Distribution and characterization of the equine type I IFN family

The interferon (IFN) was described as a substance secreted by virally infected cells and endowed with the ability to render cells capable of interfering with a subsequent viral infection. In eutherian mammals, type I IFNs comprises seven major homologous subgroups including IFNA, IFNB, IFND, IFNE, IFNK, IFNW and IFNX. They are the key cytokines orchestrating host antiviral defense and other physiological processes. All type I IFNs are clustered together in specific regions in humans, mice and bovine genomes. Two mammalian IFN genes of ancient origin, IFNB and IFNE, define the outer limits of the locus, with all the other genes, except IFNK, distributed between these two markers. Twenty four type I IFN genes were identified in the genome of horse (Equus caballus). They included eight IFNWs (two are pseudogenes), six IFNAs, three IFNBs and IFNXs, two IFNDs, one IFNK and IFNE. Interestingly, IFND, which has been discovered only in pigs to date, was also found in equine genome. However, IFNT, which was identified in bovine genome, did not exist in equine genome. The specific distribution of type I IFN in equine genome suggested these IFNs were required in immune defense against particular pathogens affecting horse itself. The equine type I IFN locus also had two sub-loci (1 & 2) distributed in different chromosomes. The larger sub-loci encompassing 530 kb was located on chromosomes 23. Another smaller sub-loci encompassing 86 kb was located on chromosomes Un0111. It seemed that equine type I IFN locus showed its specific property different from those of mouse, human, pig and bovine genome, which were located on the same chromosomes.