It has long been appreciated that a drop in eosinophils is associated with acute bacterial infection while, in contrast, blood levels of monocytes and neutrophils rise. The initial eosinopenic response is believed to be secondary to margination of circulating eosinophils to blood vessel walls, but the mechanism for prolonged eosinophil depletion with bacterial infection remains undefined. In a culture system developed to differentiate mature eosinophils from low density bone marrow progenitors, IL-5 stimulation of progenitors resulted in induced expression of six TLRs with highest expression of TLR2 and TLR4 noted throughout eosinophil development. Surface expression of TLR4 was confirmed on CD34+ progenitors by FACS. Stimulation of progenitors with LPS markedly inhibited IL-5-mediated eosinophil production. Further, LPS administration in vivo specifically reduced numbers of eosinophil progenitors in the bone marrow. Treatment of developing eosinophils with LPS induced the generation of pro-inflammatory mediators, including IL-6, IL-10, IFN-γ and TNF-α, but mechanistic studies demonstrated that suppression of eosinophil production by LPS was independent of these mediators. Taken together, these findings suggest a direct effect of LPS on eosinophil progenitors as an explanation for eosinopenia following bacterial infections and a potentially novel therapeutic strategy for depleting eosinophil progenitors and inhibiting peripheral eosinophilia in eosinophil-associated diseases.
- Copyright © 2011 by The American Association of Immunologists, Inc.