Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• Immunological memory

• Innate lymphoid cells

• Gut helminth infection

• Small intestine adaptation

• IL-25

Main Findings

Helminths have evolved complex pathways to create favourable host environments. Through a well-characterized immunoregulatory axis, helminths stimulate gut epithelial tuft cells to produce IL-25, which activates group 2 innate lymphoid cells (ILC2s). This drives epithelial proliferation and mucus production, which prevent excessive damage to the intestinal barrier. Prior studies have shown that epithelia can have ‘memory’ of previous responses, marked by sustained transcriptional and epigenetic changes. However, whether helminth-associated protective pathways can lead to lasting immune memory in the gut remains unclear. In this preprint (not peer-reviewed), Cortez et al. explore how IL-25 rewires mouse ILC2s to promote durable host-protective adaptations.

The authors first confirmed that intraperitoneal IL-25 injection induces small intestinal lengthening and epithelial proliferation, which persisted for at least 50 days. At the +50-day ‘memory’ time point, epithelial cells from IL-25-treated mice had greater expression of IL-4 and IL-13 gene targets than controls. Notably, ILC2s were more numerous in these mice and had sustained IL-13 and IL-5 production. Rag2-/-IL2rg-/- mice, which lack all lymphocytes including ILCs, had no small intestine changes after IL-25 treatment, in contrast to Rag1-/-mice, which form ILCs. Furthermore, mice lacking IL-4 and IL-13, Il4ra (IL-4/IL-13 receptor), or Il17rb (IL-25 receptor) also had no intestinal changes in response to IL-25, which shows the crucial role of type 2 cytokines produced by IL-25-activated ILC2s. Importantly, IL-25 pretreatment protected rec against secondary helminth infection.

Single-cell RNA sequencing and bulk chromatin accessibility profiling suggested that ILC2s develop memory-effector states after IL-25 exposure, characterized by marked epigenetic alterations and increased expression of Il4, Il13 and Il5 as well as transcription factors such as Zeb2 and Maf. Strikingly, exogenously administered IL-25 led to ILC2 and intestinal adaptation without requiring endogenous IL-25, tuft cells or canonical type 2 alarmins, indicating that IL-25 has a direct effect on ILC2s. Further experiments ruled out impacts of epithelial stem cells or the gut microbiota.

Conditional deletion of IL-25 receptor from IL-5-expressing cells or of all Il4ra+ cells after IL-25 treatment reduced epithelial adaptation. This suggests the requirement for IL-25-responsive IL-5+ ILC2s and type 2 epithelial signalling in intestinal adaptation. Finally, transplanting ILC2s from IL-25-conditioned mice into Rag2-/-IL2rg-/- mice protected against helminth challenge.

In summary, these findings lay a strong foundation for elucidating how innate immune cells balance tissue protection with mucosal pathogen defence by retaining ‘memory’ of prior infection.

Limitations

Followup work could delve deeper into transcriptomic and epigenomic landscapes to dissect gene-regulatory networks underlying ILC2 memory. This includes further characterising the roles of transcription factors like Zeb2, Hlf, Maf, and others which are implicated in memory and effector states. The single-cell transcriptomic data also revealed distinct ILC2 subpopulations, including those expressing Il17a or Gzma. Further research will be needed to bioinformatically validate their existence, and experimentally determine if all these populations contribute to the memory state or if they have specialized roles.

Continued in vivo studies may explore why IL-25-activated ILC2s do not heighten gut allergic sensitivity, and if memory ILC2s are truly independent of alarmin signaling. Such exploration may help explain why activated ILC2s do not fully recapitulate the same phenotypes when transferred into naive hosts, or if other factors such as a longer required experiment time frame are at play.

Finally, the majority of the study utilizes intraperitoneal IL-25 injections to induce helminth-like pathology in mice. While it is important to isolate the activation of the tuft cell-ILC2 circuit from the wide immune effects of helminths, it may be interesting to validate and further characterize the immune memory of ILC2s in full parasite exposure.

Significance/Novelty

Cortez et al. advances our understanding in the formation of innate immune memory in the context of ILC2s and mucosal immunity. In particular, the study characterizes a distinct form of memory ILC2 cells induced by IL-25 which are sustained long-term and independent of alarmin or tuft cell signaling. This is different from previously described epigenetically-mediated “trained immunity” and IL-33-dependent ILC2 memory found in the lungs. Furthermore, the tissue adaptation is present specifically in immune ILC2 cells and not found in epithelial cells (where immunological memory has been previously reported). Because activated ILC2s were found not just in the gut but also at distal sites like the lung and adipose tissue, the findings also suggest a pivotal role for ILC2s in mediating a broad immune and mucosal barrier to helminth infection. As such, Cortez et al. sets the groundwork for leveraging these non-specific responses that are mediated by protective and tolerance pathways rather than inflammatory mechanisms. Novel therapeutic strategies may attempt to raise mucosal immunity without additional allergic sensitization or chronic inflammation.

Credit

Reviewed by Brian Soong as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute and the University of Toronto. 

The author declares no conflict of interests in relation to their involvement in the review.