Endogenous interleukin-12 is not required for resolution of Chlamydophila abortus (Chlamydia psittaci serotype 1) infection in mice

A Th1 immune response involving gamma interferon (IFN-gamma) production is required to eliminate Chlamydophila abortus infections. In this study, the role of interleukin-12 (IL-12) in protecting against C. abortus infection w as investigated using IL-12(-/-) and wild-type (WT) C57BL/6 mice to determi ne the role of this Th1-promoting cytokine. IL-12(-/-) mice were able to el iminate the C. abortus infection in a primary infection. However, there was a delay in the clearance of bacteria when IL-12(-/-) mice were infected wi th a sublethal dose of C. abortus, the delay being associated with a lower production of IFN-gamma. The low level of IFN-gamma was essential for survi val of IL-12(-/-) infected mice. Both WT and IL-12(-/-) mice developed a Th 1 immune response against C. abortus infection, since they both produced IF N-gamma and immunoglobulin G2a antibody isotype. In addition, when mice wer e given a secondary infectious challenge with C. abortus, a protective host response which resolved the secondary infection was developed by both WT a nd IL-12(-/-) mice. The lack of IL-12 resulted in few infiltrating CD4(+) T cells in the liver relative to the number in WT mice, although the number of CD8(+) T cells was slightly higher. The more intense Th1 response presen ted by WT mice may have a pathogenic effect, as the animals showed higher m orbidity after the infection. In conclusion, these results suggest that alt hough IL-12 expedites the clearance of C. abortus infection, this cytokine is not essential for the establishment of a protective host response agains t the infection.