Immune Cells Mistake Heart Attacks for Viral Infections

A study led by Kevin King, a bioengineer and physician at the University of California San Diego, has found that the immune system plays a surprising role in the aftermath of heart attacks.  The research could lead to new therapeutic strategies for heart disease.

The team, which also includes researchers from the Center for Systems Biology at Massachusetts General Hospital (MGH), Brigham and Women’s Hospital, Harvard Medical School, and the University of Massachusetts, presents the findings in the Nov. 6 issue of Nature Medicine.

Ischemic heart disease is the most common cause of death in the world and it begins with a heart attack. During this process, heart cells die, prompting immune cells to enter the dead tissue, clear debris and orchestrate stabilization of the heart wall.

But what is it about dying cells in the heart that stimulates the immune system? To answer this, researchers looked deep inside thousands of individual cardiac immune cells and mapped their individual transcriptomes using a method called single cell RNA-Seq. This led to the discovery that after a heart attack, DNA from dying cells masquerades as a virus and activates an ancient antiviral program called the type I interferon response in specialized immune cells. The researchers named these “interferon inducible cells (IFNICs).”

When investigators blocked the interferon response, either genetically or with a neutralizing antibody given after the heart attack, there was less inflammation, less heart dysfunction, and improved survival. Specifically, blocking antiviral responses in mice improved survival from 60 percent to over 95 percent. These findings reveal a new potential therapeutic opportunity to prevent heart attacks from progressing to heart failure in patients.

“We are interested to learn whether interferons contribute to adverse cardiovascular outcomes after heart attacks in humans,” said King, who did most of the work on the study while he was a cardiology fellow at Brigham and Women’s Hospital and at the Center for Systems Biology at MGH in Boston.

The immune system has evolved innate antiviral programs to defend against a diverse range of invading pathogens. Immune cells do this by detecting molecular fingerprints of pathogens, activating a protein called IRF3, and secreting interferons, which orchestrate a defense program mediated by hundreds of interferon-stimulated genes. Investigators found that surprisingly, the antiviral interferon response is also turned on after a heart attack despite the absence of any infection. Their results point to dying cell DNA as the cause of this confusion because the immune system interprets it as the molecular signature of a virus.

Surprisingly, the immune cells participating in the interferon response were a previously unrecognized subset of cardiac macrophages. These cells could not be identified by conventional flow sorting because unique markers on the cell surface were not known. By using single cell RNA Seq, an emerging technique that combines microfluidic nanoliter droplet reactors with single cell barcoding and next generation sequencing, the researchers were able to examine expression of every gene in over 4,000 cardiac immune cells and found the specialized IFNIC population of responsible cells.

Future studies will aim to better understand the interferon response and the IFNIC cell type and explore their roles in the infarcted and remodeling heart. The team is also working to understand the interferon response in other tissues and diseases where cell death occurs.

Tissue Engineering Advance Reduces Heart Failure in Model of Heart Attack

Researchers have grown heart tissue by seeding a mix of human cells onto a 1-micron-resolution scaffold made with a 3-D printer. The cells organized themselves in the scaffold to create engineered heart tissue that beats synchronously in culture. When the human-derived heart muscle patch was surgically placed onto a mouse heart after a heart attack, it significantly improved heart function and decreased the amount of dead heart tissue.

“Our novel technique is the first to achieve resolution of 1 micrometer or less,” the researchers reported in the journal Circulation Research. This tissue engineering advance is an important step toward the goal of preventing heart failure after a heart attack. Such heart failures account for nearly half of the 7.3 million worldwide heart disease-related deaths each year.

The heart cannot regenerate muscle tissue after a heart attack has killed part of the muscle wall. That dead tissue can strain surrounding muscle, leading to a lethal heart enlargement. It has long been the dream of heart experts to create new tissue that could replace damaged muscle and protect the heart from dilatation after a heart attack.

The researchers, led by Jianyi “Jay” Zhang, M.D., Ph.D., the University of Alabama at Birmingham, and Brenda Ogle, Ph.D., the University of Minnesota, modeled the scaffold after a three-dimensional scan of the extracellular matrix of a piece of mouse myocardial tissue. Extracellular matrix is the collection of compounds secreted by cells that gives structural support and cushioning to hold the tissue together.

Using multiphoton three-dimensional printing, the team then created crosslinks among extracellular proteins dissolved in a photoreactive gelatin. When the uncrosslinked gelatin was washed away, the photopolymerized extracellular protein scaffold that remained replicated the shape of the extracellular matrix, with hollows where cells had been.

This native-like scaffold was seeded with a mix of 50,000 cardiomyocytes, smooth muscle cells and endothelial cells derived from human-induced pluripotent stem cells, or hiPSCs. This cardiac muscle patch, about four one-thousandths of an inch thick and eight one-hundredths of an inch square began beating within one day of seeding, and the speed and strength of contractions increased significantly over the next week.

Researchers found that the scaffold had aligned the muscle cells properly, similar to native heart tissue, and the cells showed a smooth wave of electrical signal moving across the patch, a vital part of the electrophysiology that propagates contraction of the heart across the atria or ventricles. It appeared that the native-like structure of the scaffold contributed to the healthy electrical and mechanical function of the cells.

When two of the patches were transplanted onto an infarcted mouse heart, there was significant improvement in measures of cardiac function, blood vessel density and cell proliferation, and reduced infarct size and programmed cell death, or apoptosis.

“Thus, the hiPSC-derived cardiac muscle patches produced for this report may represent an important step toward the clinical use of 3-D-printing technology,” Zhang, Ogle and colleagues wrote. They also said, “To our knowledge, this is the first time modulated raster scanning has ever been successfully used to control the fabrication of a tissue-engineered scaffold, and consequently, our results are particularly relevant for applications that require the fibrillar and mesh-like structures present in cardiac tissue.”