Researchers headed by a team at The Ohio State University’s College of Medicine have shown for the first time, in mice, that heart problems associated with flu infection are not caused by inflammation in the lungs, as has long been predicted. The investigators’ study found that the electrical malfunctions and heart scarring seen in some of the most severely ill flu patients are caused by direct influenza infection of cardiac cells.

The scientists had seen flu viral particles in cardiac cells of infected mice in previous work, but couldn’t say for sure their presence in the heart was driving cardiac damage. In the new work, through which mice were infected with a genetically altered flu virus that wasn’t able to replicate in heart cells, animals developed classic inflammatory flu symptoms—but no cardiac complications.

“We showed that even when you have a very severe infection in the lungs, if you’re using that virus that can’t replicate in the heart, you don’t get those cardiac complications,” said Jacob Yount, PhD, associate professor of microbial infection and immunity in The Ohio State’s College of Medicine. “It proves it’s direct infection of the heart that’s driving these complications. Now we need to figure out what direct infection does: Is it killing heart cells? Does it have long-term ramifications? Do repeated infections have heart complications that build up over time? There are a lot of questions now for us to answer.”

Yount is senior author of the team’s published report in Science Advances, which is titled “Influenza virus replication in cardiomyocytes drives heart dysfunction and fibrosis.”

Seasonal flu remains a major contributor to human mortality, and the potential for new pandemic strains remains “an ever-present worldwide concern,” the authors wrote. It has been established for some time that hospitalized flu patients can develop heart problems. A 2020 study found that about 12% of adults in the United States hospitalized with the flu over eight years developed sudden, serious heart complications. However, as the researchers pointed out, whether flu-associated heart dysfunction occurs due to direct infection of cardiac tissue, or indirectly through systemic lung inflammation, hasn’t been clear.

“There is a debate within the clinical literature as to whether influenza virus directly or indirectly causes cardiac complications,” the team noted. And while live virus has been detected in human and nonhuman primate heart samples, direct infection of the heart has rarely been investigated. “Instead, current dogma states that severely infected lungs produce a cytokine storm with systemic cardiotoxic inflammation, which indirectly drives cardiac dysfunction.”

Yount has studied flu for years, and his lab developed a mouse model lacking IFITM3, the gene that codes for a key protein in the innate immune system’s clearance of viral infections. His team found in a 2019 study that flu-infected mice lacking the IFITM3 gene— IFITM3 knockout (KO) mice—were at higher risk for developing cardiac issues. “… mice lacking the interferon-induced transmembrane protein 3 (IFITM3) suffer from severe cardiac electrical dysfunction and fibrosis upon influenza virus infection, thus providing a long-sought model for influenza-associated cardiac complications,” Yount and investigators wrote in their Science Advances paper.

These mice not only are highly susceptible to flu, but are also deficient in the same antiviral protein that some people are lacking, too: About 20% of Chinese people and 4% of Europeans have a genetic variant that causes a deficiency of IFITM3. “We know those people are more susceptible to severe flu infections, and our mouse research would suggest they’re also more susceptible to heart complications with the flu,” said Yount, who is also a program co-director of the viruses and emerging pathogens program in The Ohio State Unversity’s Infectious Diseases Institute.

While the previous research with the IFITM3 KO mice suggested the flu virus replication in the heart might play a key role in cardiac dysfunction, as the team pointed out in their most recent paper, “… the observed cardiac phenomena could not be decoupled from the severe lung infection and heightened inflammation that also occurs in IFITM3 KO mice.”

To help address this question, for their newly reported study the researchers altered the genome of an H1N1 flu strain so that the virus could infect and cause inflammation in lung tissue, but could not replicate in heart cells. “ … we generated a novel recombinant heart-attenuated influenza virus via genome incorporation of target sequences for miRNAs [microRNAs] expressed in cardiomyocytes,” they explained. The investigators then injected this altered virus, and a control virus, into normal mice and the IFITM3 KO animals.

They confirmed that both the unaltered flu virus and the modified strain caused lung and systemic inflammation and generated high concentrations of viral particles in the mice, the altered virus was undetected in normal mouse heart cells and was also found in significantly lower concentrations in the IFITM3-deficient mouse hearts. These results allowed for direct comparisons between the hearts of mice with and without robust virus replication.

The researchers detected less heart muscle damage, lower biomarkers for cell injury, less scarring, or fibrosis, of heart tissue, and decreased electrical signaling problems in the hearts of mice that received the genetically altered virus. “Using this novel heart-attenuated influenza virus, we found that severe lung inflammation during influenza virus infection, even in highly infected IFITM3 KO mice, was not sufficient to drive cardiac dysfunction in the absence of virus replication in cardiomyocytes,” they reported. “Thus, direct infection and replication of influenza virus in cardiomyocytes is a primary determinant of cardiac pathology associated with severe influenza.”

“We have this mouse model and this virus that allowed us to distinguish between the severe lung inflammation and the direct replication of the virus in the heart,” Yount said. “We hadn’t been able to separate those two things in the past. If you don’t have the virus replicating strongly in the heart, you don’t see the same electrical abnormalities or the same fibrotic response.”

There’s also still a lot to learn. Influenza tends to focus most of its efforts on infiltrating the lungs, but generally isn’t present in the blood or other organs. But it does get to the heart—and finding out how this happens is part of continuing work in Yount’s lab. “Several key issues remain to be addressed by future approaches in dissecting cardiac pathogenesis of influenza virus,” the authors stated. “Of particular interest is the mechanism by which the virus spreads from the primary site of infection (respiratory tract/lungs) to the heart and how productive infection in the heart is achieved thereafter.” In addition, they noted, “… there is much to be learned about the clinical role of cardiac infection in humans, particularly in individuals with deleterious IFITM3 single-nucleotide polymorphisms, who may have a greater risk for direct influenza virus infection of the heart and cardiac pathology.

While it’s too soon to tell how the team’s reported research might influence the treatment of hospitalized flu patients with cardiac complications, Yount suggests the new findings indicate that clearing the viral infection could be key to reducing flu’s problematic effects on the heart. “One thing this tells us is that this is another reason to get your flu shot, because you don’t want your heart to get infected by the flu—and it is a possibility,” he said.

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