Type 2 Innate Lymphoid Cells Accumulate in the Brain After Hypoxia-Ischemia but Do Not Contribute to the Development of Preterm Brain Injury

Abstract

Background: The immune system of human and mouse neonates is relatively immature. However, innate lymphoid cells (ILCs), commonly divided into the subsets ILC1, ILC2, and ILC3, are already present in the placenta and other fetal compartments and exhibit higher activity than what is seen in adulthood. Recent reports have suggested the potential role of ILCs, especially ILC2s, in spontaneous preterm labor, which is associated with brain damage and subsequent long-term neurodevelopmental deficits. Therefore, we hypothesized that ILCs, and especially ILC2s, play a role in preterm brain injury. Methods: C57Bl/6J mice at postnatal day 6 were subjected to hypoxia-ischemia (HI) insult induced by left carotid artery ligation and subsequent exposure to 10% oxygen in nitrogen. The presence of ILCs and ILC2s in the brain was examined at different time points after HI. The contribution of ILC2s to HI-induced preterm brain damage was explored using a conditionally targeted ILC2-deficient mouse strain (Rorα fl/fl IL7r Cre), and gray and white-matter injury were evaluated at 7 days post-HI. The inflammatory response in the injured brain was assessed using immunoassays and immunochemistry staining. Results: Significant increases in ILCs and ILC2s were observed at 24 h, 3 days, and 7 days post-HI in the injured brain hemisphere compared with the uninjured hemisphere in wild-type mice. ILC2s in the brain were predominantly located in the meninges of the injured ipsilateral hemispheres after HI but not in the brain parenchyma. Overall, we did not observe changes in cytokine/chemokine levels in the brains of Rorα fl/fl IL7r Cre mice compared with wild type animals apart from IL-13. Gray and white-matter tissue loss in the brain was not affected after HI in Rorα fl/fl IL7r Cre mice. Correspondingly, we

Single-cell atlas reveals meningeal leukocyte heterogeneity in the developing mouse brain

“…We found that the majority of meningeal immune cells were BAMs, microglia-like cells, and monocytes, whereas other cell populations, such as neutrophils, T cells, B cells, and ILCs, were present in smaller numbers. This is in agreement with previous studies in which all observed cell populations were described in the meninges of mouse embryos (Tanabe and Yamashita 2018;Utz et al 2020), neonatal mouse pups (Albertsson et al 2018;Nazmi et al 2018;Tanabe and Yamashita 2018;Zelco et al 2020), and adult mice (Gadani et al 2017;Mrdjen et al 2018;Van Hove et al 2019). BAMs are newly defined macrophages found in the meninges, choroid plexus, and perivascular spaces of brain vessels, and only a few studies have characterized the meningeal BAM populations at different developmental ages (Mrdjen et al 2018;Van Hove et al 2019;Utz et al 2020).…”

“…We identified neutrophils, ILCs, T cells, and B cells in the neonatal mouse meninges, which agreed with previous findings (Albertsson et al 2018;Nazmi et al 2018;Zelco et al 2020). These clusters were much smaller than BAM and microglia-like cell populations and contained only a few hundred cells (Fig.…”

“…Six hours is generally considered to be the limit of the therapeutic window after HI because this is when the initial injury, caused mainly by the accumulation of reactive oxygen species due to mitochondrial failure, becomes irreversible and leads to neuronal death. Previously, we found that γδT cells and ILC2s are increased in the meninges and in the brain parenchyma at 6 h after HI in a preterm brain injury mouse model (Albertsson et al 2018;Zelco et al 2020). In the current study, we found that most of the meningeal immune cell populations were not affected.…”

“…After transcardiac saline perfusion, the meninges of PND4 mice were peeled off from the brain, each hemisphere separately, and positioned on a glass slide. The staining was performed as described (Zelco et al 2020) in the Supplemental Material.…”

“…After insult in the neonatal brain, γδT cells have a detrimental effect by enhancing brain damage, more so than conventional T and B cells (Zhang et al 2017;Albertsson et al 2018;Nazmi et al 2018), which differs from the adult brain where both γδT cells (Shichita et al 2009) and conventional T cells (Yilmaz et al 2006;Hurn et al 2007) play important roles in ischemic injury. The γδT cells tend to concentrate in the meninges after brain injury (Albertsson et al 2018;Alves de Lima et al 2020) similar to innate lymphoid cells (ILCs) (Gadani et al 2017;Zelco et al 2020). However, we still know little about the immune landscape of the meninges in newborns, including whether or not the meninges represent an important site harbouring immune cells that take part in the inflammatory response and contribute to injury processes after insult to the developing brain.…”