Type I interferons differentially modulate maternal host immunity to infection by Listeria monocytogenes and Salmonella enterica serovar Typhimurium during pregnancy

Abstract

Problem: IFN-alpha receptor deficiency (IFNAR^-/- ) enhances immunity to Listeria monocytogenes (LM) and Salmonella enterica serovar Typhimurium (ST) in the non-pregnant state by inhibiting pathogen-induced immune cell death. However, the roles of IFNAR signaling in modulating immunity to infection during pregnancy are not well understood.

Method of study: C57BL/6J wild-type (WT) and IFNAR^-/- mice were infected systemically with LM or ST. Bacterial burden in spleen and individual placentas was enumerated at day 3 post-infection. Immune cell numbers and percentages were quantified in spleen and individual placentas, respectively, through flow cytometry. Cytokine expression in serum, spleen, and individual placentas was measured through cytometric bead array.

Results: IFNAR^-/- mice exhibited decreased splenic monocyte numbers in non-pregnant and pregnant state, and an altered distribution of placental immune cell types in the non-infected state. IFNAR^-/- mice controlled LM infection more effectively than WT mice even during pregnancy. This correlated with enhanced serum IL-12 expression, despite reduced splenic monocyte numbers relative to WT controls. In contrast, pregnant IFNAR^-/- mice unlike their non-pregnant counterparts exhibited increased susceptibility to ST infection, which was associated with decreased serum IL-12 expression.

Conclusion: Type I IFN responses differentially impact host resistance to LM and ST infection during pregnancy through modulation of immune cell distribution and cytokine responses.

Keywords: Listeria; Salmonella; Type I Interferon; cytokines; pregnancy.

Figure 1:. IFNAR^−/− mice exhibit altered distributions of splenic and placental immune cell types relative to WT mice.

Spleen, individual placentas and serum were collected from non-pregnant and pregnant (day 14–15 of gestation) WT and IFNAR^−/− mice in the non-infected state. (A) Total splenic cell numbers were determined by counting live cells using the Cellometer. N = 6 mice per group. Immune cell populations in the spleen and individual placentas were identified through flow cytometry. (B) Splenic numbers of monocytes, macrophages, DCs, B cells, T cells and NK cells. N = 6 mice per group. (C) Placental percentages of monocytes, macrophages, DCs, B cells, T cells and NK cells. N = 51–52 individual placentas per pregnant group. (D) MCP-1 expression levels were determined through CBA. N = 4–5 mice per group. Data are presented as mean ± SEM. Statistical significance was analyzed by Mann Whitney U test. *: p ≤ 0.05, **: p <0.01, ***: p < 0.001, ****: p < 0.0001. NPNI, non-pregnant non-infected; PNI, pregnant non-infected.

Figure 2:. IFNAR^−/− mice retain increased resistance to LM infection from the non-pregnant to pregnant state.

Non-pregnant and pregnant (day 11–12 of gestation) WT and IFNAR^−/− mice were infected with LM 5×10⁴ CFUs i.v. Spleens, individual placentas and serum were collected at day 3 post-infection. (A and B) Bacterial burdens in spleens and individual placentas were enumerated. N = 6–8 mice per group; 47–63 individual placentas per pregnant group. (C) Fetal resorption rates were determined using the formula R/(R + V) × 100, where R is the number of resorbing fetuses and V is the number of viable fetuses per mouse. N = 3–11 mice per group. Immune cell populations in the spleen and individual placentas were identified through were evaluated through flow cytometry. (D) Monocyte numbers in the spleen in non-pregnant and pregnant WT and IFNAR^−/− mice at day 3 post-infection. N = 4 mice per group. (E) Percentages of neutrophils, monocytes, DCs and NK cells in the placenta at day 3 post-infection. N = 24–27 individual placentas per pregnant group. Cytokine expression levels in the serum (F) and spleen (G) at day 3 post-infection. N = 4–5 mice per group. Data are presented as mean ± SEM. Statistical significance was analyzed by Mann Whitney U test. *: p ≤ 0.05, **: p <0.01, ***: p < 0.001, ****: p < 0.0001. NPI, non-pregnant infected; PNI, pregnant non-infected; PI, pregnant infected.

Figure 3:. Pregnant IFNAR^−/− mice exhibit enhanced susceptibility to ST infection relative to non-pregnant controls.

Non-pregnant and pregnant (day 11–12 of gestation) WT and IFNAR^−/− mice were infected with ST 10³ CFUs i.v. (A) Mice were monitored every day following infection. Mouse deaths were recorded upon observing >20% loss in body weight and/or 2 or more severe signs of clinical illness, including piloerection, lack of grooming and morbidity. N = 7–8 mice per group. Spleen, individual placentas and serum were collected at day 3 post-infection. (B and C) Bacterial burden in spleen and individual placentas were enumerated. N = 7–13 mice per group; 35–70 individual placentas per pregnant group. (D) Fetal resorption rates were determined using the formula R/(R + V) × 100, where R is the number of resorbing fetuses and V is the number of viable fetuses per mouse. N = 5–13 mice per group. Immune cell populations in the spleen and individual placentas were identified through were evaluated through flow cytometry. (E) Monocyte numbers in the spleen in non-pregnant and pregnant WT and IFNAR^−/− mice at day 3 post-infection. N = 4–7 mice per group. (F) Percentages of neutrophils, monocytes, macrophages, DCs, B cells, T cells (CD4⁺ and CD8⁺) and NK cells in the placenta at day 3 post-infection. N = 38–40 individual placentas per pregnant group. Cytokine expression levels in the serum (G), spleen (H) and placenta (I) at day 3 post-infection. N = 5–9 mice per group; 10 individual placentas per pregnant group. Bacterial burden, resorption rates, cell numbers and cytokine expression levels are presented as mean ± SEM. Statistical significance was analyzed by Mann Whitney U test. *: p ≤ 0.05, **: p <0.01, ***: p < 0.001, ****: p < 0.0001. NPI, non-pregnant infected; PNI, pregnant non-infected; PI, pregnant infected.