Biology Of Vaccinia

Poxviruses are classified into 2 subfamilies, chordopoxvirinae (vertebrate poxviruses) and entomopoxvirinae (insect pox viruses), and at least 46 species (11). The classification scheme is based on host range, sequence homology, and antigenicity. Vaccinia virus is a member of the orthopoxvirus genus. It is genetically distinct from both cowpox virus and variola virus (smallpox). All members of the orthopoxvirus genus have immune cross-reactivity and are genetically stable. This allowed for the complete eradication of variola virus in 1977 (last case of endemic smallpox).

Multiple strains of vaccinia viruses exist (Table 1). As vaccination became widespread throughout the world, numerous centers produced and maintained the vaccine in different ways, resulting in numerous strains, which differ in characteristics, pathogenicity, and host range. The New York City Board of Health strain was obtained from England in 1856 and was originally used for smallpox vaccination in the United States (5). The Western Reserve (WR) strain is a laboratory derivative of this strain and appears to be one of the more virulent strains in laboratory animals and nonhuman primates.

It has not been used in patients to date. Another derivative, the Wyeth strain, was produced by Wyeth as a smallpox vaccine and is the backbone commonly used for experimental vaccines in clinical trials. The modified vaccinia ankara (MVA) strain was developed through multiple rounds of infection in avian cells. This strain is highly attenuated and does not replicate in human cells (12).

As with all poxviruses, the vaccinia virus is a double-stranded DNA virus whose entire life cycle exists within the cytoplasm of eukaryotic cells. The virus contains an outer envelope, as well as an internal membrane, and it carries the enzymes required for initiation of transcription. The genome of the Copenhagen strain of vaccinia virus was completely sequenced and reported in 1990 (13); other strains have been sequenced subsequently (14). The genome consists of double-stranded DNA with inverted terminal repeats and a terminal hairpin loop, which mimics a large, circular, single-stranded DNA. The genome consists of 191,636 base pairs encoding approximately 2063 proteins of 65 or more amino acids in length. It is among the largest viruses in size, averaging 270 x 350 nm in the shape of a brick (15).

Vaccinia has two infectious forms: the intracellular mature virus (IMV) and the extracellular enveloped virus (EEV) (16). The infectious form responsible for cell-to-cell spread in vivo is the EEV. IMV is the form recovered during viral purification in vitro, as it is released on cellular disruption. Purification of EEV is quite difficult because the EEV envelope is too fragile to withstand the purification process. The process of cell entry by vaccinia is not well understood and is complicated by the fact that there are multiple forms of infectious virus, and multiple cell types and species infected. EEV and IMV bind different but unknown cellular receptors as evidenced by different binding efficiencies among cell lines, and the observations that the enzymatic digestion of cell surface molecules and the binding of specific monoclonal antibodies to cells affects binding of EEV and IMV differentially (17). Consistent with this, confocal microscopy has shown that IMV and EEV enter cells by different mechanisms (18). The IMV contains several proteins on its outer envelope, including A17L, A27L, and D8L (19-25). D8L was one of the first membrane proteins identified in the IMV. It is nonessential in the viral life cycle but may mediate IMV binding to cell surface chondroitin sulfate (23,24). A27L may mediate vaccinia interaction with cell surface heparan sulphate. Virus infection is inhibited by 60% in the presence of soluble heparin

Table 1 Vaccinia Strains Reported in the Literature
0 0

Post a comment