Unexpectedly, overexpression of IL-4 led to total suppression of mouse cell-mediated immune response to the mousepox virus. All ten mice in the experimental sample died. Furthermore, when mice already vaccinated against mousepox were inoculated with the IL-4 expressing mousepox virus, they suffered a mortality rate of over 60 percent. This suggested that even immune memory responses instilled through vaccination were inhibited by the virus-coded IL-4.
As explained by Dr. Ron Jackson: When you co-express IL-4 in a thymaidine kinase positive background virus, what happens is the virus background is partially pathogenic, but the action in IL-4 on top of that with all the other viral virulence factors, youre already pushing the immune response to a T-Helper-2 response. Its just too much, and the whole system just flips, and you get a total suppression of the response. So, the virus replicates uncontrolled, and the mice die of mousepox.