LPS-induced interaction between Cd300b/DAP12 and TLR4/CD14 results in the amplification of inflammation and increased mortality from sepsis.

Abstract

Abstract LPS, present in gram-negative bacteria membranes, causes a strong immune cell activation following detection by TLR4, which initiates a pro-inflammatory cytokine storm that leads to a more severe immunopathology, like septic shock and, subsequently, death. Recently, the myeloid-specific Cd300b receptor was implicated in regulating the immune response to bacterial infection by an unknown mechanism. Here, we identified LPS as a ligand for Cd300b and found that wild-type (WT) mice, unlike Cd300b−/− mice, were highly susceptible to septic shock. WT but not Cd300b−/− mice show increased serum levels of pro-inflammatory cytokines (e.g. TNFa) and a reduced level of the anti-inflammatory cytokine, IL-10. Neutralization of IL-10 in Cd300b−/− mice diminishes their survival advantage over WT mice. In vivo depletion and adoptive transfer studies identify Cd300b-expressing macrophages as the key cell type augmenting septic shock, suggesting that Cd300b amplifies the TLR4-LPS induced immune response thereby causing lethal inflammation. Indeed, Cd300b and its adaptor, DAP12, associated with TLR4/CD14 upon LPS binding, promoting MyD88/TIRAP dissociation and the recruitment and activation of Syk and PI3K. This results in the activation of AKT, which subsequently leads to a reduced IL-10 production, via a PI3K/AKT-dependent inhibition of the MEK1/2-ERK1/2 signaling pathway. In sum, these findings describe a previously unidentified LPS-induced signaling complex consisting of Cd300b/DAP12/TLR4/CD14/Syk/PI3K that effectively amplifies TLR4/CD14-mediated inflammation. Furthermore, our data change the paradigm of how LPS mediates TLR4 signaling in myeloid cells and identify potential targets for future clinical intervention.