The RNA-binding protein tristetraprolin schedules apoptosis of pathogen-engaged neutrophils during bacterial infection
Florian Ebner, … , Michael Sixt, Pavel Kovarik
Florian Ebner, … , Michael Sixt, Pavel Kovarik
Published June 1, 2017; First published May 15, 2017
Citation Information: J Clin Invest. 2017;127(6):2051-2065. https://doi.org/10.1172/JCI80631.
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Research Article Immunology Infectious disease

The RNA-binding protein tristetraprolin schedules apoptosis of pathogen-engaged neutrophils during bacterial infection

Abstract

Protective responses against pathogens require a rapid mobilization of resting neutrophils and the timely removal of activated ones. Neutrophils are exceptionally short-lived leukocytes, yet it remains unclear whether the lifespan of pathogen-engaged neutrophils is regulated differently from that in the circulating steady-state pool. Here, we have found that under homeostatic conditions, the mRNA-destabilizing protein tristetraprolin (TTP) regulates apoptosis and the numbers of activated infiltrating murine neutrophils but not neutrophil cellularity. Activated TTP-deficient neutrophils exhibited decreased apoptosis and enhanced accumulation at the infection site. In the context of myeloid-specific deletion of Ttp, the potentiation of neutrophil deployment protected mice against lethal soft tissue infection with Streptococcus pyogenes and prevented bacterial dissemination. Neutrophil transcriptome analysis revealed that decreased apoptosis of TTP-deficient neutrophils was specifically associated with elevated expression of myeloid cell leukemia 1 (Mcl1) but not other antiapoptotic B cell leukemia/lymphoma 2 (Bcl2) family members. Higher Mcl1 expression resulted from stabilization of Mcl1 mRNA in the absence of TTP. The low apoptosis rate of infiltrating TTP-deficient neutrophils was comparable to that of transgenic Mcl1-overexpressing neutrophils. Our study demonstrates that posttranscriptional gene regulation by TTP schedules the termination of the antimicrobial engagement of neutrophils. The balancing role of TTP comes at the cost of an increased risk of bacterial infections.

Authors

Florian Ebner, Vitaly Sedlyarov, Saren Tasciyan, Masa Ivin, Franz Kratochvill, Nina Gratz, Lukas Kenner, Andreas Villunger, Michael Sixt, Pavel Kovarik

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Figure 1

Myeloid-specific deletion of Ttp increases immune response and resistance against invasive infection with S. pyogenes.

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Myeloid-specific deletion of Ttp increases immune response and resistanc...
(A) Survival of TTPfl/fl and TTPΔM mice (n = 8/genotype) during s.c. infection with 3 × 108 CFU S. pyogenes. Animals were monitored for 5 days. Kaplan-Meier survival curves are shown. **P < 0.01, by Mantel-Cox test. (B) The area of inflammation was determined 48 hours p.i. Two independent experiments were pooled (n = 11 TTPfl/fl and n = 16 TTPΔM mice in total). Error bars represent the mean. **P < 0.01, by unpaired Student’s t test. (C) H&E-stained tissue sections of flanks from TTPfl/fl and TTPΔM mice 24 and 48 hours p.i. were graded for leukocyte infiltration, necrotic tissue abundance, inflammation, PMN infiltration, and vascular congestion. Scores were combined for overall grading. Error bars indicate the median. *P < 0.05, by Mann-Whitney U test. (D) Representative H&E- (left panel), Gr-1– (middle panel), and S. pyogenes–stained (right panel) tissue sections of flanks from TTPfl/fl (top) and TTPΔM (bottom) animals 48 hours p.i. Arrows indicate Gr-1–stained cells. Note an increased infiltrate and containment of bacteria in tissues from TTPΔM mice. Original magnification, ×2.5 and ×20 (insets in H&E-stained images). Scale bars: 200 μm. (E) Bacterial loads in the blood, liver, and lesions from TTPfl/fl and TTPΔM mice infected with 3 × 108 CFU S. pyogenes. Samples were taken 48 hours p.i., and the bacterial load was determined. Dot plots represent a pool of 2 independent experiments (n = 11 TTPfl/fl and n = 14 TTPΔM mice for organs; n = 13 TTPfl/fl and n = 11 TTPΔM mice for lesions). Error bars indicate the median. *P < 0.05 and **P < 0.01, by Mann-Whitney U test.