In vitro studies with lymphocytes from sheep orally inoculated with an aromatic-dependent mutant of Salmonella typhimurium.

It was previously shown that a live aroA-strain of Salmonella typhimurium of ovine origin was a safe and effective vaccine against salmonellosis in sheep. The protective effect was observed in the apparent absence of a detectable, systemic T cell response. In the present study, populations of B and T cells from the peripheral blood of sheep vaccinated with S25/1aroA were separated and their responsiveness in vitro to Salmonella was examined. The purified T cells proliferated very weakly in response to Salmonella in the absence of interferon-gamma and interleukin 2/4 production. However, whole peripheral blood mononuclear cells and purified B cells proliferated strongly in response to Salmonella, and Salmonella-specific IgM antibodies could be detected in cell supernatants. Furthermore, Salmonella-specific IgM-producing cells were detected at low frequency by enzyme linked immunospot techniques. These observations extend the earlier findings that oral vaccination with S25/1aroA primes predominantly antigen-specific B cells in the absence of strong Salmonella-specific T cell responses.