Group B Streptococcus engages an inhibitory Siglec through sialic acid mimicry to blunt innate immune and inflammatory responses in vivo - PubMed (original) (raw)

Group B Streptococcus engages an inhibitory Siglec through sialic acid mimicry to blunt innate immune and inflammatory responses in vivo

Yung-Chi Chang et al. PLoS Pathog. 2014 Jan.

Abstract

Group B Streptococcus (GBS) is a common agent of bacterial sepsis and meningitis in newborns. The GBS surface capsule contains sialic acids (Sia) that engage Sia-binding immunoglobulin-like lectins (Siglecs) on leukocytes. Here we use mice lacking Siglec-E, an inhibitory Siglec of myelomonocytic cells, to study the significance of GBS Siglec engagement during in vivo infection. We found GBS bound to Siglec-E in a Sia-specific fashion to blunt NF-κB and MAPK activation. As a consequence, Siglec-E-deficient macrophages had enhanced pro-inflammatory cytokine secretion, phagocytosis and bactericidal activity against the pathogen. Following pulmonary or low-dose intravenous GBS challenge, Siglec-E KO mice produced more pro-inflammatory cytokines and exhibited reduced GBS invasion of the central nervous system. In contrast, upon high dose lethal challenges, cytokine storm in Siglec-E KO mice was associated with accelerated mortality. We conclude that GBS Sia mimicry influences host innate immune and inflammatory responses in vivo through engagement of an inhibitory Siglec, with the ultimate outcome of the host response varying depending upon the site, stage and magnitude of infection.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Figure 1

Figure 1. Augmented bactericidal activity in bone marrow-derived macrophages from Siglec-E KO mice.

(A) Wild-type Group B Streptococcus (GBS WT) showed Sia-dependent binding to mSiglec-E. GBS ΔSia mutant is an isogenic mutant lacking Sia expression. Background binding from hIgG1-immobilized wells, which served as a negative control, were subtracted from data shown. (B) Whole blood from Siglec-E KO mice showed better control of GBS growth; results pooled from two independent experiments. Siglec-E KO macrophages showed greater phagocytic activity (C) and bactericidal ability (D). GBS WT but not ΔSia mutant induced higher TNF-α (E) and IL-6 (F) secretion in macrophages from Siglec-E KO animals. Differences between different groups were calculated by two-way ANOVA with Bonferroni posttest (A–B, D–F) or unpaired t test (C). *** P<0.001; ** P<0.01; * P<0.05. (G) WT GBS stimulated macrophages recruited more SHP-1 to Siglec-E than GBS ΔSia mutant. (H) Enhanced ERK activation and IκB degradation in Siglec-E KO macrophages after GBS stimulation. Representative data from two reproducible experiments are shown.

Figure 2

Figure 2. Skewed cytokine responses in Siglec-E KO mice challenged in a GBS intranasal pneumonia model.

Mice were infected intranasally with 5×107 CFU WT GBS and cytokine levels in cell-free BAL fluid or lung homogenates collected 6 h post infection. (A) Bacterial load in lung tissue was calculated by dilution plating for CFU enumeration. (B) Cells from BAL were counted and stained with mAb to F4/80 and Gr-1 to quantitate infiltrating cell populations. IL-1β (C) and IL-6 (D) in the BAL and IL-10 in lung homogenates (E) were examined by ELISA analysis. Data shown are means ± SEM and each circle denotes 1 mouse (n = 8 for WT and n = 7 for mSiglec-E KO mice). The difference between different groups was calculated by Mann-Whitney test.

Figure 3

Figure 3. Absence of Siglec-E exacerbates inflammation and accelerates mortality in a systemic lethal dose GBS challenge.

WT and Siglec-E KO mice were challenged intravenously with 3×108 CFU of WT GBS. (A) Kaplan-Meier plot is shown for survival analysis (n = 10 for each group). Blood and brains were collected 18 h after GBS infection to measure IL-6 (B), serum amyloid A (C) and bacteria loads (D and E). Data shown are means ± SEM and each circle denotes 1 mouse (n = 6 for WT and n = 8 for Siglec-E KO mice). Differences between different groups were calculated by Mann-Whitney test (B–E).

Figure 4

Figure 4. Reduced brain dissemination and enhanced bactericidal responses in Siglec-E deficient mice upon sublethal GBS challenge.

Comparison of bacterial counts (expressed in CFU) recovered from the kidney (A) and brain (B) of WT mice and Siglec-E KO mice 48 h after intravenous challenge with 1×108 CFU of WT GBS. (C) Brain bacterial counts were corrected for blood contamination (brain/blood ratio) using a conservative estimate of the mouse cerebral blood volume (2.5 ml per 100 g tissue). mRNA expression of IL-1β (D) and IL-12 (E) in lung and IL-10 in spleen (F) was examined by quantitative real time RT-PCR analysis. Results pooled the data from two independent experiment with final numbers of n = 14 for each group. Each circle denotes 1 mouse (A–F). Siglec-E KO microglia cells showed greater bactericidal ability (G) and produced higher levels of TNF-α (H) after GBS challenge. Statistical analysis was performed by Mann-Whitney test (A–F), two-way ANOVA with Bonferroni posttest (G) and one-way ANOVA with Tukey's multiple comparison test (H). *P<0.05.

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