Brain endothelial CXCL12 attracts protective natural killer cells during ischemic stroke

Abstract

Background The innate lymphoid cell (ILC) family consists of NK cells, ILC type 1, 2, 3 and lymphoid tissue inducer cells. They have been shown to play important roles in homeostasis and immune responses and are generally considered tissue resident. Not much is known about the presence of ILC members within the central nervous system and whether they are tissue resident in this organ too. Therefore, we studied the presence of all ILC members within the central nervous system and after ischemic brain insult. Methods We used the photothrombotic ischemic lesion method to induce ischemic lesions within the mouse brain. Using whole-mount immunofluorescence imaging, we established that the ILCs were present at the rim of the lesion. We quantified the increase of all ILC members at different time-points after the ischemic lesion induction by flow cytometry. Their migration route via chemokine CXCL12 was studied by using different genetic mouse models, in which we induced deletion of Cxcl12 within the blood–brain barrier endothelium, or its receptor, Cxcr4, in the ILCs. The functional role of the ILCs was subsequently established using the beam-walk sensorimotor test. Results Here, we report that ILCs are not resident within the mouse brain parenchyma during steady-state conditions, but are attracted towards the ischemic stroke. Specifically, we identify NK cells, ILC1s, ILC2s and ILC3s within the lesion, the highest influx being observed for NK cells and ILC1s. We further show that CXCL12 expressed at the blood–brain barrier is essential for NK cells and NKp46+ ILC3s to migrate toward the lesion. Complementary, Cxcr4-deficiency in NK cells prevents NK cells from entering the infarct area. Lack of NK cell migration results in a higher neurological deficit in the beam-walk sensorimotor test. Conclusions This study establishes the lack of ILCs in the mouse central nervous system at steady-state and their migration towards an ischemic brain lesion. Our data show a role for blood–brain barrier-derived CXCL12 in attracting protective NK cells to ischemic brain lesions and identifies a new CXCL12/CXCR4-mediated component of the innate immune response to stroke.

Innate lymphoid cells in neuroinflammation

“…Even though ILCs are a minor population in healthy CNS and their functions overlap with those of T-cells, they can play an important role in CNS homeostasis and the development of CNS pathologies. ILCs can promote pathological CD4 + T-cell responses in the CNS during autoimmune neuroinflammation ( Grigg et al, 2021 ), pathogenesis of ischemia ( Gan et al, 2014 ; Zhang et al, 2014 ) and neurodegenerative diseases such as AD ( Zhang et al, 2020 ) and ALS ( Garofalo et al, 2020 ), and neurodevelopmental delay during CMV infection ( Kveštak et al, 2021 ), but can also confer tissue protection ( Gadani et al, 2017 ; Earls et al, 2020 ; Fung et al, 2020 ; Wang et al, 2023 ). Understanding ILC development and functions, as well as characterization of ILC subsets is continuously evolving.…”

“…In the case of ischemic disease, NK cells have been reported to have both beneficial and detrimental roles ( Gan et al, 2014 ; Zhang et al, 2014 ; Wang et al, 2023 ). NK cells accumulate in brain infarction in both humans and mice ( Gan et al, 2014 ; Zhang et al, 2014 ).…”

“…In another study, the CXCL10-CXCR3 axis contributed to NK cell accumulation in ischemic brain tissue, and NK cells were found to promote the necrosis of neural cells via IFN-γ ( Zhang et al, 2014 ). In contrast, NK cells had a protective role following the induction of photothrombotic ischemia ( Wang et al, 2023 ). ILC1s, ILC2s and ILC3s were also located within the lesion, but the highest influx was observed for NK cells and ILC1s.…”

“…C-X-C motif chemokine ligand 12 (CXCL12) expression at the BBB was needed for the recruitment of NK cells toward the lesion in a CXCR4-dependent manner. Importantly, NK cells alleviated neurological deficits of mice as observed by beam-walk sensorimotor test ( Wang et al, 2023 ). The underlying cause for the different roles of NK cells in between the studies is not currently clear.…”

“…The underlying cause for the different roles of NK cells in between the studies is not currently clear. However, different models were used, characterized with different kinetics of NK cells which could potentially explain the discrepancies ( Gan et al, 2014 ; Zhang et al, 2014 ; Wang et al, 2023 ). Interestingly, NK cells contribute to the clearance of injured sensory axons in peripheral nerves.…”