Having a regular or irregular heartbeat may come down to moonlighting immune cells that surprisingly help power blood-pumping pulses, a new study in Cell suggests.

In a series of experiments, Harvard researchers caught immune cells hanging around and helping heart cells conduct electricity for their rhythmic beats. The immune cells, called macrophages, are best known for surveilling the body and devouring invading germs and debris. But in the heart, they snuggled up to heart cells and formed pores through which electrical current could pulse through the organ, allowing for synchronous heart muscle contractions that pump blood. The macrophages also helped neighboring heart cells recharge between pulses.

In genetically engineered mice, a lack of macrophages in the heart led to irregular heartbeats that, in humans, would warrant implanting a pacemaker, the researchers found. In all, the finding suggests that macrophages are unexpectedly key to normal heart functioning—and could be behind some mysterious heart problems.

Further research, the authors write, could help physiologists better understand how the heart works and develop new types of therapies.

"This work opens up a completely new view on electrophysiology; now, we have a new cell type on the map that is involved in conduction," lead author Matthias Nahrendorf, a systems biologist at Harvard, said in a press release.

Maarten Hulsmans & Matthias Nahrendorf

Maarten Hulsmans & Matthias Nahrendorf

Maarten Hulsmans & Matthias Nahrendorf

Maarten Hulsmans & Matthias Nahrendorf

The researchers got the idea to look into macrophages after recent research caught them taking up second jobs in organs. Some macrophages rove widely through the body, gobbling garbage and germs as they go, but others patrol specific organs and tissues. Those stationary guards seem to do their best at blending in and helping out. For instance, in the liver and spleen, macrophages take on the task of iron recycling.

The researchers peeked into the hearts of mice and autopsied humans and found macrophages mingling with heart cells. There, the macrophages took on heart-specific cell shapes, allowing them to form electrical pores that connected them with their neighbors. In experiments, the macrophages unexpectedly displayed fluctuating electrical charges, which just happened to be synchronized with those of the heart cells.

With the mouse data, the researchers speculate that macrophages could play a role in conduction abnormalities, such as atrial fibrillation and certain arrhythmias. Further study on these cells may steer researchers to new therapies, and it may explain why anti-inflammatory drugs can sometimes help with heart disease.

Cell, 2017. DOI:10.1016/j.cell.2017.03.050 (About DOIs).