Bone Marrow Derived Mesenchymal Cells Secrete Granulopoietic Cytokines upon Danger Signaling

Granulopoietic homeostasis is regulated at steady-state to supply sufficient numbers of pooled and circulating neutrophils to maintain barrier function against commensal flora. In addition, upon pathogenic microbial challenge, an increased formation of neutrophils is induced, termed ‘emergency granulopoiesis’.

Antibody-mediated reduction of neutrophil numbers in steady-state induces a feedback loop leading to an increase of bone marrow granulopoiesis with expansion of hematopoetic stem and progenitor cells. This feedback loop was demonstrated to depend on TLR4 and TRIF, but not MyD88 signaling (Bugl et al. Blood 2013). In contrast, emergency granulopoiesis was shown to be dependent on MyD88 signaling in endothelial cells (Boettcher et al. Blood 2014).

Bone marrow mesenchymal stromal cells (MSC) are niche-forming cells, harboring and regulating hematopoiesis. Upon steady-state neutropenia an increase of niche size was observed. Here we investigated, whether niche-forming MSC act as sensors of pathogen-associated molecular patterns (PAMPs) and induce granulopoietic cytokines to stimulate expansion of adjacent hematopoietic stem and progenitor cells.

MSC of C57BL/6 and TLR4-KO mice were cultured in vitro and treated with LPS for 24 hours. Cells were harvested and qRT-PCR for G-CSF, TLR4, MyD88, TRIF, GM-CSF, IL-1β, IL-18 and Casp-1 was performed

After treatment with LPS, RNA of granulopoietic cytokines G-CSF and GM-CSF were massively up regulated in MSC of WT mice. Upstream regulating, inflammasome components IL-1ß and caspase-1 RNA levels increased as well, with little changes in IL-18, TLR4, MyD88 and TRIF.

Unexpectedly, TLR4-KO MSC up regulated transcription of IL-1β and G-CSF upon LPS stimulation as well, and caspase-1 was found to be strongly up-regulated in unstimulated TLR4-KO compared to WT MSC.

In summary, bone marrow stromal cells are found to be PAMP-sensing and secrete cytokines that regulate granulopoiesis. TLR4-independent sensing of LPS by MSC might correspond to the alternative noncanonical inflammasome pathway recently described (Kayagaki et al. Science 2013).