THE RELATIONSHIP BETWEEN DECREASED COGNITIVE FUNCTIONS AND THE LEVEL OF PROINFLAMMATORY CYTOKINES IN PATIENTS WITH ALZHEIMER’S DISEASE, VASCULAR DEMENTIA, AND MILD COGNITIVE DISORDER.

IL-17 and neurocognitive domains in patients with MCD and VD. Correlation analysis confirmed the relationship between the severity of cognitive impairment and the level of proinflammatory markers, suggesting that inflammation can lead to cognitive decline in AD patients. The results of the study indicated that IL-23 may ha ve a more complex relationship with the progression of this disease which gives reason to consider IL-23 as a marker of inflammatory activity.

This work is licensed under Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/ 248 Correlation analysis confirmed the relationship between the severity of cognitive impairment and the level of proinflammatory markers, suggesting that inflammation can lead to cognitive decline in AD patients. The results of the study indicated that IL-23 may have a more complex relationship with the progression of this disease which gives reason to consider IL-23 as a marker of inflammatory activity.

Introduction/Вступ
Alzheimer's disease (AD) is a degenerative disease that leads to dementia symptoms [1,2]. Histopathological signs of AD are amyloid plaques in the brain, mainly consisting of fibrillary forms of amyloid β-peptide-40 (Aβ-40) and amyloid β-peptide-42 (Aβ-42). Insoluble Aβ peptides formed in the central nervous system (CNS) play a decisive role in the pathogenesis of AD, activating the complement pathway, stimulating microglia to produce proinflammatory cytokines and chemokines. This pro-inflammatory process mediated by microglia leads to neurodegeneration [3]. Aβ peptides also increase the production of reactive nitrogen and oxygen species by microglial cells, which leads to the development of oxidative stress, stimulation of Th17 cells, and production of interleukin (IL-17) [4]. Apparently, the main role of IL-17 in the pathogenesis of AD is the recruitment of neutrophils and the stimulation of their function. Neutrophils are the main targets for IL-17 in the CNS, promote inflammation and damage to CNS tissues, and may play an important role in the development of AD [5]. Since several studies [6,7] indicate an important role for IL-17 and IL-23 in the pathogenesis of AD and VD, the aim of the study was to investigate the relationship between IL-17 and IL-23 levels and neurocognitive scales in patients with AD, VD and mild cognitive disorder (MCD).

Materials and methods
In accordance with the set goal on the basis of the neurological department of Municipal noncommercial organization «Clinical Hospital № 4» of Sumy city council, 89 patients were examined, of which 59 had cognitive impairment (32 men and 27 women, the average age was 66.8 ± 8. (Parkinson's disease, fronto-temporal degeneration, dysmetabolic encephalopathies, demyelinating diseases, traumatic brain injury, tumors of the brain and its membranes, neuroinfections, intoxications, alcohol abuse, severe post-stroke deficit, drugs use (neuroleptics, benzodiazepines, antidepressants, barbiturates, antiepileptic), inability to have sufficient verbal contact.
All patients were tested with a comprehensive neuropsychological examination using the following tests and scales: Mini Statistical analysis was performed using Microsoft Excel 2016 software. Comparisons of categorical variables for demographic and clinical characteristics were made using the χ 2 test, and Student's t-test was applied for continuous variables. The Mann-Whitney U-test and the Kruskal-Wallis test were used to compare the concentrations between groups. The Spearman correlation test was used to ascertain the associations between the cytokines concentrations and the score for the MMSE, MoCA, FAB and ADAS-cog. The differences were considered statistically significant at р < 0.05.

Results
Comparison of the results of neurocognitive scales (Table 1) showed that the mean scores of MMSE were significantly lower in patients with AD vs. patients with VD and MCD (p 1 ˂ 0.0001), (p 3 = 0.0028).
MoCA total score test also was significantly lower in patients with AD vs. patients with VD and MCD (p 1 ˂ 0.0001), (p 3 = 0.0005). The mean scores of ADAS˗cog test was significantly higher in patients with AD vs. patients with VD and MCD (p 1 ˂ 0.0001), (p 3 = 0.0063). Levels of detectable interleukins (IL-17 and IL-23) were significantly higher in patients with AD vs. patients with VD (p 3 = 0.0481), (p 3 = 0.0027),) and MCD (p 1 = 0.0003), (p 1 = 0.0065). We detected higher serum concentrations of IL-23 in patients with higher ADAS-cog scores (ADAS-cog >30), as well as the predomination of pro-inflammatory IL-23 over IL-17 in the peripheral blood of patients with AD.
The use of correlation analysis revealed statistical association between the serum concentration of IL-17, IL-23 and neurocognitive scales in patients with AD, VD and MCD ( Table  2). The level of proinflammatory interleukins (IL-17, IL-23) in the control group was within normal limits.
Our results revealed a significant positive correlation between the serum concentrations of IL-17 and IL-23 for all patients with AD and MCD (r = 0.723; p = 0.002), (r = 0.432; р = 0.017). An examination of the possible connection between the serum concentration of IL-23 and ADAS-cog scores in patients with MCD revealed the following results: positive correlation was established between IL-23 and individual domains of the ADAS-cog scale (r = 0.423; p = 0.020). There was a positive relationship between IL-23 and sections «word recognition tasks» (r = 0.466; p = 0.030), «understanding» (r = 0.306; p = 0.059) and «strike out numbers» (r = 0.301; p = 0.061).

Discussion
Numerous recent studies demonstrated a correlation between proinflammatory cytokines (such as IL-2, IL-8, IL-9, IL-10, TNF-α, IL-17, IL-18, IL-21) and cognitive deficits, especially with AD and MCD [10,11,12,13]. In patients with Alzheimer's disease, elevated levels of IL-6 were moderately correlated with decreased cognitive function, lower memory, and lower verbal velocity on the MMSE scale [14]. Subsequently, several researchers confirmed that under inflammatory conditions, excessive IL-6 through activation of neuronal NADPH-oxidase induced by inflammation impair cognitive processes, such as spatial learning and memory [14,15,16]. Saresella et al. found that IL-23 was significantly increased in the brain, but that IL-17 was not augmented in AD, MCD and did not correlate with measures of progression or dementia severity [17]. Other studies showed that level of IL-17 was significantly increased in the brain and in the cerebrospinal fluid of individuals with AD and correlated with cognitive impairment [18].
We investigated for the first time the peripheral cytokine profile in AD, VD, and MCD. In particular, it was noted that the concentrations of IL-17 and IL-23 are interrelated, and the content of each of them increased linearly with the progression of the disease. In our study, IL-23 correlated with cognitive functions in patients with AD. This study found an association between severity of cognitive impairment and the level of pro-inflammatory markers, suggesting that inflammation may cause cognitive decline in patients with AD. Recent studies confirmed that improved cognitive performance was seen after blocking IL-23 signaling p40 subunit by antibody [19]. These findings suggest that IL-23 may show a more complex relationship with disease progression. In conclusion, this gives grounds to consider the level of IL-23 in the serum as marker of inflammatory activity. However, understanding the complete interaction process of the two cytokines in the pathogenesis of AD requires further studies.

Conclusions/Висновки
Our results showed that levels of detectable pro-inflammatory cytokines (IL-17 and IL-23) were significantly higher in patients with AD as compared with VD and MCD. Such more pronounced changes in the production of IL-23 in patients with AD may indicate the activity of the inflammatory process. The level of interleukin 23 in all examined patients with AD had the highest correlations with the neurocognitive scales, which indicated its important role in the pathogenesis of this disease. There were no significant correlations between the serum concentration of IL-17 and neurocognitive domains in patients with MCD and VD.

Prospects for future research/Перспективи подальших досліджень
Future investigation may elucidate a potential role of interleukin 23 as additional biomarker for early predicting of MCD progression in AD.
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