Abstract
A preliminary study was conducted to determine the cellular immune status of neonatal, colostrum-deprived calves following inoculation with either attenuated or virulent bovine coronavirus (BCV). Uninfected calves served as controls. To determine the intestinal and systemic cellular immune status, we performed MHC-restricted cytotoxic lymphocyte (CTL) assay on mesenteric lymphocytes, enumerated T cell subsets in peripheral blood lymphocytes, and examined histopathological alterations in mesenteric lymph nodes and gut-associated lymphoid tissue. Target cells for the CTL assay were autologous testicular cells and effector cells were mesenteric lymphocytes from calves infected with BCV. No appreciable specific lysis was observed in any group of calves indicating the absence of demonstrable CTL responses. The TC/TS population was severely depressed in the calf inoculated with the virulent virus but not in those inoculated with either the attenuated virus or placebo. The mesenteric lymph nodes and Peyer's patches of calves inoculated with the virulent virus showed severe depletion of lymphocytes. These calves developed intestinal antibody responses in the acute phase of infection (1 week after infection) but were immunosuppressed in the later stage of infection.
Keywords: Bovine coronavirus, cellular immunity, T lymphocytes, calf scours, immunosuppression
Résumé
Une étude préliminaire a été conduite pour déterminer le statut immunitaire de veaux nouveau-nès privés de colostrum, et subissant une inoculation d'un coronavirus bovin (BVC) soit virulent, soit atténué. Les veaux non infectés ont servi de témoins. Pour déterminer le statut immunitaire cellulaire, systémique et intestinal, on a mis au point un test de lymphotoxicité (CTL) efficace sur les lymphocytes mésentériques. On a aussi dénombré la sous-population cellulaire T parmi les lymphocytes du sang périphérique, et on a examiné les altérations histopathologiques touchant les noeuds lymphatiques mésentériques et le tissu lymphoīde de drainage associé. Les cellules cibles pour le test CTL étaient des cellules autologiques de testicules et les cellules effectrices étaient les lymphocytes mésentériques des veaux infectés par le BCV. Quelque soit le groupe de veaux, aucune lyse spécifique significative n'a été observée, indiquant l'absence de réponse au CTL. Alors que veaux inoculés avec le virus virulent présentaient une dépression sévère des populations Tc/Ts, ce n'était pas le cas chez ceux inoculés avec le virus atténué ou le placebo. Les noeuds lymphatiques mésentériques et les plaques de Peyer des veaux ayant reçu le virus virulent montraient une déplétion marquée en lymphocytes. Ces veaux ont développé des réponses en anticorps intestinaux lors de la phase aigüe de l'infection, une semaine après l'inoculation, mais étaient immunodéprimés par la suite.
Mots-clé: Coronavirus bovin, immunité cellulaire, lymphocytes T, veau, immunodépression
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