Activation of toll-like receptors 2 and 4 on CD34 cells increases human megakaryo/thrombopoiesis induced by thrombopoietin.

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-induced 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 suppressed 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 supernatants 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 provision during infection episodes by an autocrine IL-6 loop triggered by PI3K/NF-κB axes.