A pathogen-derived small RNA modulates host immunity for chronic parasitism.

Os-milRNA2, a fungal microRNA-like RNA from Ophiocordyceps sinensis, is delivered into Thitarodes xiaojinensis immune tissues and represses attacin-1 during mid-stage infection.

• Why it matters: Chronic parasitism requires immune evasion without killing the host too early, but how host immunity is stabilized during months-long exploitation is unclear.
• What they did: The study identified Os-milRNA2 as a cross-kingdom regulator that directly targets attacin-1, then compared its mid-stage accumulation with effects of Os-milRNA2 inhibition on immune gene expression and fungal proliferation.
• The result: Inhibiting Os-milRNA2 restores host immunity and restricts fungal proliferation, and lineage-specific retention of the Os-milRNA2 binding site in the T. xiaojinensis attacin-1 3'UTR suggests regulatory coevolution.

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