A NOVEL INHIBITORY RECEPTOR (ILT3) EXPRESSED ON MONOCYTES, MACROPHAGES, AND DENDRITIC CELLS INVOLVED IN ANTIGEN-PROCESSING

Immunoglobulin-like transcript (ILT) 3 is a novel cell surface molecul e of the immunoglobulin superfamily, which is selectively expressed by myeloid antigen presenting cells (APCs) such as monocytes, macrophage s, and dendritic cells. The cytoplasmic region of ILT3 contains putati ve immunoreceptor tyrosine-based inhibitory motifs that suggest an inh ibitory function of ILT3. Indeed, co-ligation of ILT3 to stimulatory r eceptors expressed by AI)Cs results in a dramatic blunting of the incr eased [Ca2+](i) and tyrosine phosphorylation triggered by these recept ors Signal extinction involves SH2-containing protein tyrosine phospha tase 1, which is recruited by ILT3 upon cross-linking. ILT3 can also f unction in antigen capture and presentation. It is efficiently interna lized upon cross-linking, and delivers its ligand to an intracellular compartment where it is processed and presented to T cells. Thus, ILT3 is a novel inhibitory receptor that can negatively regulate activatio n of APCs and can be used by APCs for antigen uptake.