Induction and regulation of Th1-inducing cytokines by bacterial DNA, lipopolysaccharide, and heat-inactivated bacteria

Th1 immune responses, characterized by production of gamma interferon (IFN- gamma), are associated with protective immunity to viruses and intracellula r bacteria. Heat-killed Brucella; abortus promotes secretion of Th1-inducin g cytokines such as interleukin-12 (IL-12) and IFN-gamma and has been used as a carrier to induce Th1 responses to vaccines. To explore which bacteria l constituents could mediate this response and how it is regulated, murine spleen cells were cultured with B. abortus derived DNA, lipopolysaccharide (LPS), or whole killed organisms. Each constituent induced similar, substan tial amounts of IL-10. However, only B. abortus and B. abortus DNA induced high levels of IFN-gamma and IL-12. B. abortus and B. abortus DNA-stimulate d IL-12 production was maximal by 6 to 18 h, while IL-10 production steadil y accumulated over this time period. These kinetics suggested that IL-10 ma y eventually downmodulate the Th1-like cytokine response to B. abortus and B. abortus DNA, which was confirmed by using neutralizing antibody. In the absence of IL-10, B. abortus LPS induced strong IFN-gamma responses, but IL -12 p70 levels were still undetectable from BALB/c spleen cells. LPS induce d IL-12 if the spleen cells were primed with IFN-gamma and IL-10 was neutra lized, indicating that LPS can stimulate IL-12 production under the most fa vorable conditions. Responses to Escherichia coil LPS and DNA mirrored the responses to B. abortus components, suggesting that immune effects observed with these constituents may be generalizable to many microbial species. In vivo experiments demonstrated the same hierarchy of responses for IL-12 pr oduction. These findings support the likelihood that microbial components, if used as carriers or adjuvants, can differ substantially in their ability to effect a Th1 response.