CELL-MEDIATED CYTOLYSIS OF EQUINE HERPESVIRUS-INFECTED CELLS BY LEUKOCYTES FROM YOUNG VACCINATED HORSES

The objective of this study was to determine whether the administratio n of modified-live equine herpesvirus (EHV-1) to young horses with res idual maternal antibodies stimulated EHV specific cytolytic responses, and whether these responses were crossreactive between EHV-1 and EHV- 4. Eighteen clinically normal Belgian cross-foals were used in the stu dy and were commingled in two adjacent pens. Skin biopsies were harves ted from 16 foals within 24 h of birth and fibroblast cultures were es tablished, expanded and cryopreserved. Beginning at approximately 10 w eeks of age, 10 randomly chosen foals were inoculated on days 0, 21, a nd 43 of the study with a vaccine containing modified-live EHV-1. Bloo d mononuclear leukocytes were obtained on days 0, 32, and 50 for the a ssessment of EHV-specific cytolytic activity using 5 h and 18 h chromi um release assays. EHV-l-specific antibodies were assessed by enzyme-l inked immunosorbent assay using serum collected on days -21, 0, 32, an d 50 of the study. Lymphocyte blastogenic tests and bioassays for inte rferon activity were conducted on day 50. After two vaccinations, mono nuclear leukocytes from seven of ten vaccinated foals had cytolytic ac tivity against autologous EHV-1 cells and leukocytes from six of ten l ysed EHV-4-infected cells when tested in an 18 h assay. This activity was enhanced by exogenous interleukin 2 and was markedly reduced using target cells from unrelated horses. Cytotoxicity was not detected in a 5 h assay following in vitro stimulation of leukocytes. After three vaccinations, blood leukocytes from 6/6 vaccinated foals and 0/6 unvac cinated foals had proliferative responses EHV-1. There were no signifi cant differences in interferon production by leukocytes from these foa ls. Twelve foals tested had low concentrations of (maternal) EHV-l-spe cific antibody prior to vaccination. Five of eight foals tested had in creases in EHV-specific antibodies, while 4/4 commingled unvaccinated foals had a decrease or no change in EHV-specific antibodies. These re sults demonstrate cytotoxic cellular immune responses can be induced i n young horses with maternal antibodies following administration of mo dified-live vaccine. (C) 1997 Elsevier Science B.V.