Compromised T-cell immunity in turkeys may lead to an unpredictable avian metapneumovirus vaccine response and variable protection against challenge

Abstract

Avian metapneumovirus (aMPV) is an important respiratory pathogen of turkeys with considerable economic impact on poultry production. Although vaccination is widely used for the control of the disease, questions regarding vaccine safety and efficacy remain to be elucidated. This report describes the problems associated with reproducibility of the aMPV-vaccine response, comparing T-lymphocyte-compromised and T-cell-intact turkeys. In three consecutive experiments, turkeys partially depleted of T-lymphocytes by treatment with cyclosporin A as well as untreated turkeys were vaccinated with a commercial live aMPV subtype A (aMPV-A) vaccine at 2 weeks of age. Two weeks later they were challenged with a virulent aMPV-A strain. Despite similar genetic background of the turkeys, comparable housing conditions under isolation and the application of the same aMPV-A vaccine, considerable variation was observed among the experiments regarding replication of the vaccine virus, vaccine-induced clinical signs and protection against challenge infection. The results indicate that differences in the outcome of aMPV-A vaccination may be associated with T-lymphocyte suppression and additionally with an interfering aMPV-B vaccine exposure at the hatchery in two of the experiments. Our study provides possible explanations for the variable protection provided by aMPV vaccines under field conditions.

Development of vaccine-induced immunity against TRT in turkeys depends remarkably on the level of maternal antibodies and the age of birds on the day of vaccination

“…Despite the above, the upper respiratory tract immune system was strongly stimulated in those groups after vaccination. Those results partially corroborate the findings of Rubbenstroth and Rautenschlein [ 16 ] who reported very high levels of vaccine-induced immunity against aMPV infections despite the fact the aMPV/A RNA was not regularly detected in nasal turbinates of turkeys vaccinated against TRT on 13 or 14 DOL. Differences between birds vaccinated at different age suggest that the replication of vaccine aMPV/A is limited during the maturation of the immune and/or respiratory system, but this does not influence the development of vaccine-induced immunity.…”

“…Humoral immunity is strongly stimulated by vaccination or aMPV infection [ 7 , 10 - 13 ], but antibodies do not play a key role in protection against TRT and should not be considered as indicators of immunity against aMPV infections [ 7 , 9 , 11 , 12 , 14 - 16 ]. It has been shown, however, that high antibody titers suppress aMPV replication in the upper respiratory tract, thus alleviating the clinical course of TRT [ 6 , 11 , 12 , 14 ].…”