ERYTHROPOIESIS SUPPRESSION BY BACTERIAL LIPOSACCHARIDES IS EXTRINSICALLY MEDIATED INDEPENDENTLY OF G-CSF

The central macrophage (MΦ) in erythroblastic islands (EI) is critical to erythropoiesis by providing iron and growth factors, and mediating enucleation of the maturing erythroblasts. As MΦ are key effectors of inflammation, we investigated the effect of lipopolysaccharides (LPS) on erythropoiesis in vivo. LPS (2.5mg/kg/d for 2 days) in C57BL/6 mice caused the whitening of the bone marrow (BM) with dramatically decreased erythroblast and reticulocyte numbers. Imaging flow cytometry was used to visualize structural changes and quantify numbers of EI. LPS treatment in vivo caused a marked loss of EI in the BM. This suppressive effect of LPS on BM erythropoiesis was TLR4-dependent as it was absent in TLR4 KO mice. By qRT-PCR and flow cytometry, TLR4 is not expressed by BM erythroblasts but by myeloid cells including EI MΦ. Together with the fact that addition of 100ng/mL LPS into BFU-E assays does not inhibit BFU-E colony growth, these suggest that the suppressive effect of LPS on erythroblasts is indirectly mediated. It is known that LPS mobilizes HSC in a G-CSF-dependent manner. As MΦ express the G-CSF receptor (GCSFR), we explored LPS effects in GCSFR KO mice. Surprisingly, although HSC did not mobilize in response to LPS in GCSFR KO mice, BM erythropoiesis was still suppressed with reduced numbers of erythroblasts. Unexpectedly however, the BM from LPS-treated GCSFR KO mice was still red with high numbers of erythrocytes. The explanation of this paradox is that LPS treatment causes BM vascular leakage in GCSFR KO mice with blood plasma volume in the BM 2.9-fold higher compared to LPS-treated WT mice (measured by i.v. injection of Evans Blue). In conclusion our data show that LPS suppresses medullar erythropoiesis indirectly in a G-CSF-independent manner but GCSFR-mediated signaling is necessary to maintain the integrity of the BM vasculature in response to LPS.