Abstract
The gammaherpesvirus subfamily has long been the study of intensive investigation
owing to the association between infection and development of lymphoproliferative
disease. Well-known members ofthe Gammaherpesvirinae include Epstein-Barr virus
(EBV) and Kaposi's Sarcoma-associated herpesvirus (KSHV). Common properties of
gammaherpesviruses include a narrow host range of infection and limited productive
growth in vitro, and these factors make the study of acute infection problematic.
Murine gammaherpesvirus-68 (MHV-68) is able to undergo lytic replication in a
range of cell types in vitro and can infect inbred strains of mice. These properties
make MHV-68 an excellent model for the study of gammaherpesvirus pathogenesis.
Herpesviruses have been indicated in development of diseases in the lung, including
pneumonia and idiopathic pulmonary fibrosis. MHV-68 allows investigation of
gammaherpesvirus infection of and persistence in the lung - following intranasal
inoculation the virus establishes a
life-long infection in this organ, with virus
persisting in epithelial cells and/or B cells. Identification of key viral genes required
for persistence may allow for development of vaccination and/or treatment strategies.
Using real-time PCR the long-term viral load in the lungs was reduced following the
deletion of key genes from the viral genome. Genes identified are the thymidine
kinase gene, previously shown to play a role during acute infection of the lung and
ORF73, a homologue of the KSHV LANA-1 gene. Initial data also suggests that the
ORF72 and Mil genes, both involved in reactivation from latency, may play a role in
maintaining viral load at late time points post-infection.
In vivo investigation of the Ml gene of MHV-68 has demonstrated a potential role in
control of viral reactivation from latency in the spleen. A novel MHV-68 mutant,
MIA, lacking 1171 bp of the Ml ORF, was used to study the role of Ml in
pathogenesis. Initial data suggests that in vivo infection with MIA results in increased
viral titres during acute infection of the lung, indicating a potential role in control of
initial infection. The major role of Ml appears to be during acute phase latency in the
spleen, with the MIA virus failing to drive splenomegaly and establishing latency at
lower levels. Despite the presence of fewer latently infected splenocytes, MIA
reactivates at significantly higher levels, indicating that a function of Ml is to control
viral reactivation from latency.
A viral mutant (M4Inl) was created that carries a stop codon inserted at genome co¬
ordinate 8386 in the region between the M3 and M4 genes. The mutation is thought to
be in an untranscribed region of the genome, potentially in the promoter region of the
M3 or M4 genes. Studies demonstrated that the virus is attenuated following infection
of both wild-type and IFNyR " mice with respect to lung pathology scores. The
lethality of M4Inl in juvenile IFNyR." mice is reduced compared with wild-type
MHV-68 infection. Despite the location of the mutation within potential promoter
regions, M4Inl transcribes both M3 and M4 at wild type levels in vitro, and in vivo in
the spleen. This evidence suggests an apparently untranscribed region of the MHV-68
genome is able to influence pathogenesis in the lung independent of the neighbouring
genes.