When tissues are injured, the blood vessels that feed them are often shocked and damaged as well. “Vasoactivity” refers the ability of blood vessels to dilate or constrict. When tissues are harmed, blood vessels tend to shrink in order to squelch blood loss at the site of damage. This same response, however, and deprive the damaged tissues of much-needed oxygen and lead to “ischemia,” which is the insufficient supply of blood and oxygen to an organ.
James B. Hoying and his colleagues at the University of Louisville in Kentucky used the “stromal vascular fraction” or SVF from fat in order to treat damaged blood vessels to determine if they could mitigate the decrease in vasoactivity as a result of injury.
The SVF refers to the stem fraction from fat after the fat has been minced, digested with enzymes, and centrifuged (it’s more complicated than that, but this is a short summary). The cells that remain include mesenchymal stromal cells, growth factors, immune cells, pre-fat cells and fat cells, blood-cell-making stem cells, and blood vessel-making cells (endothelial cells). The SVF, therefore, contains a cocktail of cell types and growth factors that are available for regenerative medicine.
Hoying and his team discovered that when fluorescent SVF cells were injected into a laboratory mouse, they cells distributed to a variety of tissues. Further and more detailed examinations showed that these cells were finding their ways into organs and tissues because they traveled through the circulatory system and could be found in the walls of blood vessels.
Next, the composition of the SVF was examined. About 25% of the cells in the SVF were endothelial cells, 22% were various types of blood cells, 20% were “CD11b” cells, which means that these cells had a protein called CD11b on their cell surfaces. That protein was formerly canned “Mac-1” and is was normally found on the surfaces of phagocytic cells called macrophages. Therefore, this CD11b faction could very well be macrophages, but other cell types have this protein on their surfaces as well.
Next, Hoying and others injected these SVF-derived cells into the large leg vein (saphenous) of the leg. Such injections consistently caused these vessels to relax and dilate. Secondly, the SVF-derived cells caused the vessels to relax in a CD11b-dependent manner. In other words, the more CD11b cells there were in the SVF preparation, the greater the amount of vasoactivity they induced. If fractions were depleted of their CD11b, they could not induce vasoactivity.
When Hoying and others examined the SVF-treated vessels, they saw CD11b+ cells lining the inner layer of the vessels. Thus these cells were getting right up against the inside of the vessel and signaling to the underlying smooth muscle to relax.
Finally, Hoying and others clamped the saphenous veins of laboratory mice. Such clamping will induce tissue ischemia and inflammation in the vessels. Can SVF cells calm the inflammation and make the vessels more vasoactive? The answer is an unqualified yes. See below. The veins from SVF-treated animals show signficantly greater dilation than those from untreated or CD11b-depleted SVF-treated animals.
This an interesting and exciting finding not only because of the ability of these fat-based cells to maintain vasoactivity even under pro-inflammatory conditions, but because it is the macrophage cell population that is doing the work. In most stem preparations, macrophages are excluded. This paper shows that macrophages have greater therapeutic capabilities than previously thought, and should also be tested for sanative properties.