How Inflammation Suppresses Stem Cell Function

Wang et al. PLoS One;5(12):e14206.

Low grade inflammation is well known to correlate with development of numerous disease conditions such as heart failure, kidney failure, and diabetes. It is generally accepted that oxidative stress caused by inflammation is one of the means by which disease evolution occurs. Inflammatory conditions usually generate oxygen free radicals that damage cells and cause the cells of the body to lose function. Importance of reducing inflammation in terms of preventing diseases, such as heart disease, is seen by the beneficial effects of antiinflammatories such as aspirin.

A recent paper (Wang et al. TLR4 Inhibits Mesenchymal Stem Cell (MSC) STAT3 Activation and Thereby Exerts Deleterious Effects on MSC-Mediated Cardioprotection. PLoS One. 2010 Dec 3;5(12):e14206.) suggests that inflammation may actually inhibit the activity of stem cells, and through suppressing the body’s repair processes, causes various diseases to appear.

The mesenchymal stem cell is a type of stem cell found in the bone marrow, fat, heart, and other tissues, that is activated in response to injury and acts to heal damaged tissues. Particularly in the case of heart attacks, it has been demonstrated that administration of bone marrow mesenchymal stem cells causes accelerated healing both in humans and animals. The therapeutic effects of mesenchymal stem cells seem to be mediated by production of growth factors, as well as proteins that support creation of new blood vessels, a process called angiogenesis. Currently several companies are currently developing mesenchymal stem cell based drug candidates including Osiris Therapeutics, Athersys Inc, Mesoblast, and Medistem.

Given the fact that these cells are not a “laboratory experiment” but have actually been used in more than a 1000 patients, understanding conditions that affect their activity, as well as means of making them more effective is important. Inflammatory mediators are believed to influence activity of mesenchymal stem cells, since the protein toll like receptor 4 (TLR4), which recognizes tissue inflammation is found in high concentrations on mesenchymal stem cells. TLR-4 was originally found on cells of the “innate” immune system as a molecule that recognizes “danger signals”.

In order to determine the function of TLR4 on bone marrow mesenchymal stem cells, scientists at Indiana University used mice that have been genetically engineered not to have expression of this protein. Bone marrow mesenchymal stem cells from the mice lacking TLR-4 were demonstrated to function in a similar manner to normal mesenchymal stem cells in the test tube. However when these mesenchymal stem cells were administered to mice after a heart attack, the cells were capable of generating a highly significant improvement in heart function as compared to normal mesenchymal stem cells. The scientists concluded that inflammatory signals “instruct” mesenchymal stem cells to produce less therapeutic factors than they normally would.

These data are very interesting since other reports have suggested that inflammatory mediators actually stimulate mesenchymal stem cells to produce higher amounts of anti-inflammatory factors such as interleukin-10. One of the reasons for the discrepancy may be that inflammation in the context of a heart attack may be different than the inflammatory signals used by other studies.