In this study, we aimed to identify changes in IFN-related cytokines in AD patients using a U-Plex MSD multiplex panel. This was the first study to demonstrate that the main subtype of IFNs, IFN-β, is significantly higher in the CSF of Aβ+ MCI and AD patients compared to cognitively normal Aβ- individuals. Additionally, we demonstrated that CSF IFN-β levels significantly correlate with CSF AD core biomarker levels. Interestingly, we found that CSF IFN-β levels are inversely associated with cognitive performance and AD specific brain atrophy in MRI scans. Consistently, we observed upregulated levels of IFN-β in post-mortem brain tissue of AD patients, particularly in regions with amyloid plaques and neurofibrillary tangles. Overall, our study provides pathological and clinical evidence suggesting that CSF IFN-β may contribute to AD pathology and disease progression.
Prior research has explored the role of type I IFNs in cognitive function in various diseases. Blocking age-induced type I IFN-dependent gene expression in the choroid plexus promoted neurogenesis and partially rescued cognitive function [22]. Studies have also shown that type I IFN-driven microglia-dependent synapse injury leads to cognitive impairment in CNS lupus [36]. Furthermore, other forms of cognitive impairment have been linked to microbiota-gut-brain axis dysregulation driven by microbiome-dependent intestinal IFN-γ from Th1 cells of the peripheral system [37].
In this study, we also measured several pro-inflammatory (TNF-α, IL-6, MCP-1, and CXCL-10) and anti-inflammatory (IL-10) cytokines that can be regulated by IFNs [38]. However, we found no difference in the expression levels of these cytokines between the Aβ+ and Aβ- groups, except for higher CSF IL-6 in the Aβ- CI group compared to Aβ- NC and Aβ+ groups.
Higher CSF IL-6 levels were observed in the Aβ- CI group compared to Aβ- NC and Aβ+ groups, which is consistent with previous findings [39-41]. Longitudinal cohort studies have indicated an association between elevated plasma IL-6 levels and cognitive decline over a 10-year fellow-up period [39]. Our data also showed that CSF IL-6 negatively correlated with cognition in non-AD individuals and was associated with some brain MRI features of VCI/VaD. This is consistent with previous studies that found high levels of IL-6 can cause dysfunction in vascular endothelium[42], higher WMH volumes[43] and involvement in the progression of VCI[44]. These results highlight the potential role of CSF IL-6 in the development and progression of VCI.
APOE, the strongest genetic risk factor for AD, has been linked to different inflammatory responses in APOE ε4 carriers and noncarriers [45]. Our data showed that CSF IFN-β, IFN-γ and serum IFN-β levels were significantly higher in APOE ε4 carriers than in other genotypes. However, including AD core pathology biomarkers as mediators altered the results, suggesting that the observed differences in CSF IFN-β and IFN-γ between APOE ε4 carriers and non-carriers are driven by Aβ and tau pathology. In contrast, changes in serum IFN-β with APOE genotypes were not affected by Aβ or tau-related biomarkers, indicating that this difference was independent of AD pathology.
We also examined the relationships between IFN-related cytokines and cognition performance using MMSE and CDR scores. After adjusting for age, sex, APOE ε4 status, and education years, we found an inverse association between CSF IFN-β and cognitive performance. The association is consistent with finding from studies animal model studies, which showed that type I IFN signalling activation occurs in AD mouse models [7, 23], while blocking IFNAR can rescue memory and synaptic deficits [7].
Additionally, we observed an inverse association between CSF IFN-β and typical cortical atrophy in AD patients, such as posterior-dorsal part of the cingulate gyrus, supramarginal gyrus, superior parietal lobule, hippocampus and amygdala. In contrast, CSF IL-6 is associated with motor cortex atrophy, thalamus and pallidum volume, and white matter hyperintensity volume. These associations are comparable to findings from community volunteers, showing higher peripheral inflammation (IL-6) associated with lower cortical gray and white matter, hippocampus volumes [40].
Our study identified subtypes of interferon and related cytokines in Aβ+ MCI and AD patients. We discovered that CSF IFN-β is causally associated with AD pathology and cognitive impairment. In line with our CSF findings, IFN-β was primarily upregulated, especially in regions near amyloid plaques and neurofibrillary tangles in post-mortem AD brain tissues. This finding suggests that CSF IFN-β may be involved in AD-related cortical atrophy, accelerating cognitive impairment. The monoclonal antibody antagonist of IFNAR, anifrolumab, has been approved for the treatment of systemic lupus erythematosus [46] but have not yet been tested in AD patients.
The study has certain limitations. To minimize the impact of confounding factors, we strictly screened the participants in our CANDI cohort. However, the subgroups in this cross-sectional study were relatively small. Further research is needed to confirm the role of CSF IFN-β in AD pathogenesis using large populations and longitudinal cohorts. Additionally, it is crucial to identify the specific cells that contain and secrete high levels of IFN-β. A thorough investigation into how IFN-β impacts AD pathology, such as plaque formation and tau phosphorylation, is also essential for future research.
Our study pinpointed IFN-β as the primary isoform of interferon in the brain and CSF related to AD. We demonstrated that CSF IFN-β level was significantly higher in Aβ+ MCI and AD patients. Importantly, we discovered that IFN-β levels are associated with AD core pathology, cognitive performance, and brain atrophy. Our findings support the hypothesis of targeting the type I IFN response, particularly the IFN-β signalling pathway, in AD patients with high CSF IFN-β concentrations to potentially reverse or slow down cognitive decline.