Key Words tolerance, antigen processing, DEC-205s Abstract Dendritic cells (DCs) have several functions in innate and adaptive immunity. In addition, there is increasing evidence that DCs in situ induce antigenspecific unresponsiveness or tolerance in central lymphoid organs and in the periphery. In the thymus DCs generate tolerance by deleting self-reactive T cells. In peripheral lymphoid organs DCs also induce tolerance to antigens captured by receptors that mediate efficient uptake of proteins and dying cells. Uptake by these receptors leads to the constitutive presentation of antigens on major histocompatibility complex (MHC) class I and II products. In the steady state the targeting of DC antigen capture receptors with low doses of antigens leads to deletion of the corresponding T cells and unresponsiveness to antigenic rechallenge with strong adjuvants. In contrast, if a stimulus for DC maturation is coadministered with the antigen, the mice develop immunity, including interferon-γ -secreting effector T cells and memory T cells. There is also new evidence that DCs can contribute to the expansion and differentiation of T cells that regulate or suppress other immune T cells. One possibility is that distinct developmental stages and subsets of DCs and T cells can account for the different pathways to peripheral tolerance, such as deletion or suppression. We suggest that several clinical situations, including autoimmunity and certain infectious diseases, can be influenced by the antigen-specific tolerogenic role of DCs.

Dendritic cells (DCs) have the capacity to initiate immune responses, but it has been postulated that they may also be involved in inducing peripheral tolerance. To examine the function of DCs in the steady state we devised an antigen delivery system targeting these specialized antigen presenting cells in vivo using a monoclonal antibody to a DC-restricted endocytic receptor, DEC-205. Our experiments show that this route of antigen delivery to DCs is several orders of magnitude more efficient than free peptide in complete Freund's adjuvant (CFA) in inducing T cell activation and cell division. However, T cells activated by antigen delivered to DCs are not polarized to produce T helper type 1 cytokine interferon γ and the activation response is not sustained. Within 7 d the number of antigen-specific T cells is severely reduced, and the residual T cells become unresponsive to systemic challenge with antigen in CFA. Coinjection of the DC-targeted antigen and anti-CD40 agonistic antibody changes the outcome from tolerance to prolonged T cell activation and immunity. We conclude that in the absence of additional stimuli DCs induce transient antigen-specific T cell activation followed by T cell deletion and unresponsiveness.

Evidence suggests that regulatory T cells expressing the transcription factor Foxp3 develop extrathymically and intrathymically. Mechanisms of extrathymic induction require further scrutiny, especially as proliferation and/or phenotypic changes of preexisting suppressor cells must be distinguished from true de novo generation. Here we report the conversion of truly naive CD4(+) T cells into suppressor cells expressing Foxp3 by targeting of peptide-agonist ligands to dendritic cells and by analysis of Foxp3 expression at the level of single cells. We show that conversion was achieved by minute antigen doses with suboptimal dendritic cell activation. The addition of transforming growth factor-beta or the absence of interleukin 2 production, which reduces proliferation, enhanced the conversion rate. In addition, regulatory T cell populations induced in subimmunogenic conditions could subsequently be expanded by delivery of antigen in immunogenic conditions. The extrathymic generation and proliferation of regulatory T cells may contribute to self-tolerance as well as the poor immunogenicity of tumors and may be exploited clinically to prevent or reverse unwanted immunity.

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.