Plasmacytoid dendritic cells (pDCs) represent a specialized immune cell population that produces large amounts of type I interferons (IFN) in response to viruses and function as a critical linkage between innate and adaptive immunity. Our long-term goal is to study the functions of pDCs pertaining to specific physiological environments or conditions. The central hypothesis is that unique pDC receptors play key roles in the biological functions of pDCs through interactions with their ligands and activating specific intracellular pathways. We based that hypothesis on the observation that 1) human pDC receptor ILT7 activates an ITAM-mediated signaling pathway to inhibit IFN responses by pDCs, 2) the potential ligand of ILT7 is expressed by a group of human breast cancer cell lines, and 3) human pDC receptor BDCA2, a C-type lectin with unknown ligand, potentially utilizes the ITAM-mediated mechanism to regulate pDCs'IFN responses. The specific aims are to: 1) Determine the detailed signaling mechanism of ILT7 in human pDCs and study the underlining mechanism how ITAM-mediated pathway intersects with the Toll-like receptor (TLR)-mediated innate immune responses. 2) Identify the natural ligand of ILT7, which is expressed by human breast cancer cell lines, and study its tissue expression in relation with pDC infiltration. The function of the ligand on pDCs through interaction with ILT7 will be thoroughly elucidated. 3) Determine the detailed signaling mechanism used by BDCA2. The natural ligand for BDCA2 is to be screened using a reporter cell system capable of sensing surface BDCA2 engagement. A wide range of carbohydrate structures, microbial agents and cell-associated factors will be tested in this aim. The altered presence and functions of pDCs have been implicated in human ailments such as autoimmune diseases, infectious diseases and cancer. By focusing on the two surface receptors uniquely expressed by human pDCs through pursuing their ligands and shared intracellular signaling mechanism, the proposed studies will greatly advance research on the physiological functions of pDCs and may generate direct molecular targets with therapeutic potential.