How does the new technique help?

Single-cell RNA sequencing reads the activity level of genes inside thousands of individual cells at once.

That resolution matters because a patch of skin contains many cell types — keratinocytes, fibroblasts, blood-vessel cells and several immune populations — that share genes in bulk measurements and can hide subtle, disease-specific signals.

Why does sunlight hit lupus patients so hard?

The team pinned the difference on interferons, signaling proteins the body releases to fight viruses. Lupus skin, the researchers found, cranks out far more interferon after UV exposure than healthy skin does. The excess drives the overzealous dendritic-cell response.

"This study is the first to dig into lupus sun sensitivity at the single-cell level," said Michelle Kahlenberg, M.D., Ph.D., vice chair of research for the Department of Internal Medicine at U-M Health and the Michael H. and Maricia S. Klein professor of rheumatic disease.

"It identifies that lupus skin exhibits a hyperinflammatory response to ultraviolet light centered around too much production of interferons," she said.

What happens inside the body after UV exposure?

That interferon surge sets off two linked events. First, blood vessels in the skin switch into an activated state and start recruiting more immune cells. Second, the incoming dendritic cells take on an inflammatory profile that can ignite rashes and, in some patients, spill over into the bloodstream as a systemic flare.

"This leads to vascular activation and recruitment and inflammatory skewing of monocyte-derived dendritic cells, which may be an important step in causing lupus rashes," Kahlenberg said.

"This study identifies some critical pathways that can be targeted to prevent photosensitivity and the rashes and systemic flares that follow."

What could this change for patients?

The findings point to specific molecular pathways that drug developers could try to block. Researchers in the Kahlenberg Lab say they will continue to refine single-cell profiling as a tool to guide lupus care.

For now, the study has not produced a new treatment. The work is observational, based on a limited patient sample, and centers on the skin's response to a single UV wavelength. Larger trials will need to confirm whether blocking interferon signaling or dendritic-cell recruitment can prevent flares.

Still, the study offers clinicians a sharper picture of why an ordinary walk outside can be medically risky for people with lupus. Future work from the lab will likely extend the single-cell approach to other triggers and to lesional skin.