LONDON - African swine fever virus produces a protein that mimics the actions of the immunosuppressant drug cyclosporin A, U.K. researchers have shown.
The discovery could explain how the virus manages to evade its host's immune system and could lead to new strategies for developing a vaccine against this economically important disease of pigs.
African swine fever was first identified in Kenya early this century, when domestic pigs came into contact with warthogs and bush pigs. In the 1950s, the disease spread to parts of Europe and South America. Although it now has been eradicated from Spain and Portugal, it persists in Europe in Sardinia, which has wild pigs.
The economic impact of the disease is enormous. It causes death among a high proportion of those animals which become infected; furthermore, the only means of control is by slaughtering pigs thought to have been exposed to the virus.
In Malta and the Dominican Republic, the disease was eradicated only after programs which involved slaughtering the entire pig population were instituted.
Attempts to develop a vaccine against the disease have failed. Now researchers at the Institute for Animal Health at the Pirbright Laboratory, in Pirbright, U.K., have gained insight into why the search for a vaccine has so far drawn a blank.
Linda Dixon, head of the institute's molecular biology group working on African swine fever, told BioWorld International, “We have identified a novel mechanism for virus immune evasion - a viral protein which acts in the same way as cyclosporin A. This might in the long term help us to develop a vaccine strategy to protect against this disease.“
Dixon, James Miskin and other colleagues at the Institute for Animal Health reported their finding in a paper in Science, July 24, 1998, titled “A viral mechanism for inhibition of the cellular phosphatase calcineurin.“ Miskin is first author of the paper.
Publication Of Genome Sequence Spurs Research
The group initially started to investigate the role of certain genes of African swine fever virus following the publication of the entire genome in 1995. It predicted certain genes might be involved in evading the immune system of the host because the proteins they encoded resembled host proteins or other viral proteins known to play a role in modulating the immune system.
The scientists focused on a protein called A238L, as this was similar to a host protein that inhibits activation of a host transcription factor called nuclear factor kappa B (NF-kappaB). Normally, the host protein stays bound to NF-kappaB. In the event of a viral infection, however, the inhibitory host protein becomes degraded, freeing NF-kappaB to trigger transcription of a group of genes involved in the activation of the immune response.
As reported in an earlier publication, Dixon's group showed, as it had expected, that A238L also could inhibit activation of NF-kappaB. While trying to find out more about precisely how A238L did this, the group realized A238L also had a second function, which was very similar to the action of cyclosporin A.
“We were using a yeast two-hybrid screen,“ Dixon said, “which is basically a genetic method for identifying proteins which bind to each other, and we found that A238L binds to the same proteins as cyclosporin A - namely, cyclophilin and the cellular phosphatase, calcineurin. This suggested to us that A238L might function as a protein analogue of cyclosporin A. A238L is the only protein so far known to do that, and calcineurin is the only known target of cyclosporin A.“
The data reported in Science show, using three different methods, that A238L binds to calcineurin, and that it inhibits the activity of calcineurin.
Cyclosporin A is known to act by binding to calcineurin, thus inhibiting signaling pathways in cells that are activated by calcineurin.
This phenomenon has been well studied in T cells, which have a transcription factor known as NFAT (nuclear factor of activated T cells). Normally, a subunit of NFAT is found in the cytoplasm and is phosphorylated. When the T cell becomes activated, this subunit is dephosphorylated by calcineurin and it moves to the nucleus, where it plays a part in activating transcription of a number of different immunomodulatory genes, including those coding for various cytokines.
Cyclosporin A, by inhibiting the activity of calcineurin, inhibits this dephosphorylation and the translocation of the transcription factor subunit to the nucleus.
Dixon told BioWorld International, “We postulated that, as African swine fever virus does not replicate in T cells but in macrophages, there should be an NFAT transcription factor in macrophages, and we subsequently isolated a gene encoding NFAT from macrophages. We showed, using a reporter gene assay which included a construct that was dependent on NFAT transcription factor, that A238L inhibits activation of NFAT-dependent gene transcription.“
The implication is that there may be host proteins that act in the same way, she said. The finding might allow researchers to uncover new mechanisms of immune system regulation.
“By studying how A238L binds to calcineurin and how it inhibits the activation of NF-kappaB,“ she added, “we might be able to design new drugs to act in the same way.“
The group next plans to study what genes A238L suppresses and what this means for the host in terms of viral pathogenesis. *