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., 2019; Dansokho et al., 20016; Zenaro 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
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