Identification of host factors which restrict African swine fever virus replication

African swine fever (ASF) is a fatal haemorrhagic disease of domestic pigs. The etiological

agent of ASF is African swine fever virus (ASFV), a large DNA virus and only member of

the Asfarviridae family. The virus replicates in macrophages and infection has previously

been suggested to correlate with expression of cell surface markers, including CD163,

which is a characteristic marker of intermediate and late stages of macrophage

differentiation. ASFV cell tropism and host factors which modulate infection have not

previously been studied extensively in macrophages. Macrophages retain a plasticity of

function in vivo and in vitro and alter their phenotype in response to various stimuli. This

allows them to respond to a variety of situations. The impact of priming macrophages by

different stimuli on ASFV infection rates was investigated. The results provide information

relevant to identifying cellular factors which modulate infection in vitro and in vivo.

To characterise the impact of macrophage activation on ASFV replication, adherent porcine

bone marrow cells were stimulated to form regulatory, classically or alternatively activated

macrophages using media supplemented with IL10, IFN? or IL4 respectively. Cell surface

marker expression was assessed using FACS analysis to confirm the predicted macrophage

phenotypes were induced. The percentage of macrophages infected was shown to vary

significantly, dependent upon virus isolate, treatment and duration of infection. Differences

were also observed in production of infectious progeny virus and cell survival following

infection of macrophages in different activation states. The results indicate that the

activation state of macrophages is important for ASFV infection and that treatment with IL4

to stimulate alternative activation may increase persistence of ASFV infection. Infection of

differentially activated cells with porcine reproductive and respiratory syndrome virus

(PRRSV) was also investigated to allow comparison to ASFV infection. Interestingly,

despite similar cell tropisms of ASFV and PRRSV, PRRSV infection of alternatively

activated macrophages was severely inhibited unlike ASFV infection.

The requirement of the cell surface marker CD163 for ASFV infection was investigated as

it is a marker of alternatively activated macrophages and has also been suggested to be a

receptor for ASFV. However, ASFV infection rate did not correlate with its presence on the

cell surface and the data indicated that ASFV infection rates were not increased in cells

stably transformed to express CD163.