Brigham and Women’s Hospital (BWH) is a Harvard University-affiliated institution with a robust research program. In particular, several BWH are interested in mesenchymal stem cells and their ability to suppress inflammation and mediate healing in injured organs.
To that end, a research team led by Robert Sackstein from BWH’s Departments of Dermatology and of Medicine and Reza Abdi from BWH’s Department of Medicine and Transplantation Research Center, has published a stupendous report in the journal Stem Cells. In this paper, Sackstein and his coworkers used mesenchymal stem cells (MSCs) to successfully treat laboratory animals that suffered from type 1 diabetes.
Type 1 diabetes is, to a large extent, a disease of the immune system, since a large majority of type 1 diabetes patients have immune cells that recognize the insulin-secreting beta cells as foreign and these immune cells attack and obliterate them. MSCs are a type of adult stem cell that has shown potent immune-suppressing and anti-inflammatory effects in animal and human clinical studies. Previous preclinical trials with diabetic-prone mice have demonstrated that intravenous administration of MSCs can tamp down pancreatic injury and reduce the blood sugar levels without insulin administration. However, these effects were modest and temporary.
Sackstein and his team suggested that if more MSCs could be inserted into the pancreatic islets, then more islets would be spared from immune destruction. This would yield a more complete reversal of diabetes.
MSCs tend to lack a key cell surface adhesion molecule called HCELL. HCELL mediates the homing of cells in the bloodstream to inflammatory sites. Unfortunately, direct injection of MSCs directly into pancreatic islets is not clinically feasible because the pancreas is fragile and the damage caused by injection would cause the release of hydrolytic enzymes that would degrade the rest of the pancreas and other tissues as well. In order to move intravenously administered MSCs to the sites of the immune attack, Sackstein and others engineered MSCs that expressed the HCELL homing molecule. The presence of HCELL on the surfaces of these MSCs directed them to the inflamed pancreatic islets.
The BWH team found that administering these HCELL-bearing MSCs into diabetic mice caused the MSCs to lodge in the islets. These cells decreased inflammation in the pancreas and durably normalized blood sugar levels in the mice, which eliminated the need for insulin administration; in other words they caused a sustained reversal of diabetes
Sackstein concluded that while further studies of the effects of MSCs are warranted, this preclinical study represents an important step in the potential use of mesenchymal stem cells in the treatment of type 1 diabetes and other immune-related diseases.
A study at the Texas Health Science Center has shown that stem cell treatments that quash inflammation soon after traumatic brain injury (TBI) might also offer lasting cognitive gains.
TBI sometimes causes severe brain damage, and it can also lead to recurrent inflammation of the brain. This ongoing inflammation can extend the damage to the brain. Only a few drugs help (anti-inflammatory drugs for example). Up to half of patients with serious TBI need surgery, but some stem cells like a sub group of mesenchymal stem cells called multipotent adult progenitor cells (MAPCs) can reduce short-term inflammation, and induce functional improvement in mice with TBI. Unfortunately, few groups have gauged the long-term effects of MAPCs on TBI.
In an article that appeared in the journal Stem Cells Translational Medicine, a research team led by the Director of the Children’s Program in Regenerative Medicine, Charles Cox, reported the use of human MAPCs in mice that had suffered TBI.
In this study, Cox and his colleagues infused MAPCs into the bloodstream of two groups of mice 2, and 24 hours after suffering a TBI. The first group of mice received two million cells per kilogram, and mice in the other group received an MAPC dose five times stronger.
Four months after MAPC administration, those mice that had received the stronger dose continued to experience less brain inflammation and better cognition. Spatial learning was increased and motor deficits had decreased.
According to Cox, the intravenously administered MAPCs did not cross the blood/brain barrier. Since immune cells can cross the blood/brain barrier for a short period of time after a TBI and cause autoimmunity, this result shows that the MAPCs are quelling inflammation through “paracrine” mechanisms (paracrine means that molecules are secreted by the cells and these secreted molecules elicit various responses from nearby cells). Cox made this clear: “We spent 18 months looking for them in the brain. There was little to no engraftment there.”
Rather than entering the brain, the MAPCs “set up shop in the spleen, a giant reservoir of T and B cells. The MAPCs change the spleen’s output to anti-inflammatory cells and cytokines, which communicate with immune cells in the brain—microglia—and change their response to injury from hyper-to-anti- inflammatory. The cells alter the innate immune response to injury. We have shown this in a sequence of papers.”
University of Cambridge neurologist, Stefano Pluchino, has worked with immune regulatory stem cells. Pluchino said that Cox’s study shows a “good dose response” on disability and behavior “after hyperacute, or acute, intravenous injection of MAPCs.” However, Pluchino noted that the description of the effects of MAPCs on microglia (white blood cells in the brain that gobble up foreign matter and cell debris) is “speculative.” Pluchino continued: “It is not clear whether these counts have been done on the injured brain hemisphere, and whether MAPC effects were observable on the unaffected hemisphere. The distribution and half-life of these MAPCs is not clear” and has never been demonstrated convincingly in Athersys papers (side note: Athersys is the company that isolates and grows the human MAPCs). “It is also not clear if effects in the Cox study were a ‘false positive,’ secondary to a paradoxical immune suppression the xenograft modulates.” That is, a false positive could occur because human cells in animal bodies rouse immune reactions. “It is not clear where in the body these MAPCs would work, either out or into the injured brain.” Additionally the mechanism by which these cells act does not seem to be clear, according to Pluchino.
But, Pluchino added: “Athersys is already in clinic with MAPCs in graft vs. host disease, myocardial infarction, stroke, progressing towards a phase I/II clinical trial in multiple sclerosis, and completing the pre-clinical work in traumatic brain and spinal cord injuries. Everything looks great. The company is solid. The data is convincing in terms of behavioral and pathological analyses. But the points I have raised are far from clarified.”
Cox admitted that Pluchino’s points are valid. He pointed out that human cells were used in rodents, since the FDA wants pre-clinical studies in laboratory animals in order to first evaluate the safety and efficacy of the exact cells to be used in a proposed therapy before they head to the clinic. “As we are not seeking engraftment of these cells, and would not plan to immunosuppress a trauma patient, we have not pursued animal models that use immunosuppression. Our study was designed with translationally relevant end-points, recognizing the limitations of not having a final mechanism of action determined. The growing consensus is there are many mechanism(s) of action in cell therapies.”
Cox also agreed that the suggested effects of MAPCs on microglia, “is not truly a proof of mechanism.” However, Cox and his co-workers have developed a protocol that can potentially more accurately quantify microglia in mice. “We ultimately plan more mechanistic studies to define endogenous microglia versus infiltrating microglia and the effects of various cell therapies. “
Additionally, Cox also said that: “We have published work showing the majority of acutely infused MSCs and MAPCs are lodged in the lung after intravenous delivery. This was an acute study in non-injured animals, but others have shown similar data.” In another study, Cox’s research group showed that the cells cluster in the spleen, which corroborates work by other research groups that have used umbilical cord cells to treat stroke.
Finally, Cox disagrees that the suppression of immune cell function in animals by human cells is appropriately characterized as “a false positive.” Cox explained that the infused cells induce a “modulation of the innate immune response, and typically, the immune rejection of a transplant is associated with immune activation, not suppression. So it well may be a ‘true positive.’”
In order for MAPCs to make to the clinical trial stage, Cox will need to investigate the mechanisms by which MAPCs suppress inflammation and if their purported effects on microglia in the central nervous system are real. He will also need to show that these cells work in other types of laboratory animals beside mice. Rats will probably be next, and after that, my guess is that the FDA would allow Athersys to apply for a New Drug Application.
Researchers from Johns Hopkins University have examined wound healing in older mice and discovered that increasing blood flow to the wound can increase the rate of wound healing. Increasing blood flow to the wound requires a combination of gene therapy and the same stem cells the body already uses to heal itself.
John W. Harmon is professor of surgery at Johns Hopkins School of Medicine, and in a presentation to the American College of Surgeons’ Surgical Club, made the case that harnessing the power of bone marrow stem cells can increase the rate at which older people heal.
As we age, our wounds do not heal as fast. However, Harmon thinks that harnessing the power of bone marrow stem cells can remedy this disparity in healing rates.
To heal burns or other wounds, stem cells from the one marrow rush into action and home to the wound where they can differentiate into blood vessels, skin, and other reparative tissues. Stem cell homing is mediated by a protein called Hypoxia-Inducible-Factor-1 (HIF-1). According to Harmon, in older patients, few of these stem cells are released from the bone marrow and there is a deficiency of HIF-1. HIF-1 was actually discovered about 15 years ago by one of Harmon’s collaborators, a Johns Hopkins scientist named Gregg J. Semenza.
Harmon’s first strategy was to boost HIF-1 levels by means of gene therapy. This simply consisted of injecting the rodents with a copy of the HIF-1 gene that yielded higher levels of HIF-1 expression.
Even though higher levels of HIF-1 improved wound healing rates, burns were another story. To accelerate burn healing, Harmon and his co-workers used bone marrow stem cells from younger mice combined with the increased levels of HIF-1. This combination of HIF-1 and bone marrow stem cells from younger mice led to accelerated healing of burns so that after 17 days, almost all the mice had completely healed burns. These animals that healed so fast showed better blood flow to the wound and more blood vessels supplying the wound.
Harmon said that while this strategy is promising, he think that a procedure that uses a patient’s own bone marrow cells would work better since such cells would have a much lower chance of being rejected by the patient’s immune system. In the meantime, HIF-1 gene therapy has been successfully used in humans with a sudden lack of blood flow to a limb (see Rajagopalan S., et al., Circulation. 2007 Mar 13;115(10):1234-43). Harmon postulated that “it’s not a stretch of the imagination to think this could someday be used in elderly people with burns or other difficult wounds.”
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.
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.
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.