Aging is a major risk factor for the development of nervous system functional decline, even in the absence of diseases or trauma. The axon–myelin units and synaptic terminals are some of the neural structures most vulnerable to aging-related deterioration1,2,3,4,5,6, but the underlying mechanisms are poorly understood. In the peripheral nervous system, macrophages—important representatives of the innate immune system—are prominent drivers of structural and functional decline of myelinated fibers and motor endplates during aging7. Similarly, in the aging central nervous system (CNS), microglial cells promote damage of myelinated axons and synapses8,9,10,11,12,13,14,15,16,17,18,19,20. Here we examine the role of cytotoxic CD8+ T lymphocytes, a type of adaptive immune cells previously identified as amplifiers of axonal perturbation in various models of genetically mediated CNS diseases21 but understudied in the aging CNS22,23,24,25. We show that accumulation of CD8+ T cells drives axon degeneration in the normal aging mouse CNS and contributes to age-related cognitive and motor decline. We characterize CD8+ T-cell population heterogeneity in the adult and aged mouse brain by single-cell transcriptomics and identify aging-related changes. Mechanistically, we provide evidence that CD8+ T cells drive axon degeneration in a T-cell receptor- and granzyme B-dependent manner. Cytotoxic neural damage is further aggravated by systemic inflammation in aged but not adult mice. We also find increased densities of T cells in white matter autopsy material from older humans. Our results suggest that targeting CD8+ CNS-associated T cells in older adults might mitigate aging-related decline of brain structure and function.
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The sequencing data are available from the Gene Expression Omnibus under accession number GSE138891. Source data are provided with this paper. Other data that support the findings of this study are available from the corresponding author.
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We thank M. Hörner and S. Popp for initial advice regarding the NOR analysis and D. Klein for fruitful discussions and suggestions. We thank H. Blazyca, S. Loserth and B. Meyer for technical assistance and J. Schreiber, A. Weidner and T. Bimmerlein for their attentive care of mice. We thank the Edinburgh Brain and Tissue Bank for providing the postmortem brain tissue. This work was supported by the German Research Foundation (grant no. MA1053/6-2 to R.M. and grant no. GR5240/1-1 to J.G.), the Roman, Marga und Mareille Sobek Foundation (to R.M.), the Charitable Hertie Foundation (grant no. P1150084 to J.G.) and the Interdisciplinary Centre for Clinical Research of the University of Würzburg (grant no. A-302 to R.M. and grant no. Z-6 to P.A.).
The authors declare no competing interests.
Peer review information Nature Aging thanks the anonymous reviewers for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended Data Fig. 1 Aging results in myelin alterations and T cell-associated axon degeneration in the CNS.
a, Representative electron micrographs of optic nerve cross-sections from adult (12-month-old) and aged (24-month-old) mice. Redundant myelin and myelin fragments (open arrowheads) as well as axonal spheroids typically showing swelling and accumulation of organelles (asterisks) or neurofilaments (hashtag) are detectable in aged but not adult nerves. Additionally, periaxonal vacuoles (circle) as well as degenerating fibres (arrowhead) are occasionally present in aged nerves. Scale bars, 2 μm. b, Z-stack projections of YFPtg axons in optic nerves and c, YFPtg axon terminals in the superior colliculi of adult and aged Thy1-YFP-H transgenic mice. Irregular and discontinuous axons with spheroids (arrows) and reduced density of axon terminals are observed in aged mice. Scale bar, 20 μm. d, Automated optokinetic response analysis of visual acuity (c/d, cycles per degree) shows no significant difference between adult and aged mice. Only at a more advanced age (28-month-old), there is a significant decline of visual acuity. (n = 5 mice per group, one-way ANOVA with Bonferroni’s multiple comparisons test, F (2, 11) = 12.95, P = 0.001). e, Immunofluorescent detection and quantification of CD4+ T lymphocytes (arrow) in optic nerves from mice at distinct ages and aged Rag1−/− mice. Densities of CD4+ T cells are significantly increased in aged compared with adult mice (n = 5 mice per group, one-way ANOVA with Bonferroni’s multiple comparisons test, F (5, 24) = 16.58, P < 0.001). Scale bar, 20 µm. f, Immunofluorescent detection and Z-stack 3D reconstruction of CD8+ T lymphocytes and SMI32+ axonal spheroids in an aged optic nerve by confocal microscopy (see also Supplementary Video 1). Higher magnification of individual optical slices and quantifications show proximity (< 10 µm) of CD8+ T cells to SMI32+ axonal spheroids and vice versa (n = 150 CD8+ cells and 211 SMI32+ profiles from 5 aged mice). Scale bars, 10 µm. g, Electron micrograph of a longitudinal optic nerve section and quantification shows that most damaged axons (axonal spheroids or degenerating axons; n = 102 from 5 aged mice) are ensheathed by myelin. Scale bar, 1 µm.
a, Optical coherence tomography (OCT) analysis of the innermost retinal composite layer (NFL/GCL/IPL, nerve fibre layer, ganglion cell layer, inner plexiform layer) in peripapillary circle scans at distinct ages demonstrates retinal thinning in aged (24-month-old) compared with adult (12-month-old) mice (n = 5 mice per group, repeated measures one-way ANOVA with Bonferroni’s multiple comparisons test, F (2.039, 8.154) = 31.52, P < 0.001, each circle represents the mean value of one mouse), v- blood vessel. b, Retinal area is not changed during aging (n = 5 mice per group, one-way ANOVA, F (4, 20) = 1.757, P = 0.18). c, Histological and d, immunofluorescent visualization of neurons in the ganglion cell layer (GCL) of retinal flat mount preparations. Scale bars, 50 μm. e, f, Quantification of respective cell densities in adult and aged mice in the middle region of the retina. Densities of Nissl-stained GCL neurons are not significantly reduced (n = 5 mice per group, one-way ANOVA, F (4, 20) = 2.217, P = 0.10), whereas RBPMS+ retinal ganglion cells (of which 91% were also Brn3a+) but not ChAT+ displaced amacrine cells are significantly decreased in aged compared with adult mice (n = 4 mice per group, two-sided Student’s t-test, RBPMS+: t = 4.339 df = 6, ChAT+: t = 1.008, df = 6). g, Brain weights of aged mice are mildly reduced compared with adult mice (n = 5 mice per group, two-sided Student’s t-test, t = 2.466, df = 8).
a, Representative example of immunohistochemical identification of YFPtg SMI32+RBPMS+ alpha-RGCs (arrow) in retinal flat mount preparations from Thy1-YFP-H transgenic mice for subsequent neurite tracing and Sholl analysis. Scale bar, 50 μm b, Tracing and Sholl analysis of the complete dendritic arbor (purple) in z-stack projections of YFPtg alpha-RGCs (Extended Data Fig. 1b) demonstrates a significant reduction in total dendrite length and ramification index of aged (24-month-old) compared with adult (12-month-old) Thy1-YFP-H transgenic mice (n = 9 RGCs from 3 mice per group, two-sided Student’s t-test, dendrite length: t = 9.867 df = 4, ramification index: t = 4.501, df=4. Each circle represents one RGC and each diamond represents the mean value of one mouse). Scale bar, 50 μm. c, Representative plots of Sholl analysis for traced dendritic arbors of alpha-RGCs from adult and aged mice.
Extended Data Fig. 4 Aging-related accumulation of CD8+ T cells in the CNS is specific to the white matter.
a, Immunofluorescence-based quantification of CD8+ T cells and SMI32+ axonal spheroids in distinct CNS white matter regions of adult (12-month-old) and aged (24-month-old) mice (each circle represents the mean value of one mouse). Densities of CD8+ T cells and SMI32+ axonal spheroids are significantly increased in optic nerve (also shown in Fig. 1e,f, n = 5 mice per group), corpus callosum (n = 4 mice per group), and cerebellar white matter (n = 4 mice per group) of aged mice (two-sided Student’s t-test, CD8+: optic nerve: t = 12.95, df = 8, corpus callosum: t = 4.849, df = 6, cerebellar white matter: t = 4.025, df = 6, SMI32+: optic nerve: t = 6.247, df = 8, corpus callosum: t = 9.003, df = 6, cerebellar white matter: t = 5.846, df = 6). b, Quantification of CD8+ T cells in distinct CNS gray matter regions of adult and aged mice. Densities of CD8+ T cells are not significantly increased in retina (n = 5 mice per group), frontal cortex (n = 4 mice per group), and cerebellar cortex (n = 4 mice per group) of aged mice (two-sided Student’s t-test, retina: t = 1, df = 8, frontal cortex: t = 0.6547, df = 6, cerebellar cortex: t = 1.163, df = 6). c, Immunofluorescent detection of MHC-I on Iba1+ microglia (arrowheads) or d, on APC+ oligodendrocytes (arrows) in optic nerves from adult and aged mice. Scale bars, 20 µm. e, Quantification reveals increased percentages of MHC-I+ oligodendrocytes and MHC-I+ microglia in aged nerves (n = 5 mice per group, two-sided Student’s t-test, oligodendrocytes: t = 5.425, df = 8, microglia: t = 8.794, df = 8).
Extended Data Fig. 5 Aging-related CD8+ T cell-driven axon degeneration responds to systemic challenge.
a, Electron microscopy-based quantification of profiles showing axonal spheroid formation or undergoing degeneration in optic nerves (each circle represents the mean value of one mouse). Scale bar, 2 μm. Absence of adaptive immune cells attenuates axon damage in aged (24-month-old) mice (n = 4 mice per group, one-way ANOVA with Bonferroni’s multiple comparisons test, F (2, 9) = 10.7, P = 0.004). b, Axonal damage is reestablished to Wt levels in aged Rag1−/− BMC Wt mice but not in Rag1−/− BMC Cd8−/−, BMC Gzmb−/−, or BMC OT-I mice (n = 4 mice per group, one-way ANOVA with Bonferroni’s multiple comparisons test, F (3, 12) = 13.2, P < 0.001). c, LPS injection aggravates axonal damage in aged Wt but not aged Cd8−/− or adult (12-month-old) Wt mice (n = 4 mice per group, one-way ANOVA with Bonferroni’s multiple comparisons test, F (3, 12) = 49.17, P < 0.001). d, OCT analysis of the innermost retinal composite layer (NFL/GCL/IPL) in peripapillary circle scans. Aging-related retinal thinning is attenuated in Rag1−/− mice (n = 5 mice per group, one-way ANOVA with Bonferroni’s multiple comparisons test, F (2, 12) = 46.27, P < 0.001). e, LPS injection enhances retinal thinning in aged Wt but not aged Cd8−/− or adult Wt mice (n = 5 mice per group, one-way ANOVA with Bonferroni’s multiple comparisons test, F (3, 16) = 95.37, P < 0.001). f, Quantification of RBPMS+ RGCs in retinae from adult, aged, and aged Rag1−/− mice. As opposed to normal aged mice, there is no significant loss of RGCs detectable in mice lacking lymphocytes (n = 4 mice per group, one-way ANOVA with Bonferroni’s multiple comparisons test, F (2, 9) = 9.356, P = 0.006). Scale bar, 20 μm. g, Quantification of RBPMS+ RGCs in aged Rag1−/− BMC Wt and Rag1−/− BMC Cd8−/−, BMC Gzmb−/−, or BMC OT-I mice mice (n = 4 mice per group, one-way ANOVA with Bonferroni’s multiple comparisons test, F (3, 12) = 6.973, P = 0.006). h, Electron microscopy-based estimation of total axonal numbers in optic nerves of adult and aged Cd8−/− mice (n = 5 mice per group, two-sided Student’s t-test, t = 4.107, df = 10). Scale bar, 2 μm. i, Novel object recognition test shows no aging-related impairment of memory in Cd8−/− mice (n = 5 mice per group, two-sided Student’s t-test, t = 1.036, df = 8).
Extended Data Fig. 6 scRNA-seq reveals heterogeneity and activation of CD8+ T lymphocytes in the aged CNS.
a, UMAP visualization of CD8+ T cells freshly sorted from adult (12-month-old, n = 5) and aged (24-month-old, n = 4) mouse brains and analyzed by scRNA-seq. Combined (top, 8,809 cells) and separate visualization of cells from adult (middle, 4,338 cells) and aged (bottom, 4,471 cells) brains are displayed. b, Heatmap of top 10 cluster-specific genes. The color scale is based on a z-score distribution from -2 (purple) to 2 (yellow). c, Heatmaps of differentially expressed genes comparing cells isolated from adult and aged brains across all clusters (left) or within cluster 2 (right) as identified in panel a. d, Contribution of the samples to each cluster is displayed in percent and e, absolute numbers extrapolated to total cells per brain. f, Heatmap of differentially expressed genes between cluster 1 and cluster 3 as annotated in panel a. g, Violin plots of Lag3 (top) and Gzmb (bottom) gene expression in clusters 1 to 5. IST, interferon-stimulated T cells; TEFF, effector T cells; TCM, central memory T cells.
a, UMAP visualizations of CD8+ T cells from brains of adult and aged mice showing the expression of selected control genes (Cd3e, Cd8a) and cluster-enriched marker genes (as annotated in Extended Data Fig. 6a). Transcript levels are color-coded: gray, not expressed; red, expressed. Complete lists of cluster-specific marker genes can be found in Supplementary Table 1. b, UMAP visualizations of CD8+ T cells showing the expression of selected marker genes that indicate effector/memory function and are likely involved in tissue recruitment, residency, or egress.
a, Immunofluorescent localization of CD8+ T cells (arrows) and laminin+ blood vessels in longitudinal optic nerve and coronal brain sections of adult (6-month-old) CXCR3-GFP reporter mice. CD8+ T cells associated with the CNS parenchyma, meninges, choroid plexus and cerebrospinal fluid express CXCR3, while intravascular and circulating T cells are negative for CXCR3. Scale bar, 10 μm. b, Immunohistochemical characterization of CD8+ T cells (arrows) in distinct CNS-compartments of aged (24-month-old) Wt and adult CX3CR1-GFP or CXCR3-GFP reporter mice. Parenchymal (Paren) and meningeal (Men) CD8+ T cells (clusters 1 to 5) show heterogeneous expression of Lag3, CD103 and PD-1. A20 and high Stat1 expression is detected in meningeal and choroid plexus (CP)-resident CD8+ T cells (IST). CD8+CD62L-CX3CR1+ TEFF and CD8+CD62L+CXCR3- TCM2 are detected in blood and CD8+CD62L+CXCR3+ TCM1 are detected in meninges and cerebrospinal fluid (CSF; inset). Scale bar, 10 μm. Micrographs represent three independent experiments with similar results.
a, Violin plots showing the expression of selected genes with significant differences between CD8+ T cells from brains of adult (12-months-old) and aged (24-month-old) mice when comparing cells from all clusters (as annotated in Extended Data Fig. 6a). Upper panels show genes enriched in CD8+ T cells from adult mice, lower panels show genes enriched in CD8+ T cells from aged mice. b, Heatmaps showing differentially expressed genes between adult and aged mice in each cluster. c, Violin plots showing the expression of selected genes with significant differences between CD8+ T cells from brains of adult and aged mice within cluster 2. Upper panels show genes enriched in CD8+ T cells from adult mice, lower panels show genes enriched in CD8+ T cells from aged mice. d, Violin plots showing the expression of selected genes with significant differences between CD8+ T cells from cluster 1 and cluster 3. Upper panels show genes enriched in cluster 1, lower panels show genes enriched in cluster 3. e, Overlap of significantly decreased (top) or enriched (bottom) transcripts when comparing cells from aged with adult mice over all clusters (upper-left circle), aged with adult mice within cluster 2 (upper-right circle) and cells from cluster 1 against cluster 3 (lower circle). There are global and cluster-specific transcriptional changes in CD8+ T cells during brain aging. Complete lists of genes with significant differences in expression for all comparisons can be found in Supplementary Table 1.
a, Representative plots of general flow cytometric gating of single, viable CD45highCD8+ T cells freshly isolated from brains. b, Gated CD8+ T cells from adult (12-month-old) mice or c, from aged (24-month-old) mice are analyzed for expression of CD62L, CX3CR1, IL7R, PD-1 and CD103. Percentages and identity of the respective cells are indicated in the quadrants or below the gate. CD8+ T cells comprise small populations of CD62L+CX3CR1- cells (TCM1 and 2) and CD62L-CX3CR1+ cells (TEFF). TCM1 and 2 show high IL7R and mostly low PD-1 expression. Among the remaining CD62L-CX3CR1- cells, some are IL7R-CD103- (cluster 3) and IL7R+CD103+ (cluster 2), but the majority are IL7R+CD103- cells which can be further subdivided by PD-1 expression (clusters 1, 5, and IST being PD-1+). There is an increase in PD-1 expression on CD8+ CNS-associated T cells during aging (n =3 mice per group).
Supplementary Fig. 1 and Table 2.
Cluster-specific marker genes, differentially expressed genes and GO lists for the scRNA-seq data.
Z-stack 3D reconstruction of CD8+ T lymphocytes and SMI32+ axonal spheroids in an aged optic nerve by confocal microscopy.
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Groh, J., Knöpper, K., Arampatzi, P. et al. Accumulation of cytotoxic T cells in the aged CNS leads to axon degeneration and contributes to cognitive and motor decline. Nat Aging 1, 357–367 (2021). https://doi.org/10.1038/s43587-021-00049-z
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