By using a multiplex assay and assessing 30 inflammatory markers in CSF in 498 people with PD and 67 people with DLB we show:
Cross-sectionally
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Higher CSF levels of ICAM-1, Interleukin-8, MCP-1, MIP-1 beta were associated with lower MoCA scores in the total PD cohort and also after stratification by sex.
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Higher CSF levels of ICAM-1, Interleukin-8, MMP3, MIP-1 beta, SCF and VEGF were associated with higher CSF levels of neurodegenerative/PD-specific biomarkers, namely Aβ1-42, t-Tau, p181-Tau, NFL, and α-synuclein in the total PD cohort and also after stratification by sex. Similarly, higher CSF levels of ICAM-1, Interleukin-8, MMP3, SCF and VEGF were associated with higher CSF levels of neurodegenerative/PD-specific biomarkers in the DLB cohort
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PD patients with GBA mutations show similar levels of inflammatory CSF markers when compared to PD patients with GBA wildtype status, even when stratified by mutation severity
Longitudinally
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PD patients who developed cognitive impairment during the study had higher CSF levels of TNF-alpha at baseline compared to patients without development of cognitive impairment and compared to patients who presented with cognitive impairment already at baseline
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Higher CSF levels of VEGF were associated with a longer duration until development of cognitive impairment in PD patients with GBA wildtype status
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Higher CSF levels of MIP-1 beta were associated with a longer duration until development of cognitive impairment in PD patients with GBA mutation
Using a different multiplexed immunoassay by Myriad RBM, Austin, TX, USA (http://rbm.myriad.com) with assessment of 41 inflammatory markers in CSF/serum pairs in 453 sporadic PD patients, we could previously show that the most important inflammatory mediators associated with motor and cognitive function and with neurodegenerative/PD-specific biomarkers were FABP, ICAM-1, IL-8, MCP-1, MIP-1-beta, and SCF. At that point, results were more robust for CSF than for serum 21. Except for FABP which was not part of the current assay and VEGF which was not included in the first assay all other inflammatory markers from the current assay were also included in the Myriad assay. Therefore, the finding of relevant associations of higher CSF levels of ICAM-1, Interleukin-8, MCP-1, MIP-1-beta, and SCF with cognitive dysfunction and with higher CSF levels of neurodegenerative/PD-specific biomarkers seem consistent in sporadic PD and also relevant when including PD patients with GBA mutation and patients with DLB. Other cross-sectional studies in sporadic PD patients reported higher CSF levels of CRP, Interleukin-6 and Interleukin-8 22,23 to be associated with worse motor function 23,24. One longitudinal study found that higher CSF levels of MCP-1 and MMP10 were associated with more severe motor impairment 25. Similarly, several cross-sectional studies reported higher CSF levels of CRP, Interleukin-6, Interleukin-8, MCP-1, SCF and YKL-40 to be related with worse cognitive function 23,24,26,27. Higher CSF levels of CRP and MCP-1 were also associated with depression and fatigue 23,26. While the type of assays (single vs. multiplex, platforms), assessed cytokines and cohorts’ characteristics (samples size, disease duration) are quite variable, inflammatory markers of the monocyte-macrophage signalling and chemotaxis pathway (ICAM-1, Interleukin-8, MCP-1, MIP-1 beta, SCF) seem to play a relevant role in PD-associated inflammation. ICAM-1 has been demonstrated in sustaining neuroinflammation via activated microglia in PD brains, MPTP-treated monkeys and rats 28,29. Interleukin-8 is produced by macrophages and promotes chemotaxis causing granulocytes to migrate toward sites of infectious/injured tissue where, as a second function of Interleukin-8, phagocytosis is induced. Secretion of Interleukin-8 is increased by oxidative stress which promotes inflammation and thereby further enhances oxidative stress in a vicious circle. MCP-1 has a chemotactic function on monocytes while MIP-1 is produced by macrophages and promotes chemotaxis and synthesis of other pro-inflammatory cytokines such as Interleukin-1, Interleukin-6 and TNF-alpha 30. In-vivo and in-vitro experiments show an up-regulation of SCF in neurons of injured brain tissue paralleled by neural stem/progenitor cell migration highlighting that SCF is involved in self-renewal and cell survival 31. A central role in maintaining chronic inflammation upon α-synuclein aggregation and cell death is the bi-directional loop between activated microglia and activated inflammasome32. Both further induce secretion of inflammatory cytokines such as Interleukin-1 beta, Interleukin-6 and MCP-1. Studies in mouse models of Alzheimer’s disease, brain injury, myocardial infarction, and inflammatory bowel disease could demonstrate a reduction in expression levels of Interleukin-1 beta, Interleukin-6, MCP-1 and NLRP3 upon treatment with pterostilbene and prebiotics 33–36. These findings offer the chance to accumulate knowledge on mechanistic aspects and treatment options of inflammation across different (neurodegenerative) diseases.
While one small study reported increased plasma levels of IFN-γ, Interleukin-1 beta, Interleukin-2 and TNF-alpha in eight PD patients carrying heterozygous mutations in the GBA gene 37, two larger studies did not find any differences in blood and CSF levels between PD patients with versus without heterozygous GBA mutations which is in line with our findings 38,39.
Our finding of a longer duration until development of cognitive impairment in patients of the highest quartile of VEGF levels is in line with recent studies. VEGF, a canonical angiogenic factor, plays a key role in promoting hippocampal synaptic plasticity and memory consolidation. Studies in mice could show that VEGF overexpression improves cognitive function and memory performance 40,41 and sustained activity in the hippocampus triggers a rapid release of VEGF suggesting that activity-dependent secretion of VEGF is involved in synaptic plasticity 42. In humans, higher CSF levels of VEGF have been associated with increased hippocampal volume and improved cognitive performance over time 43. Further, if typical Alzheimer’s disease biomarkers signatures in CSF are present, elevated VEGF levels are associated with less cognitive decline 43 highlighting a potential neuroprotective role for VEGF.
We conclude that CSF levels of inflammatory markers are associated with clinical rating scales of motor and cognitive function as well as with levels of neurodegenerative CSF markers but that the majority of these inflammatory CSF markers is limited in robustly predicting longitudinal trajectories of developing cognitive impairment, at least in our two cohorts of PD patients with and without GBA mutation. Further longitudinal analyses in de-novo PD patients with a long follow-up time until clinical endpoints (e.g. cognitive impairment, falls, malignant subtype) are reached in at least 50% of the cohort are needed in order to support our findings.
Importantly, we detected a relevant interaction of sex, age and disease duration with the CSF levels of inflammatory markers in the cross-sectional but also in the longitudinal analyses. Therefore, we recommend to define age and sex specific cut-off values for inflammatory markers. This will be highly necessary before planning and interpreting clinical trials.
The strength of the present study is the large monocentric collection of high-quality CSF samples according to standardized procedures which minimizes variance in sample collection and processing as often seen in multicenter studies.
Limitations of our study are as follows: (I) The single measurement of inflammatory markers limits the evaluation of variation in repeatedly assessed intra-individual measurements. (II) The mean storage-time until measurement of inflammatory markers was 6 years which might impact detectability of markers that are present at low concentrations. (III) Although spanning a wide spectrum between 1–30 years, the heterogeneous disease duration at study baseline limits homogenous longitudinal data analyses in all patients from diagnosis on.