Vaccinia Virus K1L and C7L Inhibit Antiviral Activities Induced by Type I Interferons

Abstract

Cellular tropism of vaccinia virus (VACV) isVaccinia virus (VACV) is the prototypical member of the poxvirus family of large, complex, double-stranded DNA viruses (21). VACV has a very broad host range and is capable of infecting many vertebrate animal species. Its host range, however, can be significantly narrowed by deleting from its genome some of the so-called host range genes, the most important of which are E3L, K1L, and C7L (17). VACV mutants deleted of E3L (⌬E3L) or both K1L and C7L (⌬K1L⌬C7L) replicate abortively and express only a subset of viral genes in most mammalian cell lines (3, 24). These mutants are highly attenuated in animal hosts but are capable of eliciting immune responses, making them attractive vaccine vectors for infectious diseases and cancers (27,28). NYVAC, a VACV strain derived through deletion of 18 genes, including both K1L and C7L (27), has been used as the vector for an AIDS vaccine (2).The functions of E3L and the host factors that restrict the replication of the ⌬E3L mutant have been studied extensively. E3L encodes a 20-kDa and a 25-kDa protein that bind doublestranded RNA (dsRNA) and Z form DNA (6, 15). The E3 proteins antagonize the dsRNA-dependent protein kinase PKR (5), which exists as an inactive form in the cells and undergoes autophosphorylation and activation upon binding to dsRNA. The activated PKR phosphorylates the ␣ subunit of eukaryotic initiation factor 2 (eIF2␣), resulting in a block in protein translation at the initiation step. The infection of most mammalian cells by the ⌬E3L mutant leads to the activation of PKR and a block in translating viral mRNAs (16). The replication of the ⌬E3L mutant in nonpermissive HeLa cells can be rescued by silencing PKR expression (32), while its replication in permissive Huh7 (human hepatoma) cells can be blocked by upregulating PKR expression with interferon (IFN) treatment (1). In addition to affecting PKR, E3 has also been shown to inactivate IFN-stimulated gene 15 (ISG15) (14), another IFN effector that plays a role in host defense against VACV.Like the replication of the ⌬E3L mutant, the replication of the ⌬K1L⌬C7L mutant in nonpermissive HeLa cells is blocked at the translation of viral mRNA. However, the host factors that restrict the replication of the ⌬K1L⌬C7L mutant and the molecular functions of K1L or C7L remain a mystery. K1 and C7 share no amino acid sequence homology, but either K1L or C7L can complement the replication defect of the ⌬K1L⌬C7L mutant in most cell lines. The exception is rabbit RK13 cells, where K1L but not C7L can complement (24). K1L is present in only a few orthopoxviruses, while C7L or a functional homologue of C7L is present in almost all mammalian poxviruses (18). K1 comprises multiple ankyrin repeats, a protein motif that is involved in protein-ligand interaction. It was shown to prevent the degradation of IB␣ and to thus inhibit host . C7L has no homologue outside the poxvirus family, and its molecular function remains unknown. It may play a role in inhibiting cellular apoptosis in respons...

C7L Family of Poxvirus Host Range Genes Inhibits Antiviral Activities Induced by Type I Interferons and Interferon Regulatory Factor 1

“…We previously showed that both K1L and C7L antagonize antiviral activities induced by type I IFN in Huh7 cells (13). However, it has not been determined whether C7L homologues from other mammalian poxviruses can also antagonize type I IFN activities.…”

“…Our previous work suggested that K1L and C7L function by antagonizing products of ISGs (13). To find out which ISG product is the target of K1L and C7L, we screened an expression library consisting of over 350 ISGs for inhibitors of vK1L…”

“…Ϫ , K1L-rev, C7L-rev, vVACV-C7L, vYLDV-67R, vMYXV-M62R, vMYXV-M63R, vMYXV-M64R, and vCPXV-020 were described previously (12)(13)(14).…”

“…The sensitivity of the viruses to human IFN-␤ was tested with Huh7 cells as described previously (13). Briefly, Huh7 cells were treated with 200 U/ml of human IFN-␤ (PBL Biomedical Laboratories) for 24 h and then infected with wild-type (WT) or mutant VACV at an MOI of 5.…”

“…VACV requires either K1L or C7L for productive replication in most mammalian cells (3,17), but it requires neither for replication in avian cells and in a few specific mammalian cell lines, such as human hepatoma Huh7 cells (13). When Huh7 cells are pretreated with type I interferon (IFN), however, K1L or C7L is required for replication beyond early gene expression (13), suggesting that K1L and C7L support VACV replication by antagonizing some antiviral factors that are expressed constitutively in most mammalian cells but are induced by IFN only in some cell lines, such as Huh7 (13).…”