Penetration of viral genetic material into host cell

Mechanism of neutralization of budded Autographa californica nuclear polyhedrosis virus by a monoclonal antibody: Inhibition of entry by adsorptive endocytosis

Autographa californica nuclear polyhedrosis virus (AcNPV) is characterized by two different phenotypes, each with a specific role in the life cycle of the virus in nature. They differ widely in infectivity both in vivo and in vitro, and are neutralized by different populations of antibodies. A monoclonal antibody designated AcV1 neutralizes one phenotype, BV, by reacting with a major envelope antigen not present on the other phenotype. BV can functionally enter cells by two different pathways and AcV1 neutralizes BV by preventing it from using its preferred pathway, adsorptive endocytosis. Further, a "nonneutralizable" fraction of BV in the presence of antibody excess enters by the alternative pathway. The difference in infectivity between the two phenotypes of AcNPV can be attributed to mechanisms of functional entry, as only BV can enter by the more efficient adsorptive endocytosis.

Chapter 12. Antiviral Agents

This chapter discusses the agents with activity primarily against RNA viruses. The communicable diseases of the respiratory tract are probably the most common cause of symptomatic human infections. The viruses that are causative agents for human respiratory disease comprise the five taxonomically distinct families: orthomyxoviridae, paramyxoviridae, picornaviridae, coronaviridae, and adenoviridae. The influenza viruses, which consist of types A, B, and C, belong to the family orthomyxoviridae. Types A and B have been associated with significant increases in mortality during epidemics. The disease may be asymptomatic or cause symptoms ranging from the common cold to fatal pneumonia. Immunization against influenza has been recommended for high-risk groups and antiviral chemotherapy (amantadine) is available for the treatment and prophylaxis of all influenza A infections. There is both a great need for and interest in developing a chemotherapeutic agent for the treatment of these two viral, respiratory tract pathogens. The family picornaviridae contains the genus Rhinovirus that is composed of over a hundred distinct serotypes. Amantadine and rimantadine are specifically active against influenza A virus infections. The amantadine recipients reported a higher incidence of side effects largely attributed to the central nervous system (CNS) symptoms. This difference in side effects may be a pharmacokinetic phenomenon that results in higher plasma concentrations of amantadine. Significant progress continues to be made in the clinical use and development of agents active against DNA viruses. Acyclovir (9-(2-h droxyethoxymethyl)guanine) has been the subject of several reviews and of a syrnposium. Considerable progress has been made in evaluating the clinical promise of acyclovir; however, there remains much to be learned concerning the best use of this drug in clinical practice. Significant strides have been made in the development of clinically useful antiviral agents, especially against the DNA viruses of the herpes family. Most of these agents are directed against viral nucleic acid synthesis and require activation by a virus-induced thymidine kinase. Researchers have begun to focus on other strategies that may produce broader spectrum anti-viral agents with different mechanisms of action.

Charged polymers modulate retrovirus transduction via membrane charge neutralization and virus aggregation

The specific mechanisms of charged polymer modulation of retrovirus transduction were analyzed by characterizing their effects on virus transport and adsorption. From a standard colloidal perspective two mechanisms, charge shielding and virus aggregation, can potentially account for the experimentally observed changes in adsorption behavior and biophysical parameters due to charged polymers. Experimental testing revealed that both mechanisms could be at work depending on the characteristics of the cationic polymer. All cationic polymers enhanced adsorption and transduction via charge shielding; however, only polymers greater than 15 kDa in size were capable of enhancing these processes via the virus aggregation mechanism, explaining the higher efficiency enhancement of the high molecular weight molecules. The role of anionic polymers was also characterized and they were found to inhibit transduction via sequestration of cationic polymers, thereby preventing charge shielding and virus aggregation. Taken together, these findings suggest the basis for a revised physical model of virus transport that incorporates electrostatic interactions through both virus-cell repulsive and attractive interactions, as well as the aggregation state of the virus.

Viral multiplicity of attachment and its implications for human immunodeficiency virus therapies

The multiplicity of attachment (MOA) of a virion in any particular time interval is the average number of cellular attachment opportunities that must be blocked to keep the virion in suspension. MOA is usually proportional to incubation time and cell concentration. Low MOA (like low multiplicity of infection) is required for reproducible assay of adsorptive blockers, and high MOA by itself can produce spurious synergies between adsorptive blockers, e.g., soluble CD4 (sCD4) and some antibodies. Poliovirus and human immunodeficiency virus (HIV) data show that viral neutralization conforms quantitatively to MOA and kinetic theory over large ranges of incubation times and target cell concentrations. Extrapolating sCD4 data beyond conditions achievable in vitro to those in vivo predicts that sCD4 concentrations above the strain-specific sCD4-gp120 dissociation constant are required to block lymphoid HIV significantly, in at least semiquantitative agreement with clinical results. The extrapolation is applicable to humoral neutralization data as well. MOA analysis also indicates that although completely stopping the attachment of individual virions to cells may still be an effective therapeutic strategy against established HIV infection, merely retarding attachment probably is not. The concept of MOA holds great promise for improving the therapeutic relevance of in vitro data and can be applied to any infectious agent, to many processes that impair or enhance infection steps, and to many assay end points, not just infection.

General analysis of receptor-mediated viral attachment to cell surfaces

Viruses are multivalent particles that attach to cells through one or more bonds between viral attachment proteins (VAP) and specific cellular receptors. Three modes of virus binding are presented that can explain the diversity in binding data observed among viruses. They are based on multivalency of attachment and spatial versus receptor saturation effects which are easily distinguished based upon simple criteria. Mode 1 involves only monovalent virus/receptor binding. Modes 2 and 3 involve multivalent bonds between the virus and cell; however, in mode 3 space on the cell surface becomes saturated before receptors. A model is developed for viral attachment that accounts for nonspecific binding, receptor/virus interactions, and spatial saturation effects. The model can describe each mode in different limits and can be applied to virus binding data to extract key physical information such as receptor number and affinity. These values are used to postulate the type of VAP/receptor interaction involved and to predict binding at different parameter values. For the mode 2 binding of Adenovirus 2, the model predicts a receptor number of 4-15 x 10(3) on HeLa cells and an affinity of 2-6 x 10(7) M-1 which closely approximate experimental estimates. For the binding of three, broad-host-range, enveloped viruses, Semliki Forest virus, Vesicular Stomatitis virus, and the baculovirus, Autographa californica nuclear polyhedrosis virus, the model predicts receptor numbers of 10(5) or greater and affinities in the range of 10(4) to 10(5) M-1. These values are indicative of a VAP/oligosaccharide interaction which has been documented for a number of other viruses. Experimental evidence is presented that is the first to demonstrate that baculovirus binding is mediated by a cell surface receptor.

Interaction of African swine fever virus with macrophages

Morphological data obtained by electron microscopy have shown that African swine fever virus adapted to VERO cells enters swine macrophages, its natural host cell, by a mechanism of receptor-mediated endocytosis. Binding studies with 3H-labeled virus and competition experiments with UV-inactivated virus have shown that the virus entry that leads to a productive infection in swine macrophages is mediated by saturable binding sites on the plasma membrane. The virus also penetrated into rabbit macrophages that do not produce infectious virus and initiated the synthesis of some early viral proteins; however, the viral replication cycle was aborted since viral DNA synthesis did not occur. The interaction of ASF virus particles with rabbit macrophages was mediated by nonsaturable binding sites, suggesting that the lack of specific receptors in these cells may be related to the absence of a productive infection. A similar abortive infection was detected in macrophages from other virus-resistant animal species.

Unusual features of protein interaction in human immunodeficiency virus (HIV) virions

The treatment of HIV-1 virions with ionic and nonionic detergents (NP 40, octylglucoside, Na deoxycholate) resulted in an effect unusual for enveloped viruses: instead of solubilization of glycoproteins, the core protein p24 was solubilized while envelope glycoproteins with other structural proteins were found in subviral particles. These data are consistent with a model of HIV structural organization in which glycoproteins are included in the matrix formed by the protein p 17 and suggest that p24 is neither involved in the matrix nor closely bound to any viral proteins.

The function of the vaccinia hemagglutinin in the proteolytic activation of infectivity

The vaccinia virus hemagglutinin (HA) has specific affinity for the structural protein, VP37K. The nature of this affinity and its relationship to the function of the HA were analyzed using HA mutants. The VP37K reactive site of the HA molecule is located in its transmembrane region, and the vaccinia virus HA associates with the viral particle via the VP37K-HA affinity. The viruses possessing an HA with fusion inhibitor activity were largely of the low infectivity form, whereas the viruses that associated mutant HAs defective in the activity were of the high infectivity form. D1 mutant virus does not produce HA. When it was incubated with the HA of the IHD-J strain, the HA associated with the virus particle. The HA-loaded D1 mutant virus acquired a high affinity not only for chick erythrocytes but also for KB and Vero cells. At the same time, the infectivity for Vero cells was decreased. The original high infectivity was recovered by treatment with trypsin. The virion-associated vaccinia HA has two functions; the HA protects the infectivity of the virus by the fusion inhibitor activity and exhibits affinity against host cells. Vaccinia virus first adsorbs to the cell via HA, and then proteolysis of the HA activates the second adsorption site which seems to be the fusogenic site of the virus. Proteolytic activation represents removal of the fusion inhibitor activity of the HA.

Towards the development of antimicrobial drugs acting by inhibition of pathogen attachment to host cells: a need for polyvalency

The development of inhibitors of microbial attachment to target cells has been proposed recently as a possible novel approach to antimicrobial chemoprophylaxis and treatment. In this paper an attempt is made to contend that such artificial inhibitors must be polyvalent, i.e. capable of binding to the pathogen or its target by multiple bonds.

The entry of African swine fever virus into Vero cells

The entry of African swine fever virus into Vero cells has been investigated by both biochemical and morphological techniques. A quantitative electron microscopy analysis of the early steps of the infection has shown that African swine fever virus enters Vero cells by a receptor-mediated endocytosis mechanism. The internalization of virus particles is a temperature- and energy-dependent process, since it did not take place at 4 degrees or in the presence of NaF and 2,4-dinitrophenol. To determine the involvement of acidic intracellular vacuoles in the virus entry pathway we have tested the effect of lysosomotropic agents in the infection. Chloroquine, dansylcadaverine, amantadine, methylamine, and ammonium chloride inhibited African swine fever virus production in Vero cells. Dansylcadaverine and chloroquine did not inhibit virus adsorption and internalization; however, in the presence of these drugs, virus particles were retained in cytoplasmic vacuoles and early viral RNA and protein synthesis were not detected, indicating that these compounds inhibit an early step in the infectious cycle, probably the uncoating of the virus particle.

Ultrastructural features of the intestinal absorption of mouse mammary tumor virus in newborn BALB/cfRIII mice

The retrovirus mouse mammary tumor virus is present in mouse strains with a high incidence of mammary tumors as a causative agent. It is produced mainly in the mammary glands of sexually mature females and is milk-transmitted to newborns. The fate of the mouse mammary tumor virus is almost unknown. Where it enters, how it is distributed, and where it remains latent, remain unresolved problems. This study tries to answer the first of these questions. Viruses are for the most part digested in the stomach. Very few well-preserved B particles, i.e., the infective particles, are allowed to enter through a process of endocytosis, mainly in the newborn-type epithelial cells. These are epithelial cells with a very rich absorptive apparatus, characteristic of newborn rodents. The adult-type absorptive cells and the M cells of the Peyer's patches might be partly involved.

Interaction of influenza virus with gangliosides and liposomes containing gangliosides

It has already been shown that influenza virus binds unspecifically to liposomes containing ganglioside GM1 wheras with gangliosides GD1b and GT1b binding occurs in a specific and saturable manner [Slepushkin et al. (1986) Biol. Membr. 3, 229-235]. In the present study the mode of interaction between influenza virus and various gangliosides or phospholipid liposomes containing cholesterol and gangliosides has been investigated. The influence of exogenous gangliosides on the structure of the viral envelope was studied using fluorescent and photoactivatable phospholipids incorporated into the viral membrane. With both types of probes maximal effects of gangliosides were caused by GT1b. Addition of that ganglioside resulted in a marked decrease in the fluorescence polarization (P) of fluorescent labeled virus as well as in substantial changes in the binding of photoactivatable analogues of sphingomyelin and phosphatidylcholine to virus proteins, mainly hemagglutinin. The effects of GT1b and GD1b on P value were comparable, whereas gangliosides with other oligosaccharide chains caused much smaller changes in P. Furthermore GT1b but not GM1 influenced phospholipid-hemagglutinin cross-linking. Interaction of the virus with large unilamellar liposomes was monitored by two fluorescence assays based on resonance-energy transfer from the tryptophans and tyrosines of viral proteins to vesicles labeled with a triacylglycerol (anthrylvinyldioleoylglycerol) or from these labeled vesicles to virions labeled with a perylenoyl derivative of galactosylcerebroside (PGalSph). A third fluorescence assay was based on relief of self-quenching in PGalSph-labeled virions, upon low-pH-induced virus-liposome fusion. With all three fusion assays the changes of fluorescence caused by GT1b were more pronounced than those induced by GM1. On the other hand, virus-induced release of [14C]glucose from multilamellar liposomes was enhanced by GM1 but not by GT1b or GD1b. It is concluded that the interaction of GT1b or GD1b with virus hemagglutinin induces a rearrangement of the viral lipids rendering lipid bilayer areas of the viral envelope significantly fluid, which in turn promotes fusion of the virus with target membranes. Probably virus-liposome fusion and virus-induced liposome leakage are brought about by different mechanisms depending on specific or unspecific binding of the virions to the target.

The surface receptor is a major determinant of the cell tropism of influenza C virus

N-Acetyl-9-O-acetylneuraminic acid (Neu5,9Ac2) has been shown to be a high-affinity receptor determinant for attachment of influenza C virus to erythrocytes (G. N. Rogers, G. Herrler, J. C. Paulson, and H-D. Klenk, 1986, J. Biol. Chem. 261, 5947-5951). In this report the nature of the cell surface receptor for influenza C virus on tissue culture cells was analyzed. Pretreatment with either neuraminidase or neuraminate 9-O-acetylesterase was found to render LLC-MK2 cells resistant to infection by influenza C virus as evidenced by the failure to detect virus release into the medium by hemagglutination titration. Susceptibility to infection was fully restored after incubation of neuraminidase-treated cells with bovine brain gangliosides known to contain Neu5,9Ac2. These results indicate that (i) Neu5,9Ac2 is the primary receptor determinant required for influenza C virus to attach to tissue culture cells and to initiate infection and (ii) gangliosides containing this type of sialic acid are potential receptors for influenza C virus. Several cell lines which are resistant to infection by this virus were able to release influenza C virus into the medium provided they were incubated with bovine brain gangliosides prior to virus infection. This result indicates that lack of appropriate receptors on the cell surface is a major reason for the restricted cell tropism of influenza C virus.

Effects of inhibitors of the cytoplasmic structures and functions on the early phase of infection of cultured cells with simian virus 40

To obtain information about cytoplasmic structures and functions involving the entry of simian virus 40 virions into cells, we examined whether the inhibitors that affect the functions and/or structure of lysosomes, cell membrane, and cytoskeletons inhibit expression of nuclear T antigen in the SV40-inoculated rat 3Y1 and monkey CV-1 cells. Chloroquine, methylamine, and butylamine did not inhibit T-antigen expression, suggesting that lysosomal acidification is not required for establishment of infection. Cytochalasin B had no effect, suggesting that microfilaments are not involved. Monensin, colcemid, and amantadine each inhibited T-antigen expression at doses causing no obvious cytotoxicity. Maximal inhibition was seen when these inhibitors were added to the cultures within 1 hr (monensin), within 4 hr (colcemid), or within 12 hr (amantadine) after virion adsorption to the cell surface. When the inhibitor was present in the virus-inoculated cultures for 24 hr and then removed, nuclear T antigen began to be expressed at 4 hr (monensin), 9 hr (colcemid), or 1 hr (amantadine) after removal of the inhibitors. Results of SDS-PAGE analysis of immunoprecipitated radiolabeled proteins of infected cells revealed that amantadine inhibited synthesis of large and small T antigens as well as general protein synthesis. Inhibition by colcemid may be due to disruption of microtubules, because other microtubule-disrupting agents (colchicine, vinblastine, nocodazole, and podophyllotoxin) also inhibited appearance of nuclear T antigen but lumicolchicine and taxol did not. Electron microscopy revealed that, in the presence of colcemid, although the adsorbed virions were readily internalized to form pinosomes, vectorial movement of the pinosomes to the nucleus appeared to be inhibited. Results of electron microscopy also suggest that inhibition by monensin may occur mainly in internalization of adsorbed virions and that the inhibition is leaky such that the early steps of infection proceed slowly in the presence of monensin. We conclude that monensin, colcemid, and amantadine interfere with mutually different early events of SV40 infection.

Modification of vaccinia virus penetration proteins analyzed by monoclonal antibodies

Modifications induced in structural vaccinia virus proteins that elicit the high infectious state by virus activating treatments involving trypsin and phosphatidylserine were analyzed using antivaccinia monoclonal antibodies (MABs). MABs reactive against each of the five outer layer proteins (VP54K, 34K, 32K, 29K, and 17K-25K) neutralized infectivity. VP54K possesses at least two neutralizing epitopes. Treatment with trypsin or with isolated plasma membrane cleaved VP54K into TVP41K carrying epitope A and removed a fragment containing epitope B from the virus. MABs against either of the epitopes could neutralize the virus. The exposure of epitope A concomitantly activated virus infectivity, and it was an essential step of penetration. MABs against VP17K-25K reacted more efficiently with trypsin-treated virus than with untreated virus, but the size of VP17K-25K was not affected by trypsin; this finding indicated that trypsin treatment rendered the VP17K-25K epitopes more accessible to antibody and hence to neutralization. MABs against VP32K and VP29K neutralized infectivity to the same extent irrespective of the state of activation. Virus treated with phosphatidylserine (PS) was neutralized more efficiently by MAB against VP34K than untreated virus, but the amount of antibody that reacted with the virus was the same before and after treatment with PS. Phosphatidylserine did not modify epitope structure itself, but it activated the function of VP34K. It was concluded that blocking of the functions attributed to any of the five proteins resulted in neutralization of virus infectivity, and treatment with trypsin and phosphatidylserine activates infectivity of vaccinia virus by modifying three of them (VP54K, VP34K, VP17K-25K) with characteristic behavior for each protein.

The mode of entry of herpes simplex virus type 1 into Vero cells

The mode of entry of herpes simplex virus type 1 (HSV-1) into Vero cells was investigated quantitatively with biological techniques. The entry of virus occurred rapidly when the virus-adsorbed cells were incubated at 37 C. The kinetics of virus entry was found to be similar to that of the process of uncoating, indicating that the uncoating of HSV-1 occurs simultaneously with the entry of virus into the cell. Experiments with ammonium chloride revealed that acidity in endosomes is not necessary for the entry or uncoating of HSV-1, in contrast with the cases of enveloped RNA viruses. In addition, endocytosis of the virus seems to be one of the processes of entry for HSV-1. However, the kinetics of endocytosis showed that the cell-bound virus is endocytosed gradually and suggested that the endocytosis of HSV-1 does not lead the virus to an uncoating process. These results are most consistent with a mechanism of entry for HSV-1 involving fusion of the viral envelope with the plasma membrane of the host cell.

Early interactions between animal viruses and the host cell: relevance to viral vaccines

Viral recognition of specific receptors in the host cell plasma membrane is the first step in virus infection. Attachment is followed by a redistribution or capping of virus particles on the cell surface which may play a role in the uptake process. Certain viruses penetrate the plasma membrane directly but many, both enveloped and non-enveloped viruses, are endocytosed at coated pits and subsequently pass into endosomes. The low pH environment of the endosome facilitates passage of the viral genome into the cytoplasm. For some viruses the mechanism of membrane penetration is now known to be linked to a pH-mediated conformational change in external virion proteins. As a consequence of infection there are alterations in the permeability of the plasma membrane which may contribute to cellular damage. Recent advances in the understanding of these processes are reviewed and their relevance to the development of new strategies for vaccines emphasised.

Monoclonal antibody that inhibits infection of HeLa and rhabdomyosarcoma cells by selected enteroviruses through receptor blockade

BALB/c mice were immunized with HeLa cells, and their spleen cells were fused with myeloma cells to produce hybridomas. Initial screening of culture fluids from 800 fusion products in a cell protection assay against coxsackievirus B3 (CB3) and the CB3-RD virus variant yielded five presumptive monoclonal antibodies with three specificities: protection against CB3 on HeLa, protection against CB3-RD on rhabdomyosarcoma (RD) cells, and protection against both viruses on the respective cells. Only one of the monoclonal antibodies (with dual specificity) survived two subclonings and was studied in detail. The antibody was determined to have an immunoglobulin G2a isotype and protected cells by blockade of cellular receptors, since attachment of [35S]methionine-labeled CB3 was inhibited by greater than 90%. The monoclonal antibody protected HeLa cells against infection by CB1, CB3, CB5, echovirus 6, and coxsackievirus A21 and RD cells against CB1-RD, CB3-RD, and CB5-RD virus variants. The monoclonal antibody did not protect either cell type against 16 other immunotypes of picornaviruses. The monoclonal antibody produced only positive fluorescence on those cells which were protected against infection, and 125I-labeled antibody confirmed the specific binding to HeLa and RD cells. The results suggest that this monoclonal antibody possesses some of the receptor specificity of the group B coxsackieviruses.

Fusion of influenza virus membranes with liposomes at pH 7.5

Influenza virus X-31 (H3N2) membranes fuse with liposomes containing ganglioside GD1a at pH 7.5. Fusion was demonstrated by electron microscopy and also can be measured by counting the labeled virus proteins incorporated into liposomes after bound virus has been removed. Liposomes composed of lipids that have no net charge behave as reported by other investigators and do not fuse with influenza X-31 membranes at neutral pH, but they do fuse at low pH. Therefore, the liposomal composition is a factor in whether liposomes fuse with influenza virus membranes at neutral pH, probably by determining whether binding occurs. The liposomal composition necessary for fusion at neutral pH needs to be individualized for each influenza subtype. To establish that a virus requires low pH for membrane fusion, it is first necessary to establish that fusion does not occur at neutral pH under conditions where adequate binding occurs.

Entry of adenovirus 2 into HeLa cells

Adenovirus 2 (Ad2) uncoating was analyzed as the destabilization of virions which renders the parental genome sensitive to DNase treatment. This event demonstrated a strong temperature dependence, and an Arrhenius plot of initial uncoating rates revealed an inflection point at around 16 degrees C. Activation energies of 331 kJ/mol below and 88 kJ/mol above this temperature were obtained for the uncoating process. Penetration of Ad2 through the plasma membrane was completely inhibited by sodium azide, whereas uncoating was only slightly influenced. This indicated that uncoating had already taken place at the outside of the plasma membrane. Incubations of Ad2 with isolated plasma membranes and cell homogenates showed that intact and metabolizing cells were required for uncoating. We further suggest, based on the inhibitory patterns of EDTA, EGTA, dansylcadaverine, and dithiothreitol, that this destabilization of virions follows upon reorganization in the plasma membrane. In the electron microscope the involvement of coated vesicles was shown for the initial uptake of virions, possibly followed by the engagement of acidic vesicles as judged from the effects of lysosomotropic agents on gene expression. The vectorial transport of virions from the plasma membrane to the nucleus was not affected by reagents interfering with the cytoskeletal system. Consequently, we propose that Ad2 virions are internalized by adsorptive endocytosis.

Inhibition of sendai virus-induced hemolysis by long chain fatty acids

A number of fatty acids were found to inhibit Sendai virus-induced hemolysis. cis-Unsaturated fatty acids such as oleate, as well as the methyl-branched isostearate, completely inhibited viral hemolysis at concentrations as low as 5-10 micrograms/ml, whereas the saturated, normal acids such as palmitate and stearate were comparably inhibitory only at 2-5 times those concentrations. trans-Unsaturated acids, as well as several other amphiphilic compounds, were either not or only weakly inhibitory. In contrast to their disparate effects on viral hemolysis, cis- and trans-unsaturated acids lysed erythrocytes in the same concentration range, which is several times higher than that at which the former compounds inhibited viral hemolysis. The mechanism of inhibition of viral hemolysis by isostearate involves the inactivation of viral hemolytic activity per se, since isostearate neither inhibited viral hemagglutination nor rendered erythrocytes significantly less susceptible to hemolysis. Furthermore, the concentration dependence of hemolysis inhibition by isostearate was biphasic, increasing sharply at the critical micelle concentration from a linear relationship below that concentration. Finally, an inhibitory concentration of isostearate was well below that at which amphiphiles dissolved membranes and did not dissolve Sendai virus, as shown by sucrose gradient centrifugation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It was concluded that low concentrations of fatty acids--particularly cis-unsaturated or fluid-phase types--could block the fusion, as opposed to agglutination, step of viral hemolysis by perturbing hydrophobic regions of the Sendai virus membrane.

Characterization of a virus that causes transient aplastic crisis

Transient aplastic crisis in children with congenital hemolytic anemias has been linked epidemiologically to infection with a serum parvovirus-like virus (SPLV). The virus is found in the blood in the early stages of the crisis, and serum containing SPLV inhibits erythroid colony formation in vitro. After sedimentation of virus-containing sera through a sucrose density gradient, colony inhibitory activity is present in the particulate fraction and separate from serum immunoglobulins. No inhibitory activity can be recovered from convalescent-phase sera after similar fractionation procedures. Inhibition of erythroid colony formation in vitro is not a feature of sera from other viral infections. The pattern of resistance of SPLV activity to chemicals and enzymes is compatible with it being a parvovirus. By using replating techniques, a target of SPLV has been identified as a late erythroid progenitor cell. Neither SPLV antigen nor anti-SPLV IgM was present in the sera of patients with other forms of bone marrow failure.

Virus receptor interaction in the adenovirus system. II. Capping and cooperative binding of virions on HeLa cells

Adenovirus type 2 attachment to HeLa cells was analyzed under controlled conditions. The temperature-dependent attachment kinetics revealed an inflection point at around 20 degrees C, and above this temperature the increase of the rate was doubled. In multiplicity dependence experiments, the attachment exhibited positive cooperative binding at 37 degrees C. This binding pattern was inhibited by low temperatures and prefixation of cells with 0.015% glutaraldehyde. Attachment of rhodamine-labeled virions revealed capping of the particles on 15% of the cells at 37 degrees C. Capping was inhibited by low temperatures, glutaraldehyde fixation of cells, and treatment with cytochalasin B, azide, and 2-deoxyglucose. Consequently, we propose that the adenovirus type 2 attachment to cells leads to rearrangements in the plasma membrane, resulting in cooperative binding and capping of the virus particles.