What stimulates the maturation of monocytes? The research implicated α interferon, which is produced by one class of dendritic cells and is elevated in people with lupus. According to Banchereau, the results suggest that hyperactive dendritic cells, driven by high levels of α interferon, may cause lupus.

That still leaves a puzzle, however. How to explain the high levels of interferon, especially when the interferon-producing dendritic cells are often scarce in the blood of lupus patients? Banchereau thinks that armies of dendritic cells have deployed from the blood into the tissues. There, if they are simulated by a virus, the errant cells may start pumping out α interferon, leading to the self-destructive attacks that produce symptoms like vasculitis and rashes.

Patients could soon benefit from this work, Banchereau predicts. Steroids and immunosuppressants can ease symptoms of lupus, but they cannot cure the disease. “This provides...

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