Infections can trigger hematopoiesis at sites outside the bone marrow in the liver, the spleen or the skin.Researchers of Ludwig-Maximilians-Universitaet (LMU) in Munich now show that a specific type of immune cell facilitates such "extra medullary" formation of blood cells.
Balanced hematopoiesis is essential for the function of the immune system. During fetal development, hematopoiesis takes place mainly in the liver and the spleen. Later the process is delegated to the bone marrow, and this tissue normally serves as the sole source of blood cells for the rest of one's lifetime. However, certain infections can reactivate hematopoiesis at sites other than the bone marrow, a process which is referred to as "extramedullary" hematopoiesis. One of the best known inducers is the so-called cytomegalovirus (CMV), a member of the herpesvirus family, which is widespread in human populations worldwide, and can lead to serious illness in individuals with immature or otherwise compromised immune systems.
An international team led by Professor Ulrich Koszinowski at LMU's Max von Pettenkofer-Institute has now examined how this virus activates hematopoiesis in tissues other than the bone marrow. "Herpesviruses are highly species-specific," explains Dr. Stefan Jordan, the lead author on the new paper. "So, in order to study the phenomenon of extramedullary hematopoiesis in an animal model, we were forced to turn to the mouse virus." The murine CMV induces extramedullary hematopoiesis principally in the spleen.
Killing of infected cells paves the way
The new findings reveal a hitherto unsuspected link between natural killer (NK) cells and hematopoiesis. NK cells play an important role in combating CMV infections, because they are the immune system's first line of defense against the virus. In the first place, they are able to recognize and eliminate CMV-infected cells and, secondly, they synthesize and secrete signal molecules that mobili
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