Inked mice hint at how tattoos live on

Tattoos are pretty much permanent. Their ink can live on in the skin for decades. But biologists have been curious about how tattoos manage that. As inked skin cells die, the trash-collecting cells — part of the body’s immune system — should remove the ink along with the dead skin cells. Now French researchers think they know why that isn’t happening.

The body’s trash collectors pick up the ink, then remain in place, holding the ink there. When these immune cells die, they release the ink, only for new immune cells to slurp it up. Throughout, the tattoo’s image remains the same.

There have been other ideas for why tattoos are permanent. Some researchers have proposed that the ink stays in connective cells. Others think it remains in immune cells that simply live a long time. Sandrine Henri, who works in France at the Immunology Center of Marseille-Luminy, was skeptical.

She wanted to probe the role of immune cells known as macrophages (MAK-roh-fayj-es). “Macrophages will scavenge everything. That’s their job,” Henri says. “If they could do their job properly, tattoo ink would be removed rapidly.” But the ink wasn’t. She decided to investigate why.

She and her colleagues tattooed the tails of mice with green ink. Then they watched to see how macrophages in the skin responded.

Like Roomba vacuums, macrophages suck up cellular debris in the body. And here, they gobbled up the ink. But they did not go on to digest and remove it. Instead, the immune cells held onto the ink until they were killed by researchers.

Some 90 days later, new macrophages had moved in and reabsorbed the ink. And throughout, there was no difference in the look of the tattoo.

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Henri’s team described its findings March 6 in the Journal of Experimental Medicine. Her team now proposes that macrophages likely explain how tattoos persist in people, too.

But they can’t be sure — yet, says Desmond Tobin. A mouse study simply won’t settle that, says this skin expert in England at the University of Bradford. Macrophages may live longer in people than in mice, he says. Long-lived macrophages, for instance, might preserve tattoos in humans. If true, they wouldn’t need to adopt some ink-release-and-recapture tactic.

Whatever the final answer, the new findings might still help improve tattoo removal, Henri’s team says. Tattoos are usually removed with lasers. Combining lasers with some treatment that also focuses on getting rid of nearby skin macrophages might make that removal more effective, the researchers say.