Frequency of synaptic antigen-specific CD4+ T cells in dementia is age-dependent but not correlated with cognitive impairment

Abstract

Neurodegenerative dementias including Alzheimer disease severely impair cognitive and social abilities and are a major cause of mortality with no causal treatment yet. Autoimmune mechanisms have been increasingly considered to contribute to disease progression, e.g. by enhancing protein misfolding or pro-inflammatory immune responses. Understanding this contribution may lead to novel treatment options beyond removing neurodegeneration-associated proteins. We hypothesized that CD4⁺ T_H cells against synaptic proteins may play a role in dementia, given the profound changes of synaptic proteins in the disease. We investigated T_H cell frequencies and phenotypes after antigen-reactive T cell enrichment (ARTE) using three important synaptic antigens known to play a role in cognitive function, N-Methyl-D-Aspartate receptor (NMDAR), Leucine-rich, glioma inactivated 1 (LGI1) and metabotropic glutamate receptor 5 (mGluR5). Our data revealed that synaptic autoantigen-specific T_H cells occurred in all cohorts and were similarly frequent in patients with dementia and sex- and age-matched controls. However, they were significantly reduced compared to young healthy subjects, indicating strong age-related effects ('immune senescence'). Compared to the ubiquitously available Candida albicans antigen, synaptic autoantigen-specific T_H cell responses were strongly driven by IFNγ-producing T cells, expression of which markedly decreased with age. Patients with dementia had significantly less IL-17-producing synaptic autoantigen-specific T_H cells than aged healthy controls. This first direct ex vivo quantitative and qualitative analysis of circulating T cells autoreactive to three synaptic autoantigens in dementia shows no correlation with cognitive impairment. It suggests that synaptic autoantigen-specific T_H cells decline with age and are not a major driver of dementia development.

Keywords: Dementia; Immune senescence; LGI1; MGluR5; NMDA receptor; Synaptic autoantigens; T cells.

Fig. 1

Normal CD4⁺ T_H cell frequencies in dementia patients and healthy controls. A Flow cytometry gating strategy used in this project. Lymphocytes were selected from PBMCs, gated for live CD4⁺ T_H cells and then for CD154⁺ T_H cells followed by differentiation of memory and naïve T cell subsets. B Without antigen stimulation, PBMCs showed similar frequencies of CD4⁺ T_H cells in all cohorts. C Naïve T cells and memory T cells (including memory T subsets) did not differ between all three cohorts in unstimulated condition

Fig. 2

CD154⁺ T_H cells targeting neuronal antigens decrease with age. A Frequencies of CD154⁺ T_H cells after stimulation with the respective antigen (Candida, NR1, LGI1, mGluR5) were similar in dementia patients and aged controls, but markedly lower compared to young controls. B No differences between CD154⁺ T_H cell frequencies after stimulation with neuronal surface antigens among different dementia subgroups. C Significant age-dependent decrease of neuronal antigen-specific CD154⁺ T_H cells. The reduction was particularly obvious in the aged group, where no decrease was observed following stimulation with Candida albicans (right). D Regression analyses illustrating the relationship between CD154⁺ T_H cell frequencies and MMSE score, displaying a significant decrease of neuronal antigen-specific, but not Candida-specific CD154⁺ T_H cells in patients with reduced MMSE scores. E In regression analyses between MMSE scores and age, MMSE scores did not significantly correlate with age

Fig. 3

T_memory cells were most strongly linked to the age-dependent reduction in T_H cell frequency after synaptic antigen stimulation. A Naïve T_H cells against synaptic antigens were equally frequent in all cohorts, in contrast to increased frequencies in the Candida-stimulated group of young controls. B Memory T_H cells were significantly more frequent in young controls in both, the Candida-stimulated and synaptic antigen-stimulated groups. C-E Within the memory compartment, T_EM were more frequent than T_CM and T_EMRA cells. Within those subsets, no differences were found between cohorts after stimulation with neuronal antigens

Fig. 4

Differential cytokine profiles in CD154⁺ T_H cells. A TNFα was the main inflammatory cytokine in Candida-specific T_H cells. Lower frequencies were observed in T_H cells after stimulation with neuronal antigens. B IFNγ production predominated in synaptic antigen-specific CD4⁺ T_H cells, in particular in young healthy controls with nearly twice as much IFNγ production compared to aged controls and dementia patients. C IL-17 production after stimulation with synaptic antigens was significantly increased in healthy aged subjects compared to dementia patients and young controls