Background: A growing body of research links environmental factors with neurodegeneration and premature mortality. However, the biological mechanisms through which pollutants affect early Alzheimer's disease (AD) pathology in asymptomatic stages are largely unknown. We aimed to assess the association between air pollution and changes in cerebrospinal fluid (CSF) biomarkers of AD and neuroinflammatory processes in cognitively unimpaired (CU) individuals at risk of AD dementia. Method: We included 225 middle-aged CU participants from the ALFA+ study, many within the Alzheimer's continuum, with baseline and one follow-up measurement of CSF biomarkers of primary pathology of AD (e.g. Aβ42/40 ratio, phosphorylated tau181 and total tau). Markers of neurodegeneration (e.g NfL), astrocytic (e.g. GFAP, YKL40), and microglial (e.g. sTREM 2) reactivity, and general inflammation (e.g. IL-6) were also included. Land use regression models were used to estimate individual levels of air pollution, including nitrogen oxide (NO2), particulate matter (PM2.5, PM10), and PM2.5 absorbance, at the participants' residential address. We conducted sex-specific analyses using general linear models adjusted for age and time between measurements, with an interaction term for sex and environmental exposure. The influence of AD-related factors, like genetic predisposition and baseline amyloid pathology, on the relationship between air pollution and CSF biomarkers was also assessed. Moreover, we performed mediation analysis to investigate the pathways through which air pollution affected a 3-year rate of change in cognition and biological brain age via significant biomarkers. Results: Results indicated sex-specific responses to air pollution. Women showed increases in IL-6 and GFAP, markers linked to neuroinflammation and astroglial activity, while men experienced impacts at baseline GFAP levels. The findings were consistent regardless of genetic predisposition to AD and amyloid pathology. Mediation analysis showed significant effects of GFAP on the relationship between air pollution and rate of change of attention and executive functions in women, highlighting primary influence pathways dependent on GFAP mediation. No significant mediation, neither direct effect was found for brain age. Conclusions: Our findings highlight air pollution's significant role in contributing to sex-specific neuroinflammatory and astrocytic response to air pollutants and its involvement in cognitive performance, underscoring the need for further research to elucidate these mechanisms.

Conflicts of Interest GK is a full time employee of Roche Diagnostics GmbH, Penzberg, Germany. CQ-R is a full-time employee of Roche Diagnostics International Ltd, Rotkreuz, Switzerland. MS-C has given lectures in symposia sponsored by Almirall, Eli Lilly, Novo Nordisk, Roche Diagnostics, and Roche Farma; received consultancy fees (paid to the institution) from Roche Diagnostics; and served on advisory boards of Roche Diagnostics and Grifols. He was granted a project and is a site investigator of a clinical trial (funded to the institution) by Roche Diagnostics. In-kind support for research (to the institution) was received from ADx Neurosciences, Alamar Biosciences, Avid Radiopharmaceuticals, Eli Lilly, Fujirebio, Janssen Research & Development, and Roche Diagnostics. JDG has received research support from GE Healthcare, Roche Diagnostics, and Hoffmann - La Roche, speaker/consulting fees by Biogen, Roche Diagnostics, Philips Nederlands and Life Molecular Imaging, and serves in the Molecular Neuroimaging Scientific Advisory Board of Prothena Biosciences. HZ has served at scientific advisory boards and/or as a consultant for Abbvie, Acumen, Alector, Alzinova, ALZPath, Amylyx, Annexon, Apellis, Artery Therapeutics, AZTherapies, Cognito Therapeutics, CogRx, Denali, Eisai, Merry Life, Nervgen, Novo Nordisk, Optoceutics, Passage Bio, Pinteon Therapeutics, Prothena, Red Abbey Labs, reMYND, Roche, Samumed, Siemens Healthineers, Triplet Therapeutics, and Wave, has given lectures in symposia sponsored by Alzecure, Biogen, Cellectricon, Fujirebio, Lilly, Novo Nordisk, and Roche, and is a co-founder of Brain Biomarker Solutions in Gothenburg AB (BBS), which is a part of the GU Ventures Incubator Program (outside submitted work). The NeuroToolKit is a panel of exploratory prototype assays designed to robustly evaluate biomarkers associated with key pathologic events characteristic of AD and other neurological disorders, used for research purposes only and not approved for clinical use (Roche Diagnostics International Ltd, Rotkreuz, Switzerland). Elecsys Phospho-Tau (181P) CSF and Elecsys Total-Tau CSF assays are approved for clinical use. COBAS and ELECSYS are trademarks of Roche. All other product names and trademarks are the property of their respective owners.

The research leading to these results has received funding from la Caixa Foundation (ID 100010434), under agreement LCF/PR/GN17/50300004, and the environMENTALment project, funded by the Ajuntament de Barcelona and la Caixa Foundation (project code: 23S06083-001). Additional support has been received from the Universities and Research Secretariat, Ministry of Business and Knowledge of the Catalan Government under the grant no. 2021 SGR 00913. NVT was supported by the Spanish Ministry of Science and Innovation State Research Agency (IJC2020-043216-I/MCIN/AEI/10.13039/501100011033) and the European Union NextGenerationEU/PRTR and currently receives funding from the Spanish Research Agency MICIU/AEI/10.13039/501100011033 (grant RYC2022 038136-I cofunded by the European Union FSE+ and grant PID2022-143106OA-I00 cofunded by the European Union FEDER). Additionally, NVT is supported by the William H. Gates Sr. Fellowship from the Alzheimer's Disease Data Initiative. GSB is supported by the Agencia Estatal de Investigacion AEI/10.13039/501100011033 through the project PID2020-119556RA-I00 and by Instituto de Salud Carlos III (ISCIII) through the project CP23/00039 (Miguel Servet contract), co-funded by the European Union (FSE+). MSC receives funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant agreement No. 948677); ERA PerMed (ERAPERMED2021-184); Project PI19/00155 and PI22/00456, funded by Instituto de Salud Carlos III (ISCIII) and co-funded by the European Union; and from a fellowship from la Caixa Foundation (ID 100010434) and from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 847648 (LCF/BQ/PR21/11840004). JDG receives research support from the Innovative Health Initiative (IHI) of the European Commission (Grant agreement No. 101112145), BrightFocus Foundation (A2022034S), Instituto de Salud Carlos III (PMP22/00022), and Fundacio La Marato de TV3 (202318-30-31-32). All CRG authors acknowledge the support of the Spanish Ministry of Science, Innovation, and Universities to the EMBL partnership, the Centro de Excelencia Severo Ochoa, and the CERCA Programme/Generalitat de Catalunya. HZ is a Wallenberg Scholar and a Distinguished Professor at the Swedish Research Council supported by grants from the Swedish Research Council (2023-00356; 2022-01018 and 2019-02397), the European Union's Horizon Europe research and innovation programme under grant agreement No 101053962, Swedish State Support for Clinical Research (ALFGBG-71320), the Alzheimer Drug Discovery Foundation (ADDF), USA (201809-2016862), the AD Strategic Fund and the Alzheimer's Association (#ADSF-21-831376-C, #ADSF-21-831381-C, #ADSF-21-831377-C, and #ADSF-24-1284328-C), the Bluefield Project, Cure Alzheimer's Fund, the Olav Thon Foundation, the Erling Persson Family Foundation, Familjen Ronstroms Stiftelse, Stiftelsen for Gamla Tjanarinnor, Hjarnfonden, Sweden (#FO2022-0270), the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 860197 (MIRIADE), the European Union Joint Programme Neurodegenerative Disease Research (JPND2021-00694), the National Institute for Health and Care Research University College London Hospitals Biomedical Research Centre, and the UK Dementia Research Institute at UCL (UKDRI-1003).

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