Cross-cohort NULISAseq identifies tau-anchored CSF and plasma-specific signatures across the amyloid-positive Alzheimer's disease

INTRODUCTION: ATN-stratified cerebrospinal fluid (CSF) proteomic signals may partly reflect classifier tau immunoassays; reproducible non-tau signals and CSF-plasma differences remain unclear. METHODS: We profiled CSF from 60 A⁺ Beijing patients with Alzheimer's disease (AD) and 875 A⁺ Alzheimer's Disease Neuroimaging Initiative (ADNI) participants using the NULISAseq CNS Disease Panel. Differential expression was assessed using a false discovery rate (FDR) < 0.05 and |log₂FC|≥0.5 across three ATN contrasts. We also analyzed ADNI plasma, protein-cognition correlations, tau-adjusted associations, and nested models comparing non-tau proteins with tau biomarkers. RESULTS: Eight CSF proteins met the cross-cohort criterion within at least one matched contrast: pTau-217, pTau-181, pTau-231, MAPT, SMOC1, MDH1, VSNL1, and FABP3. NULISA tau correlated with INNOTEST (ρ = 0.75-0.85) and Elecsys (ρ = 0.65-0.68). T and N classifications agreed for 94.6% of ADNI participants (κ = 0.880); after adjustment for CSF pTau-217, all 12 associations involving the four non-tau proteins retained their direction and FDR support. In CSF, Elecsys p-tau181 and NULISA tau-family analytes had the strongest cognitive correlations (mean |ρ| = 0.35 and 0.29-0.34, respectively), followed by FABP3 (0.24). Adding the four-protein block after tau increased cross-validated R² across all three cross-sectional outcomes in models without diagnosis (+0.0101 to +0.0265), most clearly for the Alzheimer's Disease Assessment Scale-Cognitive Subscale, 13-item version (ADAS-Cog13); diagnosis-conditioned models showed no stable increment beyond tau and clinical characteristics. In plasma, pTau-217 was the only analyte that met both thresholds in all three contrasts. DISCUSSION: CSF NULISAseq identified a reproducible tau-associated signature across A⁺ cohorts that extended beyond the tau family. The four non-tau proteins extend the molecular profile associated with tau-defined disease state and merit further evaluation for biological stratification in independent cohorts.