New research reveals that blood stem cells are an active part of the immune system, and fight against invaders that enter the body. But in this process do they expose the body to cancer?
The most familiar human stem cells are the hematopoietic stem cells, also known as blood stem cells. These are the stem cells that reside in the bone marrow, and create the red blood cells that carry oxygen in the bloodstream, and the white blood cells that fight infections. Due to the great importance of the hematopoietic stem cells, the body takes care to keep them well inside the inner area of large bones, called the 'bone marrow'. Until recently, scientists believed that their entire function was to make and release red blood cells and white blood cells into the bloodstream. But a recent Harvard study showed that hematopoietic stem cells have the ability to contribute by themselves to protecting the body from invaders.
the riddle
Professor Ulrich von Andrien from the Pathology Laboratory at Harvard Medical School says that the hematopoietic stem cells can leave the bone marrow and travel in the bloodstream. This fact has been known for almost fifty years, but no explanation has yet been found for it. Hematopoietic stem cells are one of the body's most important assets, and if given the wrong chemical 'instructions', they can develop cancer. Apparently, the body has no reason to let these cells wander alone in the body.
the answer
According to Professor von Andrien, during the wandering of the hematopoietic stem cells in the body, they enter internal organs and there they scan for disease-causing invaders. If they encounter such an invader, they immediately start defense processes: they divide again and again and create a large amount of white blood cells that fight the infection on the spot.
"This process changes our perspective on blood stem cells," says von Andrien.
The lymphatic system - the body's drainage system
The study began when researchers took samples of the lymph fluid from the thoracic ducts of a mouse. The thoracic ducts are an important part of the lymphatic system, which drains excess fluid from the body's organs and back into the bloodstream. The lymphatic system is used in many cases as a kind of 'emergency opening': if there are too many fluids in a certain area, they will drain through the lymphatic ducts.
The researchers discovered in the samples from the lymph fluids a population of cells that exhibited markers of hematopoietic stem cells. The number of these cells in the samples was extremely small, and it was difficult to diagnose them well. In order to verify the test, the researchers created genetically modified mice, whose hematopoietic stem cells glowed with fluorescent light under the microscope lens. In this way, the researchers were able to discover that a small amount of hematopoietic stem cells is found in all internal organs, including the liver, heart and lungs.
"When you look at the overall picture, you realize that the cells migrated from the bone marrow into the bloodstream, and from there they reached the tissue," says Stefan Masberg, a researcher and cardiologist in von Anderien's group. "Then, they were drained through the lymph ducts back into the bloodstream, and from there they returned to their home in the bone marrow. But the real question is why. Why would they do that?"
The stem cells - the guardians of the body
The team found that the stem cells remain in the tissue for 36 hours before draining through the lymph vessels. This pattern of action has raised the hypothesis that they are there to protect the tissue. In order to test the hypothesis, Masberg and his colleagues injected a bacterial toxin into the mouse tissue. Within a few days, they discovered clusters of immune system cells formed in the infected areas.
"Typical responses of the immune system deplete local stores of the cells responsible for fighting the infection," says Masberg. He believes that the hematopoietic stem cells bring about an initial immune response, then divide and create many new cells of the immune system. In this way, the local white cell pool is not depleted, and the local immune response is stronger.
The final proof
In an article published in the prestigious journal Cell on November 30, the researchers describe the discovery of the molecular mechanism that allows the stem cells to protect the internal organs.
It turns out that after the stem cells finish guarding a certain organ, they receive a signal - a signal that is a fatty molecule - instructing them to leave their guard and drain back into the bloodstream. The presence of the bacterial toxin prevents the signal from reaching the cell. On the surface of the stem cells there are receptors that are able to distinguish the presence of the toxin. When the stem cell becomes aware of the toxin, it is unable to respond to the signal that tells it to leave the organ it is in. The cell is actually trapped inside the tissue, and the following signals it receives tell it to divide into cells of the immune system that will be able to fight the bacterial infection in the area.
"Stem cells are actually an active part of the immune system," says Van Andrien. "This provocative way of thinking allows us to conceive of new ways in which the body can fight pathogens."
The fear
The discovery is indeed groundbreaking, but raises questions about the way different diseases, such as cancer, break out. It is likely that during the stay of the sensitive stem cells outside the bone marrow, they are much more exposed to the environment. They may undergo mutations, receive wrong signals or even be infected with a virus. Any such disruption in their activity may cause them to become cancerous cells. Although the protection of the stem cells on the body helps in the short term, there is concern that it increases the risk of cancer in the long term.
One response
I believe that now they will start researching in different directions, but I have a feeling that any change in the predetermined dose of the stem cells will cause unwanted results in the human structure in the future.
complicated?