LOS ANGELES – Evidence is building for the hypothesis that impairments in the skin’s microbiome promote Staphylococcus aureus colonization and drive atopic dermatitis, Dr. Donald Y.M. Leung said at the annual meeting of the American Academy of Allergy, Asthma, and Immunology.
The link between the bacteria and atopic dermatitis has long been discussed, but its role in pathogenesis still needs definition, he said.
Dr. Donald Y.M. Leung
“We’ve never been able to look at the total bacterial composition of the skin, but now with next-generation sequencing it’s finally possible to look at all the phylla and species. Other investigators have shown that during flares of atopic dermatitis there’s a reduction in bacterial diversity and an increase in staph, with S. aureus being particularly abundant. Then, post-flare, you see see a drop in S. aureus; this clearly suggests (it’s) important,” according to Dr. Leung, head of the division of pediatric allergy and immunology at National Jewish Health in Denver and professor of pediatrics at the University of Colorado.
Staphylococcus aureus is known to secrete virulence factors including cytotoxins, superantigens, lipases, and proteases that activate inflammatory cells and can cause significant skin barrier dysfunction.
The discovery that filaggrin mutations result in structural abnormalities in the skin barrier and are associated with sharply increased rates of atopic dermatitis and peanut allergy have strengthened the association, but filaggrin can’t be the whole story. Mutations in filaggrin are largely confined to individuals of Northern European ancestry; African Americans don’t have filaggrin mutations.
Yet atopic dermatitis is a global phenomenon. Further, a skin barrier defect is not enough to cause atopic dermatitis, Dr. Leung said. But such a defect, whether caused by a filaggrin mutation or something else, allows S. aureus to attach to and colonize the skin. Staph overgrowth or infection then activates an inflammatory cell cascade involving natural killer T cells, mast cells, cytokines, and Langerhans cells. That’s why the most effective treatments for atopic dermatitis address both the need to rebuild the skin barrier as well as the counterproductive immune response, he added.
Elsewhere at the AAAAI meeting, Dr. Andrea L. Jones, of National Jewish Health, presented an analysis of 718 children and adolescents with atopic dermatitis, all of whom had been cultured for S. aureus, in that institution’s database. Methicillin-resistant S. aureus (MRSA) was found in 19%; 57% were positive for methicillin-sensitive S. aureus (MSSA) and 23% lacked S. aureus. Of note, the prevalence of peanut allergy was highest at 78% in the group with MRSA; the prevalence was 39% in those with MSSA and 4% in those without S. aureus.
The prevalence of allergies to wheat, egg, milk, or soybeans in the youths with atopic dermatitis was unrelated to MRSA colonization.
“Our hypothesis – although we need to do a prospective study – is that staph colonization may lead to barrier dysfunction and thus allow environmental allergens to invade through the skin. Interestingly enough, people who weren’t colonized by staph had a very low level of sensitization to peanut,” said Dr. Leung, who was the senior investigator in the study.
Dr. Leung was a coauthor on another study that points to a potential new avenue of treatment in atopic dermatitis. Presented by investigators at the University of California, San Diego, at a recent meeting of the Society for Investigative Dermatology, the study showed that atopic dermatitis is marked by a defect in the commensal skin bacteria which normally keep S. aureus in check.
In that study, the amount of S. aureus growing on a defined area of lesional skin of atopic dermatitis patients was nearly 10-fold greater than that on nonlesional skin and the skin of controls without atopic dermatitis.
Commensal bacteria on lesional skin may possess markedly reduced antimicrobial activity. The NIH-sponsored Atopic Dermatitis Research Network plans to conduct clinical trials to see if transplanting beneficial commensal bacteria will reduce staph colonization in atopic dermatitis patients and thereby result in therapeutic benefit, Dr. Leung noted.
He reported serving on scientific advisory boards for more than half a dozen pharmaceutical companies and receiving numerous research grants from the NIH.