To identify endocytic receptors that allow dendritic cells (DCs) to capture and present antigens on major histocompatibility complex (MHC) class I products in vivo, we evaluated DEC-205, which is abundant on DCs in lymphoid tissues. Ovalbumin (OVA) protein, when chemically coupled to monoclonal αDEC-205 antibody, was presented by CD11c+ lymph node DCs, but not by CD11c− cells, to OVA-specific, CD4+ and CD8+ T cells. Receptor-mediated presentation was at least 400 times more efficient than unconjugated OVA and, for MHC class I, the DCs had to express transporter of antigenic peptides (TAP) transporters. When αDEC-205:OVA was injected subcutaneously, OVA protein was identified over a 4–48 h period in DCs, primarily in the lymph nodes draining the injection site. In vivo, the OVA protein was selectively presented by DCs to TCR transgenic CD8+ cells, again at least 400 times more effectively than soluble OVA and in a TAP-dependent fashion. Targeting of αDEC-205:OVA to DCs in the steady state initially induced 4–7 cycles of T cell division, but the T cells were then deleted and the mice became specifically unresponsive to rechallenge with OVA in complete Freund's adjuvant. In contrast, simultaneous delivery of a DC maturation stimulus via CD40, together with αDEC-205:OVA, induced strong immunity. The CD8+ T cells responding in the presence of agonistic αCD40 antibody produced large amounts of interleukin 2 and interferon γ, acquired cytolytic function in vivo, emigrated in large numbers to the lung, and responded vigorously to OVA rechallenge. Therefore, DEC-205 provides an efficient receptor-based mechanism for DCs to process proteins for MHC class I presentation in vivo, leading to tolerance in the steady state and immunity after DC maturation.

The prevention and treatment of prevalent infectious diseases and tumors should benefit from improvements in the induction of antigen-specific T cell immunity. To assess the potential of antigen targeting to dendritic cells to improve immunity, we incorporated ovalbumin protein into a monoclonal antibody to the DEC-205 receptor, an endocytic receptor that is abundant on these cells in lymphoid tissues. Simultaneously, we injected agonistic α-CD40 antibody to mature the dendritic cells. We found that a single low dose of antibody-conjugated ovalbumin initiated immunity from the naive CD4+ and CD8+ T cell repertoire. Unexpectedly, the αDEC-205 antigen conjugates, given s.c., targeted to dendritic cells systemically and for long periods, and ovalbumin peptide was presented on MHC class I for 2 weeks. This was associated with stronger CD8+ T cell–mediated immunity relative to other forms of antigen delivery, even when the latter was given at a thousand times higher doses. In parallel, the mice showed enhanced resistance to an established rapidly growing tumor and to viral infection at a mucosal site. By better harnessing the immunizing functions of maturing dendritic cells, antibody-mediated antigen targeting via the DEC-205 receptor increases the efficiency of vaccination for T cell immunity, including systemic and mucosal resistance in disease models.

The avoidance of autoimmunity requires mechanisms to actively silence or tolerize self reactive T cells in the periphery. During infection, dendritic cells are not only capturing microbial antigens, but also are processing self antigens from dying cells as well as innocuous environmental proteins. Since the dendritic cells are maturing in response to microbial and other stimuli, peptides will be presented from both noxious and innocuous antigens. Therefore it would be valuable to have mechanisms whereby dendritic cells, prior to infection, establish tolerance to those self and environmental antigens that can be processed upon pathogen encounter. In the steady state, prior to acute infection and inflammation, dendritic cells are in an immature state and not fully differentiated to carry out their known roles as inducers of immunity. These immature cells are not inactive, however. They continuously circulate through tissues and into lymphoid organs, capturing self antigens as well as innocuous environmental proteins. Recent experiments have provided direct evidence that antigen-loaded immature dendritic in vivo silence T cells either by deleting them or by expanding regulatory T cells. In this way, it is proposed that the immune system overcomes at least some of the risk of developing autoimmunity and chronic inflammation. It is proposed that dendritic cells play a major role in defining immunologic self, not only centrally in the thymus but also in the periphery.

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