Page 892 - Veterinary Immunology, 10th Edition

Viruses can block interferon activity. Methods range from blocking
VetBooks.ir interferon receptor signal transduction to synthesizing soluble

interferon receptors. Some viruses inhibit IFN-γ production by
blocking the activities of IL-18 and IL-12, both of which are required

for its production. Myxoma and poxviruses produce a protein
related to the IFN-γR. By binding free IFN-γ, this prevents it
binding to cell receptors. Equine herpesvirus-1 suppresses IFN-β
production, and as a result, expression of viperin was also

suppressed. Interestingly, it does not suppress IFN-α production.
Some viruses make viral versions of cytokines and chemokines
and their receptors. These have been called virokines or
immunoevasins. For example, equine herpesvirus makes CCR3, the

receptor for CCL11. Marek's disease virus makes a protein related
to CXCL8. Poxviruses make a version of the immunosuppressive
cytokine IL-10. Cowpox virus also makes an IL-1β-binding protein
that reduces its availability.

Interference With Antigen Processing

Pathways

Many viruses interfere with the expression of MHC class I
molecules and so inhibit antigen presentation. They use many

different suppressive techniques, including reducing transcription
of MHC genes, blocking transporter protein function and the
transfer of peptides into the endoplasmic reticulum, inhibiting
proteasomal degradation of viral proteins, inhibiting the
intracellular transport of MHC class I α chains, preventing delivery

of the loaded MHC to the cell surface, and ubiquinating and hence
destroying MHC molecules. Thus bovine herpesvirus-1 suppresses
the expression of MHC class I molecules by interfering with

transporter protein functions and downregulating the expression of
mRNA for MHC class I molecules. Other viruses may cause MHC
class I molecules to be retained within a cell; they may prevent
peptide binding to transporter proteins, prevent proteasomal
degradation, redirect MHC molecules to lysosomes for degradation,

or even encode inhibitors that block caspase activity. Influenza A
viruses can block macrophage differentiation into dendritic cells.
Other viruses may downregulate the expression of co-stimulating

892