A conserved African swine fever virus IκB homolog, 5EL, is nonessential for growth in vitro and virulence in domestic swine

An African swine fever virus (ASFV) gene with similarity to the cellular inhibitor of NFκB (IκB) was described in the pathogenic African isolate Malawi Lil-20/1 (ORF 5EL) and a cell-culture-adapted European virus, BA71V (ORF A238L). Recently, this gene was shown to be a functional IκB homolog capable of downregulating NFκB-regulated gene expression. This observation suggests the gene may be of significance to aspects of ASFV pathogenesis and virulence in domestic swine by interfering with a normal antiviral host response. Here we show, using nucleotide sequence analysis, that 5EL is highly conserved among Various African and European pathogenic field isolates and that in all cases its similarity to IκB genes is limited to the presence of four low complexity ankyrin repeats in the ASFV gene. The 5EL gene of Malawi Lit-20/ 1 encodes a 28-kDa protein which was expressed early in virus-infected macrophage cell cultures with maximum levels observed at 3 to 5 hr postinfection. To study gene function, a Malawi Lil-20/1 5EL gene deletion mutant (Δ5EL) was constructed. Growth characteristics of Δ5EL in porcine macrophage cell cultures were indistinguishable from those of the parental virus. And, Δ5EL exhibited an unaltered parental Malawi Lil-20/1 disease and virulence phenotype in domestic swine. Thus, although highly conserved among ASFV isolates, 5EL is nonessential for growth in porcine macrophages in vitro and for viral virulence in domestic swine. A possible role for this gene in transmission of ASFV in nature, a setting which involves the cycling of ASFV between two highly adapted hosts, Ornithodoros ticks and warthogs or bush pigs, in sub-Saharan Africa is discussed.