Revaccination with Marek's disease vaccines induces productive infection and superior immunity.

The most common lymphoproliferative disease in chickens is Marek's disease (MD), which is caused by the oncogenic herpesvirus Marek's disease virus (MDV). The emergence of hypervirulent pathotypes of MDV has led to vaccine failures, which have become common and which have resulted in serious economic losses in some countries, and a revaccination strategy has been introduced in practice. The mechanism by which revaccination invokes superior immunity against MD is unknown. After field trials which showed that revaccination provided protection superior to that provided by a single vaccination were performed, experiments were conducted to explore the interaction between revaccinated chickens and MDV. The results showed that the chickens in the revaccination groups experienced two consecutive productive infections but that the chickens in the single-vaccination groups experienced one productive infection, demonstrating that revaccination of viruses caused the chickens to have productive and then latent infections. Revaccination of the virus induced in the chickens a higher and a longer temporary expansion of the CD8(+), CD4(+), and CD3(+) T-lymphocyte subpopulations, stronger peripheral blood lymphocyte proliferative activity; and higher levels of neutralizing antibody than single vaccination. These findings disagree with the postulate that MDV antigens persist, stimulate the immune system, and maintain a high level immunity after vaccination. The suppression of productive infection by maternal antibodies in chickens receiving the primary vaccination and a lower level of productive infection in the revaccination groups challenged with MDV were observed. The information obtained in this study suggests that the productive infection with revaccinated MDV in chickens plays a crucial role in the induction of superior immunity. This finding may be exploited for the development of a novel MD vaccine that results in the persistence of the antigen supply and that maintains a high level of immunity and may also have implications for other viral oncogenic diseases in humans and animals.

Infectious bursal disease virus: strains that differ in virulence differentially modulate the innate immune response to infection in the chicken bursa.

Little is understood about the immune responses involved in the pathogenesis of infectious bursal disease virus (IBDV). Strains of IBDV differ in their virulence: F52/70 is a classical virulent strain (vIBDV), whereas UK661 is a very virulent strain (vvIBDV) that causes greater pathology and earlier mortality. The exact causes of clinical disease and death are still unclear. Pro-inflammatory cytokines such as interleukin (IL)-1beta and IL-6, produced by activated macrophages, could play a role, as could cytokines produced by T and natural killer (NK) cells, such as interferon (IFN)-gamma, which stimulate macrophages. We quantified mRNA transcription in bursal tissue, by real-time quantitative reverse transcription- polymerase chain reaction (RT-PCR), for the type I IFN (IFN-alpha and IFN-beta), pro-inflammatory cytokines (IL-1beta, IL-6, and CXCLi2), the anti-inflammatory cytokine transforming growth factor (TGF)-beta4, and Th1 cytokines (IFN-gamma, IL-2 [and the closely related IL-15], IL-12, and IL-18) for the first 5 days after infection of 3-week-old chickens with F52/70 or UK661 and compared these with levels in bursal tissue from uninfected age-matched controls. Both strains induced a pro-inflammatory response, evidenced by increased mRNA transcription of IL-1beta, IL-6, and CXCLi2, and down-regulation of TGF-beta4, of similar magnitude and timing. IFN-gamma mRNA was induced by both strains, although to a greater degree by the vvIBDV strain, indicating that a cell-mediated response is induced. Neither virus initially induced high levels of type I IFN. F52/70 seems to use a "stealth" approach by not inducing the type I IFNs, whereas UK661 down-regulates their expression. This suggests that both viruses modulate the host immune response, although probably by using different mechanisms.

DNA vaccines for poultry: the jump from theory to practice.

DNA vaccines could offer a solution to a number of problems faced by the poultry industry; they are relatively easy to manufacture, stable, potentially easy to administer, can overcome neonatal tolerance and the deleterious effects of maternal antibody, and do not cause disease pathology. Combined with this, in ovo vaccination offers the advantage of reduced labor costs, mass administration and the induction of an earlier immune response. Together, this list of advantages is impressive. However, this combined technology is still in its infancy and requires many improvements. The potential of CpG motifs, DNA vaccines and in ovo vaccination, however, can be observed by the increasing number of recent reports investigating their application in challenge experiments. CpG motifs have been demonstrated to be stimulatory both in vitro and in vivo. In addition, DNA vaccines have been successfully delivered via the in ovo route, albeit not yet through the amniotic fluid. Lastly, a recent report has demonstrated that a DNA vaccine against infectious bronchitis virus administered via in ovo vaccination, followed by live virus boost, can slightly improve on the protective effect induced by the live virus alone. Therefore, DNA vaccination via the in ovo route is promising and offers potential as a poultry vaccine, however, efficacy needs to be improved and the costs of production reduced before it is likely to be beneficial to the poultry industry in the long term.

Use of Marek's disease vaccines: could they be driving the virus to increasing virulence?

Marek's disease (MD) is an economically important neoplastic disease of poultry. MD almost devastated the poultry industry in the 1960s but the disease was brought under control after Marek's disease herpesvirus (MDV) was identified and vaccines were developed. This is the first effective use of an antiviral vaccination to prevent a naturally occurring cancer in any species. MDV infection has many effects. Initially causing a cytolytic infection in B-lymphocytes, MDV infects activated T-lymphocytes where it becomes latent. In susceptible chicken genotypes MDV transforms CD4+ lymphocytes, causing visceral lymphomas and/or neural lesions and paralysis. Fully productive infection and shedding of infectious virus only occurs in the feather-follicle epithelium. Vaccination of newly-hatched chicks with live vaccines has been widely used to successfully control MD since the early 1970s. However, vaccinated chickens still become infected and shed MDV. Vaccine breaks have occurred with regularity and there is evidence that the use of MD vaccines could be driving MDV to greater virulence. MD continues to be a threat and a number of strategies have been adopted such as the use of more potent vaccines and vaccination of the embryonic stage to provide earlier protection. Recombinant MD vaccines are useful vectors and are being exploited to carry both viral and host genes to enhance protective immune responses. The future aim must be to develop a sustainable vaccine strategy that does not drive MDV to increased virulence.

Differential cytokine responses following Marek's disease virus infection of chickens differing in resistance to Marek's disease.

The production of cytokine mRNAs, in addition to viral DNA, was quantified by real-time quantitative reverse transcription-PCR (RT-PCR) (cytokines) or PCR (virus) in splenocytes during the course of Marek's disease virus (MDV) infection in four inbred chicken lines: two resistant (lines 6(1) and N) and two susceptible (lines 7(2) and P). Virus loads were only different after 10 days postinfection (dpi), increasing in susceptible lines and decreasing in resistant lines. Gamma interferon (IFN-gamma) mRNA was expressed by splenocytes from all infected birds between 3 and 10 dpi, associated with increasing MDV loads. For other cytokines, differences between lines were only seen for interleukin-6 (IL-6) and IL-18, with splenocytes from susceptible birds expressing high levels of both transcripts during the cytolytic phase of infection, whereas splenocytes from resistant birds expressed neither transcript. These results indicate that these two cytokines could play a crucial role in driving immune responses, which in resistant lines maintain MDV latency but in susceptible lines result in lymphomas.