Transcriptional and translational analyses of the UL2 gene of equine herpesvirus 1: a homolog of UL55 of herpes simplex virus type 1 that is maintained in the genome of defective interfering particles

The Antiviral and Antitumor Effects of Defective Interfering Particles/Genomes and Their Mechanisms

Defective interfering particles (DIPs), derived naturally from viral particles, are not able to replicate on their own. Several studies indicate that DIPs exert antiviral effects via multiple mechanisms. DIPs are able to activate immune responses and suppress virus replication cycles, such as competing for viral replication products, impeding the packaging, release and invasion of viruses. Other studies show that DIPs can be used as a vaccine against viral infection. Moreover, DIPs/DI genomes display antitumor effects by inducing tumor cell apoptosis and promoting dendritic cell maturation. With genetic modified techniques, it is possible to improve its safety against both viruses and tumors. In this review, a comprehensive discussion on the effects exerted by DIPs is provided. We further highlight the clinical significance of DIPs and propose that DIPs can open up a new platform for antiviral and antitumor therapies.

The early UL3 gene of equine herpesvirus-1 encodes a tegument protein not essential for replication or virulence in the mouse

The UL3 gene of equine herpesvirus-1 (EHV-1) is retained in the genome of defective interfering particles and encodes a ~33kDa myristylated protein. Further characterization showed that the UL3 gene is trans-activated only by the sole immediate early (IE) protein and encodes an early protein that is dispensable for EHV-1 replication and localizes in the tegument of purified virions. UL3-deleted EHV-1 (vL11ΔUL3) exhibits properties of host cell tropism, plaque size, and growth kinetics similar to those of the parental virus. Expression levels of EHV-1 proteins representative of all three gene classes in vL11ΔUL3-infected cells were identical to those in cells infected with parental virus. Mice intranasally infected with vL11ΔUL3 and parental virus showed no significant difference in mortality or virus lung titers. These findings suggest that the UL3 protein does not play a major role in the biology of EHV-1 in cell culture or virulence in the mouse.

The UL4 protein of equine herpesvirus 1 is not essential for replication or pathogenesis and inhibits gene expression controlled by viral and heterologous promoters

Defective interfering particles (DIP) of equine herpesvirus 1 (EHV-1) inhibit standard virus replication and mediate persistent infection. The DIP genome is comprised of only three genes: UL3, UL4, and a hybrid gene composed of portions of the IR4 (EICP22) and UL5 (EICP27) genes. The hybrid gene is important for DIP interference, but the function(s) of the UL3 and UL4 genes are unknown. Here, we show that UL4 is an early gene activated solely by the immediate early protein. The UL4 protein (UL4P) was detected at 4hours post-infection, was localized throughout the nucleus and cytoplasm, and was not present in purified virions. EHV-1 lacking UL4P expression was infectious and displayed cell tropism and pathogenic properties in the mouse model similar to those of parental and revertant viruses. Reporter assays demonstrated that the UL4P has a broad inhibitory function, suggesting a potential role in establishing and/or maintaining DIP-mediated persistent infection.

The ICP0 protein of equine herpesvirus 1 is an early protein that independently transactivates expression of all classes of viral promoters

To assess the role of the equine herpesvirus type 1 (EHV-1) ICP0 protein (EICP0) in gene regulation, a variety of molecular studies on the EICP0 gene and gene products of both the attenuated cell culture-adapted Kentucky A (KyA) strain and the Ab4p strain were conducted. These investigations revealed that (i) the ICP0 open reading frame (ORF) of the KyA virus strain is 1,257 bp in size and would encode a protein of 419 amino acids, and in comparison to the ICP0 gene (ORF63) of the Ab4p strain of 1,596 bp (E. A. Telford, M. S. Watson, K. McBride, and A. J. Davison, Virology 189:304-316, 1992), it has an internal in-frame deletion of 339 bp; (ii) one early transcript of 1.4 kb predicted to encode the EICP0 protein and a late transcript of 1.8 kb are detected in Northern blot analyses using probes containing the EICP0 ORF; (iii) the KyA EICP0 protein (50 kDa) and the Ab4p EICP0 protein (80 kDa) are expressed as several species of early proteins that are first detected at 3 to 4 h postinfection by Western blot analyses of infected-cell polypeptides, using an antiserum generated to a TrpE fusion protein that harbors amino acids 46 to 153 of the EICP0 protein; and (iv) the EICP0 protein of both EHV-1 strains is a potent transactivator of EHV-1 genes. Transient expression assays using a simian virus 40 expression construct of the EICP0 protein of the KyA strain showed that the EICP0 protein independently transactivated chloramphenicol acetyltransferase reporter constructs under the control of the immediate-early promoter (3.9-fold), the early thymidine kinase promoter (95-fold), the late (gamma1) IR5 promoter (85-fold), and the late (gamma2) glycoprotein K promoter (21-fold). The finding that the EICP0 protein of the KyA virus can function as an activator of gene expression indicates that amino acids corresponding to residues 319 to 431 of the Ab4p EICP0 protein are not essential for EICP0 transactivation of EHV-1 promoters.

Expression of an equine herpesvirus 1 ICP22/ICP27 hybrid protein encoded by defective interfering particles associated with persistent infection

Defective interfering (DI) particles of equine herpesvirus type 1 (EHV-1) are capable of mediating persistent infection (S. A. Dauenhauer, R. A. Robinson, and D. J. O'Callaghan, J. Gen. Virol. 60:1-14, 1982; R. A. Robinson, R. B. Vance, and D. J. O'Callaghan, J. Virol. 36:204-219, 1980). Sequence analysis of cloned DI particle DNA revealed that portions of two regulatory genes, ICP22 (IR4) and ICP27 (UL3), are linked in frame to form a unique hybrid open reading frame (ORF). This hybrid ORF, designated as the IR4/UL3 gene, encodes the amino-terminal 196 amino acids of the IR4 protein (ICP22 homolog) and the carboxy-terminal 68 amino acids of the UL3 protein (ICP27 homolog). Portions of DNA sequences encoding these two regulatory proteins, separated by more than 115 kbp in the standard virus genome, were linked presumably by a homologous recombination event between two identical 8-bp sequences. Reverse transcriptase-PCR and S1 nuclease analyses revealed that this unique ORF is transcribed by utilizing the transcription initiation site of ICP22 and the polyadenylation signal of ICP27 in DI particle-enriched infection. Immunoprecipitation and Western blot (immunoblot) analyses with antisera to the ICP22 and ICP27 proteins demonstrated that a 31-kDa hybrid protein was synthesized in the DI particle-enriched infection but not in standard virus infection. This 31-kDa hybrid protein was expressed at the same time as the ICP22 protein in DI particle-enriched infection and migrated at the same location on polyacrylamide gel electrophoresis as the protein expressed from a cloned IR4/UL3 expression vector. These observations suggested that the unique IR4/UL3 hybrid gene is expressed from the DI particle genome and may play a role in DI particle-mediated persistent infection.

Regulatory function of the equine herpesvirus 1 ICP27 gene product

The UL3 protein of equine herpesvirus 1 (EHV-1) KyA strain is a homolog of the ICP27 alpha regulatory protein of herpes simplex virus type 1 (HSV-1) and the ORF 4 protein of varicella-zoster virus. To characterize the regulatory function of the UL3 gene product, a UL3 gene expression vector (pSVUL3) and a vector expressing a truncated version of the UL3 gene (pSVUL3P) were generated. These effector plasmids, in combination with an EHV-1 immediate-early (IE) gene expression vector (pSVIE) and chimeric EHV-1 promoter-chloramphenicol acetyltransferase (CAT) reporter constructs, were used in transient transfection assays. These assays demonstrated that the EHV-1 UL3 gene product is a regulatory protein that can independently trans activate the EHV-1 IE promoter; however, this effect can be inhibited by the repressive function of the IE gene product on the IE promoter (R. H. Smith, G. B. Caughman, and D. J. O'Callaghan, J. Virol. 66:936-945, 1992). In the presence of the IE gene product, the UL3 gene product significantly augments gene expression directed by the promoters of three EHV-1 early genes (thymidine kinase; IR4, which is the homolog of HSV-1 ICP22; and UL3 [ICP27]) and the promoter of the EHV-1 late gene IR5, which is the homolog of HSV-1 US10. Sequences located at nucleotides -123 to +20 of the UL3 promoter harbor a TATA box, SP1 binding site, CAAT box, and octamer binding site and, when linked to the CAT reporter gene, are trans activated to maximal levels by the pSVIE construct in transient expression assays. Results from CAT assays also suggest that the first 11 amino acids of the UL3 protein are not essential for the regulatory function of the UL3 gene product.

Identification and characterization of the ICP22 protein of equine herpesvirus 1

The equine herpesvirus 1 (EHV-1) homolog of herpes simplex virus type 1 ICP22 is differently expressed from the fourth open reading frame of the inverted repeat (IR4) as a 1.4-kb early mRNA and a 1.7-kb late mRNA which are 3' coterminal (V. R. Holden, R. R. Yalamanchili, R. N. Harty, and D. J. O'Callaghan, J. Virol. 66:664-673, 1992). To extend the characterization of IR4 at the protein level, the synthesis and intracellular localization of the IR4 protein were investigated. Antiserum raised against either a synthetic peptide corresponding to amino acids 270 to 286 or against a TrpE-IR4 fusion protein (IR4 residues 13 to 150) was used to identify the IR4 protein. Western immunoblot analysis revealed that IR4 is expressed abundantly from an open reading frame composed of 293 codons as a family of proteins that migrate between 42 to 47 kDa. The intracellular localization of IR4 was examined by cell fractionation, indirect immunofluorescence, and laser-scanning confocal microscopy. These studies revealed that IR4 is localized predominantly in the nucleus and is dispersed uniformly throughout the nucleus. Interestingly, when IR4 is expressed transiently in COS-1 or LTK- cells, a punctate staining pattern within the nucleus is observed by indirect immunofluorescence. Cells transfected with an IR4 mutant construct that encodes a C-terminal truncated (19 amino acids) IR4 protein exhibited greatly reduced intranuclear accumulation of the IR4 protein, indicating that this domain possesses an important intranuclear localization signal. Western blot analysis of EHV-1 virion proteins revealed that IR4 proteins are structural components of the virions. Surprisingly, the 42-kDa species, which is the least abundant and the least modified form of the IR4 protein family in infected cell extracts, was the most abundant IR4 protein present in purified virions.

Characterization of the myristylated polypeptide encoded by the UL1 gene that is conserved in the genome of defective interfering particles of equine herpesvirus 1

Equine herpesvirus 1 (EHV-1, Kentucky A strain) preparations enriched for defective interfering particles (DIPs) can readily establish persistent infection. The UL1 gene, which is conserved in the genome of DIPs that mediate persistent infection, maps between nucleotides 1418 and 2192 (258 amino acids) from the L (long) terminus. UL1 has no homology with any known gene encoded by herpes simplex virus type 1 but has limited homology to open reading frame 2 of varicella-zoster virus and the "circ" gene of bovine herpesvirus type 1. Previous work showed that the EHV-1 UL1 gene belongs to the early kinetic class and is transcribed as a 1.2-kb polyadenylated mRNA (R. N. Harty, R. R. Yalamanchili, and D. J. O'Callaghan, Virology 183:830-833, 1991). In this report, the UL1 protein was identified and characterized as a 33-kDa polypeptide in EHV-1-infected cells by using rabbit polyclonal antiserum raised against a TrpE-UL1 fusion protein (amino acids 7 to 258 of UL1) synthesized in Escherichia coli. Results from Western blot (immunoblot), immunoprecipitation, indirect immunofluorescence, and biochemical analyses indicated that the UL1 polypeptide (i) is more abundant in cells infected with DIP-enriched virus than in cells infected with standard EHV-1, (ii) is synthesized as early as 3 h postinfection (p.i.) in infection with standard virus or in infection with DIP-enriched virus preparations and increases in abundance up to 12 h p.i., (iii) appears to be associated with the rough endoplasmic reticulum-Golgi apparatus early in infection (3 to 4 h p.i.), while a diffuse cytoplasmic pattern of fluorescence is observed late in infection (7 to 8 h p.i.), (iv) is modified by myristic acid as it contains a consensus N-terminal myristylation site and is readily labeled with [3H]myristic acid, and (v) is associated with mature EHV-1 virions.