Differences in Local and Systemic Immune Responses to Primary and Secondary Immunizations with the Herpesvirus of Turkeys (HVT) Vaccine in Layer Pullets

Marek’s disease, caused by a herpesvirus, is a major health issue in poultry production. In chickens, vaccination with live herpesvirus of turkeys (HVT) provides effective protection. Little is known about the immune responses to the HVT vaccine. In this study, the immune responses to primary (V1) or secondary (V2) intradermal (i.d.) HVT vaccinations were examined. V1 and V2 were i.d. injected into growing feather (GF) pulps of six 9-week-old and six 19-week-old Light-brown Leghorn (LBL) pullets, respectively. Growing feather pulps and blood were collected before (0 h) and periodically through 10 d or 28 d post pulp injection (p.i.), respectively, to determine pulp- and blood-leukocyte profiles and HVT-specific antibody levels. Data were analyzed by two-way analysis of variance (ANOVA) or two-way Repeated-Measures ANOVA (P ≤ 0.05). In GF pulps, V1 and V2 recruited leukocytes by 1 d p.i., reaching peak levels (% pulp cells) at 3 d p.i. that were greater in V1 than V2 (54% vs. 24%). The V1 T-cell response primarily involved CD4+ T-helper cells with peak levels at 3 d p.i., whereas the V2 T-cell response was dominated by CD8+ cytotoxic-T cells, with highest levels at 1 d to 3 d p.i. In the blood, heterophil concentrations were increased at 6 hours p.i. in V1 and V2. In V1, the relative HVT-specific IgM plasma levels were elevated at 10 d and 28 d p.i., while IgG steadily increased to the greatest levels at 28 d p.i.; in V2, IgM- and IgG-levels did not change significantly post pulp injection. Overall, V1 and V2 resulted in temporal, qualitative, and quantitative differences in leukocyte profiles suggestive of primary and secondary cell-mediated, antiviral immune responses.