News: Pox researchers point to bioterrorism solutions

The way certain cytokines regulate the immune response to mousepox has potential to help with poxvirus infections, including smallpox, according to new research at the ANU.

The mousepox virus, Ectromelia, is the focus of work by Dr Gunasegaran Karupiah and team from the immunology and genetics division at the John Curtin School of Medical Research at the ANU in Canberra.

Mousepox is a close relative of the smallpox virus and a member of the vaccinia group of viruses -- the vaccine used to eradicate smallpox was a vaccinia virus, thus these viruses are in the same antigenic group. Mousepox causes the same symptoms in mice as smallpox does in humans -- fever and rashes.

“Smallpox was one of the few viruses we were able conquer with a vaccine,” said Karupiah. “But there was not much interest in understanding how it produced its effects.”

Smallpox was considered to be eradicated in the mid-1970s in the developed world, but vaccination continued in some parts of the developing world. Today, the US army still vaccinates its personnel.

According to Karupiah, the smallpox vaccine causes quite debilitating side-effects in about 1 in 10,000 people. Because the vaccine is, in effect, the vaccinia virus, it cannot be given to immunocompromised people as it causes generalised vaccinia, and it can cause CNS complications. Karupiah’s team is using two mouse models, one genetically resistant and one genetically susceptible to mousepox virus, to study their immune response to infection.

“We have bred mice with a particular genetic background in order to study the immune system,” said Karupiah. “We have done a very systematic study looking at these mice and have identified proteins, or cytokines, that are present in the resistant mice and not in susceptible mice.”

Karupiah said susceptible mice die very quickly from the mousepox infection, whereas resistant mice are able to resist infection. The team has found that resistant mice generate three specific cytokines that act in concert with T-cells to generate immunity. Firstly, they produce interferon (IFN) gamma, an important cytokine for protection against many viruses. T-cells produce IFN-gamma, which is necessary to activate T-cells to seek out the virus.

“Gamma interferon may not be effective against certain viruses, but it is critical for poxviruses,” said Karupiah.

Secondly, resistant mice produce interleukin-2 (IL-2), a T-cell growth factor that is involved in the development and expansion of the immune system. And thirdly, they produce tumor necrosis factor (TNF), which also has anti-viral effects, although not as dramatic as IFN-gamma.

Susceptible mice do not produce these cytokines, or produce very little of them. They therefore do not produce killer T-cells, which would detect and destroy virus infected cells –- epithelial cells, stromal cell and some macrophages are infected by the mousepox virus.

Along with resistant mice, the susceptible mice do produce interleukin-4 (IL-4).

Karupiah said their research findings could address some of the issues associated with the smallpox vaccine, such as side effects or if the vaccine was not working in an individual, an individual could be treated in IFN-gamma.

“A lot of the world’s population is not vaccinated for smallpox now,” said Karupiah. “With talk of use of smallpox by terrorists, the use of a vaccine would be too late, but the use of an anti-viral, like IFN-gamma, could help with infection.”

Poxviruses are DNA viruses that replicate inside their host cell cytoplasm. Over the course of time these viruses have picked up genes from their host organisms that produce proteins that can bind to host IFN-gamma, IFN-gamma receptors, as well as other interferons.

“These are very clever viruses,” said Karupiah. “They can pick up DNA from the host and modify it, and they can target interferons causing inactivation or mimic the action of interferon themselves.”