Macrophage Pro-Inflammatory Response to Francisella novicida Infection Is Regulated by SHIP

Abstract

Francisella tularensis, a Gram-negative facultative intracellular pathogen infecting principally macrophages and monocytes, is the etiological agent of tularemia. Macrophage responses to F. tularensis infection include the production of pro-inflammatory cytokines such as interleukin (IL)-12, which is critical for immunity against infection. Molecular mechanisms regulating production of these inflammatory mediators are poorly understood. Herein we report that the SH2 domain-containing inositol phosphatase (SHIP) is phosphorylated upon infection of primary murine macrophages with the genetically related F. novicida, and negatively regulates F. novicida–induced cytokine production. Analyses of the molecular details revealed that in addition to activating the MAP kinases, F. novicida infection also activated the phosphatidylinositol 3-kinase (PI3K)/Akt pathway in these cells. Interestingly, SHIP-deficient macrophages displayed enhanced Akt activation upon F. novicida infection, suggesting elevated PI3K-dependent activation pathways in absence of SHIP. Inhibition of PI3K/Akt resulted in suppression of F. novicida–induced cytokine production through the inhibition of NFκB. Consistently, macrophages lacking SHIP displayed enhanced NFκB-driven gene transcription, whereas overexpression of SHIP led to decreased NFκB activation. Thus, we propose that SHIP negatively regulates F. novicida–induced inflammatory cytokine response by antagonizing the PI3K/Akt pathway and suppressing NFκB-mediated gene transcription. A detailed analysis of phosphoinositide signaling may provide valuable clues for better understanding the pathogenesis of tularemia.

Francisella Targets Cholesterol-Rich Host Cell Membrane Domains for Entry into Macrophages

“…In some instances of bacterial uptake, PI3K activity is not required for entry, but the kinase is activated and leads to signaling events that are essential for successful intracellular survival of the invading bacterium or proper closure of the phagosome (27,46,47). Infection of macrophages by F. novicida leads to an activation of PI3K (48). We show herein that PI3K does not appear to play a role for entry of Francisella LVS into macrophages.…”

Phosphatidylinositol 3-Kinase Activation Attenuates the TLR2-Mediated Macrophage Proinflammatory Cytokine Response toFrancisella tularensisLive Vaccine Strain

“…novicida (44,82). Those studies found that pharmacological inhibition of the PI3K/Akt pathway during infection with F. tularensis subsp.…”

Subversion of Host Recognition and Defense Systems by Francisella spp

“…In accordance, activation of the PI3K/Akt pathway during F. novicida infection blocks phagosomal escape and cytosolic replication (181). Interestingly, F. novicida infection of macrophages activates the SH2 domain-containing inositol phosphatase (SHIP) by an unknown mechanism that is dependent on live bacteria (167). SHIP antagonizes activation of the PI3K/Akt pathway, leading to rapid escape from the phagosome and robust cytosolic replication (167).…”

“…Interestingly, F. novicida infection of macrophages activates the SH2 domain-containing inositol phosphatase (SHIP) by an unknown mechanism that is dependent on live bacteria (167). SHIP antagonizes activation of the PI3K/Akt pathway, leading to rapid escape from the phagosome and robust cytosolic replication (167). Additionally, the PI3K/Akt pathway can be activated by cell surface receptors, including Fc␥R (143) and TLRs (121).…”