182: The leukocyte immunoglobulin-like receptor B4 differentially regulates FcγRI-dependent TNF production via its three intracellular immunoreceptor tyrosine-based inhibitory signalling motifs (ITIMs)

Abstract

The leukocyte immunoglobulin-like receptor (LILR) B4 belongs to a family of activating and inhibitory cell surface receptors that have been increasingly recognised as key regulators of the threshold and amplitude of leukocyte activation. LILRB4 which is highly expressed on monocytes and macrophages transduces inhibitory signals through its three intracellular immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Clinically, increased expression is associated with better clinical outcome in renal transplant recipients contrasted with impaired immune effector cell functions in HIV infection. However, mechanisms of how LILRB4 regulates monocyte activation and the exact contribution of the three ITIMs for its immune-regulatory functions are poorly characterised. In this study we show that co-ligation of LILRB4 with FcγRI on THP-1 cells and primary monocytes markedly inhibited the production of TNF, a key pro-inflammatory cytokine. Cross-linking of FcγRI on THP-1 cells increased phosphorylation key protein tyrosine kinase signaling molecules and an adaptor protein that targets protein tyrosine kinases for degradation (c-Cbl). Co-ligation of LILRB4 and FcγRI considerably reduced FcγRI-mediated phosphorylation of these signaling molecules and terminated cellular activation by recruiting Src homology region 2 domain-containing phosphatase-1 (SHP-1)-like molecule. Importantly, using site-targeted mutagenesis we found for the first time that the middle (Y412) and distal (Y442) tyrosine residues in its ITIM motifs were critical for the inhibitory functions of LILRB4 and Y412 alone was sufficient for maximal inhibition. By contrast, the proximal tyrosine residue at Y360 showed unexpectedly enhanced Fc γ RI-mediated TNF production. Collectively, our results suggest that LILRB4 is a potent immune regulatory receptor in which combined contribution of its multiple ITIMs may fine tune innate immune responses.