[The rhabdovirus program at the Istituto Pasteur--Fondazione Cenci Bolognetti]

Studies carried out under the research program on Rhabdovirus sponsored by the Istituto Pasteur--Fondazione Cenci Bolognetti are reviewed. The program was mainly focused to investigate biologically relevant characteristics of this virus and to evaluate epidemiological and prophylactic aspects involved in sylvatic rabies control measures.

Effect of normal mosquito extracts upon arbovirus recoveries from mosquito pools

Titers of Turlock (TUR) and Hart Park (HP) viruses were reduced to undetectable levels when virus was mixed with a triturated suspension of uninfected (normal) 4th instar larvae of Culex tarsalis prior to plaque assay in cell culture. There was a linear relationship between the number of larvae in the pool and the titer of virus recoverable. Virus was undetectable when 1,000-10,000 PFU of either agent was added to pools that contained 25 or more larvae. Suspensions of up to 25 adult male or female Cx. tarsalis had little effect on detectable viral titers while pupal suspensions had an intermediate effect. The inhibitory effect of normal larval extracts on viral infectivity could be counteracted by use of polycations or a high pH buffer. A similar reduction in titer of TUR virus was observed with extracts of larvae of Aedes melanimon or Anopheles franciscanus. Larval extracts of Cx. tarsalis similarly reduced titers of California and St. Louis encephalitis viruses but not western equine encephalomyelitis virus. These findings may have significant bearing on the interpretation of transovarial transmission attempts in which pooled larvae are assayed for virus.

Biological characteristics of rhabdoviruses in different host cells

Rhabdoviruses are a group of viruses capable of infecting vertebrates, invertebrates and plants and showing a wide range of host-parasite relationships. Biological characteristics of viruses can vary according to the type of cells in which they grow. The Authors analyze differences in viral structure and multiplication related to the host. Some final considerations on the evolution of this group of viruses are also reported.

Membrane fusion proteins of enveloped animal viruses

In a living cell membrane-bound compartments are continuously either separated or united through fusion reactions, and literally thousands of such reactions take place every minute. The formation of membrane vesicles from pre-existing membranes, and their fusion with specific acceptor membranes, constitute a prerequisite for the transport of most impermeant molecules and macromolecules into the cell by endocytosis, out of the cell by exocytosis, and between the cellular organelles (Palade, 1975; Silverstein, 1978; Evered & Collins, 1982). Less frequent, but equally crucial, are fusion events in fertilization, cell division, polykaryon formation, enucleation, etc. (for reviews see Poste & Nicholson, 1978). Although a great deal is known about the properties and consequences of individual forms of membrane fusion in cellular systems, and about fusion in artificial lipid membranes, the molecular basis for the reactions remain largely unclear.

Replication of non-respiratory viruses in tracheal organ cultures

Employing Hoorn's technique, tracheal explant cultures were set from adult hamsters, rabbits and human foetuses. To determine the susceptibility of these cultures they were infected with nine different mainly non-respiratory viruses. Assay of virus was carried out in susceptible cell lines. The results of these studies indicated that herpes simplex type I (HSV-1) and vaccinia viruses multiplied in these cultures and caused ciliostasis. Herpes simplex type 2 (HSV-2) although multiplying in all, produced ciliostasis only in human foetal tracheal cultures. However, Chandipura (CHP), West Nile (WN), sandfly fever (SF-N) and polio-1 viruses multiplied without ciliostasis. These cultures did not support multiplication of Japanese encephalitis (JE) and Chikungunya (CHIK) viruses. The studies indicated that mammalian tracheal cultures support replication of the non-respiratory viruses. The continued and undiminished multiplication of viruses over long periods without ciliostasis suggests a role for the trachea in the transmission of viral infections by aerosol.

Viral hemorrhagic septicemia of rainbow trout: relation between the G polypeptide and antibody production in protection of the fish after infection with the F25 attenuated variant

A nonpathogenic variant of viral hemorrhagic septicemia virus has been selected which immunizes fish against a subsequent challenge with the wild-type virus strain. In this paper, we demonstrate that both the variant and the wild-type virus strains multiplied in spleen and kidney of infected fish, but the virus yield was lower for the variant and soon dropped below the sensitivity of our titration technique, which indicates that an early mechanism prevents the establishment of septicemia. This early mechanism could also be responsible for early protection since fish were already immunized 48 h postinfection with the variant. In a second step antibodies relayed that first defence mechanism to ensure long-lasting immunity. Antisera collected after immunization by the variant or the wild-type virus strain cross-reacted poorly in neutralization tests, which is in agreement with results obtained with plant lectins and proves that the variant is modified in its antigenic properties.

Measurement of surface antigen by specific bacterial adherence and scanning electron microscopy (SABA/SEM) in cells infected by vesiculovirus ts mutants

Temperature-sensitive (ts) mutants of the rhabdoviruses vesicular stomatitis virus and Chandipura virus have been used to measure the appearance of virus antigen on the surface of infected cells by the technique of surface analysis by bacterial adherence and scanning electron microscopy (SABA/SEM). The number of staphylococci specifically adhering to antiserum-treated infected PTK-2 or BSC-1 cells at permissive (31 degrees C) and restrictive (39 degrees C) temperatures was followed in time-course experiments and a close correspondence was observed between the proportion of staphylococci bound at 39 degrees C and the known phenotypic properties of the ts mutants. Virus surface antigen was undetected in cells infected by transcription- and replication-defective ts mutants with thermolabile L proteins under restrictive conditions up to an input multiplicity of infection of 50, and in cells infected by a replication-defective NS protein mutant. Some surface antigen was detected late in infection in PTK-2 cells infected by a replication-defective N protein mutant. Surface antigen accumulated normally in maturation-defective mutants with lesions in envelope proteins. These results establish the suitability of the SABA/SEM technique for quantitative estimation of virus antigen on the surface of infected cells.

Carbon dioxide sensitivity in mosquitoes infected with sigma, vesicular stomatitis, and other rhabdoviruses

Carbon dioxide, usually an innocuous narcotic for insects, kills mosquitoes infected with rhabdoviruses. Such toxicity was originally observed in Drosophila harboring a hereditary virus, sigma, and has been considered unique to Drosophila. The new findings support the possibility that insects with piercing and sucking mouthparts harbor similar hereditary viruses and transmit some of them to vertebrates or plants.

Rhabdoviridae. Report of the Rhabdovirus Study Group, International Committee on Taxonomy of Viruses

The family Rhabdoviridae comprises approximately 75 viruses infecting vertebrates, invertebrates and plants. The main characteristics of the member viruses are: (i) the viruses infecting vertebrates and invertebrates are bullet-shaped and the viruses infecting plants are usually bacilliform; (ii) the viruses have particle lengths varying from 130 to 380 nm and widths varying from 60 to 95 nm; (iii) the viruses possess unit-membrane envelopes from which protrude spikes 5 to 10 nm long; (iv) the viruses have precisely coiled helical nuecleocapsids with a diameter of approx. 50 nm; (v) most of the viruses which have been studied contain 5 proteins; the prototype, vesicular stomatitis virus, contains proteins designated L (large), G (glycoprotein), N (nucleoprotein), NS (nonstructural) and M (matrix); N or NS is phosphorylated in most members which have been studied; (vi) the viruses contain single-stranded RNA which is transcribed into several messenger RNA species with sizes corresponding to the structural proteins; (vii) the nucleocapsid contains the RNA-dependent RNA polymerase and is infectious; and (viii) many of the viruses produce morphologically distinct defective-interfering (T) particles.