Characterising and targeting poxviral Spi-2 proteins

Apoptosis can be used to eliminate virally infected host cells, but many viruses express apoptosis inhibitors that prevent apoptosis of infected host cells, thus permitting the virus to replicate and spread. We are investigating the function and specificity of "Spi-2" proteins expressed by poxviruses like variola virus (which caused smallpox). Ectromelia virus causes a fatal disease resembling smallpox in susceptible strains of mice, but animals infected with a mutant ectromelia virus lacking the Spi-2 gene can mount an effective immune response and survive. Hopefully, drugs that antagonise Spi-2 proteins could treat poxviral infections by facilitating immune-mediated clearance of infected cells.

In order to develop and test such drugs, we need to understand the function of Spi-2 proteins. Previous research into the Spi-2 protein from cowpox virus ("CrmA") revealed that it potently inhibits host proteases of the "caspase" family. Two particular caspases have been proposed to be crucial host targets of CrmA: caspase-1, which generates inflammatory cytokines, and caspase-8, which is responsible for immune cell-mediated apoptosis. However, it is not known whether Spi-2 proteins from all poxviruses share this specificity, nor whether both caspases must be disabled for poxviruses to evade immune destruction.

We have defined the specificity of Spi-2 proteins from a panel of poxviruses. Interestingly, viruses closely related to variola virus all encode Spi-2 proteins that share CrmA's dual specificity. To explore the molecular determinants of Spi-2 protein specificity, we have also created CrmA variants with altered caspase inhibition profiles, including some that can inhibit caspase-1 but not caspase-8.