Activation of toll‐like receptors 2 and 4 on CD 34 + cells increases human megakaryo/thrombopoiesis induced by thrombopoietin

Abstract

Background: Platelet Toll‐like receptor (TLR)2/4 are key players in amplifying the host immune response; however, their role in human megakaryo/thrombopoiesis has not yet been defined. Objectives: We evaluated whether Pam3CSK4 or lipopolysaccharide (LPS), TLR2/4 ligands respectively, modulate human megakaryocyte development and platelet production. Methods: CD34+ cells from human umbilical cord were stimulated with LPS or Pam3CSK4 with or without thrombopoietin (TPO). Results: CD34+ cells and megakaryocytes express TLR2 and TLR4 at both RNA and protein level; however, direct stimulation of CD34+ cells with LPS or Pam3CSK4 had no effect on cell growth. Interestingly, both TLR ligands markedly increased TPO‐in‐ duced CD34+ cell proliferation, megakaryocyte number and maturity, proplatelet and platelet production when added at day 0. In contrast, this synergism was not observed when TLR agonists were added 7 days after TPO addition. Interleukin‐6 (IL‐6) release was observed upon CD34+ or megakaryocyte stimulation with LPS or Pam3CSK4 but not with TPO and this effect was potentiated in combination with TPO. The increased proliferation and IL‐6 production induced by TPO + LPS or Pam3CSK4 were sup‐ pressed by TLR2/4 or IL‐6 neutralizing antibodies, as well as by PI3K/AKT and nuclear factor‐κB inhibitors. Additionally, increased proplatelet and platelet production were associated with enhanced nuclear translocation of nuclear factor‐E2. Finally, the su‐ pernatants of CD34+ cells stimulated with TPO+LPS‐induced CFU‐M colonies. Conclusions: Our data suggest that the activation of TLR2 and TLR4 in CD34+ cells and megakaryocytes in the presence of TPO may contribute to warrant platelet provi‐ sion during infection episodes by an autocrine IL‐6 loop triggered by PI3K/NF‐κB axes.