T cells mingle among the immune cells that monitor the meninges, the layered membranes cradling the brain. As mice age, more and more of these cells accumulate there, according to a study published May 21 in Science Advances by Jonathan Kipnis and colleagues at Washington University in St. Louis. The researchers reported that those T cells had turned down expression of chemokine receptor 7, and taken on an immunosuppressive phenotype. These lingering inflammation dousers caused a host of problems. They impeded the movement of fluids through the brain, and skewed the transcriptomes of multiple cell types, including microglia. In a mouse model of amyloidosis, these meningeal squatters appeared to somehow exacerbate Aβ plaque deposition and memory loss.

  • T cells in aging mouse meninges downregulate chemokine receptor 7.
  • Without CCR7, T cells stall, glymph slows, memory falters.
  • CCR7 deficiency also changes neurovasculature and Aβ deposition.
  • Reducing T-regs with anti-CD25 antibodies counters some of these changes.

“This paper supports the emerging concept that the meninges are an immune organ within the brain,” said Costantino Iadecola of Weill Cornell Medical College in New York City.

Akin to satellites communicating with the Earth below, immune cells in the meninges liaise extensively with the brain parenchyma. They communicate via cytokines and other signals that researchers are just starting to understand. For example, one recent study found that meningeal gamma-delta T cells boost memory by producing IL-17 (Oct 2019 news). 

The meninges also serve as an interface with the world beyond the brain. In 2015, Kipnis and colleagues discovered the existence of a system of lymphatic channels there, which drain the brain of refuse and also serve as a transport network for immune cells (Oct 2017 news). For their part, meningeal T cells use the lymphatic channels to travel to the deep cervical lymph nodes located in the carotid sheath in the neck. At these dCLNs, the T cells touch base with other immune cells. Previously, Kipnis reported that the lymphatic system becomes sluggish with age, and that purposefully ablating it derails immune function, Aβ clearance, and cognition in mice (Jul 2018 news). 

Besides lymphatic drainage flagging with age, do meningeal T cells change their travel habits over time, as well? To investigate, first author Sandro Da Mesquita and colleagues compared the numbers and types of T cells in the meninges and dCLNs in young adult mice versus old mice. They found that compared to 2- to 3-month-old mice, 24- to 25-month-old animals had more T cells in their meninges and fewer in the dCLNs. This suggested T cells increasingly stayed put in the meninges. Furthermore, a greater proportion of T cells in both the meninges and in the dCLNs were T regulatory cells, a subtype known to exert an immunosuppressive effect.

The researchers had recently reported that meningeal T cells use CCR7 to shuttle to the dCLNs (Louveau et al., 2018). Lo and behold, they found that the proportion of meningeal T cells that expressed CCR7 roughly halved with age, suggesting this is why T cells accumulated in the meninges. In support of this idea, the researchers observed that T cells accumulated in the meninges in CCR7 knockout mice, and in wild-type mice that had their immune systems ablated and restored with CCR7-deficient bone marrow from a CCR7 knockout.

Intriguingly, removing CCR7 from immune cells not only triggered retention of T cells in the meninges and their decline in the dCLNs, it also grew the proportion of T-regs relative to T effector cells in both areas. This suggested that something about the retention of T cells in the meninges led to a shift in their milieu. The mechanisms driving the shift remain unclear, Kipnis said. “It could be that the T cells eventually become anergic, taking on a T-reg-like phenotype as they remain trapped in the meninges,” Kipnis offered. Anergic T cells no longer respond to antigens they encounter, perhaps as a tolerance mechanism. Alternatively, myeloid cells in the brain could sense that something is disturbed in the meninges, and send out signals to recruit T-regs to the scene, he said.

What are the consequences of this T-reg-leaning cadre of cells lazing about the meninges? Memory loss is one. Compared to young adult wild-type mice, CCR7 knockout or wild-type mice that had received CCR7-deficient bone marrow were less able to recognize new places or locate a hidden underwater platform. The researchers observed similar cognitive deficits in old, wild-type mice that lose CCR7 with age. Strikingly, treating them with anti-CD25 antibodies, which reduce numbers of T-regs in the meninges and in the dCLNs, boosted performance on the memory tests. The suggested that the abundance of T-regs was partly to blame for the animals’ waning cognition.

The researchers had previously reported that poor lymphatic drainage from the meninges also impaired the recirculation of cerebrospinal fluid throughout the brain via the so-called glymphatic system. Could T cell retention in the meninges trigger the same effect, even when the lymph vessels themselves were draining normally? To their surprise, the scientists found that, indeed, the influx of CSF into the brain parenchyma was severely reduced in 5- to 7-month-old mice lacking CCR7. In keeping with this, they also found reduced expression of aquaporin-4 throughout the brain vasculature in CCR7 knock-outs. This water channel sits on astrocytic end feet and helps push solutes through the glymphatic system. The findings suggest that changes in the meningeal T cell milieu somehow slow glymphatic flushing of the brain.

Plaque Boost. 5xFAD mice devoid of CCR7 (bottom) have more and larger plaques than do 5xFAD mice that express the chemokine receptor (top). [Courtesy of Da Mesquita et al., Science Advances, 2021.]

How might this affect amyloid pathology? Researchers led by Maiken Nedergaard, then at the University of Rochester Medical Center, New York, and Jeffrey Iliff, now at the University of Washington, Portland, reported that a weak glymph system exacerbates amyloid deposition (Aug 2012 news). Similarly, Kipnis and colleagues found ablating the meningeal lymph vessels did the same—both in the meninges and the parenchyma (Da Mesquita et al., 2018).

This time around, Da Mesquita generated 5xFAD mice lacking CCR7, finding that, as expected, more T cells were retained in the meninges, and more of them were T-regs. Knocking out the cytokine receptor also increased the size, number, and area of Aβ plaques in the parenchyma of 5xFAD mice, but did not trigger accumulation of Aβ in the meninges or blood vessels. Iadecola believes that poor glymphatic flow hindered clearance of Aβ from the parenchyma. He wondered whether Aβ accumulated around blood vessels in the CCR7-deficient 5xFAD mice. The findings dovetail with a recent report from Kipnis’ lab that damaged lymphatics—which also slows glymphatic flow—hindered Aβ immunotherapies from clearing Aβ from the brain (April 2021 news). 

Interestingly, the rising plaque burden was not accompanied by more microglia. On the contrary, fewer microglia gathered around plaques in CCR7-deficient 5xFAD mice than in control 5xFADs. Kipnis hypothesized that the lackluster response stems from the immunosuppressive environment imposed by T-regs in the meninges.

5xFAD lacking CCR7 also fared worse on tests of spatial learning and memory than their CCR7-replete counterparts.

To investigate how CCR7 deficiency influenced other cells in the brain and vasculature, Da Mesquita and colleagues sequenced the nuclear transcriptomes of blood endothelial cells (BECs) and brain myeloid cells taken from the whole brains of 5-month-old 5xFAD mice that did, or did not, carry the CCR7 gene. Five clusters emerged based on transcriptome commonalities. They represented microglia, border-associated macrophages (BAMs), arterial BECs, capillary BECs, and venous BECs.

Among microglia, the researchers detected 718 genes that were upregulated in 5xFAD mice without CCR7 relative to those expressing the chemokine receptor. These included some genes for familiar proteins—Apoe, Axl, Fth1, Lpl, Lyz2, and Trem2—many of which rev up in disease-associated microglia in mouse models of amyloidosis. Among BECs, genes involved in lysosomal function were ramped up in the absence of CCR7. BECs also turned up expression of genes for proteins involved in antigen processing and presentation, and leukocyte migration. Together, the findings point to lysosomal and vascular dysfunction in 5xFAD mice lacking CCR7, possibly reflecting a response to their higher amyloid load, and/or signals emanating from the meninges packed with T cells.

Kipnis is the first to admit that the paper raises more questions than it answers. For one, what triggers meningeal T cells to turn down CCR7 with age? How does their accumulation in the meninges lead to a shift toward regulatory T cells? How do the sequestered meningeal T cells influence glymphatic flow, neuronal function, and microglial handling of Aβ plaques? And of course, how many of these mechanisms are at play in the aging human brain?

“The study implicates loss of CCR7 in reshaping meningeal immunity, and the impact of this shift on the glymphatic system, cognitive functions, and Aβ deposition is intriguing,” commented Guillaume Dorothée of INSERM in Paris. Whether these effects are mediated by T-regs needs further investigation, Dorothée added, noting that depletion of CCR7 also boosted numbers of meningeal CD8+ T cells, B cells, macrophages, and neutrophils within the meninges. Recent reports have implicated B cells and neutrophils in exacerbating amyloidosis and cognitive deficits, while studies from Dorothée's lab have found beneficial roles for T-regs (Kim et al., 2021; Cruz Hernandez et al., 2019Dansokho et al., 20016Zenaro et al., 2015). Future studies could address how these different cell types, and their interplay, influence different processes in the brain, he said.—Jessica Shugart

Comments

  1. The meningeal lymphatic system and its neuro-immune interactions are of great importance. These regulate processes that range from learning and behavior to neuroinflammation and homeostasis. Aging has been shown to affect meningeal lymphatics and the immune system and is the greatest risk factor for the development of dementias, such as Alzheimer’s disease. Despite its importance, the role of aging in meningeal immunity has not been elucidated.

    In this exciting publication, De Mesquita et al. characterized the meningeal immune profile of old mice. Importantly, this allowed them to identify reduced CCR7 expression on T cells. The authors then discovered that the meningeal T cell composition in CCR7-deficient mice mimics that of aged mice. This deficiency also resulted in impaired glymphatic function, poorer cognition, and increased Aβ deposition in a mouse model of Alzheimer’s disease.

    The observed changes are profound and result in altered microglial and blood endothelial cell transcriptomes. This discovery will further allow us to understand the common mechanisms responsible for aging-associated meningeal disfunction and cognitive decline. This discovery has the potential to be the basis for new therapeutics that target CCR7-mediated egress of cells from the brain in neurodegenerative diseases such as Alzheimer’s disease.

  2. Within the last few years, the concept of “immune privilege” of the central nervous system has been revisited and modified. Growing evidence has demonstrated complex interactions between the brain and both the innate and adaptive immune systems that have important implications for development, homeostasis, normal aging, and disease. The meningeal lymphatic system is one major compartment enabling such interactions (Salvador et al., 2021). In this important study, Da Mesquita et al. provide further evidence that its perturbation contributes to aging-related decline of neural function in mice.

    They observed that with normal aging, meningeal CD4+ T cells reduce expression of CCR7, a receptor involved in egress and homing of lymphocytes to the CNS-draining, deep cervical lymph nodes. This was accompanied by increased numbers of CD4+ and CD8+ T cells and a higher proportion of CD4+CD25+FoxP3+ Treg cells in the meninges. Similar alterations were identified in adult-bone-marrow chimeras lacking CCR7 in hematopoietic cells, together with a reduced proportion of T-bethigh CD4+ and CD8+ effector T cells and lower IFNγ expression in the meninges.

    Intriguingly, CCR7 deficiency resulted in a decline of spatial memory and glymphatic function. They also detected downregulation of CCR7 in the 5xFAD mouse model of Alzheimer’s-like disease. CCR7 deficiency exacerbated Aβ accumulation and spatial memory defects, and resulted in transcriptional changes in brain endothelial cells and microglia in the AD model, reflecting increased neuroinflammation. Acute treatment of old mice with anti-CD25 antibodies reduced meningeal Treg numbers and improved spatial memory. Together, these findings indicate that decreased expression of CCR7 and increased Treg responses might contribute to aging-associated cognitive impairment. This has important translational implications.

    The work elegantly complements previous studies demonstrating that IFNγ-producing meningeal CD4+ T cells support cognitive function. However, it also raises important questions regarding the putative causes and mechanisms linking the described aging-related alterations. Our recent work has demonstrated that accumulation of cytotoxic CD8+ T cells in white-matter compartments of the normal aging CNS leads to axon degeneration and contributes to decline of non-spatial memory (Groh et al., 2021). Similarly, a detrimental role of CD8+ T cells has been proposed for Alzheimer’s disease based on findings in patients (Gate et al., 2020) and mice (Unger et al., 2020). Using scRNA-Seq, we also found that different subpopulations of CD8+ CNS-associated T cells expressed Ccr7 (mostly in the CSF and meninges) and downregulated Ifnγ expression with aging, in line with the results of Da Mesquita et al. Yet, IFNγ production by CD8+ T cells has been proposed to impair the proliferation of neural stem cells in the aged ependyma (Dulken et al., 2019). Since the present study showed that CCR7 deficiency and impaired meningeal lymphatic function result in cognitive deficits and neurovascular and microglial activation, it would be highly interesting to also study the effects on parenchymal infiltration of T cells and on neurodegenerative alterations. This might emphasize the distinct roles of CD4+ and CD8+ T cells in aging-related decline of brain function, and provide another example for the complexity of neuro-immune interactions.

    References:

    . Neuromodulation by the immune system: a focus on cytokines. Nat Rev Immunol. 2021 Mar 1; PubMed.

    . Accumulation of cytotoxic T cells in the aged CNS leads to axon degeneration and contributes to cognitive and motor decline. Nat Aging 1, 2021

    . Clonally expanded CD8 T cells patrol the cerebrospinal fluid in Alzheimer's disease. Nature. 2020 Jan;577(7790):399-404. Epub 2020 Jan 8 PubMed.

    . CD8+ T-cells infiltrate Alzheimer's disease brains and regulate neuronal- and synapse-related gene expression in APP-PS1 transgenic mice. Brain Behav Immun. 2020 Oct;89:67-86. Epub 2020 May 29 PubMed.

    . Single-cell analysis reveals T cell infiltration in old neurogenic niches. Nature. 2019 Jul 3; PubMed.

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References

News Citations

  1. Do Immune Cells in the Meninges Help with … Memory?
  2. Lymphatic Vessels Found in Human Brain
  3. Lymphatic Brain Drain Withers in Aging, Worsens Disease
  4. Brain Drain—“Glymphatic” Pathway Clears Aβ, Requires Water Channel
  5. Does Anti-Amyloid Immunotherapy Need the Lymphatic System?

Paper Citations

  1. . CNS lymphatic drainage and neuroinflammation are regulated by meningeal lymphatic vasculature. Nat Neurosci. 2018 Oct;21(10):1380-1391. Epub 2018 Sep 17 PubMed.
  2. . Functional aspects of meningeal lymphatics in ageing and Alzheimer's disease. Nature. 2018 Aug;560(7717):185-191. Epub 2018 Jul 25 PubMed.
  3. . Therapeutic B-cell depletion reverses progression of Alzheimer's disease. Nat Commun. 2021 Apr 12;12(1):2185. PubMed.
  4. . Neutrophil adhesion in brain capillaries reduces cortical blood flow and impairs memory function in Alzheimer's disease mouse models. Nat Neurosci. 2019 Mar;22(3):413-420. Epub 2019 Feb 11 PubMed.
  5. . Regulatory T cells delay disease progression in Alzheimer-like pathology. Brain. 2016 Apr;139(Pt 4):1237-51. Epub 2016 Feb 1 PubMed.
  6. . Neutrophils promote Alzheimer's disease-like pathology and cognitive decline via LFA-1 integrin. Nat Med. 2015 Aug;21(8):880-6. Epub 2015 Jul 27 PubMed.

Further Reading

Primary Papers

  1. . Aging-associated deficit in CCR7 is linked to worsened glymphatic function, cognition, neuroinflammation, and β-amyloid pathology. Sci Adv. 2021 May;7(21) Print 2021 May PubMed.