If the skin regenerates itself every couple of weeks, then why do tattoos last for years? Sure, we know that tattoo ink is inserted into the layer right beneath the outermost layer of skin, but even the cells there must regenerate eventually. The seeming paradox of tattoo permanence has hurt the brains of even the most science-savvy ink enthusiasts. Fortunately, on Tuesday, a team of researchers report they’ve found a solution.

In a paper published in the Journal of Experimental Medicine, French scientists showed that tattoos stay in the skin because cells in the skin actively ensure the ink pigments stay in one place. The particles of ink pigment, they write, are repeatedly passed on from old cells to the new ones that are coming to replace them, sort of like an immune system relay race baton. The key finding is the identity of those cells: the macrophages, immune system cells that encapsulate foreign bodies like bacteria or tattoo pigments.

“A lack of consensus regarding how to identify the immune cell types present in the skin has hampered the precise identification of the cells that capture the ink particles found in tattoo paste and retain them in situ for an extended period,” Sandrine Henri, Ph.D., and Bernard Malissen, Ph.D., both at the Immunology Centre of Marseille-Luminy in France, tell Inverse in a joint email. Henri and Malissen co-authored the paper along with 12 other researchers.

Macrophages are super tenacious in their efforts to hold onto ink pigments, which explains why even after laser tattoo removal surgery, traces of the ink still remain. New macrophages gobble up the scattered fragments of ink and hold them in place within the skin.

This research fills in a significant gap in scientific understanding about why tattoos remain in the skin for so long. Even though we have tattooed each other for thousands of years, we’re only now beginning to understand exactly how tattoos behave inside our bodies. Now that science is showing exactly how this process occurs beneath the surface of the skin, the study’s authors hope to improve tattoo removal techniques.

The first step to understanding what was happening was figuring out what sorts of cells were involved with the ink pigments in the first place. While conducting earlier research, the team discovered that the skin of black mice contains immune system cells called melanophages, which in turn contain pigment they consumed from dying melanocytes, the cells that produce the pigment that makes their skin and fur dark (this same pigment is responsible for the various shades of human skin). They wondered whether the same process is responsible for the persistence of tattoo ink.

“While analyzing the dynamics and turnover of melanophages, we started wondering how the pigments that are contained in tattoo ink are retained inside the skin for a long period,” say Henri and Malissen.

To investigate, they tattooed the tails of mice and then, after three weeks, when they could safely assume all the ink had been sequestered within the macrophages in the tails, they killed the macrophages in the mice’s skin with injections of diphtheri toxin. Their hypothesis was confirmed: Even though the scientists killed off the cells that contained the ink, the ink remained.

Scientists observed tattooed mouse tails before (left) and after (right) they killed dermal macrophages that hold tattoo ink. They found that these immune cells passed on the ink to new cells, which means the ink stayed put. Baranska et al., 2018

They conclude that the ink must have been recaptured by the macrophages that were coming to replace the dead ones.

This, in turn, explains why laser tattoo removal can take as many as 10 sessions to complete. The lasers used break up pigment particles but don’t destroy macrophages, so every time one is blasted at a bit of ink, new living macrophages readily swoop in, scoop up the broken pieces, and put them back every time.

Therefore, scientists suspect that effective tattoo removal will require killing off the macrophages at the same time that a laser is breaking up the pigment.

Macrophages encapsulate green tattoo ink pigment (left) and release it when the cells are killed (center). But 90 days later, new macrophages develop and swallow up the pigment again (right). Baranska et al., 2018

Henri and Malissen say they want to partner with dermatologists to develop and test this approach for human use. To do this, they’d need to first be able to ensure that their technique would destroy only macrophages and not other neighboring cells (as that would be dangerous). To do so, they will have to identify a specific antibody in human skin macrophages that they could target with an engineered antibody-toxin combination, then they’ll have to deliver this precisely targeted package to the macrophages at the same time that someone receives laser treatment.

“This approach would allow to kill simultaneously all the macrophages that are laden with tattoo ink,” say Henri and Malissen. “Therefore, all the tattoo ink will be free within the dermis at the same time and accessible to the laser to break it in small pieces.”

Abstract: Here we describe a new mouse model that exploits the pattern of expression of the high-affinity IgG receptor (CD64) and allows diphtheria toxin (DT)–mediated ablation of tissue-resident macrophages and monocyte-derived cells. We found that the myeloid cells of the ear skin dermis are dominated by DT-sensitive, melanin-laden cells that have been missed in previous studies and correspond to macrophages that have ingested melanosomes from neighboring melanocytes. Those cells have been referred to as melanophages in humans. We also identified melanophages in melanocytic melanoma. Benefiting of our knowledge on melanophage dynamics, we determined the identity, origin, and dynamics of the skin myeloid cells that capture and retain tattoo pigment particles. We showed that they are exclusively made of dermal macrophages. Using the possibility to delete them, we further demonstrated that tattoo pigment particles can undergo successive cycles of capture– release–recapture without any tattoo vanishing. Terefore, congruent with dermal macrophage dynamics, long-term tattoo persistence likely relies on macrophage renewal rather than on macrophage longevity.