Antigen presentation by dendritic cells

The stimulation of T lymphocytes requires the presentation of proteolytic a ntigen-derived peptides by major histocompatibility complex (MHC) molecules . All cells in the organism express MHC class I molecules, which present pe ptides derived from endogenous antigens to cytotoxic T cells. In contrast, only a few cell types, mainly from hematopoietic origin, express MHC class II molecules and are capable of stimulating helper CD4(+) T lymphocytes. On e of these cell types, dendritic cells (DC), have the unique capacity to st imulate naive T lymphocytes and initiating primary and secondary immune res ponses. This unique role of DCs relies on three specific attributes of this particular antigen presenting cells. First, their capacity to undergo a se ries of phenotypical and functional changes (maturation) in response to inf lammatory signals. These modifications cause the migration of DCs from peri pherical tissues and mucosa (where DCs reside in their immature state) towa rds secondary lymphoid organs, where DCs may stimulate resting T lymphocyte s. Second, a tightly regulated MHC class II antigen presentation capacity. Only immature DCs ingest and process antigens, whereas only the mature ones present peptides to CD4(+) T lymphocytes. Finally, a unique ability to pre sent peptides from exogenous antigens, internalized from the extracellular milieu, on MHC class I molecules. Together, these specific attributes confe r to DCs a central role in the initiation of both humoral and cellular immu ne responses.