A functional interleukin-4 homolog is encoded in the genome of infectious laryngotracheitis virus: unveiling a novel virulence factor

Abstract

Herpesviruses have evolved numerous immune evasion tactics, persisting within their hosts through self-perpetuating strategies. One such tactic involves acquiring functional copies of host genes encoding cytokines such as IL-6 (HHV-8), IL-8 (GaHV-2), IL-10 (HHV-4, HHV-5), and IL-17 (SaHV-2). These viral mimics, or virokines, can bind to cellular receptors, modulating the natural cytokine signaling and manipulating the immune response to favor the virus. In the course of full-length cDNA sequencing of infectious laryngotracheitis virus transcripts, a previously unknown highly-spliced gene was discovered in the viral genome predicted to encode a 147 amino acid protein with similarity to vertebrate interleukin-4. The three-intron gene structure was precisely conserved with chicken and other vertebrate IL-4 homologs, and the amino acid sequence displayed structural conservation with vertebrate homologs at the primary, secondary, and tertiary levels based on computational modeling. The viral IL-4 gene was subsequently identiied in all sequenced ILTV genomes. Phylogenetic analyses, along with the conserved gene structure, suggested direct capture from aGalliformeshost. Functionally, an LPS-stimulation assay showed that the expressed viral IL-4 homolog stimulated nitric oxide production in a macrophage cell line at comparable levels to recombinant chicken IL-4. A recombinant virus lacking vIL-4 exhibited slightly higher titers in cell culture compared to the parental strain.In vivobird studies demonstrated reduced pathogenicity of the vIL-4 knockout compared to wildtype. These results represent the irst report of a previously unknown virokine encoded in the ILTV genome expressing a functional IL-4 homolog and virulence factor.