Murine gammaherpesvirus 68 (MHV-68) is a natural pathogen of murid rodents and is closely related to
Human Herpesvirus 8, Herpesvirus saimiri and Epstein Barr virus. Intranasal infection of inbred mouse
strains with MHV-68 results in the lungs of these animals becoming productively infected with virus. In
immunocompetent mice, MHV-68 is cleared from the lungs by day 10 after infection. By this time the
virus has reached the spleen and has adopted a latent form of infection in B lymphocytes. Depletion of
CD8+ T cells from mice, prior to infection, results in uncontrolled MHV-68 replication in the lungs and
death of the animal by day 12 post-infection. Such evidence indicates that CD8+ T cells play an
important role in the immune response of mice to primary infection with MHV-68.
In this study T lymphocyte responses to MHV-68 were examined in vitro and in vivo.
In vitro work focused on developing a conventional assay to measure MHV-68-specific CTL activity. It
was found that splenocytes from MHV-68-infected mice consistently lysed 'S11' cells, a B cell lymphoma
line originally isolated from an MHV-68-infected mouse. All S11 cells are latently infected with MHV-68
and around 5% of these cells support viral replication. Treatment of S11 cells with the anti-viral drug, 4'-
S-EtdU, is known to prevent lytic MHV-68 protein expression. 4'-S-EtdU-treated S11 target cells were
killed by lymphocytes from infected mice. This indicates that the T lymphocytes responsible for killing S11
cells were specific for a latent antigen of MHV-68. No measurable cytolytic activity against any other
target cell line, infected in vitro with MHV-68, was detected.
S11 has been shown to divide and expand when implanted subcutaneously into nude (T cell-deficient)
mice. S11 does not expand when implanted into normal, immunocompetent mice, implying that T
lymphocytes play a key role in inhibiting tumour formation. To investigate this theory, S11 cells were
injected subcutaneously into nude mice. This was followed by transfer of re-stimulated lymphocyte
populations, enriched for either CD4+ or CD8+ T cells, into the animals. This protocol consistently
resulted in regression of S11 tumours. Splenocyte populations depleted of CD8+ T lymphocytes were
most effective in preventing tumour formation. This suggests that CD4+ T cells play a major role in
preventing B cell lymphoma outgrowth. Immunohistochemical analyses highlighted populations of
macrophages and CD4+ T cells in regressing tumours. It may be hypothesised that CD4+ T cells elicit
tumour regression via the initiation of a delayed-type hypersensitivity (DTH) response.
Further in vivo work involved depleting BALB/c mice T lymphocyte subsets (CD4 and CD8) at a range of
times before or after MHV-68 infection. Subsequently, virus titre in the lungs was assessed by plaque
assay. Latently-infected B cells in the spleen were quantified by infectious centre assay and visualised by
in situ hybridisation. Using these techniques it was demonstrated that both CD4+ and CD8+ T
lymphocytes contribute to clearance of MHV-68 from the lungs. CD8+ T cells also appear to limit the
extent of B lymphocyte infection in the spleen. However, CD8+ T cells do not appear to be responsible
for the decline in latently infected B cells which occurs in MHV-68 infected mice 2 to 3 weeks after
In conclusion, MHV-68 infection of inbred laboratory mouse strains has potential to model immunological
responses to primary gammaherpesvirus infection in humans. In addition, histopathology induced by the
S11 cell line implanted into nude mice could provide insight into gammaherpesvirus-associated B
lymphoproliferative disease in immunocompromised individuals.