Bispecific monoclonal antibody complexes facilitate erythrocyte binding and liver clearance of a prototype particulate pathogen in a monkey model.

We used Anger camera imaging in a monkey model to investigate the organ localization of a prototype particulate pathogen, 131I-labeled bacteriophage phi X174, after it was bound to the primate erythrocyte complement receptor and then cleared from the circulation. This 131I-labeled phi X174 was infused into the circulation of an immunized monkey, and the nascently formed immune complexes showed rapid and quantitative binding to erythrocytes via the immune adherence reaction (complement-mediated binding). Alternatively, phi X174 was infused into the circulation of a naive animal, and then cross-linked bispecific mAb complexes (heteropolymers, anti-CR1 x anti-phi X174) were infused into the circulation. The infused heteropolymers also facilitated rapid and quantitative binding of phi X174 to erythrocytes. In both cases, after a short lag period, the erythrocyte-bound phi X174 was rapidly cleared from the circulation, and the vast majority of the radiolabel was cleared to the liver, with a small amount clearing to the spleen. Further liver imaging confirmed that within 24 h most of the bacteriophage previously cleared to the liver via the heteropolymer system was phagocytosed and destroyed. The findings in this model system provide additional evidence for the potential utility of heteropolymers to facilitate the safe and rapid clearance of blood-borne pathogens as a potential treatment for infectious diseases.

Structure of a viral procapsid with molecular scaffolding.

The assembly of a macromolecular structure proceeds along an ordered morphogenetic pathway, and is accomplished by the switching of proteins between discrete conformations as they are added to the nascent assembly. Scaffolding proteins often play a catalytic role in the assembly process, rather like molecular chaperones. Although macromolecular assembly processes are fundamental to all biological systems, they have been characterized most thoroughly in viral systems, such as the icosahedral Escherichia coli bacteriophage phiX174. The phiX174 virion contains the proteins F, G, H and J. During assembly, two scaffoldingproteins B and D are required for the formation of a 108S, 360-A-diameter procapsid from pentameric precursors containing the F, G and H proteins. The procapsid contains 240 copies of protein D, forming an external scaffold, and 60 copies each of the internal scaffolding protein B, the capsid protein F, and the spike protein G. Maturation involves packaging of DNA and J proteins and loss of protein B, producing a 132S intermediate. Subsequent removal of the external scaffold yields the mature virion. Both the F and G proteins have the eight-stranded antiparallel beta-sandwich motif common to many plant and animal viruses. Here we describe the structure of a procapsid-like particle at 3.5-A resolution, showing how the scaffolding proteins coordinate assembly of the virus by interactions with the F and G proteins, and showing that the F protein undergoes conformational changes during capsid maturation.

Bispecific monoclonal antibody complexes bound to primate erythrocyte complement receptor 1 facilitate virus clearance in a monkey model.

We investigated the feasibility of using bispecific mAb complexes to redirect and improve the efficiency of the primate E complement receptor 1-based clearance reaction to remove a virus from the circulation. As an initial approach, we used bacteriophage phiX174 as an immunologic model for mammalian viruses. Bispecific complexes were prepared by chemically cross-linking a mAb specific for complement receptor 1 with a mAb specific for the bacteriophage phiX174. In a monkey model these complexes facilitate rapid and quantitative binding of the target bacteriophage to E in vitro and in vivo. Moreover, after in vivo binding to E, the complexes containing mAb and prototype virus are rapidly cleared from the circulation of rhesus and cynomolgus monkeys without loss of E. Our findings suggest that bispecific mAb complexes, in concert with primate E complement receptor 1, may have therapeutic utility in the treatment of diseases associated with blood-borne pathogens.

Calcium ion-induced structural changes in bacteriophage phi X174.

Monoclinic P2(1) crystals of the bacteriophage phi X174 have been incubated with calcium ions (Ca2+) and the induced structural conformational changes studied to 3 A resolution with X-ray crystallographic methods. Three different types of Ca2+ binding sites have been located within the asymmetric unit of the virion. Two sets of sites are associated with the F capsid protein. One set of sites associated with the F protein is in a general position near the icosahedral 3-fold axes of the virus, with the main-chain carbonyl oxygen atoms of residues Gly1321, Asp1421, Met1424 and Ser1426, and the side-chains of Gln1004 and Asp1421 as ligands. The other set of sites associated with the F protein is on the icosahedral 3-fold axes, with the symmetry-related main-chain carbonyl oxygen atoms of Ser1001 and the side-chains of Asn1002 as ligands. The bound Ca2+ induce a conformational change of the amino-terminal residues of the F proteins. A third set of sites, consisting of a pair of Ca2+ on the icosahedral 5-fold axes, are associated with the G spike protein and are concurrently liganded by the symmetry-related carbonyl oxygen side-chains of Asp2117. Concomitant with the binding of Ca2+ to the phage is the rotation of the Asp1209 side-chain of the F protein towards some additional electron density that was not observed in the absence of Ca2+. This density is situated in a shallow depression near the icosahedral 2-fold axes of the virus, and has been tentatively interpreted as a bound glucose molecule that is ordered only in the presence of Ca2+. The putative glucose binding site may be related to the attachment of the virus to cell surface lipopolysaccharides in the initial stages of Escherichia coli infection.

Structural basis for bacteriophage phi X174 assembly and eclipse as defined by temperature-sensitive mutations.

We present a screening procedure for classifying bacteriophage phi X174 mutants as defective in either "early" (genome delivery and RF DNA replication) or "late" (progeny ssDNA replication and morphogenesis) functions. When applied to 108 ts mutants, only five were classified as "early" mutants. Quantitative one-step growth curves confirmed the classification for 22 "late" mutants, while only one "early" mutant was correctly classified by the procedure. The other four "early" mutants gave burst sizes that required a third, or "intermediate," class of functional defects. The mutation in the "early" mutant mapped in gene A, which encodes an endonuclease required for RF replication. However, most of the mutations in the "late" and "intermediate" mutants mapped in genes for F, G, and H, three structural proteins found in mature virions. The amino acid substitutions for each mutation in F capsid and G "spike" genes were located in the atomic structure of wt virions. All but two replacements are on the outer surface of the virion or at interfaces between neighboring polypeptide chains, suggesting defects in protein-protein interactions involved in quaternary rearrangements during maturation. The kinetics of viral eclipse are also altered in most gene F "late" mutants, but only in one gene G "intermediate" mutant. Thus, the F capsid protein appears to have a central role in DNA ejection from the capsid.

Role of DNA-protein interactions in bacteriophage phi X174 DNA injection.

Like most bacteriophages, phi X174 transfers its DNA through the cell wall, leaving an empty capsid on the cell surface. The process begins with ejection of the genome at its host-receptor site. The rate of this event can be measured, so detailed structure/function analysis of the mechanism is possible now that an atomic structure of the phi X174 protein shell has been obtained. Amino acid substitutions at two arginine residues near the DNA-binding pocket of F capsid protein decrease the eclipse rate, while deletion of 27 bases from the J-F non-coding region increases the rate. An alanine to serine change in the N-terminal region of the phi X174 H "spike" protein has suppressor activity in that this mutation also increases the eclipse rate when the complete genome is present within both mutant and wild-type F capsids. These results suggest that a portion of H protein is inside the capsid, and disruption of DNA-protein interactions is involved in the ejection mechanism.

The three-dimensional structure of frozen-hydrated bacteriophage phi X174.

The three-dimensional structure of bacteriophage phi X174 (phi X174) was determined to approximately 2.6 nm resolution from images of frozen-hydrated 114 S particles. The outer surface of phi X174 is characterized by several prominent features: (i) 12 mushroom-shaped caps (approximately 7.1 nm wide x 3.8 nm high) are situated at each of the vertices of the icosahedral virion and extend to a maximum radius of 16.8 nm; (ii) a "collar" of density surrounds the base of each apical cap; and (iii) 20 conical protrusions (approximately 2.3 nm high) lie along the three-fold symmetry axes. The caps have a pentagonal morphology composed of five globular "subunits" and appear to be loosely connected to the underlying capsid. The distribution of the four gene products present in virions (60 copies each of gpF, gpG, and gpJ, and 12 copies of gpH), and the single-stranded DNA (ssDNA) genome cannot be directly discerned in the reconstructed density map, although plausible assignments can be made on the basis of solvent-excluded volume estimates and previous biochemical data. Thus, gpG accounts for most of the mass in the caps; gpH, a presumed cap protein, cannot be identified in part due to the symmetry-averaging procedures, but may be partially located within the interior of the capsid; and gpF and gpJ make up the remainder of the capsid. The genome appears to be less densely packaged inside the capsid compared to many dsDNA viruses whose nucleic acid is arranged in a liquid-crystalline state.

Atomic structure of single-stranded DNA bacteriophage phi X174 and its functional implications.

The mechanism of DNA ejection, viral assembly and evolution are related to the structure of bacteriophage phi X174. The F protein forms a T = 1 capsid whose major folding motif is the eight-stranded antiparallel beta barrel found in many other icosahedral viruses. Groups of 5 G proteins form 12 dominating spikes that enclose a hydrophilic channel containing some diffuse electron density. Each G protein is a tight beta barrel with its strands running radially outwards and with a topology similar to that of the F protein. The 12 'pilot' H proteins per virion may be partially located in the putative ion channel. The small, basic J protein is associated with the DNA and is situated in an interior cleft of the F protein. Tentatively, there are three regions of partially ordered DNA structure,

Differences in secondary structure between packaged and unpackaged single-stranded DNA of bacteriophage phi X174 determined by Raman spectroscopy: a model for phi X174 DNA packaging.

The single-stranded packaged genome (ssDNA) of bacteriophage phi X174 is shown by Raman spectroscopy to lack both the ordered phosphodiester backbone and base stacking, which are demonstrated for unpackaged, protein-free ssDNA. In solutions of moderate ionic strength, unpackaged ssDNA contains 36 +/- 7% of deoxyribosyl phosphate groups with conventional B-type backbone geometry [i.e., gauche- and trans orientations, respectively, for the 5'O-P (alpha) and 3'O-P (zeta) torsions], indicative of hairpin formation and intramolecular base pairing. Additionally, the bases of unpackaged ssDNA are extensively stacked. Estimates from Raman band hypochromic effects indicate that unpackaged ssDNA contains approximately 70% of the maximal base stacking exhibited in the linear, double-stranded, replicative form III of phi X174 DNA. Conversely, for the packaged phi X174 genome, ordered (B-type) phosphodiester groups are not present, and only 40% of the base stacking in RFIII DNA is observed. These results are interpreted as evidence that the substantial hairpin-forming potential of ssDNA is eliminated by specific and extensive ssDNA-protein interactions within the phi X174 virion. Comparison of the present results with studies of other packaged single-stranded nucleic acids suggests that proteins of the capsid shell (gpF + gpG + gpH) do not fully account for the conformational constraints imposed on ssDNA of phi X174. Accordingly, we propose a model for ssDNA packaging in which the small basic gpJ protein, which is packaged along with the genome, is involved stoichiometrically in binding to the ssDNA (approximately 90 nucleotides per subunit). The proposed gpJ-DNA interactions could prevent helical hairpin formation, restrict base stacking, and disfavor fortuitous base pairing within the capsid. The present analysis is based upon use of model nucleic acids of known conformation for calibration of the Raman intensity in the region 810-860 cm-1 in terms of specific secondary structures. The calibration curve allows quantitative determination of the percentage of ssDNA nucleotides for which the 5'O-P-O3' group is configured (g-,t) as in the B-form of DNA. The method proposed here is analogous to that employed by Thomas and Hartman (1973) for ssRNA and should be applicable to single-stranded DNA and to partially denatured forms of double- and multiple-stranded DNAs.

Preliminary investigation of the phage phi X174 crystal structure.

Crystals of the single-stranded DNA bacteriophage phi X174 have been grown. They have a monoclinic unit cell with space group P2(1), unit cell dimensions of a = 306.0 (+/- 0.2) A, b = 361.1 (+/- 0.2) A, c = 299.7 (+/- 0.2 degrees) A, beta = 92.91 degrees (+/- 0.02 degrees) and diffract to at least 2.7 A resolution. There are two virus particles per unit cell. Packing considerations show that the mean diameter of the virus particles is 280 A. The virus separates into two bands in a sucrose gradient. The ratio between the absorbance at 260 nm and 280 nm is 1.45 to 1.65 for the faster and 1.15 to 1.35 for the slower bands, but both bands contain intact particles. Crystals derived from these bands are isomorphous and there is no detectable difference in their structure amplitudes.

Phage phi X174 probed by laser Raman spectroscopy: evidence for capsid-imposed constraint on DNA secondary structure.

The Raman spectrum of the isometric bacteriophage phi X174 contains a number of well-resolved bands which have been assigned unambiguously to proteins of the capsid or to the single-stranded DNA (ssDNA) genome. Additional Raman bands of protein and DNA, which are partially overlapped in the spectrum of virus, have been resolution enhanced by Fourier deconvolution to permit improved semiquantitative measurement of spectral intensities and frequencies for structural conclusions. Raman conformation markers indicate that the ssDNA molecule within the capsid contains nucleosides of C2'-endo sugar pucker and anti-glycoside bond orientation, but the nucleic acid backbone lacks the geometry characteristic of B-form DNA. The Raman profile of encapsidated phi X DNA indicates a backbone more similar to heat-denatured DNA than to DNA containing hairpinlike secondary structure. This finding suggests limited interbase interactions in the packaged genome, which is presumably the result of constraints imposed by the viral capsid. Thus, the extensive pairing and stacking of bases indicated by Raman profiles from ssRNA viruses are not evident for the phi X174 chromosome. Overall, the proteins of the virion contain extensive beta-sheet and irregular secondary structures. Fourier deconvolution of the Raman amide I band provides an estimate of the percentage of total beta-sheet structure (approximately 60%) in all proteins of the virion. The amide III region of the spectrum confirms that beta-sheet and irregular domains are the predominant protein secondary structures. Samples of phi X174 concentrated for Raman spectroscopy by either ultracentrifugation or ultrafiltration exhibit nearly identical Raman spectra, indicating that either method can be employed to prepare intact virus without significant loss of DNA or protein components.

Irreversible binding of phage phi X174 to cell-bound lipopolysaccharide receptors and release of virus-receptor complexes.

At 37 degrees C, binding of phi X174 to the lipopolysaccharide receptors in the outer membrane of Escherichia coli C is followed by an irreversible ejection of its DNA. DNA ejection marks the beginning of the eclipse period in the infection cycle. Binding data with a phi X mutant Fcs70 at 15 degrees C, where the DNA ejection, or eclipse, rate is essentially zero, do not follow the law of mass action. This rules out a simple mechanism of reversible binding followed by irreversible DNA ejection. A more complex reaction model was devised to fit the data [Incardona, N. L. (1983) J. Theor. Biol. 105, 631-645]. It takes into account the fact that lipopolysaccharide-containing outer membrane fragments are continually released from infected E. coli cells, some of which have phi X bound to them. In this paper the model is shown to fit the binding data for wild-type virus at 15 degrees C and to account for the nonlinearity observed at 37 degrees C in the pseudo-first-order binding kinetics and first-order eclipse kinetics for both mutant and wild-type virus. This leads to the conclusion that phi X174 binding to cell-bound receptors is irreversible but binding to released receptors is reversible. The release of virus-receptor complexes from infected cells and the dissociation of these complexes were confirmed by electron microscopy. We propose that initially a single phi X174 vertex interacts reversibly with E. coli lipopolysaccharide but dissociation from the cell is prevented by the subsequent interaction of additional vertices with adjacent receptor molecules.

Eclipse kinetics as a probe of quaternary structure in bacteriophage phi X174.

The extracellular form of bacteriophage phi X174 consists of single-stranded DNA within an icosahedral capsid, which has short spikes at each of its vertices. Each spike is composed of gene G and H proteins, while the capsid itself consists of gene F protein. Since several molecules of gene H protein are injected into the cell along with the DNA, specific protein--protein and DNA--protein interactions must be broken when the genome exits and leaves an intact capsid structure at the receptor site. To demonstrate this we examined the eclipse (DNA ejection) reaction with two types of phi X174 mutants. The first contains missense mutations in a capsid or spike protein gene, and the second involves insertions or deletions in non-coding regions of the DNA. Using an improved procedure, the eclipse rate in vivo of the eclipse mutants Fcs70 has been redetermined over a larger temperature range than in previous studies. The three- to fivefold decrease in rate between 37 degrees C and 25 degrees C is due to an increase in both the enthalpy and entropy of activation when compared to the wild-type values of these kinetic parameters. This missence mutation also confers an increase in virus stability in 2 to 3 M-urea. In contrast to this, inserting 163 bases into the length of DNA packaged within the phi X174 capsid does not lead to a detectable change in eclipse rate over the same temperature range. yet this insertion into the J--F intercistronic region imparts a significant decrease in virus stability in urea. These results suggest that a specific set of non-covalent interactions is involved in phi X174 DNA ejection. This is supported by the small (50%), but significant, increase in eclipse rate that occurs when 27 bases are deleted from the J--F intercistronic region. The latter effect must be base-sequence-specific since no change in rate is observed when only seven of the 27 bases are deleted. Thus, the kinetics of the phi X174 eclipse reaction can be used as a sensitive probe of quaternary structure by correlating the change in reaction rate with alterations in amino acid and base sequences in the structural components of the virus.

A kinetic model for virus binding which involves release of cell-bound virus-receptor complexes.

A general kinetic mechanism is presented for reversible binding of viruses to cells followed by an irreversible step that initiates the delivery of the viral genome. A novel feature is additional pathways for the release of both virus-occupied and unoccupied receptors from cells. Due to one simplifying assumption, it does not apply at low receptor densities. However, it is sufficiently general to be applicable to ligand binding and internalization for those systems in which ligand diffusion is rate limiting. Three different versions of the model fit the usual kinetic data for the binding of an eclipse mutant of bacteriophage phi X174 to Escherichia coli. However, in each case binding to cell-bound receptors is irreversible. Therefore, this explains the apparent failure of this system to obey the Law of Mass Action. One version of the model also predicts that the release rate of lipopolysaccharide receptors from the outer membrane may be significantly lowered when virus is bound to these receptors.

Application of Arrhenius kinetic theory to viral eclipse: selection of bacteriophage phi X174 mutants.

Analysis of the bacteriophage phi X174 eclipse period in terms of Arrhenius kinetic theory suggests the following hypothesis: mutants should exist with two concomitant physiological characteristics as their phenotype. These are an eclipse rate lower than that of the wild type at permissive temperatures for plaque formation and an eclipse rate too low at lower temperatures to permit plaque development. Thus, enrichment of a mutagenized virus population for mutants that fail to eclipse during a short period at permissive temperatures should yield eclipse mutants with the cold-sensitive (cs; nonpermissive temperature, 25 degrees C), and not the temperature-sensitive (ts; nonpermissive temperature, 42 degrees C), plaque phenotype. In several trials, the frequency of the cs phenotype in the population increased from less than 0.2% to between 2 and 4% after the enrichment step, whereas the frequency of the ts phenotype remained unchanged (less than 0.2%). Moreover, 80% of these cs mutants have eclipse rates that are 3- to 40-fold lower than that of the wild type at both 37 degrees C and 25 degrees C. The successful application of the Arrhenius theory to phi X eclipse may provide insights into the molecular mechanism whereby the phi X174 genome is delivered into the host cell. Since the eclipse kinetics of other nonenveloped viruses are similar to those of phi X174, kinetic theory may be broadly applicable in the selection and characterization of viral eclipse mutants.

Mechanism of adsorption and eclipse of bacteriophage phi chi 174. 3. Comparison of the activation parameters for the in vitro and in vivo eclipse reactions with mutant and wild-type virus.

In a starvation buffer containing 10(-3) M divalent cations, phiX174 undergoes viral eclipse above 20 C when attached to intact host cells. An in vitro structural transition that is similar to that observed in this in vivo eclipse reaction occurs over the same temperature range in 0.1 M CaCl(2) (pH 7.2). Since both reactions result in a loss of infectivity, their kinetics have been compared in this report. Both exhibit a biphasic first-order loss in PFU that is a result of two competing first-order processes. However, a single type of heterogeneity in the population of virions is not the basis for both competing slower reactions. The Arrhenius plots of the faster components show that the in vitro eclipse reaction has the same activation energy of 35 kcal/mol (ca. 1.47 x 10(5) J/mol) as the in vivo reaction but a 10-fold lower Arrhenius preexponential factor. This is further evidence that certain features of the in vivo mechanism are retained in the in vitro reaction. In the case of the slower components, the in vitro reaction has an activation energy of 37 kcal/mol (1.55 x 10(5) J/mol), whereas that of the in vivo reaction is only 5 kcal/mol (2.1 x 10(4) J/mol). A similar analysis has been performed on a cold-sensitive eclipse mutant of phiX174. In vivo, the mutation is expressed by a two- to three-fold lower Arrhenius preexponential factor for both components of the eclipse reaction when compared to wt virus. The activation energies for both components are the same as wt virus. These results suggest that the mechanism of the eclipse reaction can be operationally divided into two aspects, each subject to mutational alteration.

Mechanism of adsorption and eclipse of bacteriophage phi X174. II. Attachment and eclipse with isolated Escherichia coli cell wall lipopolysaccharide.

A mixture of aqueous phenol, choloroform, and ether extracts the lipopolysaccharides (LPS) from the phiX174-sensitive strain, Escherichia coli C/1, and resistant strains, C/phiX and K12. Interaction of the C/1 LPS with phiX in a starvation buffer containing 10(-3) M CaCl(2) at 37 C, but not at 15 C, results in a first-order inactivation that is specific for C/1 LPS. After interaction for 60 min at 15 C, followed by centrifugation, 37 and 20% of a (14)C-phiX preparation are bound to the C/1 and C/phiX LPS pellets, respectively. The results for intact cells are 75 and 10%. Supporting the conclusion that this represents specific attachment of phiX to its receptor site in the LPS is the fact that EDTA-borate buffer is required to elute 85% of the (14)C-phiX from the C/1 LPS, whereas starvation buffer elutes the same amount from C/phiX LPS. Moreover, 95% of the PFU are found in the C/1 LPS pellets as compared with 50% in the resistant strain LPS pellets. When the products of interaction between phiX and LPS at 37 C are examined by sucrose density gradients in EDTA-borate, a single 60 to 90S peak is observed in the C/1 sample, and the single peak cosediments with the 120S marker phiX in the C/phiX sample. This change in S(20, w) is very similar to that reported for the eclipse of phiX in vivo. If the inactivation at 37 C is carried out on phiX-LPS complexes first formed at 15 C, the first-order kinetics are biphasic and nearly identical to that observed for the eclipse kinetics of phiX attached to intact cells. Thus, the phiX-LPS system is suitable for in vitro studies on the early events in phiX infection.

Mechanism of adsorption and eclipse of bacteriophage phi X174. I. In vitro conformational change under conditions of eclipse.

Bacteriophage phiX174 undergoes a conformational change during viral eclipse when virus-host cell complexes are incubated briefly at 37 C in a complex starvation buffer at pH 8. In this report, basically the same transition is demonstrated in vitro. Incubation of phiX alone for 2 to 3 hr at 35 C in 0.1 m CaCl(2) (pH 7.2) results in an irreversible decrease in S(20,w) because of an increase in the frictional coefficient that occurs during the change in conformation. The slower sedimenting conformation is noninfectious. These properties are remarkably similar to those of the eclipsed particles characterized by Newbold and Sinsheimer. Therefore, the key structural requirements for the molecular mechanism must reside within the architecture of the virus itself. This extremely simplified system uncovered the calcium ion requirement and pronounced dependence on pH between 6 and 7, both inherent properties of adsorption. This and the more than 10-fold greater rate of the in vivo conformational transition allude to the cooperative nature of attachment and eclipse for phiX.