Dendritic cells constitutively present self antigens in their immature state in vivo and regulate antigen presentation by controlling the rates of MHC class II synthesis and endocytosis.

Dendritic cells (DCs) change their antigen-presenting properties during maturation. Immature DCs efficiently capture antigens, but are reported to be impaired in their processing and presenting capacity. Upon an encounter with an inflammatory stimulus, DCs undergo a maturation process that leads to efficient presentation of antigens captured at the time of activation, but precludes processing of antigens encountered at later time points. The mechanisms that underlie these developmental changes are controversial. Thus, it is unclear whether immature DCs can present self antigens, and which are the checkpoints that regulate antigen presentation in immature and mature DCs. We have characterized these mechanisms using DCs derived directly from lymphoid organs. Immature lymphoid organ DCs constitutively presented self peptides bound to major histocompatibility complex class II (MHCII) molecules, but these MHCII-peptide complexes were degraded quickly after their transient expression on the cell surface. During maturation, MHC II endocytosis was down-regulated, so that newly generated MHC II-peptide complexes accumulated on the plasma membrane. Simultaneously, MHC II synthesis was down-regulated, thus preventing the turnover of the MHC II-peptide complexes that accumulated early during maturation. Our results demonstrate that immature DCs constitutively present self antigens in the lymphoid organs and characterize the molecular basis of the capacity of DCs to provide "antigenic memory" in vivo.