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

Abstract

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 was investigated using IL-12(-/-) and wild-type (WT) C57BL/6 mice to determine the role of this Th1-promoting cytokine. IL-12(-/-) mice were able to eliminate the C. abortus infection in a primary infection. However, there was a delay in the clearance of bacteria when IL-12(-/-) mice were infected with 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 survival of IL-12(-/-) infected mice. Both WT and IL-12(-/-) mice developed a Th1 immune response against C. abortus infection, since they both produced IFN-gamma and immunoglobulin G2a antibody isotype. In addition, when mice were given a secondary infectious challenge with C. abortus, a protective host response which resolved the secondary infection was developed by both WT and 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 presented by WT mice may have a pathogenic effect, as the animals showed higher morbidity after the infection. In conclusion, these results suggest that although IL-12 expedites the clearance of C. abortus infection, this cytokine is not essential for the establishment of a protective host response against the infection.

FIG. 1

Effect of C. abortus infection on weight loss of C57BL/6 (WT) mice (■) and IL-12^−/− mice (□). Groups of 10 mice were infected with a challenge dose of 10⁶ IFUs of C. abortus, and body weight was monitored daily. Experiments were performed twice with similar results. ∗, statistically significant difference (P < 0.05).

FIG. 2

IFN-γ and IL-4 production in spleen cells of C. abortus-infected C57BL/6 (WT) and IL-12^−/− (KO) mice. Spleen cells were stimulated with ConA (5 μg/ml) and purified EB of C. abortus (50 μg/ml), and the presence of IFN-γ and IL-4 in cell culture supernatants was determined 48 h later. The results are the means and standard deviation for five mice. Experiments were performed twice with similar results. Cytokine production in spleen cells from noninfected mice was below the detection limit (<50 pg/ml for IFN-γ and <30 pg/ml for IL-4). ∗, statistically significant difference (P < 0.05) in the production of IFN-γ between WT mice and IL-12^−/− mice when cells were stimulated with EB.

FIG. 3

Number of CD4⁺ and CD8⁺ T cells in liver sections of WT mice (■) and IL-12^−/− mice (□) after i.p. inoculation of 10⁶ IFUs of C. abortus. Adjacent liver sections were labeled for CD4⁺ and CD8⁺ cells. The number of cells was counted in 20 fields (17,000 μm²/field) of every liver section. The results are the means and standard deviation for five mice. Experiments were performed twice with similar results. ∗, statistically significant difference (P < 0.05).

FIG. 4

Immunophenotypic characterization of liver T cells from C57BL/6 (WT) and IL-12^−/− mice during C. abortus infection. Adjacent liver sections from WT (A and B) and IL-12^−/− (C and D) mice were labeled for CD4⁺ (A and C) and CD8⁺ (B and D) T cells. Note that CD4⁺ cells were more numerous in WT mice (A) than in IL-12^−/− mice (C), although there were more CD8⁺ cells in IL-12^−/− mice (D) than in WT mice (B). Magnification, ×200.

FIG. 5

Immunophenotypic characterization of liver T cells from WT (■) and IL-12^−/− (□) mice after a secondary infectious challenge with 10⁶ IFUs of C. abortus. Adjacent liver sections were immunolabeled for CD4⁺ and CD8⁺ cells. Cell number was counted in 20 fields (17,000 μm²/field) of every liver section. The results are the means and standard deviation for five mice. Experiments were performed twice with similar results. ∗, statistically significant difference (P < 0.05).

FIG. 6

Survival of WT and IL-12^−/− mice given anti-IFN-γ MAb (XMG-6). Groups of six mice were infected with 10⁶ IFUs of C. abortus. The MAb was administered to WT and IL-12^−/− mice 24 h before infection, at the time of infection, and 3 days p.i. Mice were monitored dairy, and mortality rates were calculated.