Toll‐like receptor‐mediated eosinophil–basophil differentiation: autocrine signalling by granulocyte–macrophage colony‐stimulating factor in cord blood haematopoietic progenitors

Eosinophils are multi‐functional leucocytes that play a role in inflammatory processes including allergy and infection. Although bone marrow (BM) inflammatory cells are the main source of eosinophil‐basophil (Eo/B) differentiation‐inducing cytokines, a recent role has been demonstrated for cytokine induction through Toll‐like receptor (TLR)‐mediated signalling in BM progenitors. Having previously demonstrated that cord blood (CB) progenitors induce Eo/B colony‐forming units (CFU) after lipopolysaccharide (LPS) stimulation, we sought to investigate the intracellular mechanisms by which LPS induces Eo/B differentiation. Freshly isolated CD34‐enriched human CB cells were stimulated with LPS (and/or pharmacological inhibitors) and assessed for alterations in haematopoietic cytokine receptor expression and signalling pathways by flow cytometry, Eo/B CFU in methylcellulose cultures, and cytokine secretion using Luminex assays. The LPS stimulation resulted in a significant increase in granulocyte–macrophage colony‐stimulating factor (GM‐CSF)‐responsive, as opposed to interleukin‐5‐responsive, Eo/B CFU, which also correlated with significant increases in CD34⁺ cell GM‐CSFRα expression. Functionally, CB CD34⁺ cells secrete abundant amounts of GM‐CSF following LPS stimulation, via a p38 mitogen‐activated protein kinase (MAPK)‐dependent mechanism; this secretion was responsible for Eo/B CFU formation ex vivo, as shown by antibody blockade. We show for the first time that LPS stimulation of CB progenitor cells results in autocrine activation of p38 MAPK‐dependent GM‐CSF secretion facilitating Eo/B differentiation ex vivo. This work provides evidence that early life exposure to products of bacterial agents can modulate Eo/B differentiation, representing a novel mechanism by which progenitor cells can respond to microbial stimuli and so affect immune and inflammatory responses.