Mice lacking the type I interferon receptor are resistant to Listeria monocytogenes

Abstract

Listeria monocytogenes is a facultative intracellular pathogen that induces a cytosolic signaling cascade resulting in expression of interferon (IFN)-beta. Although type I IFNs are critical in viral defense, their role in immunity to bacterial pathogens is much less clear. In this study, we addressed the role of type I IFNs by examining the infection of L. monocytogenes in BALB/c mice lacking the type I IFN receptor (IFN-alpha/betaR-/-). During the first 24 h of infection in vivo, IFN-alpha/betaR-/- and wild-type mice were similar in terms of L. monocytogenes survival. In addition, the intracellular fate of L. monocytogenes in macrophages cultured from IFN-alpha/betaR-/- and wild-type mice was indistinguishable. However, by 72 h after inoculation in vivo, IFN-alpha/betaR-/- mice were approximately 1,000-fold more resistant to a high dose L. monocytogenes infection. Resistance was correlated with elevated levels of interleukin 12p70 in the blood and increased numbers of CD11b+ macrophages producing tumor necrosis factor alpha in the spleen of IFN-alpha/betaR-/- mice. The results of this study suggest that L. monocytogenes might be exploiting an innate antiviral response to promote its pathogenesis.

Figure 1.

L. monocytogenes infection of BALB/c and IFN-α/βR^−/− mice. Naive BALB/c and IFN-α/βR^−/− mice (three per group) were infected with 2 × 10⁴ wild-type L. monocytogenes and bacterial growth in the spleen (A) and liver (B) was followed over 3 d. (C) BALB/c and IFN-α/βR^−/− mice were immunized with 2 × 10³ L. monocytogenes 3–4 wk before challenge with 4 × 10⁴ bacteria. Naive animals were infected with 4 × 10⁴ bacteria. Dashed line denotes limit of detection. CFUs from the spleen 48 h after inoculation are shown. Data represents two separate experiments.

Figure 2.

IFN-α/βR^−/− BMDMs support normal L. monocytogenes growth. BALB/c and IFN-α/βR^−/− BMDMs were (A) infected with wild-type L. monocytogenes and bacterial growth was monitored in the presence of 50 μg/ml gentamicin. Black diamonds, BALB/c; gray diamonds, IFN-α/βR^−/−. BMDMs were infected with an MOI of 10:1 wild-type or 100:1 Δhly L. monocytogenes, which does not enter the host cytosol and does not grow intracellularly, and ifnb (B) or tnfa (C) mRNA levels (normalized to β actin) and TNF-α supernatant protein levels (D) were quantified 6 h after inoculation. All experiments were performed at least twice.

Figure 3.

Elevated IL-12p70 and TNF-α, but not IFN-γ, in the absence of type I IFN signaling. (A) BALB/c and IFN-α/βR^−/− mice were infected with 2.5 × 10⁴ wild-type L. monocytogenes, bled 24 h after inoculation, and serum was assayed for cytokine levels. Data represents the average from seven mice per group over two separate experiments ± SEM. Splenocytes from uninfected BALB/c and IFN-α/βR^−/− mice (B) or from mice infected 48 h previously with 2 × 10⁴ L. monocytogenes (C) were cultured ± 4 × 10⁶ HKLM/ml for 18 h. Culture supernatants were assayed for TNF-α. Data represent the average from six mice per group over two separate experiments ± SEM.

Figure 4.

Increased number of CD11b⁺ TNF-α–producing cells in IFN-α/βR^−/− mice. Splenocytes from uninfected (uninf) or L. monocytogenes–infected (inf) BALB/c and IFN-α/βR^−/− mice were cultured ± 4 × 10⁶ HKLM/ml for 4 h. The absolute number of splenocytes was similar between BALB/c and IFN-α/βR^−/− mice. Cells were stained for cell surface markers and intracellular TNF-α and Mac-3. (A–C) Cells were stained for CD11b, CD3e, and TNF-α, gated on CD3e⁻ events, and expression of CD11b and TNF-α was analyzed. Data represents results from six mice per group over two separate experiments. (A) Representative dot plots from infected mouse splenocytes. Average percentage of total splenocytes ± SEM that are TNF-α–producing CD11b⁻ (top left) and CD11b⁺ (top right) cells are shown. (B) Average percentage of total splenocytes that are CD11b⁺/CD3e⁻. (C) Average percentage of total splenocytes stimulated with 4 × 10⁶ HKLM/ml that are CD11b⁺/TNF-α⁺ or CD11b⁻/TNF-α⁺. (D and E) Cells stimulated with 4 × 10⁶ HKLM/ml were stained for CD11b, Mac-3, and TNF-α. (D) Representative dot plots from one of three mice per group. Values shown are average percentage of total splenocytes ± SEM residing within the R1 and R2 populations. (E) Cells within the R1 and R2 gates were analyzed for intracellular TNF-α staining. Average percentage of total splenocytes ± SEM that are CD11b^int/Mac-3^high/TNF-α⁺ (R1) or CD11b^high/Mac-3^int/high/TNF-α⁺ (R2). Statistical significance was determined by using the Mann-Whitney nonparametric test. *, P = 0.081; **, P = 0.005.