Innate IFN-γ Is Essential for Systemic Chlamydia muridarum Control in Mice, While CD4 T Cell-Dependent IFN-γ Production Is Highly Redundant in the Female Reproductive Tract

Abstract

Protective immunity against the obligate intracellular bacterium Chlamydia has long been thought to rely on CD4 T cell-dependent gamma interferon (IFN-γ) production. Nevertheless, whether IFN-γ is produced by other cellular sources during Chlamydia infection and how CD4 T cell-dependent and -independent IFN-γ contribute differently to host resistance have not been carefully evaluated. In this study, we dissected the requirements of IFN-γ produced by innate immune cells and CD4 T cells for resolution of Chlamydia muridarum female reproductive tract (FRT) infection. After C. muridarum intravaginal infection, IFN-γ-deficient and T cell-deficient mice exhibited opposite phenotypes for survival and bacterial shedding at the FRT mucosa, demonstrating the distinct requirements for IFN-γ and CD4 T cells in host defense against Chlamydia In Rag1-deficient mice, IFN-γ produced by innate lymphocytes (ILCs) accounted for early bacterial control and prolonged survival in the absence of adaptive immunity. Although type I ILCs are potent IFN-γ producers, we found that mature NK cells and ILC1s were not the sole sources of innate IFN-γ in response to Chlamydia By conducting T cell adoptive transfer, we showed definitively that IFN-γ-deficient CD4 T cells were sufficient for effective bacterial killing in the FRT during the first 21 days of infection and reduced bacterial burden more than 1,000-fold, although mice receiving IFN-γ-deficient CD4 T cells failed to completely eradicate the bacteria from the FRT like their counterparts receiving wild-type (WT) CD4 T cells. Together, our results revealed that innate IFN-γ is essential for preventing systemic Chlamydia dissemination, whereas IFN-γ produced by CD4 T cells is largely redundant at the FRT mucosa.

Keywords: CD4 T cells; Chlamydia; IFN-γ; infection; innate.

FIG 1

IFN-γ-deficient mice and αβ T cell-deficient mice exhibit opposite phenotypes after Chlamydia muridarum intravaginal infection. B6, IFN-γ^−/−, and TCRβ^−/− mice were infected intravaginally with 1 × 10⁵ C. muridarum. Survival (A) and bacterial shedding (B) from the lower female reproductive tract (FRT) were monitored by vaginal swabs. Data are combined results of three independent experiments with 9 to 13 mice per group. Each data point represents an individual mouse. Lines represent mean log₁₀-transformed values. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001; ns, not significant.

FIG 2

Innate lymphocytes (ILCs) are essential for systemic bacterial control and prolonged survival of Rag1-deficient mice. B6, Rag1^−/−, and Rag2^−/− γc^−/− mice were infected intravaginally with 1 × 10⁵ C. muridarum organisms. (A) Survival. (B) Body weight. (C) Bacterial burdens in vaginal swabs, rectal swabs, and systemic organs determined at 12 days postinfection. Data shown are combined results of two independent experiments with 9 or 10 mice per group (A) or representative results of two independent experiments with 3 to 5 mice per group in each experiment (B and C). Each data point represents an individual mouse. Bars and error bars represent means and standard errors of the means (SEM). *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001; ns, not significant.

FIG 3

Innate IFN-γ and group 1 ILCs contribute to host resistance to lethal Chlamydia dissemination in Rag1^−/− mice. B6, Rag1^−/−, and Rag2^−/− γc^−/− mice were infected intravaginally with 1 × 10⁵ C. muridarum organisms. Groups of Rag1^−/− mice were treated with either anti-IFN-γ or anti-NK1.1 depleting Abs throughout the infection. (A) Survival. (B) Bacterial burdens in lower FRT (L-FRT), upper FRT (U-FRT), rectal swabs, and systemic organs determined at 14 days postinfection. (C) Representative flow cytometry plots showing IFN-γ secretion by CD11b⁺ NK1.1⁺ cells detected by IFN-γ secretion assay. (D) Percentages of IFN-γ-producing CD11b⁺ NK1.1⁺ cells quantified based on the flow cytometry analysis in panel C. (E) Serum IFN-γ level on days 7 and 14 after infection, as measured by IFN-γ cytokine enzyme-linked immunosorbent assay (ELISA). Data in panels A, B, D, and E are combined results of at least two independent experiments with 3 to 5 mice per group in each experiment. Each data point in panels B, D, and E represents an individual mouse. Bars and error bars represent means and SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001; ns, not significant.

FIG 4

CD4 T cell-dependent IFN-γ production is largely redundant at the FRT mucosa. CD4 T cells isolated from WT B6 or IFN-γ^−/− mice were adoptively transferred to TCRβ^−/− mice. Recipient TCRβ^−/− mice were infected intravaginally with 1 × 10⁵ C. muridarum organisms. (A) Schematic depicting the TCRβ^−/− adoptive transfer experimental setup. (B and C) Bacterial shedding from the FRT (B) and survival of TCRβ^−/− recipient mice (C) after receiving naive CD4 T cells from either WT or IFN-γ^−/− donors and intravaginal infection. Data are combined results of three independent experiments with 6 to 11 mice per group. Each data point in panels B and C represents an individual mouse. Lines represent mean log₁₀-transformed values. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001; ns, not significant.