NK-like CD8(+) cells in immunologically naïve neonatal calves that respond to dendritic cells infected with Mycobacterium bovis BCG.

Pre-exposure to environmental mycobacteria and induction of an inappropriately biased immune response may be major factors affecting the efficacy of BCG; vaccination of neonates that have not been exposed to environmental mycobacteria may induce more effective immunity. Responses of neonatal calves to mycobacterial antigens using dendritic cells (DC) as antigen-presenting cells were investigated. In nonvaccinated, immunologically naive calves as young as 1 day old, a population of CD8(+) cells proliferated and produced IFN-gamma in response to BCG-infected DC. CD3(-) CD8(+) NK-like and CD3(+) CD8(+) T cells were evident within the responding CD8(+) population. The response was not MHC-restricted. The NK-like CD3(-) cells were the major population producing IFN-gamma. The presence of mycobacteria-reactive, IFN-gamma-secreting CD8(+) NK cells in neonatal calves may have important consequences for the induction of a Th1-biased immune response.