Although dendritic cells (DCs) are known to be critical for inducing T cell immunity in immunized or infected individuals, it was recently proposed that DCs are also essential to silence potentially pathogenic self-reactive T cells that have escaped negative selection in the thymus (1, 2). One important prediction of such a model would be that DCs ingest, process, and present self-Ags in vivo under steady state conditions. Three papers published in this issue examine the role of DCs in maintaining tolerance to self-Ags.
Presentation of self-Ags by professional APCs was first demonstrated in the thymus for Ags expressed by cortical bone marrow–derived cells and for abundant soluble Ags that could gain access to this organ by trafficking through the blood (3). A number of studies have also shown that APCs can process and present self-Ags to T cells in the...
