The parapoxvirus orf virus encodes a novel soluble protein inhibitor of ovine granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2 (IL-2). The GM-CSF- and IL-2-inhibitory factor (GIF) gene was expressed as an intermediate-late viral gene in orf virus-infected cells. GIF formed homodimers and tetramers in solution, and it bound ovine GM-CSF with a Kd of 369 pM and ovine IL-2 with a Kd of 1.04 nM. GIF did not bind human GM-CSF or IL-2 in spite of the fact that orf virus is a human pathogen. GIF was detected in afferent lymph plasma draining the skin site of orf virus reinfection and was associated with reduced levels of lymph GM-CSF. GIF expression by orf virus indicates that GM-CSF and IL-2 are important in host antiviral immunity.

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The parapoxvirus orf virus was resistant to type 1 (IFN-alpha) and type 2 (IFN-gamma) interferons in cultures of ovine cells. The recently identified orf virus OV20.0L gene exhibits 31% predicted amino acid identity to the vaccinia virus E3L interferon-resistance gene, and is referred to as the (putative) orf virus interferon-resistance gene (OVIFNR). The objective of this study was to determine whether OVIFNR was involved in interferon resistance. Recombinant OVIFNR as a thioredoxin fusion protein (OVIFNR-Tx) inhibited the activation (by autophosphorylation) of an interferon-inducible, double-stranded (ds) RNA-dependent kinase (PKR) of sheep, which was shown to bind dsRNA (poly I:C). PKR in other species is involved in the inhibition of protein synthesis as part of the antiviral state in infected cells. Virus-infected cell lysates, but not control lysates, from cells grown in the presence of cytosine arabinoside also contained PKR inhibitory activity, which indicated that the inhibitory activity was associated with early viral gene expression. Significantly, the OVIFNR gene expressed in interferon-treated ovine fibroblasts protected the unrelated Semliki Forest virus from the antiviral effect of both type 1 and type 2 interferons. Taken together, the results indicate that the OVIFNR gene functions as an interferon-resistance gene, the product of which inhibits PKR in a similar way to the vaccinia virus E3L gene product.

The in vivo dynamics of differentiated cells and interleukin (1L)-1 p, IL-8, granulocyte-macrophage colony-stimulating factor (GM-CSF) and interferon (1FN)q titres in afferent lymph were compared following orf virus reinfection and inactivated virus injection of previously infected sheep. The biphasic lymphoblast and cytokine response in the lymph to virus reinfection is consistent with a response initially to orf virus as recall antigen followed by a response to viral replication. CD4 T cells increased in output over other cell types in the lymph in both groups. A rapid immune/inflammatory response was detected in lymph plasma as an increase in cytokine titres within 24 h of virus reinfection or injection. Lymph cells producing IL-1 p and IL-8 appeared prior to those producing GM-CSF in both groups. In spite of variations in the concentration of individual cytokines in lymph following reinfection, both the size of the orf lesion and the time to resolve were similar in all cases.

Protection of cattle from alcelaphine herpesvirus-1 (AlHV-1)-induced malignant catarrhal fever (MCF) has been described previously, using an attenuated virus vaccine in an unlicensed adjuvant. The vaccine was hypothesised to induce a protective barrier of virus-neutralising antibody in the oro-nasal region, supported by the observation of high titre neutralising antibodies in nasal secretions of protected animals. Here we describe further analysis of this vaccine strategy, studying the effectiveness of the vaccine formulated with a licensed adjuvant; the duration of immunity induced; and the virus-specific antibody responses in plasma and nasal secretions. The results presented here show that the attenuated AlHV-1 vaccine in a licensed adjuvant protected cattle from fatal intranasal challenge with pathogenic AlHV-1 at three or six months. In addition, animals protected from MCF had significantly higher initial anti-viral antibody titres than animals that succumbed to disease; and these antibody titres remained relatively stable after challenge, while titres in vaccinated animals with MCF increased significantly prior to the onset of clinical disease. These data support the view that a mucosal barrier of neutralising antibody blocks infection of vaccinated animals and suggests that the magnitude of the initial response may correlate with long-term protection. Interestingly, the high titre virus-neutralising antibody responses seen in animals that succumbed to MCF after vaccination were not protective.