Interaction of coxsackievirus A21 with its cellular receptor, ICAM-1

Coxsackievirus A21 (CAV21), like human rhinoviruses (HRVs), is a causative agent of the common cold. It uses the same cellular receptor, intercellular adhesion molecule 1 (ICAM-1), as does the major group of HRVs; unlike HRVs, however, it is stable at acid pH. The cryoelectron microscopy (cryoEM) image reconstruction of CAV21 is consistent with the highly homologous crystal structure of poliovirus 1; like other enteroviruses and HRVs, CAV21 has a canyon-like depression around each of the 12 fivefold vertices. A cryoEM reconstruction of CAV21 complexed with ICAM-1 shows all five domains of the extracellular component of ICAM-1. The known atomic structure of the ICAM-1 amino-terminal domains D1 and D2 has been fitted into the cryoEM density of the complex. The site of ICAM-1 binding within the canyon of CAV21 overlaps the site of receptor recognition utilized by rhinoviruses and polioviruses. Interactions within this common region may be essential for triggering viral destabilization after attachment to susceptible cells.

Genetic and biochemical studies of poliovirus cis-acting replication element cre in relation to VPg uridylylation

In addition to highly conserved stem-loop structures located in the 5'- and 3'-nontranslated regions, genome replication of picornaviruses requires cis-acting RNA elements located in the coding region (termed cre) (K. L. McKnight and S. M. Lemon, J. Virol. 70:1941-1952, 1996; P. E. Lobert, N. Escriou, J. Ruelle, and T. Michiels, Proc. Natl. Acad. Sci. USA 96:11560-11565, 1999; I. Goodfellow, Y. Chaudhry, A. Richardson, J. Meredith, J. W. Almond, W. Barclay, and D. J. Evans, J. Virol. 74:4590-4600, 2000). cre elements appear to be essential for minus-strand RNA synthesis by an as-yet-unknown mechanism. We have discovered that the cre element of poliovirus (mapping to the 2C coding region of poliovirus type 1; nucleotides 4444 to 4505 in 2C), which is homologous to the cre element of poliovirus type 3, is preferentially used as a template for the in vitro uridylylation of VPg catalyzed by 3D(pol) in a reaction that is greatly stimulated by 3CD(pro) (A. V. Paul, E. Rieder, D. W. Kim, J. H. van Boom, and E. Wimmer, J. Virol. 74:10359-10370, 2000). Here we report a direct correlation between mutations that eliminate, or severely reduce, the in vitro VPg-uridylylation reaction and produce replication phenotypes in vivo. None of the genetic changes significantly influenced translation or polyprotein processing. A substitution mapping to the first A (A4472C) of a conserved AAACA sequence in the loop of PV-cre(2C) eliminated the ability of the cre RNA to serve as template for VPg uridylylation and abolished RNA infectivity. Mutagenesis of the second A (A4473C; AAACA) severely reduced the yield of VPgpUpU and RNA infectivity was restored only after reversion to the wild-type sequence. The effect of substitution of the third A (A4474G; AAACA) was less severe but reduced both VPg uridylylation and virus yield. Disruption of base pairing within the upper stem region of PV-cre(2C) also affected uridylylation of VPg. Virus derived from transcripts containing mutations in the stem was either viable or quasi-infectious.

Identification of an RNA hairpin in poliovirus RNA that serves as the primary template in the in vitro uridylylation of VPg

The first step in the replication of the plus-stranded poliovirus RNA is the synthesis of a complementary minus strand. This process is initiated by the covalent attachment of UMP to the terminal protein VPg, yielding VPgpU and VPgpUpU. We have previously shown that these products can be made in vitro in a reaction that requires only synthetic VPg, UTP, poly(A), purified poliovirus RNA polymerase 3D(pol), and Mg(2+) (A. V. Paul, J. H. van Boom, D. Filippov, and E. Wimmer, Nature 393:280-284, 1998). Since such a poly(A)-dependent process cannot confer sufficient specificity to poliovirus RNA replication, we have developed a new assay to search for a viral RNA template in conjunction with viral or cellular factors that could provide this function. We have now discovered a small RNA hairpin in the coding region of protein 2C as the site in PV1(M) RNA that is used as the primary template for the in vitro uridylylation of VPg. This hairpin has recently been described in poliovirus RNA as being an essential structure for the initiation of minus strand RNA synthesis (I. Goodfellow, Y. Chaudhry, A. Richardson, J. Meredith, J. W. Almond, W. Barclay, and D. J. Evans, J. Virol. 74:4590-4600, 2000). The uridylylation reaction either with transcripts of cre(2C) RNA or with full-length PV1(M) RNA as the template is strongly stimulated by the addition of purified viral protein 3CD(pro). Deletion of the cre(2C) RNA sequences from minigenomes eliminates their ability to serve as template in the reaction. A similar signal in the coding region of VP1 in HRV14 RNA (K. L. McKnight and S. M. Lemon, RNA 4:1569-1584, 1998) and the poliovirus cre(2C) can be functionally exchanged in the assay. The mechanism by which the VPgpUpU precursor, made specifically on the cre(2C) template, might be transferred to the site where it serves as primer for poliovirus RNA synthesis, remains to be determined.

Development of DNA vaccines for foot-and-mouth disease, evaluation of vaccines encoding replicating and non-replicating nucleic acids in swine

We have developed naked DNA vaccine candidates for foot-and-mouth disease (FMD), an important disease of domestic animals. The virus that causes this disease, FMDV, is a member of the picornavirus family, which includes many important human pathogens, such as poliovirus, hepatitis A virus, and rhinovirus. Picornaviruses are characterized by a small (7-9000 nucleotide) RNA genome that encodes capsid proteins, processing proteinases, and enzymes required for RNA replication. We have developed two different types of DNA vaccines for FMD. The first DNA vaccine, pP12X3C, encodes the viral capsid gene (P1) and the processing proteinase (3C). Cells transfected with this DNA produce processed viral antigen, and animals inoculated with this DNA using a gene gun produced detectable antiviral immune responses. Mouse inoculations with this plasmid, and with a derivative containing a mutation in the 3C proteinase, indicated that capsid assembly was essential for induction of neutralizing antibody responses. The second DNA vaccine candidate, pWRMHX, encodes the entire FMDV genome, including the RNA-dependent RNA polymerase, permitting the plasmid-encoded viral genomes to undergo amplification in susceptible cells. pWRMHX encodes a mutation at the cell binding site, preventing the replicated genomes from causing disease. Swine inoculated with this vaccine candidate produce viral particles lacking the cell binding site, and neutralizing antibodies that recognize the virus. Comparison of the immune responses elicited by pP12X3C and pWRMHX in swine indicate that the plasmid encoding the replicating genome stimulated a stronger immune response, and swine inoculated with pWRMHX by the intramuscular, intradermal, or gene gun routes were partially protected from a highly virulent FMD challenge.

Construction and evaluation of an attenuated vaccine for foot-and-mouth disease: difficulty adapting the leader proteinase-deleted strategy to the serotype O1 virus

Over the last few years we have utilized a system to genetically engineer foot-and-mouth disease virus (FMDV) to produce live-attenuated vaccine candidates. These candidates have been generated in the genetic background of a tissue culture-adapted strain of serotype A12 virus. Based on this A12 system, we created a virus lacking the sequence encoding the leader (L) proteinase (Piccone et al., 1995), and demonstrated that this leaderless virus, A12-LLV2 was avirulent in bovine and swine, and could be used as an attenuated vaccine (Mason et al., 1997; Chinsangaram et al., 1998). The current study shows that a similar leader-deleted chimeric virus containing the genome of the type A12 virus with a substituted type O1 capsid coding region from a bovine-virulent virus can be constructed, and that the virus has low, but detectable virulence in swine. A second chimera specifying a tissue culture-adapted type O1 capsid lacking the RGD cell binding site, was avirulent in swine, but was not sufficiently immunogenic to provide protection from challenge. These results are described with respect to mechanisms of attenuation and antigen formation in live-attenuated virus-inoculated animals.

Foot-and-mouth disease virus virulent for cattle utilizes the integrin alpha(v)beta3 as its receptor

Adsorption and plaque formation of foot-and-mouth disease virus (FMDV) serotype A12 are inhibited by antibodies to the integrin alpha(v)beta3 (A. Berinstein et al., J. Virol. 69:2664-2666, 1995). A human cell line, K562, which does not normally express alpha(v)beta3 cannot replicate this serotype unless cells are transfected with cDNAs encoding this integrin (K562-alpha(v)beta3 cells). In contrast, we found that a tissue culture-propagated FMDV, type O1BFS, was able to replicate in nontransfected K562 cells, and replication was not inhibited by antibodies to the endogenously expressed integrin alpha5beta1. A recent report indicating that cell surface heparan sulfate (HS) was required for efficient infection of type O1 (T. Jackson et al., J. Virol. 70:5282-5287, 1996) led us to examine the role of HS and alpha(v)beta3 in FMDV infection. We transfected normal CHO cells, which express HS but not alpha(v)beta3, and two HS-deficient CHO cell lines with cDNAs encoding human alpha(v)beta3, producing a panel of cells that expressed one or both receptors. In these cells, type A12 replication was dependent on expression of alpha(v)beta3, whereas type O1BFS replicated to high titer in normal CHO cells but could not replicate in HS-deficient cells even when they expressed alpha(v)beta3. We have also analyzed two genetically engineered variants of type O1Campos, vCRM4, which has greatly reduced virulence in cattle and can bind to heparin-Sepharose columns, and vCRM8, which is highly virulent in cattle and cannot bind to heparin-Sepharose. vCRM4 replicated in wild-type K562 cells and normal, nontransfected CHO (HS+ alpha(v)beta3-) cells, whereas vCRM8 replicated only in K562 and CHO cells transfected with alpha(v)beta3 cDNAs. A similar result was also obtained in assays using a vCRM4 virus with an engineered RGD-->KGE mutation. These results indicate that virulent FMDV utilizes the alpha(v)beta3 integrin as a primary receptor for infection and that adaptation of type O1 virus to cell culture results in the ability of the virus to utilize HS as a receptor and a concomitant loss of virulence.

Plasmid DNA encoding replicating foot-and-mouth disease virus genomes induces antiviral immune responses in swine

DNA vaccine candidates for foot-and-mouth disease (FMD) were engineered to produce FMD virus (FMDV) particles that were noninfectious in cell culture or animals. The prototype plasmid, pWRM, contains a cytomegalovirus immediate-early promoter-driven genome-length type A12 cDNA followed by the bovine growth hormone polyadenylation site. BHK cells transfected with this plasmid produced virus, but the specific infectivity of pWRM was much lower than that achieved with in vitro-generated RNA genomes. To improve the infectivity of the plasmid, a cDNA encoding the hepatitis delta virus ribozyme was added to the 3' end of the FMDV cDNA. The resulting plasmid, pWRMH, exhibited slightly increased infectivity in cell culture and produced virus when inoculated into suckling mice. A third plasmid, pWRMHX, was created by removal of the sequences encoding the cell binding site found in capsid protein VP1 of pWRMH. Although cells transfected with pWRMHX produced viral capsids, this plasmid was not lethal in suckling mice, indicating that particles lacking the cell binding site were not able to initiate secondary infectious cycles. Swine inoculated with pWRMHX did not show any signs of disease and produced neutralizing antibodies to FMDV, and 20% of the vaccinated animals were protected from challenge. A derivative of pWRMHX, pWRMHX-pol-, harboring a mutation designed to inactivate the viral polymerase was much less immunogenic, indicating that immunogenicity of pWRMHX resulted, in part, from amplification of the viral genome in the animal.

Tissue culture adaptation of foot-and-mouth disease virus selects viruses that bind to heparin and are attenuated in cattle

Isolates of foot-and-mouth disease virus (FMDV) exist as complex mixtures of variants. Two different serotype O1 Campos preparations that we examined contained two variants with distinct plaque morphologies on BHK cells: a small, clear-plaque virus that replicates in BHK and CHO cells, and a large, turbid-plaque virus that only grows in BHK cells. cDNAs encoding the capsids of these two variants were inserted into a genome-length FMDV type A12 infectious cDNA and used to produce chimeric viruses that exhibited the phenotype of the original variants. Analyses of these viruses, and hybrids created by exchanging portions of the capsid gene, identified codon 56 in VP3 (3056) as the critical determinant of both cell tropism and plaque phenotype. Specifically, the CHO growth/clear-plaque phenotype is dependent on the presence of the highly charged Arg residue at 3056, and viruses with this phenotype and genotype were selected during propagation in tissue culture. The genetically engineered Arg 3056 virus was highly attenuated in bovines, but viruses recovered from animals inoculated with high doses of this virus had lost the ability to grow in CHO cells and contained either an uncharged residue at 3056 or a negatively charged Glu substituted for a Lys at a spatially and antigenically related position on VP2 (2134). Comparison of these animal-derived viruses to other natural and engineered viruses demonstrated that positively charged residues are required at both 2134 and 3056 for binding to heparin. Taken together, these results indicate that in vitro cultivation of FMDV type O selects viruses that bind to heparin and that viruses with the heparin-binding phenotype are attenuated in the natural host.

Characterization of synthetic foot-and-mouth disease virus provirions separates acid-mediated disassembly from infectivity

One of the final steps in the maturation of foot-and-mouth disease virus (FMDV) is cleavage of the VP0 protein to produce VP4 and VP2. The mechanism of this cleavage is unknown, but it is thought to function in stabilizing the virus particle and priming it for infecting cells. To investigate the cleavage process and to understand its role in virion maturation, we engineered synthetic FMDV RNAs with mutations at Ala-85 (A85) and Asp-86 (D86) of VP0, which border the cleavage site. BHK cells transfected with synthetic RNAs containing substitutions at position 85 (A85N or A85H) or at position 86 (D86N) yielded particles indistinguishable from wild-type (WT) virus in sedimentation and electrophoretic profiles. Viruses derived from these transfected cells were infectious and maintained their mutant sequences upon passage. However, BHK cells transfected with synthetic RNAs encoding Phe and Lys at these positions (A85F/D86K) or a Cys at position 86 (D86C) produced noninfectious provirions with uncleaved VP0 molecules. Despite their lack of infectivity, the A85F/D86K provirions displayed cell binding and acid sensitivity similar to those of WT virus. However, acid breakdown products of the A85F/D86K provirions differed in hydrophobicity from the comparable WT virion products, which lack VP4. Taken together, these studies are consistent with a role for soluble VP4 molecules in release of the viral genome from the endosomal compartment of susceptible cells.

Cloning and expression of a single-chain antibody fragment specific for foot-and-mouth disease virus

The gene for a single-chain antibody (VHK) to a conformational epitope on the type A12 foot-and-mouth disease virus (FMDV) particle was assembled and expressed in Escherichia coli. The VHK, purified from periplasmic extracts immunoprecipitated virus as efficiently as its parental monoclonal antibody (MAb) and exhibited the same binding specificity when tested against panel of natural and genetically engineered virus particles. The VHK neutralized type A12 virus in the presence of goat anti-mouse IgG; however, in the absence of the second antibody, only weak neutralizing activity was detected. Preliminary analysis of the mechanism of viral neutralization indicated that both the MAb and the VHK neutralize by the same mechanism. Small amounts of the VHK allowed infection of cells via Fc receptor-mediated adsorption in the presence of the second antibody. These data represent the first report of a single-chain neutralizing antibody for a picornavirus and provide insights into the mechanisms of viral neutralization and virus uptake.

Propagation of an attenuated virus by design: engineering a novel receptor for a noninfectious foot-and-mouth disease virus

To gain entry into cells, viruses utilize a variety of different cell-surface molecules. Foot-and-mouth disease virus (FMDV) binds to cell-surface integrin molecules via an arginine-glycine-aspartic acid (RGD) sequence in capsid protein VP1. Binding to this particular cell-surface molecule influences FMDV tropism, and virus/receptor interactions appear to be responsible, in part, for selection of antigenic variants. To study early events of virus-cell interaction, we engineered an alternative and novel receptor for FMDV. Specifically, we generated a new receptor by fusing a virus-binding, single-chain antibody (scAb) to intracellular adhesion molecule 1 (ICAM1). Cells that are normally not susceptible to FMDV infection became susceptible after being transfected with DNA encoding the scAb/ICAM1 protein. An escape mutant (B2PD.3), derived with the mAb used to generate the genetically engineered receptor, was restricted for growth on the scAb/ICAM1 cells, but a variant of B2PD.3 selected by propagation on scAb/ICAM1 cells grew well on these cells. This variant partially regained wild-type sequence in the epitope recognized by the mAb and also regained the ability to be neutralize by the mAb. Moreover, RGD-deleted virions that are noninfectious in animals and other cell types grew to high titers and were able to form plaques on scAb/ ICAM1 cells. These studies demonstrate the first production of a totally synthetic cell-surface receptor for a virus. This novel approach will be useful for studying virus reception and for the development of safer vaccines against viral pathogens of animals and humans.

Strategy for producing new foot-and-mouth disease vaccines that display complex epitopes

Widely used inactivated vaccines for foot-and-mouth disease (FMD) induce protective immunity, but vaccine production plants and residual virus in the vaccine itself have been implicated in disease outbreaks. The structure of the FMD virion has been determined, and although much of the surface of the viral particle is produced by complex folding of the three surface-exposed capsid proteins (VP1-3), some surface regions representing important linear epitopes can be mimicked by recombinant proteins or synthetic peptides. Vaccine candidates based on these products stimulate immune responses to foot-and-mouth virus (FMDV), but do not always protect livestock from disease. The basis of protective immunity to FMDV has been explored using genetic engineering to produce antigenic chimeras of the virus. Studies with these chimeras have shown that a strong and protective immune response can be generated in livestock to epitopes outside the sequential epitopes incorporated into previous subunit vaccine candidates. Genetic engineering of the virus has also been used to demonstrate that changes within the sequence encoding an arginine-glycine-aspartic acid (RGD) sequence in VP1 abrogate virus binding to cells in culture, confirming the role of RGD as the receptor binding site. Based on this information, genetically stable viruses which cannot bind to cells have been created by deleting the nucleotides coding the RGD sequence. The receptor binding site-deleted viruses have been shown to be non-infectious in tissue culture, mice, and swine. Cattle vaccinated with these viruses are protected from disease when challenged with virulent FMDV, demonstrating that they could serve as the basis for safer FMD vaccines.

Receptor binding site-deleted foot-and-mouth disease (FMD) virus protects cattle from FMD

Binding of foot-and-mouth disease virus (FMDV) to cells requires an arginine-glycine-aspartic acid (RGD) sequence in the capsid protein VP1. We have genetically engineered an FMDV in which these three amino acids have been deleted, producing a virus particle which is unable to bind to cells. Cattle vaccinated with these receptor binding site-deleted virions were protected from disease when challenged with a virulent virus, demonstrating that these RGD-deleted viruses could serve as the basis for foot-and-mouth disease vaccines safer than those currently in use. This strategy may prove useful in the development of vaccines for other viral diseases.

The foot-and-mouth disease virus leader proteinase gene is not required for viral replication

The foot-and-mouth disease virus (FMDV) leader (L) proteinase has only two known functions: (i) autocatalytic removal from the N terminus of the viral polyprotein and (ii) cleavage of the p220 subunit of the eukaryotic initiation factor 4F complex, which helps to shut off host protein synthesis. Cleavage of p220 appears to be important for picornavirus replication, since rhinoviruses and enteroviruses utilize a different proteinase (2A) to cleave p220. To explore the role of L in FMDV replication, we generated synthetic FMDV genomes lacking the L gene and tested their viability in cells. Genomes were constructed with the N-terminal Gly codon of VP4 positioned directly following either the first (Lab) or second (Lb) Met codon of the L protein. Cells transfected with synthetic RNAs lacking L and initiating with the Lab Met codon failed to produce viable virus, but cells transfected with RNAs that utilized the second AUG to drive translation of the viral polyprotein produced viable viruses. These leader-deleted viruses produced plaques on BHK cells that were slightly smaller than those produced by wild-type (WT) virus, grew to slightly lower titers than WT virus in BHK cells, shut off host protein synthesis more slowly than WT virus, and were slightly attenuated in mice. These studies indicate that the L proteinase is not essential for FMDV replication and show that in the cells and animals tested the L gene has a limited effect on virus replication.

Vaccines prepared from chimeras of foot-and-mouth disease virus (FMDV) induce neutralizing antibodies and protective immunity to multiple serotypes of FMDV

The G-H loop of VP1 (residues 132 to 159) of foot-and-mouth disease virus (FMDV) is a prominent feature on the virion surface and has an important role in vaccine efficacy, generation of antigenic variants, and cell binding. Using an infectious cDNA of FMDV, we have constructed serotype A viruses in which the G-H loop has been substituted with the homologous sequences from serotype O or C. These chimeric viruses replicated to high titer and displayed plaque morphologies similar to those of wild-type viruses, demonstrating that the functions provided by the loop can be readily exchanged between serotypes. Monoclonal antibody analyses showed that epitopes contained within the loop were transferred to the chimeras and that epitopes encoded by the type A backbone were maintained. Chemically inactivated vaccines prepared from chimeric viruses induced antibodies in guinea pigs that neutralized both type A and either type O or type C viruses. Swine inoculated with the A/C chimera vaccine also produced cross-reactive antibodies, were protected from challenge with the type A virus, and partially protected against challenge with type C. These studies emphasize the importance of epitopes outside of the G-H loop in protective immunity in swine, which is a natural host of FMDV.

Animal-derived antigenic variants of foot-and-mouth disease virus type A12 have low affinity for cells in culture

We recently have shown that binding of foot-and-mouth disease virus (FMDV) to cells in culture requires an arginine-glycine-aspartic acid (RGD) sequence in the G-H loop of the capsid protein VP1 (P. W. Mason, E. Rieder, and B. Baxt, Proc. Natl. Acad. Sci. USA 91:1932-1936, 1994). In this report, we show that FMDV type A12 viruses found in infected bovine tongue tissue (BTT) differ from their tissue culture-grown derivatives at amino acid residues near the RGD. Viruses genetically engineered to contain VP1 sequences found in animal tissue (BTT viruses) were antigenically different from their tissue culture derivatives and bound to BHK cells more poorly than did the tissue culture-adapted viruses. Passage of the genetically engineered BTT viruses in BHK cells resulted in the rapid selection of variants with cell-binding properties, antigenic characteristics, and sequences typical of tissue culture-adapted viruses. These data indicate that residues near the RGD are critical for cell binding and that interpretations of antigenic variation of FMDV can be affected by virus cultivation in vitro.

RGD sequence of foot-and-mouth disease virus is essential for infecting cells via the natural receptor but can be bypassed by an antibody-dependent enhancement pathway

Foot-and-mouth disease virus appears to initiate infection by binding to cells at an Arg-Gly-Asp (RGD) sequence found in the flexible beta G-beta H loop of the viral capsid protein VP1. The role of the RGD sequence in attachment of virus to cells was tested by using synthetic full-length viral RNAs mutated within or near the RGD sequence. Baby hamster kidney (BHK) cells transfected with three different RNAs carrying mutations bordering the RGD sequence produced infectious viruses with wild-type plaque morphology; however, one of these mutant viruses bound to cells less efficiently than wild type. BHK cells transfected with RNAs containing changes within the RGD sequence produced noninfectious particles indistinguishable from wild-type virus in terms of sedimentation coefficient, binding to monoclonal antibodies, and protein composition. These virus-like particles are defined as ads- viruses, since they were unable to adsorb to and infect BHK cells. These mutants were defective only in cell binding, since antibody-complexed ads- viruses were able to infect Chinese hamster ovary cells expressing an immunoglobulin Fc receptor. These results confirm the essential role of the RGD sequence in binding of foot-and-mouth disease virus to susceptible cells and demonstrate that the natural cellular receptor for the virus serves only to bind virus to the cell.

Sequence of the S fragment of foot-and-mouth disease virus type A12

The foot-and-mouth disease virus (FMDV) genome contains a 5' untranslated region (S fragment) capable of forming a stem-loop structure of over 350 bases, which is separated from the remainder of the genome by a homopolymeric cytidylic acid tract (poly(C)) of variable length. The sequence of the S fragment of serotype A12 appears more similar to those of type O1 or type C3 than to subtype A10. The relatively large difference between the S fragment sequences of two type A viruses suggests that the sequences per se of this region of the genome are not critical for the maintenance of function, and suggests the possibility of a relatively recent recombination event within the poly(C) region of the A12 genome.

Genetically engineered foot-and-mouth disease viruses with poly(C) tracts of two nucleotides are virulent in mice

To determine the role of the poly(C) tract found at the 5' end of the genome of foot-and-mouth disease virus, synthetic RNAs (in vitro transcripts) with poly(C) tracts of different lengths have been produced and evaluated. RNAs with poly(C) tracts of 35, 25, 16, 6, or 2 residues displayed similar specific infectivities in baby hamster kidney (BHK) cells. Viruses recovered from cells transfected with in vitro transcripts containing 6 to 35 Cs had properties similar to those of the wild-type virus in cell culture, and poly(C) tracts present in the synthetic RNA-derived viruses ranged from 75 to 140 bases in length. Viruses recovered from transcripts containing only two Cs showed very different properties. Specifically, viruses grew to much lower levels in cell culture and maintained a poly(C) tract of only two residues. The pool of viruses harvested from cells transfected with the synthetic C2 RNA also contained a small amount of a virus with a 42-base deletion in the region of the poly(C) tract, which appeared to have arisen by recombination. Taken together, these data suggest that recombination provides the mechanism of poly(C) elongation and that viruses with poly(C) tracts over 75 bases in length have a selective advantage in cell culture. Interestingly, all of the in vitro transcript-derived viruses [including viruses with poly(C) tracts of only two residues] were equally virulent in mice, indicating that poly(C) tract length has no effect on virulence in this animal model.

Is loxapine more effective than chlorpromazine in paranoid schizophrenia?

The authors compared loxapine with chlorpromazine in inpatients with paranoid schizophrenia and found no difference in clinical efficacy. Thus a previous finding, based on retrospective analyses, that loxapine was superior to other neuroleptics in the treatment of paranoid schizophrenia was not verified in this prospective study.

Autoradiographic localization of carbonic anhydrase in the developing chorioallantoic membrane

Carbonic anhydrase was localized in the chorioallantoic membrane with labeled inhibitor autoradiography of 3H-acetazolamide at 11, 14 and 19 days of incubation. At 11 days carbonic anhydrase was present in low amounts only in the undifferentiated ectoderm cells. At 14 and 19 days, the enzyme was found in increased amounts in all three germ layers of the chorioallantois. In the chorionic ectoderm the villous cavity cells contained the highest level of carbonic anhydrase. This finding lends support to the theory of H+ production to solubilize the CaCO3 of the egg shell. Sinus covering cells showed a considerable lower concentration of the enzyme than did villous cavity cells. Carbonic anhydrase in these cells may be multifunctional, assisting in calcium transport, subserving HCO3- transport from egg shell to blood, and supporting gaseous exchange. In the allantoic endoderm carbonic anhydrase was found in granule-rich cells and might be involved in the transport of Na+ and Cl- ions from allantoic fluid into the blood. The enzyme in the undifferentiated endoderm cells may have a respiratory function. In the mesoderm carbonic anhydrase was detected in the endothelium and pericytes of blood vessels where it is interpreted to support respiratory function.