Characterization of the assembly and processing of infectious laryngotracheitis virus glycoprotein B

Isolation, identification, molecular and pathogenicity characteristics of an infectious laryngotracheitis virus from Hubei province, China

Multiple outbreaks of avian infectious laryngotracheitis (ILT) in chickens, both domestically and internationally, have been directly correlate to widespread vaccine use in affected countries and regions. Phylogenetic and recombination event analyses have demonstrated that avian infectious laryngotracheitis virus (ILTV) field strains are progressively evolving toward the chicken embryo-origin (CEO) vaccine strain. Even with standardized biosecurity measures and effective prevention and control strategies implemented on large-scale farms, continuous ILT outbreaks result in significant economic losses to the poultry industry worldwide. These outbreaks undoubtedly hinder efforts to control and eradicate ILTV in the future. In this study, an ILTV isolate was successfully obtained by laboratory PCR detection and virus isolation from chickens that exhibited dyspnea and depression on a broiler farm in Hubei Province, China. The isolated strain exhibited robust propagation on chorioallantoic membranes of embryonated eggs, but failed to establish effective infection in chicken hepatocellular carcinoma (LMH) cells. Phylogenetic analysis revealed a unique T441P point mutation in the gJ protein of the isolate. Animal experiments confirmed the virulence of this strain, as it induced mortality in 6-wk-old chickens. This study expands current understanding of the epidemiology, genetic variations, and pathogenicity of ILTV isolates circulating domestically, contributing to the elucidate of ILTV molecular basis of pathogenicity and development of vaccine.

Induction of Immune Responses by Recombinant PH-1 Domain of Infectious Laryngotracheitis Virus Glycoprotein B in Chickens

Infectious laryngotracheitis virus (ILTV) is a cause of main respiratory disease of chickens controlled through live attenuated vaccines. To reduce the risk of adverse effects associated with live vaccines, a recombinant vaccine expressing PH-1 domain of viral glycoprotein B was constructed using the pET expression system under isopropylthiogalactoside (IPTG) induction. The potential immunogenicity of recombinant PH-1 (rPH-1) was evaluated in chickens. Eight-week-old specific-pathogen-free chickens were intramuscularly administered two doses of rPH-1, 25 and 50 μg, alone or with a combination of ISA70 adjuvant. The humoral immune responses were determined up to 3 months postvaccination at 2 weeks apart. The T cell proliferation response was determined on day 28 after primary immunization. The vaccinated birds with rPH-1/ISA70 developed higher and constant-specific anti-ILTV enzyme-linked immunosorbent assay (ELISA) antibodies than in those vaccinated with rPH-1 alone. Coinjection of rPH-1 and adjuvant significantly (p < 0.01) increased the T cell proliferation responses. There were no significant differences in eliciting the immune responses in chickens immunized with the higher dose of the antigen than that with the lower dose. The data indicate the immunogenic efficiency of rPH-1 against ILTV. Vaccination with recombinant proteins offers a preventing option to control the ILTV infection and could be a candidate to replace current live vaccines.

A recombinant Newcastle disease virus (NDV) expressing infectious laryngotracheitis virus (ILTV) surface glycoprotein D protects against highly virulent ILTV and NDV challenges in chickens

Infectious laryngotracheitis (ILT) is a highly contagious acute respiratory disease of chickens caused by infectious laryngotracheitis virus (ILTV). Currently, modified live ILTV vaccines are used to control ILT infections. However, the live ILTV vaccines can revert to virulence after bird-to-bird passage and are capable of establishing latent infections, suggesting the need to develop safer vaccines against ILT. We have evaluated the role of three major ILTV surface glycoproteins, namely, gB, gC, and gD in protection and immunity against ILTV infection in chickens. Using reverse genetics approach, three recombinant Newcastle disease viruses (rNDVs) designated rNDV gB, rNDV gC, and rNDV gD were generated, each expressing gB, gC, and gD, respectively, of ILTV. Chickens received two immunizations with rNDVs alone (gB, gC, and gD) or in combination (gB+gC, gB+gD, gC+gD, and gB+gC+gD). Immunization with rNDV gD induced detectable levels of neutralizing antibodies with the magnitude of response greater than the rest of the experimental groups including those vaccinated with commercially available vaccines. The birds immunized with rNDV gD showed complete protection against virulent ILTV challenge. The birds immunized with rNDV gC alone or multivalent vaccines consisting of combination of rNDVs displayed partial protection with minimal disease and reduced replication of challenge virus in trachea. Immunization with rNDV gB neither reduced the severity of the disease nor the replication of challenge virus in trachea. The superior protective efficacy of rNDV gD vaccine compared to rNDV gB or rNDV gC vaccine was attributed to the higher levels of envelope incorporation and infected cell surface expression of gD than gB or gC. Our results suggest that rNDV expressing gD is a safe and effective bivalent vaccine against NDV and ILTV.

Safety and efficacy of a turkey herpesvirus vector laryngotracheitis vaccine for chickens

Turkey herpesvirus vector laryngotracheitis vaccine (HVT/LT) expressing the glycoprotein B gene of laryngotracheitis virus (LTV) has been developed. In vitro growth kinetics of HVT/LT were similar to those of parental turkey herpesvirus (HVT), FC-126 strain. Genetic and phenotypic stabilities of HVT/LT after in vitro (in cell culture) or in vivo (in chickens) passage were confirmed by various assays, including Southern blot analysis, western blot analysis, and an indirect immunofluorescence assay. Safety of HVT/LT was assessed by an overdose study as well as by a backpassage study in specific-pathogen-free (SPF) chickens. The overdose study indicated that HVT/LT did not cause any adverse effects in chickens. The backpassage study confirmed that HVT/LT does not revert to virulence after five passages in chickens. The vaccine did not transmit laterally from vaccinated chickens to commingled nonvaccinated chickens. Efficacy of HVT/LT was evaluated in SPF layer chickens after vaccination by the subcutaneous route at 1 day of age. The majority of the vaccinated chickens (92%-100%) were protected against challenge with virulent LTV at 7 wk of age. Efficacy of HVT/LT was further evaluated in broiler chickens from a commercial source after in ovo vaccination to embryos at 18 days of incubation. After challenge with virulent LTV at 21 and 35 days of age, 67% and 87% of HVT/LT-vaccinated chickens did not develop LT clinical signs, respectively, while 100% (21 days of age) and 73% (35 days of age) of the challenge control chickens showed clinical signs of LT. These results suggest that HVT/LT is a safe and efficacious vaccine for control of laryngotracheitis (LT).

Identification of transcripts and protein products of the UL31, UL37, UL46, UL47, UL48, UL49 and US4 gene homologues of avian infectious laryngotracheitis virus

In the present study, the transcription and protein expression of seven genes of infectious laryngotracheitis virus (ILTV) were investigated: UL31 and UL37 possess homologues in all known avian and mammalian herpesviruses, whereas UL46–UL49 and US4 are only conserved in most alphaherpesviruses. A peculiarity of the ILTV genome is the translocation of UL47 from the unique long region to a position upstream of US4 within the unique short region. Northern blot analyses revealed that all of the analysed genes were transcribed most abundantly during the late (γ) phase of replication, but the only true late (γ2) gene was UL47. Using monospecific rabbit antisera, the protein products of all of the genes could be detected and localized in ILTV-infected cells. Considerable amounts of the UL31, UL47 and UL48 gene products were found in the cell nuclei, whereas the other proteins were restricted largely to the cytoplasm. Like the respective tegument proteins of other herpesviruses, the UL37 and UL46–UL49 gene products of ILTV were incorporated into virus particles, whereas the UL31 protein and the glycoprotein encoded by US4 (gG) were not detectable in purified virions. It was also demonstrated that the UL48 protein of ILTV is able to activate an alphaherpesvirus immediate-early gene promoter, which is also a typical feature of other UL48 homologues. Taken together, these results indicate that the functions of all of the investigated ILTV proteins are related to those of their homologues in other alphaherpesviruses.

Molecular biology of avian infectious laryngotracheitis virus

Infectious laryngotracheitis virus (ILTV) is an alphaherpesvirus that causes an economically important chicken disease, which results in delayed growth, reduced egg production, and also frequently in death of the animals. After acute infection of the upper respiratory tract, the virus can establish latency in the central nervous system, and subsequent reactivations can lead to infection of naive chickens. For prevention of ILT, conventionally attenuated live vaccines are available. However, these vaccine strains are genetically not characterized, and reversions to a virulent phenotype occur. Although molecular analyses of ILTV are hampered by the lack of an optimal cell culture system, the complete nucleotide sequence of the ILTV genome has recently been elucidated, and several ILTV recombinants lacking nonessential, but virulence determining genes have been constructed. Animal trials indicated that genetically engineered stable gene deletion mutants are safe alternatives to the current vaccine strains. Furthermore, since live ILTV vaccines are suitable for fast and inexpensive mass administration, they are promising as vectors for immunogenic proteins of other chicken pathogens. Thus, immunization with ILTV recombinants expressing avian influenza virus hemagglutinin was shown to protect chickens against ILT and fowl plague. Using monospecific antisera and monoclonal antibodies several virion proteins of ILTV have been identified and characterized. Since they include immunogenic envelope glycoproteins, these results can contribute to the improvement of virus diagnostics, and to the development of marker vaccines.

Evaluation of the efficacy of a live fowlpox-vectored infectious laryngotracheitis/avian encephalomyelitis vaccine against ILT viral challenge

Infectious laryngotracheitis (ILT) is caused by an alphaherpesvirus, and latency can be produced by previous exposure to vaccine virus. The main sites of latency for the ILT virus have been shown to be the trigeminal ganglion and the trachea. Reactivation of latent virus is one factor related to the production of clinical signs. The development of a genetically engineered ILT vaccine has been suggested for many years as a tool to eliminate viral latency. Several approaches have been suggested. Included among them is the development of a thymidine kinase-deficient mutant or the insertion of ILT viral glycoproteins into a viral vector such as a poxvirus. A commercially available, live, fowlpox-vectored infectious laryngotracheitis + avian encephalomyelitis (FP-LT+AE) vaccine was used in field trials in leghorn pullet flocks and evaluated by tracheal challenge in a laboratory setting with the use of the National Veterinary Services Laboratory (Ames, IA) ILT challenge virus. Interference of the pigeon pox vaccine, which is often administered concurrently with fowlpox vaccine, was also evaluated when given in conjunction with the FP-LT+AE vaccine. Overall, the results indicate that the FP-LT+AE vaccine provides adequate protection against ILT viral challenge. Proper administration is essential. In one flock, inadequate protection was most likely a result of either poor vaccine administration or previous exposure to pox virus. In addition, the simultaneous administration of pigeon pox vaccine did not appear to interfere with protection against ILT viral challenge.

Isolation and characterization of monoclonal antibodies against structural proteins of infectious laryngotracheitis virus

Thirteen infectious laryngotracheitis virus (ILTV)-specific monoclonal antibodies (MAbs) were isolated after immunization of mice with purified infectious laryngotracheitis virions. On the basis of their reactions in western blot analyses of ILTV-infected cells, the MAbs were assigned to five different virus proteins or protein groups. Two of the viral target proteins could be identified after transient expression of cloned ILTV genes in eucaryotic cells. The MAbs of group II detected a 60-kD protein that was shown to be the ILTV homologue of herpes simplex virus type 1 (HSV-1) glycoprotein (g)C. The MAbs of group I reacted with the positional homologue of HSV-1 gJ, which is encoded by the open reading frame (ORF) 5 gene within the unique short genome region of ILTV. The ORF 5 gene product of ILTV was previously described as a 60-kD glycoprotein (gp60), whereas multiple protein bands with apparent molecular masses of 85, 115, 160, and 200 kD were identified in the present study. Immunoelectron microscopy revealed that both gC and gJ of ILTV are localized in the envelope of virus particles, whereas the 15-kD protein detected by the MAbs of group III presumably represents a tegument component. Immunofluorescence analyses of infected cells demonstrated that the epitopes of the gC- and gJ-specific MAbs are conserved in all tested ILTV isolates originating from different parts of the world and that these MAbs are also suitable for in situ antigen detection in tissues of ILTV-infected chickens. The remaining ILTV-specific MAbs recognized viral proteins of 22 kD (group IV) and 38 kD (group V) that were not further characterized up to now.

DNA sequence of the UL6 to UL20 genes of infectious laryngotracheitis virus and characterization of the UL10 gene product as a nonglycosylated and nonessential virion protein

The 24 kbp KpnI restriction fragment A from the unique long genome region of infectious laryngotracheitis virus (ILTV, gallid herpesvirus-1) has been sequenced. The analysed region contains 14 open reading frames sharing homology with conserved alphaherpesvirus genes. Arrangement of the UL6 to UL20 homologues of ILTV is almost identical to that found in the herpes simplex virus type 1 genome. As in other herpesviruses the UL15 gene consists of two exons and is expressed from a spliced mRNA. However, the UL16 gene, which is usually localized within the intron sequence of UL15, is not conserved at this position of the ILTV genome. Another unique feature is the absence of any putative N-glycosylation motifs within the deduced ILTV UL10 gene product, which is the homologue of the conserved herpesvirus glycoprotein M. After preparation of a monospecific antiserum, two distinct UL10 proteins with apparent molecular masses of 36 and 31 kDa were identified in ILTV-infected cells as well as in purified virions. None of these UL10 gene products is modified by N- or O-linked glycosylation. Isolation of a green fluorescent protein-expressing UL10 deletion mutant of ILTV revealed that this gene is not required for virus replication in cell culture.

Infectious laryngotracheitis virus, an alpha herpesvirus that does not interact with cell surface heparan sulfate

Among the alpha herpesviruses studied to date, the initial stage of wild-type virus attachment involves an interaction between virally encoded structural envelope glycoproteins (predominantly glycoprotein C) and cell surface heparan sulfate proteoglycans. An analysis of the infectious laryngotracheitis virus (ILTV) glycoprotein C and glycoprotein B sequences suggested that these proteins lacked consensus heparin-binding domains. This indicated that ILTV might attach to its host cell in a heparan-independent manner, distinct from other alpha herpesviruses. The infectivity of two ILTV strains, a tissue-culture-adapted vaccine strain and a virulent field challenge strain, were found to be insensitive to the presence of exogenous heparin or chondroitin. Furthermore, infectivity was retained in chicken embryonic liver cells treated with heparinase. However, 4 degrees C attachment studies and penetration studies in the presence of citrate buffer clearly demonstrated that ILTV attaches stably to and effectively penetrates chicken embryonic liver cells. Consequently, ILTV represents an alpha herpesvirus whose initial attachment step does not involve interactions with heparan or chondroitin sulfate containing proteoglycans.

Infectious laryngotracheitis herpesvirus expresses a related pair of unique nuclear proteins which are encoded by split genes located at the right end of the UL genome region

Avian infectious laryngotracheitis virus (ILTV) possesses an alphaherpesvirus type D DNA genome of ca. 155 kbp. Completion of our previous sequence analyses (W. Fuchs and T. C. Mettenleiter, J. Gen. Virol. 77:2221-2229, 1996) of the right end of the unique long (UL) genome region revealed the presence of two adjacent, presumably ILTV-specific genes, which were named UL0 and UL[-1] because of their location upstream of the conserved UL1 (glycoprotein L) gene. Transcriptional analyses showed that both genes are abundantly expressed during the late phase of the viral replication cycle and that both mRNAs are spliced by the removal of short introns close to their 5' ends. Furthermore, the deduced gene products exhibit a moderate but significant homology of 28% to each other. The newly identified ILTV genes encode proteins of 63 kDa (UL0) and 73 kDa (UL[-1]), which both are predominantly localized in the nuclei of virus infected chicken cells. In summary, our results indicate that duplication of a spliced ILTV-specific gene encoding a nuclear protein has occurred during evolution of ILTV.