Recombinant Semliki Forest virus vector exhibits potential for avian virus vaccine development

The Semliki Forest virus (SFV) expression system was evaluated as a basis for avian vaccine development. Initial studies indicated that 1-day-old specific pathogen-free (SPF) chicks were susceptible to infection with an infectious strain of SFV, producing SFV-specific antibodies but no clinical disease. One-day-old SPF chicks immunised intramuscularly with recombinant replication-defective SFV (rSFV) particles expressing the Escherichia coli (E. coli) lacZ reporter gene developed high titres of beta-gal- specific antibodies at 4 weeks p.i. after two inoculations. In contrast, significantly lower antibody levels were elicited in chicks immunised with a recombinant SFV-based DNA construct or a conventional CMV promoter-based DNA plasmid. rSFV particles encoding the protective VP2 protein or the VP2/VP4/VP3 polyprotein of infectious bursal disease virus (IBDV) were produced and the expressed antigens were characterised in cell culture. Proteins of the correct size were generated and found to react against a range of IBDV-specific monoclonal antibodies. Immunisation of 1-day-old SPF chicks with rSFV particles encoding the IBDV proteins resulted in specific antibodies being elicited in all birds, neutralising antibodies being induced in some but not all birds.

Detection of IgM responses to bovine respiratory syncytial virus by indirect ELISA following experimental infection and reinfection of calves: abolition of false positive and false negative results by pre-treatment of sera with protein-G agarose

The IgM responses in three panels of sera generated by infection and reinfection of calves with bovine respiratory syncytial virus (BRSV) were measured by indirect ELISA (I-ELISA). The effect of depleting serum IgG by pre-treatment with protein G agarose (PGA) was evaluated. Following primary infection a weak IgM response was detected in the untreated sera of 3 out of 4 calves with maternally derived antibody (MDA). Both the magnitude and duration of the specific IgM responses in these calves were increased by pre-treatment with PGA. In addition, the fourth infected calf tested gave a single positive IgM result following PGA treatment. Transient or persistent IgM responses which were abolished by pre-treatment of sera with PGA were detected in 4/8 calves following reinfection. These were considered to be false positive results, consistent with the influence of IgM rheumatoid factor (IgM-RF). One of these calves and two additional calves showed transient increases in IgM which were resistant to PGA treatment. These were considered to represent specific IgM responses to reinfection. The results indicate the ability of PGA treatment to eliminate both false positive and false negative results and emphasise the necessity for controlling the influence of IgM-RF in IgM-specific indirect ELISAs.

Development of a blocking enzyme-linked immunosorbent assay for the serological diagnosis of chicken anaemia virus

The development and evaluation of an enzyme-linked immunosorbent assay (ELISA) for the detection of serum antibody to chicken anaemia virus (CAV) are described. This test depends on the abilities of CAV-specific antibodies present in convalescent chicken serum to block the reaction between virus antigen, adsorbed to the ELISA plate. and a CAV-specific mouse monoclonal antibody (MAb), 2A9, that has been conjugated to horseradish peroxidase. The 2A9 MAb has been shown to react with 10 geographically different field isolates of CAV, a finding which indicates that the test will find worldwide application. In comparative experiments involving 525 serum samples from specific pathogen free and commercial breeder flocks, there was 98.5% agreement between the results obtained with the blocking ELISA and those obtained with an indirect ELISA developed previously in this laboratory. The blocking ELISA was found to have advantages in terms of speed and cost compared with the indirect ELISA format.

Characterization of the genome of avian encephalomyelitis virus with cloned cDNA fragments

cDNA fragments were generated from RNA extracted from preparations of avian encephalomyelitis virus (AEV) by a reverse transcription-polymerase chain reaction (RT-PCR) strategy, which exploited the probability that AEV is a picornavirus. Rapid amplification of the 3' cDNA ends, which utilized an oligo d(T)-based primer that hybrizes to the putative Poly (A) tract at the 3' terminus of picornavirus RNA, produced a 3.8-kbp fragment (3.8-kbp 3' RACE fragment), from which a 2.5-kbp cDNA fragment specific to the extreme 3' terminal region of the AEV genome was cloned. Positive hybridization reactions between RNA from gradient-purified virus and radiolabeled probes confirmed that the cloned 2.5-kbp fragment was AEV specific. The success of the RT-PCR amplification strategy adopted and the results of northern blotting hybridization experiments indicated that the AEV genome is a polyadenylated, single-stranded RNA, approximately 7.5 kb in size. Sequence analysis of a 869-base region at the 3' terminal of the genome indicated that this region encoded a protein with close homologies to picornaviral RNA polymerase proteins. On the basis that the highest levels of protein homologies were observed with hepatitis A virus, it is likely that AEV will be reassigned to a genus other than the enterovirus genus within the virus family Picornaviridae. The AEV-specific cloned DNA fragments and nucleotide sequence information resulting from this investigation may facilitate the development of in situ hybridization and RT-PCR methods that will be useful in AEV diagnosis.

Isotype- and subclass-specific responses to infection and reinfection with parainfluenza-3 virus: comparison of the diagnostic potential of ELISAs detecting seroconversion and specific IgM and IgA

Isotype- and subclass-specific indirect enzyme-linked immunosorbent assays were developed to detect parainfluenza-3 virus-specific IgG1, IgG2, IgM, and IgA responses. Sera were treated with protein G-agarose prior to testing for specific IgM and IgA to eliminate the possibility of false-positive results due to IgM-rheumatoid factor and to remove interisotypic competition due to specific IgG. IgM and IgA absorbance values were expressed as a percentage of the absorbance values of positive reference sera included on each plate (S/P%), and respective positive/negative threshold values of 15.0% and 28.0% were determined. The mean interval between experimental infection of 3 calves and initial detection of specific IgG1 and IgG2 responses was 8.0 and 9.3 days respectively, rising rapidly to an initial plateau 13.7 and 11.0 days postinfection (dpi). Reinfection of these calves at 30 dpi resulted in further rapid increases, with higher plateau values reached 13.0 (IgG1) and 13.7 (IgG2) days later. The mean interval between infection and the first positive IgM and IgA responses was 6.7 and 12.3 days, respectively. IgM S/P% values peaked at 13.0 dpi, with all 3 calves showing a secondary anamnestic response to reinfection, peaking 4.7 days later. The IgA response to initial infection was weak, with only 2 calves showing an obvious peak response at 15.0 dpi. A strong anamnestic IgA response to reinfection occurred in 2 calves, with a peak response 9.5 days later. Apparent biphasic and triphasic IgM and IgA responses were evident in some calves. Acute and convalescent serum samples from 80 calves involved in 17 outbreaks of respiratory disease were tested for specific IgM and IgA. Positive IgM results were detected in 15 outbreaks, with 71 sera from 44 calves testing positive. Although IgA-positive results were detected in the same 15 outbreaks, only 42 sera from 31 calves were positive. In a previous study, seroconversion was detected in 21 of these calves from 10 outbreaks. Thus the diagnostic potential of the assays was in the order IgM > IgA > seroconversion. The correlations between IgM and IgA, IgM and seroconversion, and IgA and seroconversion results for each calf were 73.8%, 58.8% and 62.5%, respectively.

Evaluation of an IgM-specific indirect enzyme-linked immunosorbent assay for serodiagnosis of bovine respiratory syncytial virus infection: influence of IgM rheumatoid factor on test results with field sera

A commercially available indirect enzyme-linked immunosorbent assay for measuring bovine respiratory syncytial virus (BRSV)-specific IgG was adapted to measure virus-specific IgM. Using this assay, the development of rapid IgM responses in experimentally infected calves was observed 7-9 days postinfection, with peak absorbance values ranging from 1.698 to 2.873. When absorbance values were expressed as a percentage of a positive reference serum, a positive/negative threshold of 22% was determined by testing serum samples from 59 healthy 3-5-month-old calves. Acute and convalescent serum samples collected from 151 calves during 38 outbreaks of respiratory disease were tested, and 130 sera were positive. To determine the number of false-positive results due to the presence of IgM rheumatoid factor, a method for depleting serum IgG by pretreatment of sera with a suspension of protein-G-agarose was developed. All sera that initially tested IgM positive were retested following depletion of serum IgG. False-positive IgM reactions were detected in 23 sera (17.7%). Specific IgM responses were confirmed in 107 sera from 84 calves. Evidence of BRSV infection was detected in 34 of 38 outbreaks. In contrast, seroconversion was detected in 69 calves from 24 outbreaks, confirming the diagnostic potential of the IgM assay. Overall correlation between IgM and seroconversion results was 74.2%. Intra- and interassay reproducibility were 12.50% and 17.48%, respectively (mean coefficients of variation).

Testing of bovine sera by ELISA for IgG, IgM and IgA rheumatoid factors

Sera from 19 colostrum-deprived calves less than 1 week old, 24 colostrum-supplemented calves less than 1 week old, 36 3-5-month-old calves and 200 females greater than 9 months of age were tested by ELISA for the presence of IgM, IgG and IgA rheumatoid factors (RF). An increasing level of IgM- and IgG-RF with age was found. IgG-RF levels in the colostrum-supplemented calves were significantly higher than in the non-supplemented calves (p < 0.001). Individual IgG-RF values correlated with serum IgG levels, as determined by zinc sulphate turbidity testing (r=0.59, p < 0.01). No IgA-RF was detected. The cross-reactivity of IgM-RF with heterologous IgG was found to be greatest with rabbit IgG, followed by mouse and chicken IgG. The significance of rheumatoid factors in relation to diagnostic testing is discussed.

Evaluation of a single dilution ELISA system for detection of seroconversion to bovine viral diarrhea virus, bovine respiratory syncytial virus, parainfluenza-3 virus, and infectious bovine rhinotracheitis virus: comparison with testing by virus neutralization and hemagglutination inhibition

A single-dilution quantitative enzyme-linked immunosorbent assay (ELISA) system, based on commercial ELISA kits, for the simultaneous detection of seroconversion to bovine viral diarrhea virus (BVDV), bovine respiratory syncytial virus (BRSV), parainfluenza-3 virus (PI3V), and infectious bovine rhinotracheitis virus (IBRV) was evaluated by testing acute and convalescent serum pairs from 564 cattle in 145 outbreaks of respiratory disease. Seroconversion to BVDV, BRSV, PI3V and IBRV was detected in 8.0%, 19.0%, 13.7%, and 7.4%, respectively, of serum pairs tested. Seroconversion was detected in 60.7% of herds and 34.6% of animals tested. Infection with 2 or more viruses was found in 46.6% of these herds and in 27.2% of these animals. The majority of BVDV infections (62%) were associated with other virus infections, suggesting that BVDV may potentiate infection with other agents rather than being a primary pathogen of the respiratory tract. The results were compared with those obtained by virus neutralization and hemagglutination inhibition testing, and the sensitivity, specificity, and overall correlation were calculated. Sensitivities of 92%, 95%, 100%, and 100% were obtained for BVDV, BRSV, PI3V, and IBRV, respectively. The corresponding specificity values were 89%, 92%, 86%, and 91%. The overall correlation for each virus was 90%, 93%, 90%, and 93%, respectively. These results demonstrate that this ELISA system may be used successfully to detect seroconversion in serum pairs, highlight the frequency of multiple viral infections in outbreaks of respiratory disease, and provide further evidence of an immunosuppressive role for BVDV infections.

Standardization of enzyme-linked immunosorbent assays (ELISAs) for quantitative estimation of antibodies specific for infectious bovine rhinotracheitis virus, respiratory syncytial virus, parainfluenza-3 virus, and bovine viral diarrhea virus

Commercial enzyme-linked immunosorbent assays (ELISAs) for detection of serum antibodies to bovine viral diarrhea virus (BVDV), parainfluenza-3 virus (PI3V), respiratory syncytial virus (RSV), and infectious bovine rhinotracheitis virus (IBRV) were standardized to give a quantitative result when testing was performed at a single optimum dilution. For each test, serum samples were titrated and their end point titers calculated by an algebraic method directly from a plot of each titration series and also from a regression line fitted to this plot. The corrected optical density (COD) of each sample when tested at dilutions of 1/25, 1/50, and 1/100 was expressed as a percentage of the COD of a positive reference serum included on each plate, this value was the sample/positive (S/P) ratio. For each test, the linear relationship between the S/P ratio obtained at a dilution of 1/25, 1/50, and 1/100 and the end point titer calculated by each method was determined. In each case, the best linear relationship existed when samples were tested at a dilution of 1/100 (r = 0.973 for BVDV, 0.962 for PI3V, 0.961 for RSV, 0.947 for IBRV). From the equation of these lines, an increase in the S/P ratio between acute and convalescent serum samples of 31%, 23%, 21%, and 35% would correspond to a 4-fold rise in ELISA titer to BVDV, PI3V, RSV, and IBRV, respectively. ELISA titers calculated from S/P ratios at 1/100 were significantly related to virus neutralization titers to BVDV, RSV, and IBRV and to hemagglutination inhibition titers to PI3V (P < < 0.001 in all cases). Samples with low S/P ratios had the greatest intraassay and interassay variation. Intraassay reproducibility ranged from 3.5% to 22.3% (coefficient of variation), with a median value of 9.5%. Interassay reproducibility was lower, ranging from 6.0% to 50.6%, with a median of 17.4%.

Identification of a 24 kDa protein expressed by chicken anaemia virus

Antisera raised against oriented peptide conjugates were used to identify and partially characterize a 24 kDa protein product expressed by chicken anaemia virus (CAV). The peptides derived from the N and C termini of the protein were shown to react against the native protein, expressed within virus-infected cells, by immunofluorescence, immunoperoxidase and immunogold thin section electron microscopy techniques. The protein product was located by immunogold single labelling in intranuclear inclusions similar to those described previously for the 13 kDa CAV protein, which causes apoptosis. The 24 kDa protein was co-localized to the nuclear inclusions with the CAV 13 kDa protein by simultaneous dual labelling immunogold electron microscopy. Following isolation of the CAV proteins by nuclei isolation and SDS-PAGE, the antisera were used to probe for the protein by immunoblotting. The antisera recognized an expressed protein product of apparent molecular mass 30 kDa. An immunofluorescence time course study of CAV protein expression was carried out and the peptide antisera reacted against the protein at 12 h post-infection. Antisera against the 13 kDa protein reacted at similar times post-infection. This was in contrast to antisera raised against the 52 kDa capsid protein which is detectable by immunofluorescence only after 24 h. The 13 kDa and 24 kDa proteins thus appear to be early antigens produced by CAV during infection.

Development of an enzyme-linked immunosorbent assay for the serological diagnosis of big liver and spleen disease

An enzyme-linked immunosorbent assay (ELISA) was developed for the serological diagnosis of big liver and spleen (BLS) disease. The test utilizes a soluble, BLS-specific antigen that can be recovered from the livers of infected hens and that is known to react in the agar gel immunodiffusion (AGID) test. For use in ELISA, the BLS-specific antigen is fractionated by gel filtration chromatography and immobilized on microtiter plates using glutaraldehyde. The ELISA was evaluated using sera from infected and uninfected flocks originating in the United Kingdom and the United States. An ELISA format that incorporated control antigen recovered from the livers of uninfected birds for each serum tested was found to be more sensitive than the AGID test. A less sensitive but more cost-effective format that did not incorporate this control is considered suitable for large-scale flock screening programs.

Detection and differentiation of chicken anemia virus isolates by using the polymerase chain reaction

Complementary oligonucleotide primers which flank a 675-bp DNA fragment encompassing part of the putative gene for the capsid protein of chicken anemia virus (CAV) were used for the enzymatic amplification of CAV DNA by the polymerase chain reaction (PCR). Application of a dot blot hybridization assay by using a 32P-labeled cloned CAV DNA probe allowed PCR product amplified from as little as 0.1 fg of the target DNA sequence to be detected. When it was used for PCR amplification, DNA extracted from thymus tissue by a guanidine isothiocyanate-based method proved to be more efficient than that extracted by methods involving phenol or boiling. DNAs specified by 14 CAV isolates originating in the United Kingdom, Ireland, Germany, Sweden, the United States, Japan, and Australia were amplified. Restriction endonuclease analysis of the PCR-amplified DNAs with the enzymes HaeIII, HinfI, and HpaII indicated that the 14 CAV isolates can be assigned to seven groups, with isolates from different countries usually exhibiting the greatest number of restriction site differences.

Development of an enzyme-linked immunosorbent assay to detect serum antibody to chicken anemia agent

An enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies to chicken anemia agent (CAA) has been developed. This test utilizes a CAA-specific mouse monoclonal antibody to selectively capture virus antigen. Chicken antibodies to CAA bind to the captured antigen and are detected with horseradish peroxidase-labeled anti-chicken immunoglobulin using a conventional indirect ELISA protocol. When 388 chicken sera from specific-pathogen-free and commercial flocks from the United Kingdom, West Germany, the United States and Australia were examined, 98.5% agreement was obtained between the results of the ELISA and the indirect immunofluorescence assay. This ELISA should have worldwide application in testing SPF and commercial chicken flocks for CAA antibodies.

Production and preliminary characterization of monoclonal antibodies to chicken anemia agent

Mice were immunized with partially purified preparations of the Cux-1 isolate of chicken anemia agent (CAA), and their splenocytes were fused with NSO myeloma cells. Three patterns of staining of CAA-infected cells were recognized when the resulting hybridomas were screened by indirect immunofluorescence (IIF). Hybridomas representative of each staining pattern were cloned, and the monoclonal antibodies (MAbs) were characterized. Type 1 staining was indistinguishable from that produced by polyclonal chicken antisera to CAA. Type 2 staining was confined to large nuclear inclusions. Type 3 staining was predominantly nuclear and granular, and differed from type 1 in being more intense and occurring in a higher proportion of nuclei. Three MAbs producing type 1 staining were predominantly Cux-1-specific by IIF; they also reacted to lower titers with the Gifu-1 isolate but not at all with three other CAA isolates. These MAbs had very slight neutralizing activity against Cux-1. Another MAb giving type 1 staining reacted with all CAA isolates tested to high titers in IIF and neutralization tests. MAbs with type 2 and type 3 staining reacted by IIF with all CAA isolates tested but possessed no neutralizing activity. The availability of MABs to CAA should facilitate development of diagnostic tests for the virus.