Author: Ece Güven, Editor: Elif Duymaz , Redactor: Sema Yöşili
It is known that pro-inflammatory autoreactive T cells in the central nervous system (CNS) are responsible for autoimmune diseases with neuroinflammation, such as multiple sclerosis (MS) and also for neurodegenerative disorders like Alzheimer’s and Parkinson’s diseases. Although it has been shown that CD4+ T cells are involved in MS pathogenesis in animal models along with that major histocompatibility complex class II (MHC-II) is a risk factor in patient cohorts, the underlying mechanism for the disease has not been solved yet.
Type 3 innate lymphoid cells (ILC3s) which are a component of innate immunity have a resemblance to activated T cells, interacting with adaptive immunity and CNS. In their study, Grigg et al. investigated the role of ILC3s in autoimmunity and neuroinflammation by using a mouse model for MS with experimental autoimmune encephalomyelitis (EAE), flow cytometry to count and analyze the cell types and RNA sequencing for examining gene expression patterns.
Firstly, the researchers induced active EAE in RORγt transgenic mice labeled with enhanced green fluorescent protein (Rorc-eGFP). Following flow cytometry analysis of the brain sections of Rorc-eGFP mice, they found a significant increase in ILC3 frequencies compared to steady-state (naive); in dura mater and cervical lymph nodes (cLN) as well as CNS at all phases of EAE. These ILC3s constituted 22% of the RORγt+ cells in CNS and their numbers were elevated in the wild-type mice when neuroinflammation was induced by MOGp antigen.
Subsequently, the researchers RORγt mutant mice labeled with enhanced yellow fluorescent protein (eYFP). In EAE mice to understand the role of heterogeneity and plasticity found among ILC3s in neuroinflammation. With the intracellular staining of RORγt, eYFP+ ILC3s in CNS showed a stable expression of RORγt compared to eYFP+ ILC3s in mesenteric lymph nodes (mLN), where RORγt protein was downregulated. RNA sequencing analysis of CNS, mLN, and small intestine lamina propria (SI-LP) revealed remarkable variations in global patterns of transcription during EAE; and also proved that ILC3s in these tissues are different from AIRE-expressing cells in the periphery since none of them expressed Aire (autoimmune regulator). Moreover, cytokines (IFNγ, TNF), transcription factor (T-bet), and chemokine receptor (CCR6) making up the unique heterogeneity markers of ILC3s were found to be highly expressed in CNS-related ILC3s.
Although ILC3s are known to be tissue-resident cells in the intestine and periphery lymph nodes, their presence in CNS led the researchers to examine possible migration of these cells into CNS during neuroinflammation. Indeed, they found that cellular trafficking receptors necessary for the entrance of circulating lymphoid cells into CNS during inflammation were uniquely expressed in the CNS-associated ILC3s. Further experiments with the parabiotic partner mouse, whose circulation was surgically joined with the host mouse, proved that ILC3s in CNS were coming from the circulation as they comprised mixed origins both from the host and the parabiotic partner while ILC3s in SI-LP and cLN were of host origin. This result is consistent with the elevated levels of ILC3-like cells in blood circulation in both EAE mice and MS patients relative to naive mice and samples of control individuals (sex and age-matched), respectively.
In EAE mice, T cell activity requires MHC-II myeloid cell-mediated local activation in CNS, thus the researchers investigated the potential of CNS-associated ILC3s to present antigen and to induce CD4+ T cell response. While circulating ILC3s do not express MHCII, the ILC3s in EAE CNS were found to express high levels of transcripts for the canonical pathway of IFNγ-mediated induction of MHCII, some co-stimulatory molecules and cytokines that activate T cells, which were not expressed in tolerant tissue-resident ILC3s. Immunofluorescence staining of the brain sections that are rich in myelin from EAE Rorc-eGFP mice showed direct proximity of ILC3s with infiltrating T cells. Furthermore, CNS-associated ILC3s could capture the myelin basic protein labelled with pHrodo Red during EAE and they could process and present the MOGp antigen to the myelin-specific T cells in an MHCII-dependent manner when co-cultured with the antigen and T cells, resulting in the production of pro-inflammatory cytokines.
For testing whether the antigen presentation by ILC3s is needed for the pathogenesis due to CD4+ T cell responses in CNS during neuroinflammation, researchers induced active and passive EAE in MHCIIΔILC3 mice generated by crossing H2-Ab1-floxed mice with Rorccre mice and lacking MHC-II selectively on ILC3s, after transfer of myelin-specific T cells to track the response in vivo. In both immunizations, myelin-specific T cells in peripheral lymph nodes revealed comparable priming with the control group while the numbers of cytokine-producing myelin-specific T cells in CNS were notably decreased compared to the control group. Moreover, immune cell infiltration was reduced in the spinal cord of MHCIIΔILC3 mice and demyelinating disease was not observed.
Finally, the finding that tissue-resident ILC3s maintain tolerogenic phenotype during EAE led the researchers to test this property of ILC3s to tolerize the ILC3MOG mice against myelin-specific T cell responses in vivo. After active EAE induction, high levels of pro-apoptotic molecules were observed in myelin-specific T cells in peripheral lymph nodes. The following passive EAE stimulation revealed a remarkable reduction in the frequency of myelin-specific T cells in circulation, cLN, and CNS compared to control groups. After both passive and active induction of EAE, demyelinating disease and a significant decrease in immune cells infiltration into the spinal cord were observed, which supported the previous outcomes.
In summary, this study defines a subset of ILC3s as infiltrating cells from circulation, that can mature to present antigen in CNS to prime pro-inflammatory T cells during neuroinflammation. In contrast with this subset of ILC3s, tissue-resident ILC3s could prevent neuroinflammation by promoting T cell tolerance, thus can be manipulated to express cognate antigen through MHCII as a therapeutic approach for demyelinating disorders like MS.
Reference: Grigg, J.B., Shanmugavadivu, A., Regen, T. et al. (2021). Antigen-presenting innate lymphoid cells orchestrate neuroinflammation. Nature. https://doi.org/10.1038/s41586-021-04136-4
– Scientific News Series of Bioinfocodes-
News Articles have been written by our team members through the review and assessment of scientific researches published in 20> Impact Factor Q1 journals (click for the list).