Distinct roles for MyD88, TLR2, TLR5, and TLR9 in phagocytosis of...

Summary: The contribution of toll-like receptors (TLRs) to phagocytosis of Borrelia burgdorferi has not been extensively studied. We show that bone marrow derived macrophages (BMDM) from MyD88-/- mice or Raw cells transfected with a dominant negative MyD88 were unable to efficiently internalize B. burgdorferi. Knockouts of TLR2 and TLR9 or knockdown of TLR5 by siRNA produced no defects in phagocytosis of B. burgdorferi. Production of inflammatory cytokines was greatly diminished in MyD88-/- BMDM, but only partially affected in TLR2-/- BMDM or knockdown of TLR5 and unaffected in TLR9-/- BMDM. Cytochalasin D (cytoD) reduced cytokine induction, but not to the level of the MyD88-/-. Addition of cytoD to TLR2-/- BMDM inhibited inflammatory responses to B. burgdorferi to the level of MyD88-/- BMDM-- consistent with a role for TLR2 in both recognition of extracellular products and lysosomal sampling by TLR2 after processing of the organism. CytoD had no impact on cytokine productions in cells undergoing TLR5 knockdown. These results suggest that MyD88, but not TLR2, TLR5 and TLR9, is important for the uptake of B. burgdorferi and MyD88 affects inflammatory responses both through its effects on phagocytosis and its role in transducing signals from TLR2 and TLR5.