The discovery of cathelicidin's role came as a surprise. "We began our research wanting to understand the biology of acne and specifically looked at the role of fibroblasts, which typically provide structural support in the deeper layers of the skin," said first author Alan O'Neill, PhD, project scientist at UC San Diego School of Medicine. "What we uncovered instead was that these cells were activated to produce large amounts of an important antimicrobial, cathelicidin, in response to acne-causing bacteria called Cutibacterium acnes." The research team performed skin biopsies on acne patients treated for several months with retinoids, a class of chemicals derived from vitamin A found to promote skin health. To the researchers' surprise, the drug-enhanced cathelicidin expression after treatment, thus finding an additional, unknown mechanism for why retinoids help treat acne. To support these findings, researchers studied skin lesions on mice injected with the acne-causing bacteria and observed similar treatment responses in the mice. "Cathelicidin being so highly expressed in acne biopsy tissue was a very interesting finding to us," said Gallo. "Knowing this will be helpful in developing a more targeted therapy to treat acne."