Disruption of the murine gammaherpesvirus 68 M1 open reading frame leads to enhanced reactivation from latency

Murine gammaherpesvirus 68 (gHV68, or MHV-68) is a genetically tractable, small animal model for the analysis of gammaherpesvirus pathogenesis. The gHV68 genome is colinear with the genomes of other sequence gammaherpesviruses, containing large blocks of conserved genes interspersed by a number of putative genes without clear homologs in the other gammaherpesviruses. One of these putative unique genes, the M1 open reading frame (ORF), exhibits sequence homology to a poxvirus serine protease inhibitor, SPI-1, as well as to another gHV68 gene, M3, which we have recently shown encodes an abundantly secreted chemokine binding protein. To assess the contribution of the M1 ORF to gHV68 pathogenesis, we have generated a recombinant gHV68 in which the M1 ORF has been disrupted through targeted insertion of a lacZ expression cassette (M1.LacZ). Although M1.LacZ replicated normally in tissue culture, it exhibited decreased splenic titers at days 4 and 9 postinfection in both immunocompetent and immunodeficient mice. Despite decreased levels of acute virus replication, M1.LacZ established a latent infection comparable to wild-type (wt) gHV68, but exhibited an approximately fivefold increase in efficiency of reactivation from latency. M1.LacZ also caused severe vasculitis of the great elastic arteries in gamma interferon receptor (IFN-gR)-deficient mice with a frequency comparable to wt gHV68, but did not cause the mortality or splenic pathology observed with wt gHV68 infection of IFN-gR-deficient mice. Restoration of M1 ORF sequences into M1.LacZ (M1 marke