Prenatal inflammation reprograms hyperactive ILC2s that promote allergic lung inflammation and airway dysfunction

Background of the preprint

During embryonic and early postnatal periods of development, barrier sites including the intestinal, skin and lung epithelia are highly plastic and shaped by changes in their microenvironment. Fueling such changes are distinct waves of immune cells: they seed barrier tissues over time before becoming tissue-resident players, which modulate tissue immunity and homeostasis. These waves of immune cells, found across different tissues, have not been comprehensively mapped out over time. More importantly, the consequences of disrupting these waves through, for example, pre- or postnatal infections are poorly understood and not much is known about how such disruptions might affect tissue function and disease susceptibility.

In this preprint, the authors study allergic asthma (AA), as it is a chronic respiratory disease that often initiates in early life as a consequence of respiratory (viral) infections. Adaptive immune cells, mounting virus-specific responses, only begin to arrive in the tissue after birth. Innate lymphoid cells (ILCs), their innate antigen-independent counterpart, precede them and patrol the tissue while the adaptive immune response is limited. Particularly ILC2s, mirroring the effector functions of T helper type 2 (TH2) cells, have been implicated in asthma pathogenesis in mouse models^1-3. Tissue-specific priming of ILCs can be modulated even before birth and thus influence disease susceptibility in later life – as demonstrated in this paper.

 

Key findings of the preprint

In order to explore the role of prenatal inflammation in driving asthma susceptibility, the authors used a mouse model of prenatal exposure to poly(I:C), a viral analogue. The effects of prenatal inflammation in the offspring were assessed via a combination of flow cytometric immune profiling, histopathology, transcriptomic analysis and mouse genetics.

 

3.1 Prenatal inflammation alters the postnatal immune landscape in the lung

When profiling the postnatal immune landscape of the lung within the first two weeks of life in murine pups, ILC2s significantly expanded following poly(I:C)-induced prenatal inflammation. Additionally, ILC2s showed elevated production of the bona fide type 2 cytokines IL-5 and IL-13 and a heightened activation status as determined by surface expression of KLRG-1.

 

3.2 Prenatal inflammation acts on ILC progenitors and depends on maternal TLR3 signaling

A population of developmentally restricted hematopoietic stem cells (drHSCs) in the fetal liver gives rise to ILC lineages. Experiments included in this preprint demonstrated a skewing of drHSCs towards mature IL-5+ and IL-13+ ILC2s upon poly(I:C) treatment. Mature ILC2s would only start to arise in lung tissue close to birth, hence prenatal inflammation was concluded to directly impact ILC progenitors in embryonic tissue. Poly(I:C) was sensed by its cognate receptor TLR3 in the maternal – as opposed to the fetal – animal, since homozygous genetic loss of TLR3 in the mother prohibited ILC2 expansion in the offspring. In concert, these sets of experiments detail a maternally induced and intrinsically regulated priming of embryonic ILC progenitors towards mature ILC2s upon birth.

 

3.3 Following prenatal inflammation, ILC2s in offspring are transcriptionally reprogrammed, hyperactive and modulate T helper responses in the lung

To gain insight into the modes of transcriptional re-wiring of offspring ILC2s following prenatal inflammation, the authors performed single cell RNA-sequencing. Prenatal inflammation resulted in quantitative changes of 3 detected clusters – cells characterized by activation (Klrg1) and type 2 markers (Il5, Il13) as well as stem-like cells (Zeb1, Kit, Bcl2) expanded, while a cluster of ILC2-ILC3 intermediate states decreased.

Such transcriptional changes also translated into a hypersensitivity of ILC2s to IL-33, whose levels surprisingly were reduced in postnatal lung tissue. Within the lung tissue of pups from poly(I:C)-treated mice, in turn, ILC2s robustly skewed T cell lineages towards TH2 cells, which coincided with a reduction in regulatory T cells.

 

3.4 Prenatal inflammation-driven ILC2 expansion leads to pronounced responses to acute respiratory challenge in postnatal life

The authors next evaluated whether any of the aforementioned changes in the immune landscape of offspring mice could influence and alter disease outcome in later life. Intranasal administration of papain at P14 followed by tissue harvesting after 4 days revealed tissue pathology as well as elevated numbers of ILC2s. These effects were more pronounced in offspring from poly(I:C)-treated mice, in which the numbers of eosinophils were also significantly increased.

Similarly, acute papain allergen challenge per se caused lung pathology, mucus cell hyperplasia and immune infiltration in adult animals. Specifically in offspring exposed to prenatal inflammation, however, a general increase in eosinophils following papain administration was also observed concordant with an ILC2 expansion. Respiratory dysfunction, as measured by a FlexiVent apparatus, was already impaired in offspring from poly(I:C)-treated mice, but exacerbated upon methacholine challenge.

 

3.5 Conclusions

In conclusion, this preprint traces the repercussions of prenatal inflammation into early postnatal life and adulthood. The authors demonstrate how poly(I:C) treatment, as a proxy for a viral infection, of pregnant mice is sensed by the maternal system and subsequently reprograms embryonic ILC progenitors, which translates into a pronounced type 2 immune landscape in the lung of offspring animals. Within this remodeled immune environment, acute allergen and respiratory challenges throughout life reveal increased disease susceptibility. Overall, this work lays the groundwork for a mechanistic understanding of how prenatal infection could predispose children to respiratory disorders like asthma.

 

What I like about this preprint

The field of neonatal immunology is gaining more and more attention lately. When skimming through the pages of classic immunology books – the monumental Janeway’s for example – it may look like we have a basic understanding of how our immune system develops, which subsets seed our tissues and when. Yet, an emerging body of recent literature continues to uncover new aspects of early life immunology – be it the appearance of a new population of antigen-presenting cells in the gut around P14⁴, or long-lasting rewiring of the gut immune landscape following prenatal maternal infection⁵.

The preprint by Lopez and colleagues adds insights into how prenatal inflammation shapes the immune repertoire of another important barrier site – the lung. In a world where child-borne diseases – with allergies and asthma leading the way – become more and more prevalent, it is crucial to not only gain a better understanding of the underlying causes, but also shed light on how the immune system changes – evolves – over time. Given the highly plastic nature of several immune subsets during the neonatal window, distinct antigen exposure and infection during that period may predispose individuals to chronic respiratory disease. This preprint pinpoints prenatal inflammation and subsequent allergen exposure after birth as an example of that, highlighting ILCs in the lung as an instrumental player in establishing lung pathology. Taking a holistic approach and studying evolving immune populations as they adapt to their tissue of residence (and crosstalk with cells in their microenvironment) appears to be important in understanding chronic respiratory diseases. It also demarcates areas of research that I am personally very excited about.

 

Future directions and questions for the authors

• ILC2s exhibit maximal proliferation during the first two weeks of life in mice and peak in cellularity at P14. This time period appears to be a common window witnessing tissue seeding with and expansion of a number of immune subsets (adaptive lymphocytes, antigen-presenting cells etc.) across barrier sites. Do you think there could be overlapping mechanisms facilitating these seeding mechanisms across tissues (f.e. an increase in microbial and antigen exposure) and affecting multiple immune subsets?
• In supplementary figure 2, you show that T cells only significantly responded to papain challenge in offspring animals of saline-, but not poly(I:C)-treated mice. Do you reckon there could be compensatory mechanisms among different immune subsets upon challenge? Or in other words, do you think the increase in ILC number in the postnatal lung following prenatal inflammation could mask a typical TH2 response upon allergen exposure?
• Related to 2): In profiling the postnatal immune landscape in the lung, mostly adaptive subsets seem to be affected and skewed by ILC2 expansion following prenatal inflammation. Could this be due to a delayed arrival and hence increased plasticity of most adaptive lymphocytes in the tissue post-birth, compared to innate counterparts such as embryonically established macrophages?
• What do you think causes the downregulation of IL-33 in the lung and are epithelial cells the only source of it? You raise the intriguing possibility of ST2+ (IL33-R) ILC2s acting as a ‘sink’ for IL-33 and thus depriving other immune cells (f.e. Tregs) of it. What are the main IL-33-responding, i.e. ST2+, immune subsets in the postnatal lung?
• Are there any records of asthmatic children or patients of chronic asthma where the mother has suffered an infection (or the like) during pregnancy?
• What could be the maternal factor(s) inducing a reprogramming of ILC progenitors in the embryo upon prenatal inflammation? A paper by Lim et al. for instance has demonstrated a role for IL-6 in rewiring a baby’s gut immune system upon infection of the pregnant mother.

 

References

1          Bartemes, K. R. et al. IL-33-responsive lineage- CD25+ CD44(hi) lymphoid cells mediate innate type 2 immunity and allergic inflammation in the lungs. J Immunol 188, 1503-1513 (2012). https://doi.org:10.4049/jimmunol.1102832

2          Gold, M. J. et al. Group 2 innate lymphoid cells facilitate sensitization to local, but not systemic, TH2-inducing allergen exposures. J Allergy Clin Immunol 133, 1142-1148 (2014). https://doi.org:10.1016/j.jaci.2014.02.033

3          Klein Wolterink, R. G. et al. Pulmonary innate lymphoid cells are major producers of IL-5 and IL-13 in murine models of allergic asthma. Eur J Immunol 42, 1106-1116 (2012). https://doi.org:10.1002/eji.201142018

4          Akagbosu, B. et al. Novel antigen-presenting cell imparts Treg-dependent tolerance to gut microbiota. Nature 610, 752-760 (2022). https://doi.org:10.1038/s41586-022-05309-5

5          Lim, A. I. et al. Prenatal maternal infection promotes tissue-specific immunity and inflammation in offspring. Science 373, eabf3002 (2021). https://doi.org:10.1126/science.abf3002

Tags: allergy, asthma, ilcs, lung, neonatal immunology

doi: https://doi.org/10.1242/prelights.36444

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