V and C proteins of measles virus function as virulence factors in vivo

The measles virus (MV) P gene encodes three proteins: the P protein and two nonstructural proteins, C and V. Because the functions of both the C and V protein are unknown, we used MV C (C-) and V (V-) deletion recombinants generated by the MV reverse genetics system (F. Radecke, P. Spielhofer, H. Schnieder, K. Kaelin, M. Huber, C. Dotsch, G. Christiansen, and M. A. Billeter 1995. EMBO J. 14, 5773-5784). Compared to parental vaccine strain, Edmonston (Ed) MV, both had normal growth and cytopathic effects in Vero cells and showed similar growth kinetics in human neuroblastoma SK-N-MC cells and in primary mouse neurons expressing the MV receptor, CD46. However, in vivo, using YAC-CD46 transgenic mice as a model for MV induced CNS disease (M. B. A. Oldstone, H. Lewicki, D. Thomas, A. Tishon, S. Dales, J. Patterson, M. Manchester, D. Homann, D. Naniche, and A. Holz 1999. Cell 98, 629-640), C- and V- viruses differed markedly from wt Ed(V(+)C(+)) virus. Newborn mice inoculated with as little as 10(3) PFU of Ed strain became ill and died after 10-15 days. In contrast, those inoculated with 10(3) or 10(4) PFU of MV C- or MV V- showed significantly fewer and milder clinical symptoms and had a lower mortality. A total of 10(5) PFU V- virus were required to kill most YAC-CD46 mice, and less than half (44%) were killed with a corresponding dose of MV C-. Immunohistochemical staining for MV antigens showed similar extents of spread for MV C- and MV Ed but restricted spread for MV V- throughout the brain. Viral load and transcription were markedly reduced for V- but not for C-. Multiple cytokines and chemokines were equivalently upregulated for all three viruses. Therefore, MV C and V proteins encode virulence functions in vivo and likely operate via separate mechanisms.