DAP12 Promotes IRAK-M Expression and IL-10 Production by Liver Myeloid Dendritic Cells and Restrains Their T Cell Allostimulatory Ability

Abstract

Freshly-isolated hepatic dendritic cells (DC) are comparatively immature, relatively resistant to maturation, and can down-modulate effector T cell responses. Molecular mechanisms that underlie these properties are ill-defined. DNAX-activating protein of 12 kDa (DAP12) is an ITAM-bearing transmembrane adaptor protein, that integrates signals through several receptors, including triggering receptor expressed on myeloid cells (TREM)-1, -2; and CD200R. Notably, DC propagated from DAP12-deficient mice exhibit enhanced maturation in response to TLR ligation. Given the constitutive exposure of liver DC to endotoxin draining from the gut, we hypothesized that DAP12 might regulate liver DC maturation. We show that while DAP12 is expressed by both freshly-isolated liver and spleen myeloid DC, LPS-stimulated liver DC maintain DAP12 mRNA expression at higher levels than splenic DC. Moreover, inhibition of DAP12 expression by liver DC using small interfering (si)RNA, promotes their phenotypic and functional maturation, resulting in enhanced TNFα, IL-6 and IL-12p70 production, reduced secretion of IL-10 and enhanced CD4+ and CD8+ T cell proliferation. Furthermore, DAP12 silencing correlates with decreased STAT3 phosphorylation in mature liver DC, and with diminished expression of the IL-1R-associated kinase (IRAK)-M, a negative regulator of TLR signaling. These findings highlight, for the first time, a regulatory role for DAP12 in hepatic DC maturation, and suggest a mechanism whereby this function may be induced/maintained.

O-Acetylation of Peptidoglycan Limits Helper T Cell Priming and Permits Staphylococcus aureus Reinfection

“…S. aureus also selectively induced genes associated with DC activation ( TNFRSF1B ), negative regulators of DC function ( IRAK3 and TNIP1 ) (Sumpter et al, 2011)(Callahan et al, 2013), in addition to KCTD1 , a negative regulator of the canonical Wnt signaling pathway (Li et al, 2014) among others. On the other hand, genes selectively induced by GAS included several inflammatory cytokines, such as Il6 , IL12p40 ( IL12B ), and IL23p35 ( IL23A ) (Figure 2D).…”

Stellate Cells in Hepatic Immunological Tolerance

“…A role for LPS-induced increased expression of DNAX-activating protein of 12 kDa (DAP12), which is associated with IDO induction, is pertinent in this regard as DAP12-deficient DCs exhibit enhanced maturation upon TLR ligation [105,106]. This mechanism may be an important part of the phenomenon known as "endotoxin tolerance" [107].…”

DAP12 Deficiency in Liver Allografts Results in Enhanced Donor DC Migration, Augmented Effector T Cell Responses and Abrogation of Transplant Tolerance

“…Molecular mechanisms whereby hepatic APCs regulate/inhibit T cell responses include the expression of B7 homologue 1 (B7-H1) (14, 34, 36, 37), IL-10 (38, 39), FasL (40, 41), the Notch ligand Jagged 1 (18), CD39 (31) and DNAX-activating protein of 12kDa (DAP12) (42). DAP12 is a homodimeric immunoreceptor tyrosine-based activation motif (ITAM)-bearing transmembrane adaptor protein that is highly expressed in lymphoid tissues and the lung and to a much lesser degree in whole liver tissue (43, 44).…”

“…Recently, using small interfering RNA (siRNA) to silence DAP12, we found (42) that DAP12 promoted the expression of IL-1 receptor (R)-associated kinase (IRAK)-M, a negative regulator of Toll-like receptor (TLR) signaling and the production of IL-10 by liver mDC. Consequently, DAP12 restrained their T cell allostimulatory activity.…”