The hematopoietic stem cells: what are their origins, properties and roles ?

Hematopoietic stem cells

All cells in the blood and immune system have a common origin: hematopoietic stem cells (HSC). Housed in the bone marrow, these primitive cells guarantee the permanent renewal of our blood. The discovery of their unique properties, almost 40 years ago, made them shock agents in the treatment of various cancers and hematological diseases …

Singular properties

During cell division, a hematopoietic stem cell can give rise to another hematopoietic stem cell (self-renewal) or a progenitor (differentiation). A progenitor will actively multiply to produce all blood lines.

Also being referred to by CSH are present in very small numbers in the bone marrow. Yet they produce billions of blood cells every day, throughout an individual’s lifetime. HSCs have two main properties that allow them to achieve such a feat.

The first property is the ability to “self-renew”, that is, to reproduce identically by cell division.

The second property is their ability to differentiate into multiple blood cell lines: during successive cell divisions, HSCs “specialize” and gradually acquire the properties of red blood cells, white blood cells or platelets.

Because of this property of giving birth to several different cell types, HSCs are said to be “multipotential”. By combining self-renewal and differentiation, CSH can be maintained in constant numbers while permanently ensuring the regeneration of blood

Methodical identification

It is estimated that out of 10,000 cells from the marrow, only one is truly a stem cell. Indeed, the majority of the spinal cells are progenitors or precursors, intermediate stages of differentiation between HSCs and definitive blood cells. Since CSH has no size or shape characteristic that distinguishes them from other less primitive cells, researchers must use subterfuges to identify them in the mass of spinal cells.

The most common approach is to look for membrane markers on the surface of CSH alone. These markers are invisible under the microscope, but can be identified using complementary fluorescent probes.

However, it must be recognized that these markers are not completely specific since they are sometimes also found on the surface of more differentiated cells. But, thanks to so-called “functional” laboratory tests, it is possible to carry out a more rigorous identification of HSCs and in particular of their self-renewal and differentiation properties. Different observations allow this better identification. First, HSCs can persist in tissue culture for weeks or even months while producing many cells from different blood lines.

Only HSCs show such longevity, the more differentiated spinal cells depleting quickly. On the other hand, after injection into genetically modified mice to accept human cell transplants, the CSH will reproduce the effects of a bone marrow transplant performed in a patient: they will be able to lodge in the marrow of the mice, d ” give birth to cells belonging to all blood lines for several months.

At the source of hematopoietic stem cells

Bone marrow is the classic source of CSH. These can be harvested by aspirating the pelvic bones, hence the term bone marrow transplant which has long been given to stem cell transplants. Today, marrow transplants, strictly speaking, are only rarely performed.

The circulating blood normally contains only negligible amounts of stem cells. However, if a donor is treated with a hormone called granulocyte-colony stimulating factor or G-CSF, HSCs will migrate from the cord to the circulating blood. You can then collect CSH like a blood donation, by simply puncturing a vein in your arm.

This method of collecting CSH is the most commonly used today. Finally, the blood that flows back from the placenta after delivery and which can be collected after ligation of the umbilical cord contains a large number of CSH. For the past fifteen years or so, many centers have systematically collected CSH of cord blood from maternity hospitals.

The HSCs are then stored by freezing at very low temperatures in a “bank” of cord blood which hematologists can call on. It is thus possible for them to search according to criteria of compatibility of the CSH which can be used for the treatment of patients suffering in particular from leukemia. Since 1994, the Télévie operation has enabled the establishment of such a bank bringing together three universities in the French Community (the ULg, the UCL and the ULB).

Become the actor of his healing

Fighting to try to become the actor, and not the victim, of his cancer: this is the key that enabled Bernard Petre to get out of it. For this father of five, the trap would have been “to stand in front of the TV like a plant”.

What he always refused, compelling himself every day, even at the worst of the disease, to make a determined number of lengths between the walls of his hospital room to keep fishing. Even if it means passing “for a psychiatric patient”, as the nurses gently teased!

It was in July 2003 that cancer surprised this 45-year-old Brussels resident, already hit hard by the mourning of his eldest daughter, who died accidentally. After returning from a bike ride, Bernard notices that he is covered with little buttons.

He suffers from hay fever, which could explain this strange rash, but his allergy has never manifested itself in this way. And indeed, summer has nothing to do with it: an ultrasound reveals lymph nodes in the mesentery (intestine).

Bernard was immediately hospitalized to establish an accurate diagnosis. He is in great physical shape, but terribly anxious: he knows that these nodes can sign cancer. After a laparoscopic biopsy and then a laparotomy, the verdict falls: it is a lymphoma, fortunately detected at the early stage. No vital organ is affected, but it must be treated, and quickly.

From chemo to autograft become the actor of his healing

Chemotherapy begins in August. And badly: Bernard contracted a staphylococcus aureus during laparotomy and antibiotics go badly with cancer treatment. A total of six chemotherapies will follow one another until January of the following year.

The outpatient treatment allows her to continue working full time: “On the days when I felt ugly or anxious, having a work rhythm, seeing my clients and my colleagues gave me a dynamic and I I felt less overwhelmed by the side effects of the drugs, “he explains. “And above all, I have an extraordinary wife, who took care of many things. Thanks to Carole and the work, I had the best possible scenario.

The treatments get rid of the lymph nodes which disappear completely. But doctors are already planning a recurrence, within 3 to 4 years. They anticipate by taking a healthy stem cell sample, since they are cured. In case … And six months later, the disease proves them right.

The beginning of a lymph node has appeared. Recurrence is confirmed in the fall. A bone marrow transplant is required. For Bernard, it is the shock of being plunged back so quickly into the disease. But he feels physically fit and, still optimistic, thinks it might be the best time to do it. In addition, among his seven brothers and sisters, two are compatible.

Until the hard mini-graft, the return to normal!

In January 2005, he underwent aggressive chemo for a week. Then it’s the autograft, thanks to its own stem cells that are taken and frozen. This is the most painful period of the whole illness: already very weak, he is the victim of a new infection with staphylococcus aureus: “I had a fever.

There are whole days that I don’t remember any more. ” A month later, a new, light chemotherapy is done “to cleanse the body” before the allograft with his brother’s stem cells: it is a “mini-transplant”, by transfusion. An unpleasant step, because the liquid is cold to preserve the warming cells in the recipient’s body.

In addition, Bernard has an allergic reaction to the preservative: he trembles “to jump in his bed”. Despite this, he continues his professional activities by internet. A few hours a day, sometimes painful, but which help him to remain an actor in the face of the disease.

Hospitalized until the end of March, he highlights the psychological ordeal of the past two weeks, where he believed every other day to be able to get out, the fever ruining his hopes of returning home each time.

But Carole’s daily visits, the great humanity of the nursing staff and the support of her family and friends help her to hold on. He is very close to the nurses and discusses … cycling with the cleaning ladies! On Wednesday afternoon, Lucie (13), Anaëlle (10) and Théo (3) come to play near their dad, thanks to the hospital infrastructure designed to accommodate the children of the patients.

Zoom on the mini-graft

 The “mini-transplant” is a form of immunotherapy: preceded by less heavy chemotherapy than in traditional marrow transplant, it gives the stem cells of the donor marrow the opportunity to “track down” the recipient’s cancer cells as “foreign” cells and therefore destroy them.

Chemo weakens the recipient’s immune system, and then the graft (that is, the donor cells) completes the job of eradicating tumor cells. The “mini-transplant” reduces the risk of death (18%) and cancer recurrence, since the white blood cells in the graft continue to destroy cells of the old immune system.

The difficulty consists in correctly dosing the development of the graft to avoid possible perverse effects on the liver.

Mesenchymal stem cells: an unexpected potential!

As regards stem cells, we are today rather familiar with those called “hematopoietic”, at the origin of all blood cells, or even those called “embryonic”. However, much should be heard in the future about “mesenchymal” stem cells, whose vast differentiation potential is rich in therapeutic promise.

The bone marrow is increasingly asserting itself as a reservoir of stem cells with large resources! Stem cells called “hematopoietic” (HSC) have been studied for a long time, capable of giving birth to all blood lines. More recently, other cells have been identified which constitute a complex microenvironment which is crucial for the differentiation of HSCs.

Multi-talented cells

The microenvironment of the hematopoietic marrow has been revealed in “long-term” cell cultures (that is, lasting several weeks), in which the hematopoietic precursors at the earliest stage were observed only very locally , near a layer of cells adhering to the support.

It has appeared that these cells produce an impressive variety of cytokines (proteins involved in intercellular communication) while forming an extracellular matrix where the stem cells come to nest. These so-called “stromal” cells, which play an essential role in the renewal, differentiation and proliferation of HSCs, are of different types, but they are mainly “endothelial” cells (structural cells) and “mesenchymal cells. “(CSM). The latter have been better characterized and many functions have been recognized.

No specific marker (such as CD34 for CSH) allows the direct isolation of CSM, but they are easily characterized by a “signature” which combines expression and / or non-expression of some non-specific markers. Once isolated, the MSCs can be easily enriched and amplified (multiplied) thanks to their preferential adhesion on a plastic support. MSCs are capable of self-renewal, that is to say of maintaining their numbers while allowing a fraction of daughter cells to differentiate.

It was initially possible to describe MSCs as multipotential cells capable of giving birth to different cell types such as osteocytes, chondrocytes, myoblasts and adipocytes. But our knowledge of their potential has grown steadily. It has been shown that MSCs can be differentiated into many more cell types: thanks to specific inductions via cytokines and other chemical agents, they can form, in addition to the types mentioned above, heart, nerve cells (neurons or astrocytes), endothelial or hepatic.

Cells that we can qualify as traveling!

If the major source of MSC is the spinal microenvironment, other sources are now proven, such as adipose and muscle tissue. Fetal tissue and the dermis also contain it. MSCs are also present in cord blood and peripheral blood, suggesting that these cells are circulating and are the object of cellular mobilizations. In fact, we know that MSCs move to a site of cell deficit or tissue damage.

Thus, in mice which had been strongly irradiated, to the point where all of their marrow had been destroyed, the injection of mesenchymal cells allowed the reconstitution of hematopoiesis. It was also possible, by injecting MSCs locally, to repair bone tissue or joints whose cartilage had been damaged….

Growing applications

In humans, the first clinical trials of MSC transplantation have shown that these cells can be easily collected from the patient, amplified ex vivo and grafted without any toxicity.

The rapid normalization of platelet and neutrophil levels after a co-transplant (where the donor is also the recipient …) of CSM underlines the potential of these to regenerate the spinal microenvironment and improve graft uptake. It has also been found that MSCs grafted into children affected by a disease characterized by insufficient bone development (“osteogenesis imperfecta”) generate functional osteoblasts capable of attenuating the biochemical, structural and clinical anomalies encountered in this pathology.

Applications of MSCs should also multiply in the field of allogenic transplants (different donor and recipient), since MSCs have been shown to be able to prevent immune complications by acting directly on both T lymphocytes and cells. NK and dendritics involved in rejection of a graft by the recipient.

In summary, with MSCs, we have a cell type with enormous differentiation potential and many known functions, present in each of us, easily removed by puncture under local anesthesia, easy to multiply in vitro, dividing quickly without losing its ability to differentiate into various specialized types of cells.

We should therefore not be surprised by the current craze for the study of MSCs: both for basic research and for future clinical applications, these cells open up perspectives that were not yet suspected until recently.