Dendritic cell population and antigenic persistence
DCs are the most efficient of the APCs and are derived from haemopoetic stem cells, found in the bone marrow. These DC progenitors can follow different developmental paths, differentiating into one of several types of DCs, for example, Langerhans cells and dermal DCs found in the skin and myeloid DCs near mucosal surfaces.
In the tissues DCs undertake the role of the security guard that continuously patrol the tissue on high alert for pathogen invasion. To aid this, DCs have a unique morphology; they reach out long thin processes with which they sample the local environment. Look at the DC in the graphic. It sits right under the epithelium stretching out its process between cells, to sample large and small molecules nearby.
Even in the absence of infection, DCs capture antigens both through phagocytosis and receptor-mediated endocytosis, to detect even the smallest amounts of allergen. Up to this point, DCs sound very similar to any other APC, but they are special! When a pathogen is ingested some of the DC's PRRs are touched by the pathogenic PAMPs and this leads to activation of the DC and the start of their metamorphoses! The DC places all its MHC class II-peptide complexes on its surface, freezes them there and starts to move to naive T cells in local lymphoid organs. Imagine this like a person that has got something (peptide) to show in its hands (MHC class II) that he rises up in the air and then starts to run a marathon. Most people's arms would get tired and drop down after a few miles but not the DCs! How do you think this will impact on antigen presentation when the DC finally reaches the naive T cell? Do you agree that it provides a snap shot of what is present in the tissue when the DC set off? Let's refer to this ability to retain antigens on the surface as antigenic persistence. Antigenic persistence is one of the reasons for why DCs are such efficient APCs. DCs are also experts at migrating and surviving the long journey through the lymphatic fluid to the naive T cell. These attributes of efficient ingestion of material, antigenic persistence, migratory capacity and long survival separates DCs from other less effective APCs. But not all DC types are like this. The plasmacytoid DCs has a different story to tell you on the next page.