Regulation of monocyte/macrophage activation by Leukocyte Immunoglobulin-Like Receptor B4 (LILRB4)

Abstract

The leukocyte immunoglobulin-like receptor B4 (LILRB4) belongs to a family of cell surface receptors, primarily expressed on mono-myeloid cells. LILRB4 has been shown to inhibit FcγRI-mediated pro-inflammatory cytokine production by monocytes and induce tolerogenic dendritic cells in vitro. It is believed that LILRB4 regulates monocyte/macrophage activation through its three intracellular immunoreceptor tyrosine-based inhibitory motifs (ITIMs) by paring with activating receptor, bearing ITAM, and by dephosphorylation of non-receptor tyrosine kinases via recruitment of Src homology phosphatase-1 (SHP-1). However, the exact mechanism and the functions depending on its structure and stimuli are still unclear. In addition, regulatory functions of LILRB4 paring with non-ITAM associated activating receptors, including TLR4 is less researched. Thus, this thesis investigates, for the first time, the functions of LILRB4 in regulation of monocyte/macrophage activation depending on the position of the tyrosine residues of its ITIMs, and stimuli. Here it is shown for the first time that LILRB4 is a complex immuno-regulatory receptor that exerts dual inhibitory and activating functions in FcγRI and/or TLR4-mediated monocyte/macrophage activation including receptor-ligand internalisation, endocytosis, cytokine production, phagocytosis and bactericidal activity. Most importantly, each ITIM of LILRB4 has a different function in monocyte/macrophage activation depending on the position of its tyrosine residue. Thus, the tight regulation of these ITAM-containing and non-ITAM receptors by each LILRB4 ITIM may play a key role in fine-tuning immune responses. In addition, the results for the regulatory function of LILRB4 using various ITIM mutants in response to different stimuli provide better understanding of the signalling pathways of LILRB4 that may contribute to use LILRB4 as a potential therapeutic agent for an appropriate immune response using selective LILRB4 ITIM/s. Although no LILRB4 ligand is identified, new methodologies and the potential candidate ligands of LILRB4 which are provided here may contribute to identifying the nature of LILRB4 ligand/s in the near future.