Phagosome maturation proceeds independently of stimulation of toll-like receptors 2 and 4

Toll-like receptors modulate many aspects of the innate immune response. Recent reports suggest that the maturation of phagosomes following particle uptake is modulated through signaling of Toll-like receptors. In the current study, the kinetics of phagosome maturation was evaluated quantitatively by ratio fluorometry to determine the lumenal pH of the phagosomes and a FRET-based technique to determine the degree of phagosome/lysosome fusion. Profiles generated for phagosomes containing experimental particles with or without the TLR ligands Pam(3)Cys-Ser-(Lys)(4) or LPS failed to reveal a difference in maturation despite activating TLR-signaling pathways. Moreover, while macrophages defective in individual TLRs generated phagosome maturation profiles identical to wild-type macrophages, MyD88-deficient macrophages exhibited a marked depression in phagosome/lysosome fusion that appears independent of short-term TLR-mediated effects. The results demonstrate that the rate of maturation of phagosomes proceeds independently of TLR signaling pathways.