The Type II Secretion System of Legionella pneumophila Dampens the MyD88 and TLR2 Signaling Pathway in Infected Human Macrophages

ABSTRACT

Previously, we reported that mutants of L. pneumophila lacking a type II secretion (T2S) system elicit higher levels of cytokines (e.g., IL-6) following infection of U937 cells, a human macrophage-like cell line. We now show that this effect of T2S is also manifest upon infection of human THP-1 cell macrophages and peripheral blood monocytes but does not occur during infection of murine macrophages. Supporting the hypothesis that T2S acts to dampen the triggering of an innate immune response, we observed that the mitogen-activated protein kinase (MAPK) and nuclear transcription factor kappa-B (NF-κB) pathways are more highly stimulated upon infection with the T2S mutant compared to wild-type. By using short hairpin RNA to deplete proteins involved in specific pathogen-associated molecular pattern (PAMP)-recognition pathways, we determined that the dampening effect of the T2S system was not dependent on nucleotide binding oligomerization domain (NOD)-like receptors (NLR), retinoic-acid-inducible protein I (RIG-I)-like receptors (RLR), dsRNA dependent protein kinase receptor (PKR), or TIR-domain-containing adapter-inducing interferon-β (TRIF) signaling or an apoptosis-associated speck-like protein containing a CARD (ASC) or caspase-4 dependent inflammasome. However, the dampening effect of T2S on IL-6 production was significantly reduced upon gene knockdown of myeloid differentiation primary response 88 (MyD88), TANK binding kinase 1 (TBK1) or Toll-like receptor 2 (TLR2). These data indicate that the L. pneumophila T2S system dampens signaling of the TLR2 pathway in infected human macrophages. We also document the importance of PKR, TRIF, and TBK1 in cytokine secretion during L. pneumophila infection of macrophages.