Given the previously suggested role of XIST in restraining TLR7 and protecting against lupus, “It was not intuitive that we would find, at the bulk level, more of it around in patients who have this condition,” Dr. Antiochos says. Nonetheless, the researchers found elevated levels of XIST in the blood leukocytes of women with SLE, compared to healthy controls. Those levels correlated positively with disease activity and the interferon signature: on average, women with clinical signs of disease had higher XIST levels than those with inactive disease. Likewise, the XIST levels correlated positively with the degree of disease severity as measured by Systemic Lupus Erythematosus Disease Activity Index scores.
“We saw this relationship between interferon and XIST, and one potential explanation of that would be that XIST itself could be induced by interferon,” Dr. Antiochos says. The team’s cellular assays had suggested that XIST could bind to TLR7 to induce type I interferon. But was a reverse mechanism, in which interferon could upregulate XIST through some kind of feedback loop, also possible?
To clarify the relationship, the researchers added interferon to a variety of cell types relevant for SLE pathogenesis and measured XIST levels before and after each treatment. The experiments showed that XIST expression levels didn’t change with the addition of interferon, meaning that XIST upregulation is likely not a downstream consequence of higher interferon levels in SLE. Instead, the results suggest a one-way relationship: Increased XIST expression is more likely the upstream driver of interferon upregulation.
Dr. Antiochos and Dr. Darrah suspect that dysregulation causing an overabundance of both the XIST ligand and TLR7 receptor may be necessary for the development of SLE. “That’s exactly the sort of situation you’d need for a disease that can perpetuate itself indefinitely,” says Dr. Antiochos. “You’d expect that one minor perturbation in the system wouldn’t be enough to generate the systemic disease that can go on for years. You’d have to have pathways that can really feed forward.”
As a disease trigger, the researchers think, extracellular vesicles released during cell apoptosis could provide a key source of extracellular self-RNA, including XIST. In live human cells undergoing apoptosis, they found that programmed cell death induced the trafficking of XIST transcripts to extracellular vesicles. That selective enrichment of XIST supports a model in which the self-RNA acts as a female-specific danger signal: Its packaging into the extracellular vesicles upon cell death stimulates TLR7-dependent secretion of interferon by plasmacytoid dendritic cells, thereby contributing to SLE development and increased disease activity.
