The Type II Secretion System of Legionella pneumophila Dampens the MyD88 and Toll-Like Receptor 2 Signaling Pathway in Infected Human Macrophages

Abstract

Previously, we reported that mutants of Legionella pneumophila lacking a type II secretion (T2S) system elicit higher levels of cytokines (e.g., interleukin-6 [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 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-B) pathways are more highly stimulated upon infection with the T2S mutant than upon infection with the 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 (NLRs), retinoic acid-inducible protein I (RIG-I)-like receptors (RLRs), double-stranded RNA (dsRNA)-dependent protein kinase receptor (PKR), or TIR domain-containing adaptor inducing interferon beta (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 the 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.KEYWORDS Legionella pneumophila, TLR2, cytokine, innate immunity, macrophage, type II secretion L egionella pneumophila, a Gram-negative bacterium that is widespread in aquatic habitats, is the principal agent of Legionnaires' disease pneumonia (1-6). In the lungs, Legionella bacteria invade and grow in resident macrophages and then trigger severe inflammation (2). In macrophages, L. pneumophila evades the degradative lysosomal pathway and replicates to large numbers within a membrane-bound vacuole, the Legionella-containing vacuole (LCV) (7,8). Two protein secretion systems, Lsp type II secretion (T2S) and Dot/Icm type IV secretion (T4S), play major roles in the pathogenesis of L. pneumophila (9, 10). In T2S, protein substrates are first translocated across the inner membrane, and upon the action of the T2S pilus-like apparatus, they then exit the bacterial cell through a specific outer membrane pore (11). Using proteomics and enzymatic assays, we have shown that the T2S system of L. pneumo-

High expression of the HMGB1–TLR4 axis and its downstream signaling factors in patients with Parkinson's disease and the relationship of pathological staging

“…The injection of HMGB1 into the mouse parietal lobe resulted in the partial elevation of iNOS and IL-1β mRNA expressions 24 hr later. In addition, a study of Alzheimer's disease showed that HMGB1 stably binds to amyloid-beta (Aβ) peptides to suppress Aβ phagocytosis by microglia (Mallama, Mccoysimandle, & Cianciotto, 2017). Therefore, HMGB1 plays a major role in promoting neuritis.…”

Evasion of phagotrophic predation by protist hosts and innate immunity of metazoan hosts byLegionella pneumophila

“…Although it is more likely that the large arsenal of L. pneumophila effectors evolved through the long-term adaptation to various protist hosts (Best & Abu Kwaik, 2018), it is unlikely that these lower eukaryotes could have provided all the training necessary for successful evolution of L. pneumophila to evade the more evolved innate defence processes and proliferate in metazoan macrophages. Many findings point towards a putative role for metazoans in the evolution of L. pneumophila and its ability to replicate in macrophages effector modulation and control of NF-κB (Abu-Zant et al, 2007;Bartfeld et al, 2009;Ge et al, 2009;Losick & Isberg, 2006), including the ability to dampen TLR2 signalling (Mallama et al, 2017), and the up-regulation of antiapoptotic genes regulated by NF-κB (Abu-Zant et al, 2007;Banga et al, 2007).…”

“…In human macrophages but not murine macrophages, the type-II secretion system (T2SS) dampens signaling through the MyD88-TLR2 pathway (Fig. 1) (Mallama, McCoy-Simandle, & Cianciotto, 2017). What environmental factors selected for or conserved the ability for L. pneumophila to dampen TLR2 signaling is unknown.…”

“…instead, TLR2 is activated in response to the atypical lipopolysaccharide of the pathogen with long branched fatty acids in the lipid A moiety (Akamine et al, 2005;Girard et al, 2003). In human macrophages but not murine macrophages, the type II secretion system dampens signalling through the MyD88-TLR2 pathway (Figure 1; Mallama et al, 2017). What environmental factors selected for or conserved the ability for L. pneumophila to dampen TLR2 signalling are unknown.…”

Structure and functional analysis of theLegionellachitinase ChiA reveals a novel mechanism of metal-dependent mucin degradation

“…The T2SS is important for both intracellular and extracellular lifestyles (9). These processes include extracellular growth at low temperatures, biofilm formation, intracellular replication in amoebae and macrophages, dampening of cytokine output from infected cells and persistence in lungs (9)(10)(11)(12)(13)(14)(15).…”