The heart is one of the least regenerative organs in mammals. Damage to heart tissue, such as that resulting from a heart attack, produces a harmful inflammatory response and the formation of scar tissue rather than regeneration. Scarring disrupts normal tissue function, whether in the heart or elsewhere. The research community would like to suppress the unhelpful inflammation and scarring following injury in all types of tissue, but this phenomenon is particular problematic in the heart. Here, researchers demonstrate that the source of this inflammation may be largely the activity of B cells.
In a heart attack, blood is cut off from an area of the heart that then often dies. If the person survives, the body tries to heal the dead muscle by forming scar tissue - but such tissue can further weaken the heart. Yet another wave of damage can occur when well-intentioned immune cells try to heal the injured heart but instead drive inflammation. Pirfenidone is approved to treat a lung condition called idiopathic pulmonary fibrosis, a scarring of the lungs that has no known cause. The drug also has been known for its heart-protective effects in a number of different animal models of heart attack. Researchers had assumed that pirfenidone's protective action in the heart paralleled the reason it helps in lung disease. In the lungs, the drug slows the formation of scar tissue.
"That this drug also protects the heart is not new. But in our studies, pirfenidone didn't physically reduce scar tissue in the heart. The scar tissue is still there, but somehow the heart works better than expected when exposed to this drug. It wasn't clear why. So we set out to reverse engineer the drug to pick apart how it may be working. Since scar tissue was still present, we suspected inflammation was the main culprit in poor heart function after a heart attack." Most immune studies of the heart have focused on other types of immune cells, including macrophages, T cell lymphocytes, neutrophils, and monocytes. But the researchers found no differences in the numbers of such immune cells in the injured hearts of mice that received pirfenidone versus those that didn't. When they serendipitously measured B cells, however, they were surprised to see a huge difference.
"Our results showing B cells driving heart inflammation was quite unexpected. We didn't know that B cells have a role in the type of heart damage we were studying until our data pushed us in that direction. We also found that there isn't just one type of B cell in the heart, but a whole family of different types that are closely related. And pirfenidone modulates these cells to have a protective effect on heart muscle after a heart attack." When the researchers removed these cells completely, not only was the heart not protected, the beneficial effect of the drug went away. So the B cells are not exclusively bad, according to the scientists. "The protective effects of pirfenidone hinge on the presence of B cells. The drug may be working on other cells as well, perhaps directly or perhaps through the B cells. We're continuing to investigate the details."