Dendritic cell: Encyclopedia II - Dendritic cell - Life cycle

Dendritic cell - Life cycle

Dendritic cells start out as immature dendritic cells. These cells are characterized by high endocytic activity and low T-cell activation potential. Dendritic cells constantly sample the surroundings for pathogens such as viruses and bacteria. This is done through pattern recognition receptors (PRRs) such as the toll-like receptors (TLRs). TLRs recognize specific chemical signatures found on subsets of pathogens. Once they have come into contact with such a pathogen, they become activated into mature dendritic cells. Immature dendritic cells phagocytose pathogens and degrade its proteins into small pieces and upon maturation present those fragments at their cell surface using MHC molecules. Simultaneously, they upregulate cell-surface receptors that act as co-receptors in T-cell activation such as CD80 and CD86, greatly enhancing their ability to activate T-cells. They also upregulate CCR7, a chemotactic receptor that induces the dendritic cell to travel through the blood stream to the spleen or through the lymphatic system to a lymph node. Here they act as antigen-presenting cells: they activate helper T-cells and killer T-cells as well as B-cells by presenting them with antigens derived from the pathogen, alongside non-antigen specific costimulatory signals.

Every helper T-cell is specific to one particular antigen. Only professional antigen-presenting cells (macrophages, B lymphocytes, and dendritic cells) are able to activate a helper T-cell which has never encountered its antigen before. Dendritic cells are the most potent of all the antigen-presenting cells.

As mentioned above, MDC probably form from monocytes, white blood cells which circulate in the body and, depending on the right signal, can turn into either dendritic cells or macrophages. Activated macrophages have a lifespan of only a few days. The lifespan of activated dendritic cells, while somewhat varying according to type and origin, is of a similar order of magnitude, but immature dendritic cells seem to be able to exist in an unactivated state for much longer. The monocytes in turn are formed from stem cells in the bone marrow. However, the exact genesis and development of the different types and subsets of dendritic cells and their interrelationship is only marginally understood at the moment, as dendritic cells are so rare and difficult to isolate that only in recent years they have become subject of focused research. Distinct surface antigens that characterize dendritic cells have only become known from 2000 on; before that, researchers had to work with a 'cocktail' of several antigens which, in combination, are unique to DCs.

Adapted from the Wikipedia article "Life cycle", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki