Glial cell activation in the hippocampus of THY-Tau22 mice. (A and B) As seen using an antibody revealing pSer422 immunoreactivity, THY-Tau22 mice exhibit a high level of abnormally phosphorylated tau species in the CA1 region of hippocampus at 12 months of age. (C and D) CD11b immunostaining point out progressive microglial (C, arrows) and astroglial (D) reponses in the hippocampal CA1 area of THY-Tau22 mice (bottom) as compared to wild-type (WT) animals (top). n = 3/group. Scale bars = 500 µm (B); 100 µm (C and D).

Age-dependent upsurge of chemokines in the hippocampus of THY-Tau22 mice. (A) Quanititative PCR analysis of Ccl3, Ccl4, Ccl5 and Cxcl5 mRNAs revealed a significant and generally progressive overexpression in the hippocampus of transgenic animals as compared to wild-type (WT). Results are expressed as means ± SEM. ^*P < 0.05, ^**P < 0.01,^***P < 0.001 versus wild-type (3 months) using using one-way ANOVA followed by a post hoc Fisher’s LSD test. n = 5–13/group. (B) ELISA determinations of hippocampal CCL3, CCL4 and CCL5 levels in wild-type and THY-Tau22 mice at all ages. The three chemokines were found significantly increased in tau animals, CCL3 being the earliest upregulated chemokine. Results are expressed as means ± SEM. ^*P < 0.05, ^**P < 0.01,^***P < 0.001 versus wild-type (3 months) using using one-way ANOVA followed by a post hoc Fisher’s LSD test. n = 6–8/group. (C) Immunofluorescence analysis of CCL3 expression in the hippocampus of THY-Tau22 mice and wild-type littermates at the age of 12 months. Labelling of CCL3 (red) with the microglial marker Iba1 or the astrocyte marker GFAP (green) revealed clustered CCL3 staining solely in Iba1-positive cells (arrowheads and inset) in transgenic tau mice. To a lesser extent, such clusters are present in wild-type animals. Scale bar = 20 µm.

Hippocampal CD8⁺ T cell infiltration in the parenchyma of THY-Tau22 mice. (A–C) Top: CD8 immunostaining highlights numerous parenchymal lymphocytes in the whole hippocampus of tau transgenic animals. Infiltrated lymphocytes were clearly identified at higher magnification (red arrows in bottom panels). Scale bars = 500 µm (top) and 50 µm (bottom panels). (D) Hippocampal density of CD8⁺ T cells was found significantly increased from 7 months of age. Results are expressed as means ± SEM. ^*P < 0.05, ^**P < 0.01 versus wild-type mice using one-way ANOVA followed by a post hoc Fisher’s LSD test. n = 3/group.

Presence of parenchymal CD8⁺ cells in the cortex of patients with frontotemporal dementia (FTDP-17) carrying P301L mutations. (A) Western blot analysis performed on the cortex of patients revealed, as expected, tau hyperphosphorylation at pSer396 epitope as compared to age-matched controls (n = 3/group). (B) Immunohistochemical detection of CD8⁺ T cells in the human cortex of controls and patients. Whereas CD8⁺ T cells were exclusively distributed along the blood vessels (indicated by an asterisk) in control individuals, parenchymal (arrows) CD8⁺ cells are observed at distance from the blood–brain barrier in patients. Scale bar = 100 µm.

Blood–brain barrier integrity of THY-Tau22 mice. Immunofluorescence labelling of the tight junction marker zonula occludens-1 (ZO-1; red) and the constitutive endothelial marker von Willebrand factor (VWF; green) in the CA1 region of the hippocampus of wild-type (WT; top) and THY-Tau22 mice (bottom) at 12 months of age. DAPI staining of nuclei in blue. Similar co-stainings was observed in both genotypes, reflecting an absence of major blood–brain barrier impairment in transgenic animals. n = 3/group. Scale bar = 20 µm.

Impact of T cell depletion on spatial memory and hippocampal tau pathology in THY-Tau22 mice. (A) Effect of anti-CD3 antibody treatment on spatial memory using Y-maze task. While wild-type (WT) mice, regardless of treatment, exhibited a preference for the new arm, compared to the familiar arm, THY-Tau22 treated with the isotypic control did not demonstrate preference for the new arm, showing defective spatial memory. By contrast, anti-CD3 treated THY-Tau22 mice demonstrated a preference for the new arm similarly to control animals. Results are expressed as means ± SEM. ^***P < 0.001 Familiar/Other (O) versus New (N) arms using one-way ANOVA. n = 6–19/group. (B) Effect of T cell depletion on hippocampal markers of synaptic plasticity. Hippocampal Arc and 14-3-3 immunoreactivities were found significantly decreased in THY-Tau22 mice treated with isotypic control versus wild-type mice and return to control level in THY-Tau22 mice treated with the anti-CD3 antibody. ^*P < 0.05, versus wild-type isotype, ^#P < 0.05 and ^##P < 0.01 versus THY-Tau22 isotype group, using using one-way ANOVA followed by a post hoc Fisher’s LSD test. n = 3–4/group. Hippocampal density of CD8⁺ T cells in the different experimental groups. Density of CD8⁺ T cells was significantly increased at 9 months of age in THY-Tau22 mice treated with isotype and returned to control level in THY-Tau22 mice treated with anti-CD3 antibody. ^*P < 0.05 versus wild-type isotype, ^##P < 0.01 versus THY-Tau22 isotype group, using using one-way ANOVA followed by a post hoc Fisher’s LSD test. n = 3–10/group. (D) AT8 immunostaining indicate a similar immunoreactivity in THY-Tau22 mice regardless of treatment. n = 8/group. Scale bar = 500 µM. (E) Anti-CD3 treatment did not impact human tau expression, truncation (t-tau) and hyper-phosphorylation at several phospho-epitopes in THY-Tau22 mice. n = 6–8/group. Results are expressed as a percentage of THY-Tau22 mice injected with isotype ± SEM. (F) Effect of anti-CD3 treatment upon mRNA expression of neuroinflammatory markers in the hippocampus of THY-Tau22 mice. ^**P < 0.01, ^***P < 0.001 versus wild-type isotype, ^#P < 0.05 versus THY-Tau22 isotype group, using using one-way ANOVA followed by a post hoc Fisher’s LSD test. n = 6–9/group.