Differential activation of Brucella-reactive CD4+ T cells by Brucella infection or immunization with antigenic extracts.

In order to induce acquired cellular resistance to facultative bacterial pathogens, infection with live organisms is required. We have previously demonstrated that spleen cells from Brucella-infected mice produced gamma interferon (IFN-gamma) and interleukin-2 (IL-2) in response to Brucella antigens in vitro, while spleen cells from mice immunized with soluble Brucella proteins (SBP) produced substantial amounts of IL-2 but no detectable amount of IFN-gamma. In this study, we further analyzed the response of T cells from Brucella-infected mice and SBP-immunized mice and demonstrated that CD4(+)-enriched cells from SBP-immunized mice also produced significant amounts of IL-4, which was not detected in bulk cultures of spleen cells from infected mice. Limiting dilution analysis showed that infection resulted in a higher precursor frequency of IFN-gamma-producing CD4+ T cells and a lower precursor frequency of IL-4-producing CD4+ T cells, while immunization with SBP resulted in a higher precursor frequency of IL-4-producing cells and a very low frequency of IFN-gamma-producing cells. The precursor frequencies of IL-2-producing cells for the two groups were similar. Furthermore, IFN-gamma-producing CD4+ T cells from infected donor mice were capable of mediating resistance against challenge infection in recipient mice, but IL-4-producing CD4+ T cells from immunized mice failed to do so. These results indicate that the form of antigen has a profound influence on the outcome of the immune response. The results are discussed in light of the supposed dichotomy between Th1 and Th2 cytokine responses.