, posttranslational modifications, and nuclear transport of polyomavirus major capsid protein VP1

Polyomavirus major capsid protein VP1 synthesis was studied in infected primary baby mouse kidney cells. A standard curve of VP1 protein was used to quantitate VP1 in the cytoplasm and nucleus of infected cells during the time course of infection. Polyomavirus VP1 continued to be accumulated in the cytoplasm of the cells until 27 h postinfection, at which time the synthesis of VP1 leveled off. VP1 continued to accumulate in the nucleus of the infected cells throughout the course of infection. The presence of the six isospecies, A to F, of polyomavirus VP1 was also studied to determine the relative quantity of each species during the time course of infection. All six species were found in the cytoplasm and nucleus of infected cells at various times postinfection. However, the relative quantity of each species was different at early as compared with later times of infection. In addition, phosphorylated VP1 was found in isolated polyribosomes of infected cells, suggesting that phosphorylation of VP1 is a cotranslational modification. Examination of the effect of macromolecular synthesis on the transport of VP1 into the nucleus of infected baby mouse kidney cells as well as the rate of its nuclear accumulation during and after protein synthesis inhibition revealed that the continual transport and accumulation of VP1 in the nucleus required protein synthesis.

Characterization of simian virus 40 receptor moieties on the surfaces of Vero C1008 cells

The nature of the simian virus 40 (SV40) receptor on the surfaces of Vero C1008 cells was investigated by a virus binding assay. The optimum pH for SV40 binding to cell surfaces was found to be at 6.5; however, there was little difference in SV40 binding in the range between pH 4.5 and 7.3. The treatment of cell surfaces with several proteases or with an enzyme specific for O-linked carbohydrates significantly reduced virus binding, suggesting that the receptor for SV40 contains protein and O-linked carbohydrates. Treatment of cell monolayers with octyl glucoside removed virus-binding activity from cell surfaces. Recovery of virus-binding activity by octyl glucoside-treated cells took 2.5 h and was inhibited by cycloheximide or tunicamycin. Four polypeptides with molecular weights of 90,000, 58,000, 54,000, and 30,000 were immunoprecipitated from virus-protein complexes derived from octyl glucoside extract solutions and therefore may be components of the SV40 receptor. Competition experiments between SV40 and polyomavirus revealed that these two viruses do not share the same receptor on Vero C1008 cells.

Entry of simian virus 40 is restricted to apical surfaces of polarized epithelial cells

The uptake of simian virus 40 (SV40) by polarized epithelial cells was investigated by growth of cells on permeable supports and inoculation on either the apical or the basolateral surface. Binding of radiolabeled SV40 occurred on the apical but not the basolateral surfaces of permissive polarized Vero C1008 cells and nonpermissive polarized MDCK cells. When similar experiments were performed on nonpolarized Vero or CV-1 cells, virus binding occurred regardless of the direction of virus input. Electron micrographs of Vero C1008 cells infected at high multiplicities revealed virions lining the surfaces of apically infected cells, while the surfaces of basolaterally infected cells were devoid of virus particles. Analysis of the binding data revealed a single class of virus receptors (9 x 10(4) per cell) with a high affinity for SV40 (Kd = 3.76 pM) on the apical surfaces of Vero C 1008 cells. Indirect immunofluorescence studies revealed that synthesis of viral capsid proteins in Vero C1008 cells occurred only when input virions had access to the apical surface. Virus yields from apically infected Vero C1008 cells were 10(5) PFU per cell, while yields obtained from basolaterally infected cells were less than one PFU per cell. These results indicate that a specific receptor for SV40 is expressed exclusively on the apical surfaces of polarized Vero C1008 cells.

Characterization of anti-idiotypic antibodies generated against foot-and-mouth disease virus neutralizing monoclonal antibodies

A series of seven neutralizing monoclonal antibodies (nMAbs) against type A12 foot-and-mouth disease virus (FMDV) was used to induce polyclonal anti-idiotypic antibodies (anti-ids) in rabbits. The anti-ids were semi-purified through isotype affinity columns and assayed by solid-phase radioimmunoassay for cross-reactivity. nMAbs which map to the same epitope on the virion appear to contain a common idiotype, and the corresponding anti-ids competitively inhibited the virus-nMAb reaction. Using a modified ELISA assay, it was possible to demonstrate binding of purified anti-ids to FMDV susceptible tissue culture cells. Such antibodies however, did not interfere with the binding of virus to cells, and the binding of anti-ids to FMDV receptor-negative cells could also be demonstrated. Mice were inoculated with purified anti-ids, and two elicited anti-viral antibodies, although these antibodies were non-neutralizing. Thus anti-ids to anti-FMDV nMAbs failed to react with cellular receptors for the virus, but were able to induce anti-viral antibody and thus should be examined as an alternative vaccine strategy for this virus.

Entry of simian virus 40 is restricted to apical surfaces of polarized epithelial cells

The uptake of simian virus 40 (SV40) by polarized epithelial cells was investigated by growth of cells on permeable supports and inoculation on either the apical or the basolateral surface. Binding of radiolabeled SV40 occurred on the apical but not the basolateral surfaces of permissive polarized Vero C1008 cells and nonpermissive polarized MDCK cells. When similar experiments were performed on nonpolarized Vero or CV-1 cells, virus binding occurred regardless of the direction of virus input. Electron micrographs of Vero C1008 cells infected at high multiplicities revealed virions lining the surfaces of apically infected cells, while the surfaces of basolaterally infected cells were devoid of virus particles. Analysis of the binding data revealed a single class of virus receptors (9 x 10(4) per cell) with a high affinity for SV40 (Kd = 3.76 pM) on the apical surfaces of Vero C 1008 cells. Indirect immunofluorescence studies revealed that synthesis of viral capsid proteins in Vero C1008 cells occurred only when input virions had access to the apical surface. Virus yields from apically infected Vero C1008 cells were 10(5) PFU per cell, while yields obtained from basolaterally infected cells were less than one PFU per cell. These results indicate that a specific receptor for SV40 is expressed exclusively on the apical surfaces of polarized Vero C1008 cells.

Early events in polyomavirus infection: fusion of monopinocytotic vesicles containing virions with mouse kidney cell nuclei

The entry of polyomavirus enclosed in monopinocytotic vesicles into mouse kidney cell nuclei was studied and evidence for a fusion mechanism was obtained. In vivo studies using the fluorescent lipophilic dye diI-C16(3) as a plasma membrane label showed that polyomavirus-infected nuclei accumulate plasma membrane, while uninfected or polyoma capsid-infected nuclei do not. Further evidence for fusion was obtained with electron microscopy of thin sections of infected mouse kidney cells. These specimens showed accumulation of plasma membrane in the outer nuclear membrane as well as evidence of recent fusion events. The polyoma virions (capsid proteins) were seen to accumulate on the inner nuclear membrane and in the nucleus and were identified by immunogold staining of the thin sections. The combined results of the in vivo dye studies and thin section immunoelectron microscopy studies provide evidence for a fusion mechanism for polyomavirus entry into mouse kidney cell nuclei.

Anti-idiotypic antibodies to a polyomavirus monoclonal antibody recognize cell surface components of mouse kidney cells and prevent polyomavirus infection

Anti-idiotypic antibodies have been successfully used to identify and isolate the receptor for several cell ligands. To prepare an immunologic probe for identification of the polyomavirus receptor on mouse kidney cells, polyclonal antisera against antipolyomavirus antibodies were prepared in rabbits. Fab fragments of the previously characterized monoclonal antibody E7, which neutralizes polyomavirus infection, were used for immunization (S. J. Marriott and R. A. Consigli, J. Virol. 56:365-372, 1985). Sera containing the greatest anti-idiotype activity were identified by enzyme-linked immunosorbent assay (ELISA) and purified by a series of affinity columns. The anti-idiotypic antibodies recognized the E7 idiotope in an ELISA, and anti-idiotype binding could be inhibited by preincubation of E7 monoclonal antibody with polyomavirus virions. When mixed with anti-idiotype immunoglobulin G (IgG), E7 was no longer capable of binding or immunoprecipitating polyomavirus virions or neutralizing polyomavirus infection. Direct immunofluorescence showed anti-idiotype IgG reactivity with a cell surface component of mouse kidney cells. Anti-idiotype F(ab')2 effectively competed with polyomavirus for binding to mouse kidney cells and displayed binding kinetics similar to those of polyomavirus. Virus infection of mouse kidney cells was blocked in a dose-dependent manner following treatment of the cells with anti-idiotype IgG. The anti-idiotype identified several proteins (95, 50, and 24 to 31 kilodaltons) in an immunoblot of mouse kidney cell membrane proteins but reacted predominantly with a single 50-kilodalton protein in a radioimmunoassay. The anti-idiotype failed to react with virus proteins in three assays, including ELISA, immunoprecipitation, and immunoblotting. The implications of this work for future identification and characterization of the polyomavirus receptor on mouse kidney cells are discussed.