NF-KAPPA-B-MEDIATED INHIBITION OF APOPTOSIS IS REQUIRED FOR ENCEPHALOMYOCARDITIS VIRUS VIRULENCE - A MECHANISM OF RESISTANCE IN P50 KNOCKOUT MICE

Apoptosis is a central host defense mechanism to eliminate virus-infec ted cells. Activation of NF-KB suppresses apoptosis following some typ es of stimulation in vitro. To test the physiological importance of th is pathway in vivo, we studied murine encephalomyocarditis virus (EMCV ) infection in mice and cell lines defective in NF-kappa B1 (p50) sign aling. As previously reported, we find that all p50 knockout (p50 -/-) mice survive an EMCV infection that readily kills normal mice. By int roducing the p50 mutation into interferon (IFN) type I receptor knocko ut (IFNRI -/-) mice, we find that this resistance is not mediated by I FN-P as previously thought. While no IFNRI -/- mice survive, the doubl e-knockout mice survive 60% of the time. The survival is tightly linke d to the animals' ability to clear the virus from the heart in vivo, U sing murine embryonic fibroblasts (MEF) derived from wild-type, p50 -/ -, and p65 -/- embryos, we found that NF-KB is not required for the re plication cycle of EMCV, However, during these experiments we observed that p50 -/- and p65 -/- MEF infected with EMCV undergo enhanced, pre mature cytotoxicity, Upon examination of this cell death, we found tha t EMCV infection induced both plasma membrane and nuclear changes typi cal of apoptosis in all cell lines. These apoptotic processes occurred in an accelerated and pronounced way in the NF-kappa B defective cell s, as soon as 6 h after infection, when virus is beginning to be relea sed. Previously only the RelA (p65) subunit of NF-kappa B has been sho wn to play a role in suppressing apoptosis. In our studies, we find th at p50 is equally important in suppressing apoptosis during EMCV infec tion. Additionally, we show that suppression of apoptosis by NF-kappa B1 is required for EMCV virulence in vivo. The attenuation in p50 -/- mice can be explained by rapid apoptosis of infected cells which allow s host phagocytes to clear infected cells before the viral burst leadi ng to a reduction of the viral burden and survival of the mice.