Читать книгу Principles of Virology, Volume 2 - Jane Flint, S. Jane Flint - Страница 77

The previous sections discuss viral access to particular tissues, and entry points into the host. But there is more to initiating an infection than just access. Before describing how viruses move throughout an infected host, it will be useful to discuss briefly the concept of viral tropism: the cellular and anatomical parameters that define the cells in which a virus can reproduce in vivo. Most viruses are restricted to specific cell types in certain organs. Tropism is governed by at least four parameters. First, in order for a virus to infect a cell, the cell must be susceptible; that is, it must have receptors that the virus binds to allow entry. However, expression of an entry receptor and translocation into the cytoplasm is not sufficient to ensure virus reproduction. Successful production of viral progeny depends on cellular gene products (or other cellular components) to complete the infectious cycle, which may be synthesized by some cell types but not others. The presence of such proteins and molecules renders the cell permissive to completion of the virus reproduction cycle and release of infectious virus particles. Although a cell may be both susceptible and permissive, infection may not occur because virus particles are physically prevented from interacting with the tissue; thus, the host cells must be accessible to virus. Finally, an infection may not progress, even when the tissue is accessible and the cells are susceptible and permissive, because of intrinsic and innate immune defenses that resolve the infection before spread can occur.