To our knowledge, our study, with multiple types of cytokines, is one of the first time to characterize neuro-inflammation of Chinese elder patients in the different clinical stages of dementia, classified as LLD, and aMCI. We found that the dysfunction cytokines in patients of aMCI and LLD was markedly different, and the altered chemokines was significantly correlated with the clinical parameters, which were potential markers for cognitive impairment, depression severity, and aging with different gender. Importantly, the discriminating models based on two combined parameters(IL-6 and CCL13) could effectively distinguish LLD from aMCI patients. The novel results of this study will give new leads to further understand the aetiology of pre-clinical stage of dementia and provide therapeutic target for intervention or a marker for this disease.
Multiplex analytical technologies are quite crucial to the complex task of deciphering disease-specific biomarker patterns as they provide opportunities for an all-inclusive approach, which was not possible using traditional techniques such as enzyme-linked immunosorbent assay (ELISA). In our study, using a luminex assay, a reliable multiplex analytical technologies, it could measure multiple different cytokines simultaneously in a single run of the assay with small sample-size requirements[20]. Simultaneous measurement of multiple cytokines in the peripheral blood sample provides a cost- and time-effective strategy to resolve complex interactions among signaling molecules to obtain an objective pattern of numerous cytokines within one experiment, which provides a more inclusive and comprehensive depiction of pre-clinical stage of dementia [21] showing in Table1.
Cytokines are typically produced by cells of the immune system upon activation, which play key roles in the development and control of immune responses. Chemokines represent one of the largest subfamilies of cytokines, which have been divided into several sub-groups bases on their chemical structures. CXC and CC were two major subfamilies, depending on whether the first two N- terminal cysteines have an amino acid between them (CXC) or are adjacent (CC)[22]. Although chemokines have been relatively neglected in the investigations of mechanisms of psychiatric disorders, recent evidence has begun to demonstrate an association between chemokines and neurobiological processes which have been strongly implicated in the pathogenesis and pathophysiology of psychiatric disorders. Dysfunction cytokines have been associated with both recent AD and other psychiatric disorders, including mild cognitive impairment, schizophrenia, depression and bipolar disorder[23]. Although the exact effect of the pro-inflammatory cytokines in AD remains unknown, more and more evidence suggests that inflammation is involved in pathological process of AD, and plays a crucial role in the early stages of disease when intervention maybe most beneficial[24].
Our study benefited from LLD, aMCI and healthy control groups that were well matched for age and gender. All groups demonstrated no use of anti-inflammatory drug. Changes of cytokine levels were compared among three groups. A group of chemokines including one pro-inflammatory cytokines(IL-6),five CC chemokines (CCL2, CCL13, CCL15, CCL17, and CCL25) and
four CXCs (CXCL5,CXCL6,CXCL11 and CXCL16) were dysregulated in patients with prodromal stage of AD. We demonstrated significantly lower levels of CXCL5,CXCL6,CXCL16 and CCL25 in LLD compared with control group. CCL2,CCL13, CCL15, CCL17, and CXCL11 were significantly higher in case-aMCI than LLD, while IL-6 was significantly higher in case-LLD than in case–aMCI and control(Fig. 2).
The first dysregulated cytokine IL-6 has been extensively investigated in many preclinical and clinical studies as a pro-inflammatory cytokine which can accelerate ongoing neurodegenerative processes in AD. Pervious research found that plasma circulating IL-6 may be a useful indication for future cognitive function[25],and increased Interleukin-6 (IL-6) level was reported in late life depression patients in peripheral blood [26].Leung R show elevated IL-6 in AD patients versus controls and negatively correlated with MMSE scores[27]༌however we did not find significant associations between IL-6 concentration and cognition. Then, we briefly discuss the information about following cytokines in general. CCL2 and CCL13 belong to the monocyte chemotactic protein (MCP) family ,which exert potent pro-inflammatory actions through chemotaxis of monocyte-derived macrophages and other inflammatory leukocytes to the inflamed or injured central nervous system (CNS). CCL2 has also been proved to have effects on microglia, inducing migration and proliferation. CCL5, a target gene of NF-κB activity, is expressed by T lymphocytes, macrophages, platelets, synovial fibroblasts, tubular epithelium, and certain types of tumor cells. CCL15 is classified as a macrophage inflammatory protein and has a pro-inflammatory effect. CCL17 have been shown to be downstream mediators of the effects of systemic interferons on inflammatory state at the choroid plexus, and are correlated with hippocampal neurogenesis and hippocampal-dependent learning and memory tasks in aging. CXCL5 is a pro-inflammatory cytokine involved in both innate and acquired immune response. CXCL6 is an important inflammatory cytokine that recruits inflammatory cells to the site of inflammation by binding to the receptors CXCR1 and CXCR2. CXCL11 is a ligand of CXCR3, which have clearer detrimental pro-inflammatory effects by mediating chemotaxis of natural killer cells, Th1 cells, and their associated classically activated (M1) pro-inflammatory monocyte-derived macrophages. CXCL16 is expressed as not only a membrane-bound molecule but also a soluble chemokine, which has been identified to bind with the chemokine receptor CXCR6 to regulate immune cell chemotaxis into CXCL16-enriched environments[28–30].
Elevated cytokines have also been found in the CSF and plasma of patients with AD, particularly in mild cognitive impairment patients[31]. The reduction of some chemokines in LLD patients which promote the activation of macrophages, natural killer cells, Th1 cells in our study is not consistent with increased inflammation of the nervous system in MCI. It suggested that patients in pre-clinical stage of dementia with similar cognitive deficits showed significantly different levels of pro-inflammatory cytokines. In line with our finding, suicide people reported a similar decrease in the levels chemokines (eg.CCL1, CCL8, CCL13, CCL15, CCL17, CCL19, CCL20, CXCL11) [32]. Indeed suicidal ideation decreases with aging, but if older people have suicidal thoughts they are at a higher risk of actually committing suicide than any other age and these rates continue to rise in many countries [33]. The data indicate that these cytokines might predict who would most likely attempt or commit suicide after exposure to a depression situation.
The demographic characteristics in the LLD group, which probably would affect inflammatory feature. Personal characteristics and a series of complex factors (Low economic status, cultural, poor physical health and social isolation) would affect whether a person, group, or community lead to a late-onset major depressive disorder. In this study, the results showed that 26.1% of the elderly led a sedentary lifestyle, while 91.3% mild cognitive impairment subjects performed exercise habits. Depressive symptoms are associated with decreased physical performance in older adults in both cross-sectional and longitudinal epidemiological studies [34, 35]. However, the relationship between increasing proportion of having physical activity and decreasing symptoms in depression may be bidirectional. On the one hand, depression may lead to decreased levels of activity due to low motivation and energy. On the other hand, the decreasing exercise could be a risk factor for depression. Exercises are known to increase the secretion of multiple hormones which had antidepressant function: physical activities can change norepinephrine activity in central system temporarily, decrease the hypo-thalamopituitary–adrenocortical axis, and increase the secretion of beta-endorphins [36, 37]. To some extent, the lower levels of chemokine may also be explained by the finding that late life depression patients with a sedentary and lacking of stimulation lifestyle.
Past studies have demonstrated that sex has a significant impact on the functions of the immune system. The production of cytokines and chemokines by innate immune cells also differs between the sexes[38, 39]. In our study, potential cytokine markers for aging, depression and cognition were identified with different gender. Interestingly, there are two CXC dysregulated chemokines associated with aging process and cognition function, one CC chemokine related to depression in male. In female, we reported that seven dysregulated cytokines correlated with the parameters reflecting the cognitive function (MoCA scores), two CC chemokines and VCAM-1 concerned with depression ,and three dysregulated cytokines relevant to aging. We also found the overlap of expression of potential chemokines among the tested group(Fig. 3). Aging is the strongest risk factor for neurodegenerative diseases, it results in a significant change in inflammatory mediators [40]. In our study, CXCL5 was up-regulated in aging and inversely associated with MoCA scores in male. The alteration is particularly interesting because it played an important role in maintenance of synaptic formation in the hippocampus of male rats[41]. It is known that hippocampus is one of the most critical regions in brain, highly important to cognitive function and affective behavior. Endothelial cell VCAM-1, is a member of the immunoglobulin superfamily, upregulated on endothelium in response to inflammation. Many studies have demonstrated that plasma VCAM-1 levels increase significantly with age, and have been reported in community-dwelling elderly people with symptoms of depression. Most studies reported female gender as one of the main predictors of depression in late life, our results suggesting the differences between genders may contribute to separate dysregulated cytokine which may lead to cognitive function or depression in pre-clinical stage of dementia[42, 43].It may encourage further investigation of the mechanisms of dysregulated chemokines in cognitive impairment and depression studies considering different gender in the context of aging.
Here, we demonstrated that the using IL-6, CCL13 and together could clearly distinguish pre- dementia stage LLD from aMCI, with AUCs of 0.75,0,71 and 0.81. Treating AD can be challenging, because the pathological changes have begun in many years before the diagnosis of clinical dementia. Due to the lack of drug slow the progression of dementia, therapies should be developed to target on people at risk of dementia. More importantly, researches before investigated the efficacy of antidepressants for the treatment of depression in patients with both depression and dementia. Only two of seven studies found antidepressant pharmacotherapy to be more effective than placebo. Meanwhile, little is known about the treatment of depression in patients with MCI, rigorous identification of MCI in geriatric major depression treatment trials will help to advance effectiveness of treatments[44, 45].Thus, the distinguished cytokine will facilitate detection of people with cognitive impairment, allowing for further investigation of the pro-inflammatory cytokine effects on the pre-clinical AD and neurodegeneration diseases, adopt the individuation preventive and therapeutic measure.
The following limitations of our study have to be discussed. It is undeniable that the cross-sectional study design and small sample size limits causal conclusions regarding the relationship between inflammation and late life depression. Moreover, several methods for predicting the progression of MCI, such as positron emission tomography (PET) and analysis of biomarkers in the cerebrospinal fluid. However, these methods are unacceptable in the elderly for their high costs and invasive nature. Here, the individuals were recruited from our cohort who has been diagnosed by three different geriatric psychiatrists according to Petersen criteria in follow up for three years. The criteria was wide used and has been reported with reasonable accuracy.
Moreover, it is undeniable that further longitudinal studies combined with proteomic and genetic markers are required.