A similar infection cycle is followed by viruses belonging to Herpesviridae fa-

mily. Herpesviruses are DNA viruses that could cause infections in animals and

humans. This group of viruses largely depends on their ability of latency and re-

activation for an efficient infection. They maintain their latency period in non-

replicating cells such as neurons. The viral genome undergoes circularization,

forming an episome. During the latency period, the virus must limit its protein

expression to a minimum in order to keep evading the host immune system. This is

achieved by either generating long non-coding RNA transcripts [2] or through

modulation of chromatin insulators [3]. However, it is necessary to express those

viral proteins that would help the viral genome remain attached to the host chro-

mosome during cell division of that particular cell.

Reactivation from latency occurs on encountering extreme stress such as sup-

pression of the host immune system, neural trauma, or exposure to UV radiation [4].

Some allogenic transplantation could also induce the reactivation from latency such

as in human cytomegaloviruses [5] (Figure 2.7).

2.3.3

INFECTION IN EUKARYOTIC HOST

Viral infection in eukaryotic hosts is rather specific depending on the type of viral

genome and the host species. This is due to the fact that the host itself is a mul-

ticellular complex organism. There are several steps for the virus to undergo for a

successful infection (Figure 2.8).

1. Attachment: This is the first event where the virus recognizes the host

cell. This depends on the attachment factors present on the virus (e.g.,

glycoproteins) and host cellular receptors that facilitate the host-virus

FIGURE 2.6 Lysogenic cycle: Schematic representation of lysogenic infection cycle fol-

lowed by bacteriophage, which could either persist in the host cell for a longer duration or

lyse the host cell under environmental stress.

24

Bioprocessing of Viral Vaccines