About Dendritic Cells ( DCs )
Definition
- Dendritic Cells (DCs) are antigen-presenting immune cells that function to initiate the immune response
by activating lymphocytes and stimulating the secretion of cytokines.
What are Dendritic Cells ?
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Dendritic Cells are immune cells and form a part of the mammal immune system. They start out as immature DCs
and are characterized by high endocytic activity and weak capability to activate T-cells. An immature DC carries receptors that recognize common features of many pathogens (bacteria, viruses or fungi). In their immature
state DC can be found especially in those tissues which are in contact with the environment: the skin (where DCs
are called Langerhans’ Cells) an the coating of nose, lungs, stomach and intestines. But in their immature
state they can also be found in large amounts in circulating blood.
Immature DC constantly scan their surroundings for viruses and bacteria. As soon as they have contact with pathogens, they start to mature. This activation can also be initiated by chemokine secretion from other cells.
Immature DC phagocytose a pathogen either by receptor-mediated endocytosis (like macrophages or neutrophils do) or by receptor independent mechanism that enables DC to continually take up extracellular material. This process is called macropinocytosis.
During the process of maturation, the uptaken material is processed, degraded into small pieces and presented on the surface by MHC molecules. The presentation of pathogenic peptides goes hand in hand with several changes in DC biology as well as morphology: they up-regulate cell-surface molecules that act as co-stimulators in T-cell activation, greatly enhancing their ability to activate T-cells. The DC has now become a highly effective antigen-presenting-cell (APC). Today’s best known marker for activated and fully matured DC, which are now able to prime T-cells is a surface molecule named CD83. The selective expression, the fact that during DC maturation CD83 is strongly induced and the observation that CD83 is essential for priming T-cells make this molecule amongst many other features a research object of highest interest.
Another molecule that is up-regulated during maturation process is CCR7, a chemotactic receptor that induces the DC
to travel from the areas of antigen uptake through the blood stream to the spleen or through the lymphatic
system to a lymph node, i.e. the areas of antigen presentation
Here they act as antigen-presenting-cells: they activate CD4+ and CD8+ T-cells as well as B-cells by presenting
them the antigens derived from incorporated pathogens.
A characteristic feature of helper T-cells is their specificity to one particular antigen. Only mature
DC are able to prime a naive helper T-cell with an antigen the cell has never seen before.
DCs are not only responsible for the induction of an antigen-specific immune response, but play also an important
role by induction and maintenance of tolerance. Reports from several groups point towards an induction of specialized
regulatory T-cells by immature or semi-mature DCs.
Selected Review Articles on DCs
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Banchereau J. and Steinman R.M.
Dendritic cells and the control of immunity.
Nature 392 (6673):245-252, 1998.
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Mellman I. and Steinman R.M.
Dendritic cells: specialized and regulated antigen processing machines.
Cell 106 (3):255-258, 2001.
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Steinman R.M. and Hemmi H.
Dendritic cells: translating innate to adaptive immunity.
Curr.Top.Microbiol.Immunol. 311 17-58, 2006.
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