Journal of General Virology (2007), 88, 1667–1676 DOI 10.1099/vir.0.82748-0 Correspondence Geoffrey L. Smith glsmith@imperial.ac.uk Received 30 November 2006 Accepted 25 January 2007 INTRODUCTION Camelpox virus (CMLV) is a member of the genus Ortho- poxvirus (OPV) of the family Poxviridae, a group of large double-stranded DNA viruses that replicate in the cytoplasm (Moss, 2001). Compared with other OPVs, CMLV is poorly characterized. The genomes of CMLV strains CMS (Gubser & Smith, 2002) and M-96 (Afonso et al., 2002) were sequenced and revealed that CMLV is genetically closely related to variola virus (VARV), the cause of smallpox (Gubser & Smith, 2002). Like other OPVs, the CMLV genome has a highly conserved central region of about 100 kb encoding genes that are mostly essential for virus replication. In contrast, the genome termini encode proteins known or predicted to affect virus virulence or modulation of the host’s immune response. 3Present address: Division of Cell and Molecular Biology, Faculty of Natural Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, UK. 4Present address: Pathology, B23, University of Lie` ge, CHU Sart- Tilman, 4000 Li e`ge, Belgium. Supplementary figures are available with the online version of this paper. 0008-2748 G 2007 SGM Printed in Great Britain Camelpox virus encodes a schlafen-like protein that affects orthopoxvirus virulence Caroline Gubser,1 Rory Goodbody,1 Andrea Ecker,13 Gareth Brady,2 Luke A. J. O’Neill,2 Nathalie Jacobs14 and Geoffrey L. Smith1 1Department of Virology, Faculty of Medicine, Imperial College London, St Mary’s Campus, Norfolk Place, London W2 1PG, UK 2School of Biochemistry and Immunology, Trinity College Dublin, Dublin 2, Ireland Camelpox virus (CMLV) gene 176R encodes a protein with sequence similarity to murine schlafen (m-slfn) proteins. In vivo, short and long members of the m-slfn family inhibited T-cell development, whereas in vitro, only short m-slfns caused arrest of fibroblast growth. CMLV 176 protein (v-slfn) is most closely related to short m-slfns; however, when expressed stably in mammalian cells, v-slfn did not inhibit cell growth. v-slfn is a predominantly cytoplasmic 57 kDa protein that is expressed throughout infection. Several other orthopoxviruses encode v-slfn proteins, but the v-slfn gene is fragmented in all sequenced variola virus and vaccinia virus (VACV) strains. Consistent with this, all 16 VACV strains tested do not express a v-slfn detected by polyclonal serum raised against the CMLV protein. In the absence of a small animal model to study CMLV pathogenesis, the contribution of CMLV v-slfn to orthopoxvirus virulence was studied via its expression in an attenuated strain of VACV. Recombinant viruses expressing wild-type v-slfn or v-slfn tagged at its C terminus with a haemagglutinin (HA) epitope were less virulent than control viruses. However, a virus expressing v-slfn tagged with the HA epitope at its N terminus had similar virulence to controls, implying that the N terminus has an important function. A greater recruitment of lymphocytes into infected lung tissue was observed in the presence of wild-type v-slfn but, interestingly, these cells were less activated. Thus, v-slfn is an orthopoxvirus virulence factor that affects the host immune response to infection. Among these terminal genes, CMLV strain CMS 176R encodes a protein with sequence similarity to mammalian schlafens (slfn) (Gubser & Smith, 2002). The prototypic member of the slfn family, murine (m-) slfn1, was discovered by a subtractive hybridization be- tween transgenic mice in which T-cell maturation was + + double-positive stage and mice in halted at the CD4 8 which maturation was skewed toward CD4 single-positive selection (Schwarz et al., 1998). BLAST searches identified a further eight related mouse genes that are classified as short (slfn1 and 2), intermediate (slfn3 and 4) or long (slfn5, 8, 9, 10 and 14) m-slfns, depending on their size (Schwarz et al., 1998; Geserick et al., 2004) (Fig. 1). All m-slfns share a conserved region that contains a putative divergent ATPase associated with the cellular activities (AAA) domain (Lupas & Martin, 2002; Frickey & Lupas, 2004), whereas inter- mediate and long m-slfns have additional C-terminal sequences. In vivo, short and long m-slfns inhibit T-cell development (Schwarz et al., 1998; Geserick et al., 2004), whereas in vitro, only m-slfn1 caused arrest of fibroblast growth by inhibition of cyclin D1 (Schwarz et al., 1998; Geserick et al., 2004; Brady et al., 2005). Additionally, the expression level of m-slfns is regulated differentially after 1667