Journal of IiME Volume 3 Issue 1 www.investinme.org Human enteroviruses and chronic infectious disease (continued) HEV had never been demonstrated in humans or animals, they had been shown to exist in experimental cell culture and thus, the possibility that they might also exist in nature could not be excluded. Defective interfering HEV are viral RNA genomes that have deleted variable parts of the sequence that encode the capsid (coat) proteins of the virus (see Figure 2). This implies such viruses could never be successful, as they could never produce an intact virion. However, DI HEV exist in a dynamic equilibrium with so-called wild type HEV which do produce normal capsids. Therefore, the DI viruses are parasites upon the wild type population of HEV, using their capsids to package the mutated RNA for movement to the next cell. Using a mouse model of CVB3-induced myocarditis, in which viral RNA was detectable in the heart tissue for many days after cytopathic virus was no longer detectable in cell culture, we searched for evidence of DI forms of CVB3 RNA but were repeatedly unsuccessful. We decided to examine the entire viral RNA genome that was present in these mouse heart samples, asking the basic question: are there deletions anywhere else that might explain this odd phenomenon? When this was done, we discovered that one of the ends of the single strand of RNA that makes up the viral genome, was missing: these viral genomes were then called 'terminally deleted' or TD(8, 18, 19). What makes this discovery fascinating to virologists, is that the sequence which the virus naturally deletes, was hitherto thought to be absolutely essential for virus replication. Our results, however, showed that while this sequence was very important for efficient CVB replication, it could nonetheless be done away with, and yet have the virus survive. The cost of this survival is, however, extremely slow replication. A further cost is that this survival can occur only in cell populations that do not divide anymore or divide very infrequently as in muscle tissue. It is this reason why we were able to find these novel virus populations in heart muscle Invest in ME (Charity Nr. 1114035) of experimentally inoculated mice(19) and later, in human heart(8). There are some cell cultures that do not divide continually but stop dividing when the cells contact each other; only in such cultures can CVB-TD populations occur(18). This is very different than current cell culture models for enterovirus infections in which most aspects of the viral biology are examined in immortal continuously replicating cells. While this is a good model for the short term infection of the gastrointestinal tract in which the virus infects, rapidly produces progreny virus and is excreted to infect another individual before the immune response can curtail infection, much of the pathology associated with enterovirus infections is in other differentiated tissues in which relatively little cell turnover may occur. In these nonreplicating (or only intermittently replicating) cells, the wild type virus is at a disadvantage because these cells lack key cytoplasmic factors essential for rapid replication. However, the low level replication of the TD viruses is favored in these cells and as the intracellular portion of the virus replication cycle is much longer, it is relatively hidden to the immune system. Much of our current research focus is upon the mechanism of selection of the TD populations in such cell cultures or tissues. These results provided an answer to the conundrum of failing to find cytopathic virus in myocarditic heart samples despite the ability to find viral RNA. Indeed, virus (in TD form) does exist in such samples but because the TD populations replicate so slowly and produce so little virus, they are difficult to detect. However, because the defect is not in a part of the viral genome that makes viral proteins, they do make all the viral proteins and even virus particles. But what does the finding of TD genomes mean for HEV disease? Chronic disease associated with HEV infections The previous discussion has demonstrated that HEV can persist for longer periods of time in the immunologically-normal host Page 26/76
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