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Cytokine expression is a form of gene expression that regulates the production of proteins, called cytokines, that send signals to the immune system. This expression involves "reading" a cell's genetic code and translating it into proteins. Cells use cytokine expression to send certain instructions to immune tissues, for example, to promote inflammation near an injury. Inappropriate expression can create an immune response against one's own body, and can cause diseases like arthritis.
After an injury, white blood cells enter the area to clear away debris and prevent infection. Injured cells increase the expression of certain cytokines that attract these blood cells to the area. In turn, white blood cells initiate an inflammatory response, promoting blood flow and the arrival of more white blood cells.
During inflammation, white blood cells also increase their own cytokine expression. One important cytokine produced from the gene expression is tumor necrosis factor (TNF). TNF not only promotes inflammation, but also helps destroy tumor cells, and prevent viruses from replicating. Its proper expression near an injury site is therefore a crucial means to prevent infection.
When the body has been infected by foreign invaders, cytokine expression helps regulate the immune response. Cells express and release certain cytokines to help immune cells know that they are part of the body, and should not be attacked. Other cytokines are expressed after a cell has been infected, and pass this information on to the immune cells. These cells can then target the infected cell for destruction to keep the infection from spreading.
Occasionally, pro-inflammatory cytokines are expressed improperly, and can lead to autoimmune diseases. In rheumatoid arthritis, there is continued cytokine expression near the synovial membranes that make up the joints. Cytokines produced from this expression, such as TNF and interleukin-1 (IL-1), cause inflammation of the joints. TNF seems to be the main cytokine involved in arthritis, and its presence encourages the expression of other cytokines.
Cytokine expression is not always the direct trigger for arthritis, but allows the disease to continue. It is thought that an initial small infection near the joint can lead to a disproportionate immune response. Immune cells may attack not only the infection, but also synovial membrane cells. The cytokines released by these immune cells encourage continued inflammation and tissue damage.
Osteoarthritis is another disease that involves improper cytokine expression. TNF and IL-1 cause bone tissue to degrade, promote inflammation, and prevent the tissue from repairing itself. These cytokines also seem to prevent the expression of other factors that would encourage healing or shut off the immune response. Some recent osteoarthritis therapy studies have therefore focused on preventing the expression of these cytokines.