MAPPING PERIPHERAL IMMUNE ALTERATIONS ALONG THE ALZHEIMER’S DISEASE CONTINUUM IN DOWN SYNDROME AT SINGLE-CELL RESOLUTION

​^​​​​Down syndrome (DS) represents the largest population of genetically determined Alzheimer's disease (AD). However, no study has comprehensively examined peripheral immune alterations in individuals with DS across the AD continuum (DSAD) at single-cell resolution. We performed scRNAseq on PBMCs from 13 healthy controls (HC) and 45 individuals with DS classified as asymptomatic AD (aDS;n=23) or AD dementia (dDS;n=22). Our deep immunoprofiling was integrated with plasma NfL levels (a surrogate biomarker of neurodegeneration). Age, sex, and sequencing batch were used as covariates in all analyses. Nearly 300,000 cells passed QC and clustered into 55 immune subpopulations. CD8 naïve T cells exhibited increased exhaustion, senescence, and cytotoxicity in aDS and dDS (adj.p<0.001). Regarding AD progression, a subtype of CD8 effector memory T cells (CD8TEM2) expanded in aDS (adj.p<0.01), while CD14 monocytes and a NK subpopulation (NK2) expanded in dDS (adj.p<0.05). Widespread transcriptional alterations were observed across most cell lineages in all group comparisons (adj.p<0.05; Figure1A highlights CD8TEM2 signatures). We identified unique communication patterns in dDS, notably from CD8TEM2 cells to CD14 monocytes via the Integrin-ADGRE5/ADGRE2 axis and to NK2 cells via SIRPG-CD47 (adj.p<0.01; Figure1B). Increased exhaustion characterized a subpopulation of CD4 effector memory T cells (CD4TEM2) in dDS compared to aDS (p<0.05). Plasma NfL showed a positive association with the senescence score of CD14 monocytes in DS (beta=39.07, adj.p<0.05; with consistent trends in aDS (beta=48.19) and dDS (beta=19.77)). Our results unveil immune alterations specific to asymptomatic AD and AD dementia stages in DS, suggesting new avenues for immune-based interceptive medicine in DSAD.