Lipopolysaccharide-Induced CD300b Receptor Binding to Toll-like Receptor 4 Alters Signaling to Drive Cytokine Responses that Enhance Septic Shock

Abstract

SUMMARY Receptor CD300b is implicated in regulating the immune response to bacterial infection by an unknown mechanism. Here, we identified CD300b as an lipopolysaccharide (LPS)-binding receptor and determined the mechanism underlying CD300b augmentation of septic shock. In vivo depletion and adoptive transfer studies identified CD300b-expressing macrophages as the key cell type augmenting sepsis. We showed that CD300b, and its adaptor DAP12, associated with Toll-like receptor 4 (TLR4) upon LPS binding, thereby enhancing TLR4-adaptor MyD88- and TRIF-dependent signaling that resulted in an elevated pro-inflammatory cytokine storm. LPS engagement of the CD300b-TLR4 complex led to the recruitment and activation of spleen tyrosine kinase (Syk) and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K). This resulted in an inhibition of the ERK1/2 protein kinase- and NFκB transcription factor-mediated signaling pathways, which subsequently led to a reduced interleukin 10 (IL-10) production. Collectively, our data describe a mechanism of TLR4 signaling regulated by CD300b in myeloid cells in response to LPS.

FcγRIIIa Signaling Modulates Endosomal TLR Responses in Human CD4+ T Cells

“…TLR4 endocytosis from the cell surface requires Syk-PLCγ2 signaling (52). Another report suggested a role for the C330b-DAP12-TLR4-Syk-PI3K signaling complex in amplifying inflammatory responses in macrophages (53). A role for Syk in the regulation of TLR signaling in pDCs is also observed (54).…”

Early innate immune responses to bacterial LPS

“…Subsequently, a role for CD300b in binding LPS and associating with TLR4 and CD14 was suggested (Fig. 1B) [36]. CD300b promoted activation of the TRIF pathway and IFNβ production, and suppressed MyD88-dependent activation of ERK1/2, collectively leading to reduced production of anti-inflammatory IL-10 and increased inflammatory cytokines.…”

“…Syk activation induces the phosphorylation of Akt, and the activation of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) (Fig. 1B) [36]. Interestingly, after LPS-stimulation, TIRAP-MyD88 interaction with TLR4 was decreased in a CD300b-dependent manner, suggesting a deficiency in myddosome formation.…”

“…1B). CD300b-dependent dissociation of TIRAP and MyD88 is also likely responsible for reduction in ERK signaling and may facilitate re-directing TLR4 signaling to the endosomal TRAM-TRIF pathway, ultimately leading to lethal sepsis [36]. However, it remains unclear if the activation of PI3K or alternative mechanisms such as increased endocytosis of TLR4 are responsible for the effects of CD300b on the TLR4 pathway.…”

Efferocytosis Signaling in the Regulation of Macrophage Inflammatory Responses

“…CD300F can both stimulate and inhibit efferocytosis depending on the type of phagocyte, and CD300F-deficient mice develop symptoms of autoimmunity (87). CD300B signaling via the adaptor protein DAP12 positively regulates efferocytosis, and Voss et al recently showed that LPS treatment of macrophages causes CD300B to interact with TLR4 and that this interaction augments TLR4-dependent production of pro-inflammatory cytokines (88). Along these lines, the CD300F receptor was recently shown to interact directly with the common chain of IL-4/IL-13 receptor to enhance production of IL-4 and thus could potentially promote M2 polarization in macrophages (89).…”