11.9A: Dendritic Cells
- Discuss the mechanism of action for dendritic cells
Dendritic cells are present in lymphoid organs, the epithelia of the skin, the gastrointestinal and respiratory tracts, and in most parenchymal organs. These cells are identified morphologically by their membranous projections that resemble spines. All dendritic cells are thought to arise from bone marrow precursors. Most, called myeloid dendritic cells, are related in lineage to mononuclear phagocytes. Immature dendritic cells (e.g. Langerhans cells of the epidermis) are located in main portals of entry of microbes (skin and gut epithelia). The function of epithelial dendritic cells is to capture microbial protein antigens and to transport the antigens to draining lymph nodes. During their migration to the lymph nodes, the dendritic cells mature to become extremely efficient at presenting antigens and stimulating naive T cells, hence their classification as antigen presenting cells. Mature dendritic cells reside in the T cell zones of the lymph nodes, and in this location they display antigens to T cells. Subsets of dendritic cells can be distinguished by the expression of cell surface markers. Different subpopulations of dendritic cells may stimulate distinct types of T cell effector responses. Some may even inhibit T cell activation.
Dendritic cells are constantly in communication with other cells in the body. This communication can take the form of direct cell-to-cell contact based on the interaction of cell-surface proteins. An example of this includes the interaction of the membrane proteins of the B7 family of the dendritic cell with a CD28 cell surface molecule present on the lymphocyte. However, the cell-cell interaction can also take place at a distance via soluble factors such as cytokines. For example, stimulating dendritic cells in vivo with microbial extracts causes the dendritic cells to rapidly begin producing interleukin 12 (IL-12). IL-12 is a signal that helps differentiate naive CD4 T cells into a helper T cell phenotype. The ultimate consequence is priming and activation of the immune system for attack against the antigens which the dendritic cell presents on its surface.
Key Points
- Dendritic cells function as antigen presenting cells.
- Dendritic cells are present in small quantities in tissues that are in contact with the external environment, mainly the skin (where there is a specialized dendritic cell type called Langerhans cells) and the inner lining of the nose, lungs, stomach and intestines.
- Once activated, dendritic cells migrate to the lymphoid tissues where they interact with T cells and B cells to initiate and shape the adaptive immune response.
Key Terms
- lymphoid organs : lymph nodes, spleen, and gut-associated lymphoid tissue where lymphocytes reside.