Biomedical subjects

R L Witter

Publications and source records attributed to R L Witter.

149 records · Page 9Linked to original sources

PubMedDate not supplied

Influence of B-haplotype on the relative efficacy of Marek's disease vaccines of different serotypes.

To determine if B-haplotype differentially influences vaccinal immunity to very virulent Marek's disease (MD) virus challenge, chickens of five 15.B-congenic lines were vaccinated with vaccines representing serotypes 1, 2, and 3. B-haplotype differentially influenced vaccinal immunity to very virulent MD virus challenge using vaccines of all three serotypes, and different MD vaccines were optimal for some B-haplotypes. Regarding specific haplotypes, the 15.B-congenic chickens with B2 and B13 developed less protection against MD than chickens with B15 following vaccination with all three serotypes of MD vaccine, whereas the chickens with B5 and B21 developed variable protection with different MD vaccines. Thus, for induction of maximum MD resistance, it may be necessary to select a vaccine appropriate for the predominant B-haplotypes of the chicken flock.

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PubMedDate not supplied

Protection and synergism by recombinant fowl pox vaccines expressing genes from Marek's disease virus.

Recombinant fowl poxviruses (rFPV) were constructed to express genes from serotype 1 Marek's disease virus (MDV) coding for glycoproteins B (gB1), C (gC), and D (gD) and tegument proteins UL47 and UL48, as well as genes from serotypes 2 and 3 MDV coding for glycoprotein B (gB2 and gB3). These rFPVs, alone and in various combinations, including combinations of fowl poxvirus (FPV)/gBs with turkey herpesvirus (HVT), were evaluated for ability to protect maternal antibody-positive (ab+) and -negative (ab-) chickens against challenge with highly virulent MDV isolates. The protective efficacy was also compared with that of prototype Marek's disease (MD) vaccines. No protection was induced in ab+ chickens by rFPV expressing gC, gD, UL47, or UL48. In contrast, the rFPV/gB1 construct protected about 23% of ab+ chickens against MDV challenge compared with 26% for cell-associated HVT. Levels of protection by rFPV/gBs of different MDV serotypes was highest for gB1, intermediate for gB2, and lowest for gB3. When rFPV/gB1 was combined with cell-associated HVT, protection was enhanced by an average of 138% compared with the best component monovalent vaccine, and the mean level of protection was 59% compared with 67% for the HVT+SB-1 bivalent vaccine. Relatively high protection (50%) and enhancement (200%) were also observed between rFPV/gB1 and cell-free HVT. These results suggest a specific synergistic interaction between rFPV/gB1 and HVT, possibly analogous to that previously described between serotypes 2 and 3 viruses. Levels of protection by rFPV/ gB1 alone or by bivalent rFPV/gB1+cell-associated HVT were similar to those of conventional cell-associated MD vaccines. However, the bivalent rFPV/gB1+cell-free HVT vaccine was clearly more protective than cell-free HVT alone and, thus, may be the most protective, entirely cell-free MD vaccine thus far described.

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PubMedDate not supplied

Increased virulence of Marek's disease virus field isolates.

The continuation of an apparent evolutionary trend of Marek's disease virus (MDV) towards greater virulence may explain recent increased losses from Marek's disease (MD) in vaccinated flocks. To address this question, the virulence of 31 isolates of serotype 1 MDV obtained from layer or broiler flocks between 1987 and 1995 were characterized. Each isolate was cultured in duck embryo fibroblasts for four to six passages, and ascertained to be free from contamination with avian retroviruses, chicken anemia virus, and MDVs of other serotypes. The viruses, along with prototype viruses JM/102W and Md5, were tested for virulence by inoculation at 6 days of age into laboratory strain 15I5 x 7(1) chickens of three types: nonvaccinated, vaccinated with turkey herpesvirus (HVT) and bivalent (HVT + SB-1)-vaccinated. The results showed that three isolates did not differ from JM/102W and were classified in the virulent (vMDV) pathotype. Twenty-one isolates produced significantly higher levels of MD in HVT-vaccinated chickens than did the JM/102W control and were classified in the very virulent (vvMDV) pathotype. Seven isolates, five of which were isolated in 1994 or 1995, produced significantly higher levels of MD in bivalent-vaccinated chickens than did the Md5 (vvMDV) control. These isolates, provisionally designated as the vv+MDV pathotype, appeared to be at the high end of a virulence continuum. Several MD response parameters, including lymphoma mortality, early mortality with bursal/thymic atrophy, and frequency of visceral lymphomas or ocular lesions in nonvaccinated chickens were positively correlated with virulence. These findings support the continued evolution of MDV towards greater virulence.

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PubMedDate not supplied

Retroviral insertional mutagenesis of a herpesvirus: a Marek's disease virus mutant attenuated for oncogenicity but not for immunosuppression or in vivo replication.

Our earlier studies have shown that retrovirus insertion into herpesvirus is an efficient process that engenders recombinant herpesviruses with altered biological properties. The RM1 clone is derived from the JM strain of Marek's disease virus (MDV) through retrovirus insertional mutagenesis and contains sequences of reticuloendotheliosis virus inserted at the junction of the internal repeat and unique short regions of the genome. In previous studies, the RM1 clone appeared attenuated for oncogenicity but caused marked atrophy of the thymic lobes. The present studies represent a detailed analysis of the biological characteristics of the RM1 clone in order to better understand mechanisms of oncogenicity and gene function of MDV. RM1 was almost fully attenuated for oncogenicity but retained other in vivo properties of virulent viruses such as thymic and bursal atrophy, early immunosuppression, early cytolytic infection followed by efficient replication, and contact spread--all normally absent in attenuated strains. This suggests that, for serotype 1 MDV, oncogenicity is not tightly linked with immunodepression or viral replication and that these properties may be controlled by different genes or mechanisms. The mutation was stable through serial passage of the virus in chickens as determined by molecular analysis. None of the mutant viruses demonstrated expansion of the 132-bp repeat region of the genome, indicating that such expansion is not required for attenuation. Chickens vaccinated with RM1 clones were protected against challenge with virulent MDV, and levels of protection exceeded those of other attenuated serotype 1 vaccine viruses. Thus, attenuation by selective mutation may be an advantageous strategy for development of serotype 1 Marek's disease vaccines.

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PubMedDate not supplied

Comparative evaluation of in vitro and in vivo assays for the detection of reticuloendotheliosis virus as a contaminant in a live virus vaccine of poultry.

Indirect immunofluorescence (IFA), polymerase chain reaction (PCR), and enzyme-linked immunosorbent assay (ELISA) were used for detection of reticuloendotheliosis virus (REV) as a contaminant in a live virus fowl pox (FP) vaccine of poultry. A FP vaccine known to be contaminated with REV was tested by in vitro and in vivo assays in chicken embryo fibroblasts (CEFs) and day-old specific-pathogen-free (SPF) chicks, respectively. Using in vitro assays, IFA and PCR were more sensitive than ELISA in detection of REV in CEFs inoculated with REV-contaminated FP vaccine. However, when the vaccine was tested by in vivo assays using SPF chickens, the sensitivity of ELISA was comparable with that of IFA and PCR. Antibody to REV was not detected in SPF chickens within 4 wk postinoculation with REV-contaminated FP vaccine at hatch. Filtration of vaccine to eliminate vaccine virus from the inoculum before testing in CEFs resulted in a significant reduction in the frequency of REV detection by PCR or IFA. The data suggest that the sensitivity of IFA, PCR, and ELISA depends on the concentration of REV in the vaccine and that in vivo assays of vaccines for contamination with REV should include a test for virus because a negative antibody test may be misleading.

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