DCs are specialized antigen presenting cells found in lymphoid and epithelial tissues, which are uniquely potent inducers of primary immune responses in vitro and in vivo. In tissue culture experiments, DCs are typically two orders of magnitude more effective as antigen presenting cells than B cells or macrophages. In addition, during contact allergy and transplantation (two of the most powerful known stimuli of T cell immunity in vivo) DCs migrate from peripheral tissues to lymphoid organs, the sites for the induction of most types of primary immune responses. Based on these and similar experiments, it has been proposed that the principal function of DCs is to initiate T cell mediated immunity, but there is also indirect evidence from a number of different laboratories suggesting that DCs may play a role in maintaining peripheral tolerance. However, nearly all of these experiments involved DC purification and culture in vitro, both of which induce major alterations in the state of DC maturation and function. Thus, the physiologic role of DCs in the steady state in vivo has yet to be determined. To examine the function of DCs in vivo, we developed a method for antigen delivery to these cells using a monoclonal antibody to a DC restricted endocytic receptor, DEC-205. Our experiments show that antigen delivered to DCs is several orders of magnitude more efficient in inducing T cell activation than peptide in complete Freund's adjuvant. However, antigen delivered to DCs under physiological conditions fails to produce prolonged T cell activation, and, within 7-9 days, the number of antigen-specific T cells is reduced and residual T cells become anergic to further stimulation even by antigens delivered with strong adjuvants. The long-range goal of the proposed research is to understand the physiologic role of DCs in immunity and tolerance in vivo. The working hypothesis is that DCs maintain tolerance under steady state conditions but that they initiate immune responses when appropriately activated. The first part of the application will make use of the DEC-205-mediated antigen delivery system to define the role of DCs in tolerance and immunity to a defined peptide. In the second part of the application, DEC-205-mediated antigen delivery will be used to examine the role of DC-mediated T cell tolerance and T cell activation in experimental allergic encephalomyelitis using a defined antigenic peptide from myelin basic protein. The third part of the Project aims to understand how DC activation regulates immunity in vivo. These studies have significant potential implication for understanding the role of the innate immune system in initiation of autoimmune diseases.