Cytotoxic T-cell, delayed type hypersensitive and listeriolysin O responses in experimental bovine listeriosis.

Five different antigens of Listeria monocytogenes NCTC 11994 were assayed for their lymphoproliferative, cytotoxic, humoral and delayed type hypersensitivity (DTH) responses in experimentally infected calves. The antigens used were culture filtrate antigen (CFA), listeriolysin O (LLO), sonicated antigen (SA), live antigen (LA) and heat killed antigen (HKA). CFA was most lymphoproliferative when assayed in vitro. In an autologous monocyte cytotoxicity assay, soluble protein as well as bacterial cells elicited cytolytic responses, however, LA and LLO primed monocytes showed a higher cytotoxic effect. The expression of cell surface markers of lymphocyte subpopulations was almost identical in experimentally infected as well as non-infected calves. SA elicited the highest humoral and DTH responses. LLO being a major virulence factor with appreciable humoral as well as cellular responses may be used as a candidate antigen to develop a reliable diagnostic immunoassay.