Malignant catarrhal fever (MCF) is a fatal, incurable disease of large
ruminants. This disease is caused by two related gammaherpesviruses, Alcelaphine
herpesvirus 1 (A1HV-1) and Ovine herpesvirus 2 (OvHV-2). Lymphoproliferation
and infiltration by large granular lymphocytes (LGL) into lymphoid and nonlymphoid organs are characteristic of MCF. LGL cell lines have been established
from animals with naturally or experimentally induced MCF. Previous studies have
shown these cell lines to be highly proliferative and cytotoxic with the ability to
cause disease when inoculated into rabbits. Some cell lines have demonstrated the
ability to grow and survive without the need for exogenous IL-2. However, IL-2
mRNA was not detected. The proliferative and cytotoxic LGL phenotype observed
in the absence of exogenous IL-2 is associated with virus infection of LGL cells.
The characteristics of MCF LGL cell lines are similar to other herpesvirus-infected
cell lines, in particular Herpesvirus saimiri (HVS).
The role of virus-infected large granular lymphocytes in pathogenesis of
MCF is not clear. In the absence of defined viral antigens the main objective of this
study was to investigate how MCF viruses interfere with LGL proliferation and
In this study, IL-2 independence and a cell surface phenotype of T/ NK cells
was demonstrated in most cell lines analysed. The period of survival and growth
without exogenous IL-2 was variable but always exceeded that of uninfected control
cells. The possible role of IL-15 (a pro-inflammatory cytokine with actions similar
to IL-2) in LGL function was investigated. Results showed that IL-15 could generate
and maintain the proliferative and cytotoxic phenotype of these cell lines and may be
involved in the pathogenesis of MCF.
Virus interference with signal transduction pathways has been demonstrated
in other gammaherpesviruses. The hypothesis that the phenotype and function of the
LGL cell lines is generated through a similar mechanism was tested. The study
showed that the src kinases, Ick and Jyn, are constitutively activated in LGL cell
The identity of specific virus proteins involved in generating the observed
phenotype of the LGL cell lines had not been determined. During the course of this
study, the complete genomic sequence of the A1HV-1 genome was published. Prior
to this several open reading frames were detected in A1HV-1 that underwent genomic
rearrangement on transition from virulence to attenuation in vitro. Thus, a final
objective of this study was to determine whether these potential virulence genes are
associated with the ability of LGL cell lines to transmit disease. The proteins
encoded by these genes (ORF50, A6 and A7) were expressed in E. coli as
recombinant proteins and used to immunise rabbits. Recombinant virus proteinspecific rabbit polyclonal antibodies were generated. Using these reagents, the
expression of these proteins in LGL cell lines and A1HV-1-infected monolayer
cultures was investigated. The polymerase chain reaction (PCR) was used in parallel
experiments to determine the presence of DNA and mRNA encoding these proteins.
The results indicated that more complicated rearrangements of the A1HV-1 genome
may occur on attenuation of A1HV-1 after extensive passage than has been revealed
at present. However, the expression in LGL cells of mRNA encoded by ORF50 and
A6, that share sequence homology with the EBV R and Z transactivators, suggests
that virus replication occurs in LGL cells.
These results improve the understanding of the MCF viruses and their role in
generation of the LGL phenotype.