The toxic effect of rabbitpox virus in tissue culture

Poxvirus pathogenesis

Poxviruses are a highly successful family of pathogens, with variola virus, the causative agent of smallpox, being the most notable member. Poxviruses are unique among animal viruses in several respects. First, owing to the cytoplasmic site of virus replication, the virus encodes many enzymes required either for macromolecular precursor pool regulation or for biosynthetic processes. Second, these viruses have a very complex morphogenesis, which involves the de novo synthesis of virus-specific membranes and inclusion bodies. Third, and perhaps most surprising of all, the genomes of these viruses encode many proteins which interact with host processes at both the cellular and systemic levels. For example, a viral homolog of epidermal growth factor is active in vaccinia virus infections of cultured cells, rabbits, and mice. At least five virus proteins with homology to the serine protease inhibitor family have been identified and one, a 38-kDa protein encoded by cowpox virus, is thought to block a host pathway for generating a chemotactic substance. Finally, a protein which has homology with complement components interferes with the activation of the classical complement pathway. Poxviruses infect their hosts by all possible routes: through the skin by mechanical means (e.g., molluscum contagiosum infections of humans), via the respiratory tract (e.g., variola virus infections of humans), or by the oral route (e.g., ectromelia virus infection of the mouse). Poxvirus infections, in general, are acute, with no strong evidence for latent, persistent, or chronic infections. They can be localized or systemic. Ectromelia virus infection of the laboratory mouse can be systemic but inapparent with no mortality and little morbidity, or highly lethal with death in 10 days. On the other hand, molluscum contagiosum virus replicates only in the stratum spinosum of the human epidermis, with little or no involvement of the dermis, and does not spread systemically from the site of infection. The host response to infection is progressive and multifactorial. Early in the infection process, interferons, the alternative pathway of complement activation, inflammatory cells, and natural killer cells may contribute to slowing the spread of the infection. The cell-mediated response involving learned cytotoxic T lymphocytes and delayed-type hypersensitivity components appears to be the most important in recovery from infection. A significant role for specific antiviral antibody and antibody-dependent cell-mediated cytotoxicity has yet to be demonstrated in recovery from a primary infection, but these responses are thought to be important in preventing reinfection.

Vaccinia virus cytotoxin

Extracts of vaccinia-infected HeLa cells were rendered free from infectious virus by centrifugation followed by membrane filtration and were shown to be toxic to uninfected HeLa cells in the presence of hypertonic MgSO4, used as a macromolecular uptake inducer, under conditions which did not kill control cells. Extracts from uninfected cells were nontoxic. This biological test was adapted to a semi-quantitative assay which was used to monitor the purification of the cytotoxic factor by DEAE-cellulose and Sephadex G-100 chromatography. The cytotoxic factor was purified 100-fold, shown to be of molecular weight 30 -- 100,000 daltons, acidic and completely inactivated by soluble trypsin but not by ribonuclease under conditions believed to degrade both single- and double-stranded RNA species. It was demonstrated to be virus specific by approrpiate immunosorbent chromatography. Extracts were also prepared from vaccinia-infected HEp-2, RK and W-K cells respectively. A virus-specific factor, toxic to uninfected HeLa cells, with similar chromatographic properties to that isolated from infected HeLa cells, was isolated from these three additional cell lines. The concept of virus induced cytotoxins, substances which exert their toxic effect in the host cells in which they are made, is discussed.

Viremia and virus measurements of rabbit pox in CV-1 cells

Rabbit pox virus (RPV) produced cytopathic effect (CPE) in five types of cells grown in tissue cultures. The CPE on CV-1 cells was characterized by cell fusion and lysis. The CV-1 line is a useful and sensitive cell culture for measuring concentrations of RPV in blood and tissues of infected rabbits. Viremia was detected between the 2nd and 4th days after parenteral infection. By the 6th and 7th days, the concentration of RPV in various tissues ranged from 10(5-3) to 10(8-5) TCID(50)/g. Cross-reactivity was demonstrated by the fluorescent-antibody technique between rabbit pox, vaccinia, and monkey pox viruses.

Viral and host deoxyribonucleic acid synthesis in Shope fibroma virus-infected cells as studied by means of high-resolution autoradiography

Incorporation of (3)H-thymidine by BSC-1 cells infected with Shope fibroma virus was studied by means of high-resolution electron microscopic radioautography. One-hour pulses with the radioactive precursor were given at various times after infection, during a one-step growth cycle of the virus. In the cytoplasm of infected cells, reacted grains occurred over foci of viroplasm; these foci are believed to represent the true sites of viral deoxyribonucleic acid (DNA) replication. Shope fibroma virus DNA synthesis began before 3 hr postinfection, reached a maximum at 8 to 9 hr, and then declined rapidly. It was demonstrated that the decline in (3)H-thymidine uptake is correlated with the onset of viral morphogenesis. In comparison with the noninfected culture, the nuclear labeling, which reflects host DNA metabolism, was slightly reduced by 4 hr postinfection. Inhibition became more marked as infection progressed, and host DNA synthesis was almost completely suppressed in late stages of viral development.

The inhibition of growth of vaccinia and cowpox viruses in RK 13 cells

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Vaccinia Virus Directed RNA: Its Fate in the Presence of Actinomycin

RNA similar in base composition to viral DNA is formed in the cytoplasm of HeLa cells infected with vaccinia virus. This RNA is found both in polyribosomes and in a broad 30S to 74S region in a sucrose density gradient. When infected cells are exposed to actinomycin, the amount of this DNA-like RNA in the 30S to 74S region is unchanged. Under the same conditions, 70 percent of the polyribosome-associated DNA-like RNA is degraded to acidsoluble fragments within 2 hours, and protein synthesis is reduced by 90 percent.