NKT cells are unique T cells that share some properties with NK cells and recognize lipid antigens presented by CD1d. CD1d and NKT cells are evolutionary conserved between humans and mice, and the murine model has provided considerable insight into the biology of CD1d and CD1d- restricted T cells. Depending on the model, NKT cells either contribute to, or suppress, tissue inflammation associated with autoimmune disease. CD1d-restricted NKT cells also make a critical contribution to host immunity against many diverse pathogens, including parasites, fungi, viruses and bacteria. What antigens NKT cells recognize is unknown. Increasingly, it appears that CD1d presents self-lipid antigens. If endogenous ligands activate NKT cells, then antigen presentation by CD1d must be regulated to prevent inappropriate activation of NKT cells. The focus of this grant is to determine how CD1d expression on APC is regulated and whether its modulation influences CDId-restricted immune responses. Although little is known about CD1d regulation, we have observed that cytokines and microbial products synergize to induce CD1d expression. Just as upregulation of class I and II MHC molecules promote MHC-restricted immune responses, upregulation of CD1d may promote NKT cell activation. We hypothesize that following infection, signaling through Toll-like Receptors (TLR) and the IFNgamma-receptor leads to increased cell surface expression of CD1d on APC. This will in turn promote activation of CD1d-restricted NKT cells, which when stimulated, have antimicrobial and immunoregulatory properties. This grant will determine whether cell surface CD1d levels modulate the activation of primary NKT cells. We will go on to determine whether tissue inflammation induces CD1d in vivo, and whether CD1d levels affect the activation of NKT cells in vivo. Finally we will determine how cytokines and microbial products generate an intracellular signal to cause CD1d upregulation, and how the CD1d expression is regulated at the molecular level. Understanding how CD1 is regulated and how NKT cells become activated following infection will clarify the role of NKT cells in immunity to human pathogens, as well as provide insight into their physiological role in other immune responses.