Antigen presentation by dendritic cells

The stimulation of T lymphocytes requires the presentation of proteolytic antigen-derived peptides by major histocompatibility complex (MHC) molecules. All cells in the organism express MHC class I molecules, which present peptides 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. One of these cell types, dendritic cells (DC), have the unique capacity to stimulate naive T lymphocytes and initiating primary and secondary immune responses. This unique role of DCs relies on three specific attributes of this particular antigen presenting cells. First, their capacity to undergo a series of phenotypical and functional changes (maturation) in response to inflammatory signals. These modifications cause the migration of DCs from peripherical tissues and mucosa (where DCs reside in their immature state) towards secondary lymphoid organs, where DCs may stimulate resting T lymphocytes. 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 present peptides from exogenous antigens, internalized from the extracellular milieu, on MHC class I molecules. Together, these specific attributes confer to DCs a central role in the initiation of both humoral and cellular immune responses.