I would suggest the INFg knock-out mice (http://jaxmice.jax.org/strain/002287.html), but these mice would be confounded with the fact that all cells do not produce INFg and will have been that way since development (may play a role). The other way is to use a wild-type animal with a INFg targeted inactivating antibody. I guess it really depends on what the endpoint of your study is hoping to assess...