DISRUPTION OF M-T5, A NOVEL MYXOMA VIRUS GENE MEMBER OF THE POXVIRUS HOST-RANGE SUPERFAMILY, RESULTS IN DRAMATIC ATTENUATION OF MYXOMATOSISIN INFECTED EUROPEAN RABBITS

Myxoma virus is a pathogenic poxvirus that induces a lethal myxomatosi s disease profile in European rabbits, which is characterized by fulmi nating lesions at the primary site of inoculation, rapid dissemination to secondary internal organs and peripheral external sites, and super vening gram-negative bacterial infection. sere we describe the role of a novel myxoma virus protein encoded by the M-T5 open reading frame d uring pathogenesis. The myxoma virus M-T5 protein possesses no signifi cant sequence homology to nonviral, proteins but is a member of a larg er poxviral superfamily designated host range proteins. An M-T5(-) mut ant virus was constructed by disruption of both copies of the M-T5 gen e followed by insertion of the selectable marker p7.5Ecogpt. Although the M-T5(-) deletion mutant replicated with wild-type kinetics in rabb it fibroblasts, infection of a rabbit CD4(+) T-cell line (RL5) with th e myxoma virus M-TS- mutant virus resulted in the rapid and complete c essation of both host and viral protein synthesis, accompanied by the manifestation of all the classical features of programmed cell death. Infection of primary. rabbit peripheral mononuclear cells with the myx oma virus M-T5(-) mutant virus resulted in the apoptotic death of nona dherent lymphocytes but nut adherent monocytes. Within the European ra bbit, disruption of the M-T5 open reading frame caused a dramatic atte nuation of the rapidly lethal myxomatosis infection, and none of the i nfected rabbits displayed any of the characteristic features of myxoma tosis. The two most significant histological observations in rabbits i nfected with the M-T5(-) mutant virus were (i) the lack of progression of the infection past the primary site of inoculation, coupled with t he establishment of a rapid and effective inflammatory reaction, and ( ii) the inability of the virus to initiate a cellular reaction within secondary immune organs. We conclude that M-TS functions as a critical virulence factor by allowing productive infection of immune cells suc h as peripheral lymphocytes, thus facilitating virus dissemination to secondary tissue sites via the lymphatic channels.