Events During Penetration
The efficiency of poxvirus attachment to the host cell, as with any virus, depends directly on its collision frequency with the cellular surface. The probability of a collision increases as virion concentration goes up or it may be enhanced by the apphcation of centrifugal force (Sharp and Smith, 1960). However, the observed rates of attachment fail to reach values predicted from theoretical calculations (Smith and Sharp, 1960; Dumbell et al., 1957; Allison and Valentine, 1960 a). Perhaps this is a consequence of an instability in the initial cell-virus association, which appears to involve electrostatic forces between the predominantly acidic groups on the host and basic residues on the virus (Allison and Valentine, 1960b). While this notion that binding involves primarily electrostatic forces might be an oversimplification, at least it accounts in part for the ability of vaccinia virus and other poxviruses to become adsorbed to a broad spectrum of cell types and suitably charged non-biological materials, among them glass and metallic surfaces. Very little information is presently available concerning the next stage, when specific, irreversible binding between virus ligands and host receptois occurs, although receptors for vaccinia virus and related serotypes are ubiquitous among a great variety of mammalian and avian cells, bringing into question the nature and specificity of such receptors. Agents of the parapox group likewise demonstrate tropism for a wide range of mammalian species, as exemplified by viruses from goats and sheep such as Orf virus which can replicate in cells of the human epidermis or in culture (Andrewes and Pereira, 1972).
KeywordsVaccinia Virus Infectious Unit Cytoplasmic Matrix Avian Cell Virus Penetration
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