Discarded White Blood Cells Induce Relocation of Blood Stem Cells


Researchers at the Fundación Centro Nacional de Investigaciones Cardiovasculares or CNIC in Madrid, Spain have discovered that the clearance of the white blood cells called neutrophils induces the release of blood cell making stem cells into the bloodstream.

Our blood consists of a liquid component known as plasma and cells collectively known as “formed elements.” Formed elements include red blood cells and a whole encore of white blood cells. Red blood cells contain hemoglobin that ferry oxygen from the lungs to the tissues. White blood cells come in two flavors: granulocytes, which contain granules, and agranulocytes, which are devoid of granules.

Granulocytes are a subgroup of white blood cells characterized by the presence of cytoplasmic granules. Granulocytes are formed in the bone marrow and can be classified as basophils, eosinophils, or neutrophils. These particular cell types are named according to their distinct staining characteristics using hematoxylin and eosin (H&E) histological preparations. Granules in basophils stain dark blue, eosinophilic components stain bright red, and neutrophilic components stain a neutral pink.

Granulocytes

The most abundant white blood cells is known as a neutrophil. Neutrophils comprise 50-70% of all white blood cells and are a critical component of the immune system. When immature, neutrophils have a distinct band-shaped nucleus that changes into a segmented nucleus following maturation. Neutrophils are normally in circulating blood, but they migrate to sites of infection via chemotaxis under the direction of molecules such as Leukotriene B4. The main function of neutrophils is to destroy microorganisms and foreign particles by phagocytosis.

Granulocytes-blood smear

Because neutrophils are packed with granules that are toxic to microorganisms and our own cells, damaged neutrophils can spill a plethora of pernicious chemicals into our bodies. To prevent neutrophils from aging and becoming a problem, they live hard and die young. in the vicinity of 1011 neutrophils are eliminated every day. They are rapidly replaced, however, and the means of replacement includes stem cell mobilization from the bone marrow to the bloodstream.

Workers in the laboratory of Andrés Hidalgo have discovered what happens to the discarded neutrophils. Earlier work in mice showed that injections of dead or dying neutrophils increase the number of circulating blood cell-making stem cells. Therefore, something about dead neutrophils causes the hematopoietic stem cells to move from the bone marrow to the bloodstream. By following marked, dying neutrophils, Hidalgo and his coworkers showed that the neutrophils went to the bone marrow to die. While in the bone marrow, the dying neutrophils were phagocytosed (gobbled up) by special cells called macrophages.

Once these bone marrow-located macrophages phagocytose aged neutrophils, they begin to signal to hematopoietic stem cells in the bone marrow, and these signals drive them to move from the bone marrow to the bloodstream to replenish the neutrophil population.

Hidalgo admits that even though his research has produced some unique answers to age-old questions, it also poses almost as many questions as it answers. For example, Hidalgo and his colleagues showed that neutrophils follow a circadian or day/night rhythm and this has implications for diseases. For instance, the vast majority of heart attacks are in the morning. Does this have something to do with neutrophil aging cycles?

“Our study shows that stem cells are affected by day/night cycles thanks to this cell recycling . It is possible that the malign stem cells that cause cancer use this mechanism to relocate, for example, during metastasis,” said Hidalgo.

Daily changes in neutrophil function could be part of the reason that acute cardiovascular and inflammatory events such as heart attack, sepsis or stroke tend to occur during particular times of the day.

“Given that this new discovery describes fundamental processes in the body that were unknown before, it will now be possible to interpret the alterations to certain physiological patterns that occur in many diseases,” Hidalgo said.

See Cell 2013; 153(5): 1025 DOI:10.1016/j.cell.2013.04.040.