Herpesvirus of Turkeys (HVT) vaccine confers limited protection against the cellular changes induced by very virulent plus Marek's disease virus (vv + MDV) strains in commercial meat-type chickens.

Very virulent plus (vv+) Marek's disease virus (MDV) strains are highly immunosuppressive and cause marked alterations in splenic immune cell populations in commercial meat-type (MT) chickens, even in the presence of maternal antibodies. In this study, we evaluated the protection provided by a recombinant herpesvirus of turkeys (rHVT) vaccine delivered at either a standard dose (SD) or a high dose (HD) against splenocyte damage induced by the vv + MDV strain 686 in MT chickens. Results showed that both rHVT-SD and rHVT-HD protected against the 686-induced reduction in splenocyte lymphoproliferative responses to concanavalin A (ConA) ex vivo and modestly, albeit not significantly, reduced 686 replication in the spleen at 6 days post-infection (dpi). However, by 25 dpi, neither vaccine dose limited the 686 DNA load in the spleen. Protection against splenic immunophenotypic alterations at 25 dpi was suboptimal; neither rHVT-SD nor rHVT-HD prevented the reduction in the number of live splenocytes, B cells, or KUL01+MHC-IIhigh+ APCs. However, modest improvements were observed; the rHVT-SD/686 group showed slightly higher percentages of live splenocytes and B cells than the 686-infected group, whereas the rHVT-HD/686 group exhibited a numerical increase only in B cells. Furthermore, rHVT vaccination influenced the percentage of activated T cells. Both rHVT-SD and rHVT-HD vaccinations decreased the percentage of activated CD4+ T cells. The percentage of activated CD8α+ T cells was also reduced in both vaccine groups, but the reduction was significant only in the HD group, whereas the proportion of activated CD8β+ T cells decreased only in the SD-vaccinated group. In addition, both vaccine doses reduced the percentage of γδ+ T cells. Overall, our findings demonstrate that vaccination with rHVT provided partial protection against vv + MDV 686-induced immunophenotypic changes in the spleen, with no added benefit from using a higher rHVT-vaccine dose.