Christine F. Wu

Tissue-resident lymphocytes can recirculate, but the underlying molecular mechanism is poorly understood. During helminth infection, intestinal group 2 innate lymphoid cells (ILC2s) rapidly proliferate and give rise to inflammatory ILC2s (iILC2s), which migrate from the intestine to distal tissues. Here, we show in mice that the redistribution of iILC2s requires access to lymphatic vessels. Interleukin-25 (IL-25) induces a substantial change in the epigenetic landscape of iILC2s, with transcription factors KLF2 and ZEB2 driving increased expression of sphingosine-1-phosphate receptor 1 (S1PR1) and S1PR5, respectively. S1PR5 regulates iILC2 exit from the intestine to the lymph, whereas S1PR1 is critical for iILC2 egress from the mesenteric lymph nodes to the blood and then to distal tissues including the lung, where iILC2s contribute to tissue repair. The requirement of two S1PRs is largely due to the dynamic expression of CD69, which mediates S1PR1 internalization. Thus, S1PRs modulate iILC2 emigration from nonlymphoid and lymphoid organs in a stage-specific manner, which provides a framework for understanding the multistep migration of tissue-resident immune cells.

Loss of RNA homeostasis underlies numerous neurodegenerative and neuroinflammatory diseases. However, the molecular mechanisms that trigger neuroinflammation are poorly understood. Viral double-stranded RNA (dsRNA) triggers innate immune responses when sensed by host pattern recognition receptors (PRRs) present in all cell types. Here, we report that human neurons intrinsically carry exceptionally high levels of immunostimulatory dsRNAs and identify long 3′UTRs as giving rise to neuronal dsRNA structures. We found that the neuron-enriched ELAVL family of genes ( ELAVL2 , ELAVL3 , and ELAVL4 ) can increase (i) 3′UTR length, (ii) dsRNA load, and (iii) activation of dsRNA-sensing PRRs such as MDA5, PKR, and TLR3. In wild-type neurons, neuronal dsRNAs signaled through PRRs to induce tonic production of the antiviral type I interferon. Depleting ELAVL2 in WT neurons led to global shortening of 3′UTR length, reduced immunostimulatory dsRNA levels, and rendered WT neurons susceptible to herpes simplex virus and Zika virus infection. Neurons deficient in ADAR1, a dsRNA-editing enzyme mutated in the neuroinflammatory disorder Aicardi-Goutières syndrome, exhibited intolerably high levels of dsRNA that triggered PRR-mediated toxic inflammation and neuronal death. Depleting ELAVL2 in ADAR1 knockout neurons led to prolonged neuron survival by reducing immunostimulatory dsRNA levels. In summary, neurons are specialized cells where PRRs constantly sense “self” dsRNAs to preemptively induce protective antiviral immunity, but maintaining RNA homeostasis is paramount to prevent pathological neuroinflammation.