Uncovering a culprit behind the itch: Staphylococcus aureus

Identifying the molecular spark plug that ignites itch

 Researchers exposed the skin of mice to S. aureus. The animals developed intensifying itch over several days, and the repeated scratching caused worsening skin damage that spread beyond the original site of exposure.

Moreover, mice exposed to S. aureus became hypersensitive to innocuous stimuli that would not typically cause itch. The exposed mice were more likely than unexposed mice to develop abnormal itching in response to a light touch.

This hyperactive response, a condition called alloknesis, is common in patients with chronic conditions of the skin characterized by persistent itch. But it can also happen in people without any underlying conditions — think of that scratchy feeling you might get from a wool sweater.

To determine how the bacterium triggered itch, the researchers tested multiple modified versions of the S. aureus microbe that were engineered to lack specific pieces of the bug’s molecular makeup. The team focused on 10 enzymes known to be released by this microbe upon skin contact. One after another, the researchers eliminated nine suspects — showing that a bacterial enzyme called protease V8 was single-handedly responsible for initiating itch in mice. Human skin samples from patients with atopic dermatitis also had more S. aureus and higher V8 levels than healthy skin samples.

The analyses showed that V8 triggers itch by activating a protein called PAR1, which is found on skin neurons that originate in the spinal cord and carry various signals — touch, heat, pain, itch — from the skin to the brain. Normally, PAR1 lies dormant but upon contact with certain enzymes, including V8, it gets activated. The research showed that V8 snips one end of the PAR1 protein and awakens it. Experiments in mice showed that once activated, PAR1 initiates a signal that the brain eventually perceives as itch. When researchers repeated the experiments in lab dishes containing human neurons, they also responded to V8.

Interestingly, various immune cells implicated in skin allergies and classically known to cause itch — mast cells and basophils — did not drive itch after bacterial exposure, the experiments showed. Nor did inflammatory chemicals called interleukins, or white cells, which are activated during allergic reactions and are also known to be elevated in skin diseases and even in certain neurologic disorders.

“When we started the study, it was unclear whether the itch was a result of inflammation or not,” Deng said. “We show that these things can be decoupled, that you don’t necessarily have to have inflammation for the microbe to cause itch, but that the itch exacerbates inflammation on the skin.”

Interrupting the itch-scratch cycle

Because PAR1 — the protein activated by S. aureus — is involved in blood-clotting, researchers wanted to see whether an already approved anti-clotting drug that blocks PAR1 would stop itch. It did.

The itchy mice whose skin was exposed to S. aureus experienced rapid improvement when treated with the drug. Their desire to scratch diminished dramatically, as did the skin damage caused by scratching.

Moreover, once treated with PAR1 blockers, the mice no longer experienced abnormal itch in response to innocuous stimuli.

The PAR1 blocker is already used in humans to prevent blood clots and could be repurposed as anti-itch medication. For example, the researchers noted, the active ingredient in the medicine could become the basis for anti-itch topical creams.

One immediate question that the researchers plan to explore in future work is whether other microbes besides S. aureus can trigger itch.

“We know that many microbes, including fungi, viruses, and bacteria, are accompanied by itch but how they cause itch is not clear,” Chiu said.

Beyond that, the findings raise a broader question: Why would a microbe cause itch? Evolutionarily speaking, what’s in it for the bacterium?

One possibility, the researchers said, is that pathogens may hijack itch and other neural reflexes to their advantage. For example, previous research has shown that the TB bacterium directly activates vagal neurons to cause cough, which might enable it to spread more easily from one host to another.

“It’s a speculation at this point, but the itch-scratch cycle could benefit the microbes and enable their spread to distant body sites and to uninfected hosts,” Deng said. “Why do we itch and scratch? Does it help us, or does it help the microbe? That’s something that we could follow up on in the future.”

Additional authors included Flavia Costa, Kimbria J. Blake, Samantha Choi, Arundhasa Chandrabalan, Muhammad Saad Yousuf, Stephanie Shiers, Daniel Dubreuil, Daniela Vega-Mendoza, Corinne Rolland, Celine Deraison, Tiphaine Voisin, Michelle D. Bagood, Lucia Wesemann, Abigail M. Frey, Joseph S. Palumbo, Brian J. Wainger, Richard L. Gallo, Juan-Manuel Leyva-Castillo, Nathalie Vergnolle, Theodore J. Price, Rithwik Ramachandran, and Alexander R. Horswill.

Disclosure: Chiu serves on the scientific advisory board of GSK Pharmaceuticals. Provisional patent application Serial No. 63/438,668, in which some co-authors are listed as inventors, was filed based on these findings.

The work was funded by the National Institutes of Health (grants R01AI168005, R01AI153185, R01NS065926, R01NS102161, R01NS111929, R37AI052453, R01AR076082, U01AI152038, UM1AI151958, R01AI153185, R01JL160582, F32AI172080, T32AI049928, 1R21AG075419), Food Allergy Science Initiative (FASI), Burroughs Wellcome Fund, Drako Family Fund, Jackson-Wijaya Research Fund, Canadian Institutes of Health Research (CIHR) (grants 376560 and 469411), and ANR-PARCURE (PRCE-CE18, 2020).