Current status and future prospects in the search for protein biomarkers of immunosenescence

Complex adaptations including changes in cellular redox status, the production of high levels of pro-inflammatory cytokines and alterations in immunity occur as the result of aging of the immune system (immunosenescence). These events are thought to underlie the progression of chronic degenerative diseases of aging, such as atherosclerosis, Type 2 diabetes and Alzheimer’s disease. It is envisaged that identifying early biomarkers of immune aging would aid in identifying individuals at risk of age-related disease and would allow the discovery of novel intervention strategies. Proteomics has emerged as a rapidly expanding and innovative field, investigating protein expression, interaction and function at a global level. Several proteomic strategies, including use of mass spectrometry and non-mass spectrometry-based detection systems (including secondary antibody labeling with fluorescent tags) may be particularly advantageous in identifying biomarkers of immune health. Application of these approaches may identify factors that both contribute to (and define) age-dependent deregulation of the immune system.     

Role of persistent CMV infection in configuring T cell immunity in the elderly

Ageing is associated with declines in many physiological parameters, including multiple immune system functions. The rate of acceleration of the frequency of death due to cardiovascular disease or cancer seems to increase with age from middle age up to around 80 years, plateauing thereafter. Mortality due to infectious disease, however, does not plateau, but continues to accelerate indefinitely. The elderly commonly possess oligoclonal expansions of T cells, especially of CD8 cells, which, surprisingly, are often associated with cytomegalovirus (CMV) seropositivity. This in turn is associated with many of the same phenotypic and functional alterations to T cell immunity that have been suggested as biomarkers of immune system aging. Thus, the manner in which CMV and the host immune system interact is critical in determining the "age" of specific immunity. We may therefore consider immunosenescence in some respects as an infectious state. This implies that interventions aimed at the pathogen may improve the organ system affected. Hence, CMV-directed anti-virals or vaccination may have beneficial effects on immunity in later life.     

Epigenetic modulation of the immune function

Great efforts in the field of solid organ transplantation are being devoted to identifying biomarkers that allow a transplanted patient’s immune status to be established. Recently, it has been well documented that epigenetic mechanisms like DNA methylation and histone modifications regulate the expression of immune system-related genes, modifying the development of the innate and adaptive immune responses. An in-depth knowledge of these epigenetic mechanisms could modulate the immune response after transplantation and to develop new therapeutic strategies. Epigenetic modifiers, such as histone deacetylase (HDAC) inhibitors have considerable potential as anti-inflammatory and immunosuppressive agents, but their effect on transplantation has not hitherto been known. Moreover, the detection of epigenetic marks in key immune genes could be useful as biomarkers of rejection and progression among transplanted patients. Here, we describe recent discoveries concerning the epigenetic regulation of the immune system, and how this knowledge could be translated to the field of transplantation.     

Immune-associated biomarkers for early diagnosis of Parkinson’s disease based on hematological lncRNA–mRNA co-expression

Early stage diagnosis of Parkinson’s disease (PD) is challenging without significant motor symptoms. The identification of effective molecular biomarkers as a hematological indication of PD may help improve the diagnostic timelines and accuracy. In the present paper, we analyzed and compared the blood samples of PD and control (CTR) patients to identify the disease-related changes and determine the putative biomarkers for PD diagnosis. Based on the RNA sequencing analysis, differentially expressed genes (DEGs) were identified, and the co-expression network of DEGs was constructed using the weighted gene correlation network analysis (WGCNA). The analysis leads to the identification of 87 genes that were exclusively regulated in the PD group, whereas 66 genes were significantly increased and 21 genes were significantly decreased in contrast with the control group. The results indicate that the core lncRNA–mRNA co-expression network greatly changes the immune response in PD patients. Specifically, the results showed that Prader Willi Angelman Region RNA6 (PWAR6), LINC00861, AC83843.1, IRF family, IFIT family and calcium/calmodulin-dependent protein kinase IV (CaMK4) may play important roles in the immune system of PD. Based on the findings from the present study, future research aims at identifying novel therapeutic strategies for PD.     

Role of chemokines,innate and adaptive immunity

Polycystic Kidney Disease (PKD) triggers a robust immune system response including changes in both innate and adaptive immunity. These changes involve immune cells (e.g., macrophages and T cells) as well as cytokines and chemokines (e.g., MCP-1) that regulate the production, differentiation, homing, and various functions of these cells. This review is focused on the role of the immune system and its associated factors in the pathogenesis of PKDs as evidenced by data from cell-based systems, animal models, and PKD patients. It also highlights relevant pre-clinical and clinical studies that point to specific immune system components as promising candidates for the development of prognostic biomarkers and therapeutic strategies to improve PKD outcomes.     

Apoptosis and other immune biomarkers predict influenza vaccine responsiveness

Despite the importance of the immune system in many diseases, there are currently no objective benchmarks of immunological health. In an effort to identifying such markers, we used influenza vaccination in 30 young (20–30 years) and 59 older subjects (60 to >89 years) as models for strong and weak immune responses, respectively, and assayed their serological responses to influenza strains as well as a wide variety of other parameters, including gene expression, antibodies to hemagglutinin peptides, serum cytokines, cell subset phenotypes and in vitro cytokine stimulation. Using machine learning, we identified nine variables that predict the antibody response with 84% accuracy. Two of these variables are involved in apoptosis, which positively associated with the response to vaccination and was confirmed to be a contributor to vaccine responsiveness in mice. The identification of these biomarkers provides new insights into what immune features may be most important for immune health.     

Urinary C-type natriuretic peptide excretion: a potential novel biomarker for renal fibrosis during aging

Renal aging is characterized by structural changes in the kidney including fibrosis, which contributes to the increased risk of kidney and cardiac failure in the elderly. Studies involving healthy kidney donors demonstrated subclinical age-related nephropathy on renal biopsy that was not detected by standard diagnostic tests. Thus there is a high-priority need for novel noninvasive biomarkers to detect the presence of preclinical age-associated renal structural and functional changes. C-type natriuretic peptide (CNP) possesses renoprotective properties and is present in the kidney; however, its modulation during aging remains undefined. We assessed circulating and urinary CNP in a Fischer rat model of experimental aging and also determined renal structural and functional adaptations to the aging process. Histological and electron microscopic analysis demonstrated significant renal fibrosis, glomerular basement membrane thickening, and mesangial matrix expansion with aging. While plasma CNP levels progressively declined with aging, urinary CNP excretion increased, along with the ratio of urinary to plasma CNP, which preceded significant elevations in proteinuria and blood pressure. Also, CNP immunoreactivity was increased in the distal and proximal tubules in both the aging rat and aging human kidneys. Our findings provide evidence that urinary CNP and its ratio to plasma CNP may represent a novel biomarker for early age-mediated renal structural alterations, particularly fibrosis. Thus urinary CNP could potentially aid in identifying subjects with preclinical structural changes before the onset of symptoms and disease, allowing for the initiation of strategies designed to prevent the progression of chronic kidney disease particularly in the aging population.     

Homeostatic dysregulation proceeds in parallel in multiple physiological systems

An increasing number of aging researchers believes that multi‐system physiological dysregulation may be a key biological mechanism of aging, but evidence of this has been sparse. Here, we used biomarker data on nearly 33 000 individuals from four large datasets to test for the presence of multi‐system dysregulation. We grouped 37 biomarkers into six a priori groupings representing physiological systems (lipids, immune, oxygen transport, liver function, vitamins, and electrolytes), then calculated dysregulation scores for each system in each individual using statistical distance. Correlations among dysregulation levels across systems were generally weak but significant. Comparison of these results to dysregulation in arbitrary ‘systems’ generated by random grouping of biomarkers showed that a priori knowledge effectively distinguished the true systems in which dysregulation proceeds most independently. In other words, correlations among dysregulation levels were higher using arbitrary systems, indicating that only a priori systems identified distinct dysregulation processes. Additionally, dysregulation of most systems increased with age and significantly predicted multiple health outcomes including mortality, frailty, diabetes, heart disease, and number of chronic diseases. The six systems differed in how well their dysregulation scores predicted health outcomes and age. These findings present the first unequivocal demonstration of integrated multi‐system physiological dysregulation during aging, demonstrating that physiological dysregulation proceeds neither as a single global process nor as a completely independent process in different systems, but rather as a set of system‐specific processes likely linked through weak feedback effects. These processes – probably many more than the six measured here – are implicated in aging.     

Age and metastasis – How age influences metastatic spread in cancer. Colorectal cancer as a model

Metastasis is the major cause of death in cancer patients. Whereas colorectal cancer (CRC) incidence increases with age, metastatic spread seems to decline. Furthermore, the epidemiology of CRC is changing. There is an increase in CRC incidence in the young, presenting at an advanced stage with higher likelihood of synchronous or metachronous metastases, and a decline in CRC incidence and metastatic spread in the oldest-old. Emerging data suggest that age-related changes with regard to tumor biology (e.g. genomic instability), the tumor microenvironment (e.g. inflammaging) and the immune system (e.g. immunosenescence), complemented by interaction between the genome and exposome might contribute to the observed metastatic patterns. As aging is a key prognostic factor, this highlights the need for further studies investigating age-related patterns and underlying mechanisms of tumor growth and dissemination. Eventually, this might allow for better risk stratification, refinement of screening strategies and follow-up care as well as therapies tailored to reflect patient age and that might possibly target responsible biomarkers in a precision medicine approach. This review aims to discuss the influence of aging on metastatic spread in colorectal cancer and elucidate underlying mechanisms responsible for the observed metastatic patterns.     

Inflammatory and immune markers in HIV‐infected older adults on long‐term antiretroviral therapy: Persistent elevation of sCD14 and of proinflammatory effector memory T cells

HIV‐positive patients whose viral loads are successfully controlled by active antiretroviral therapy (ART) show no clinical signs of AIDS. However, their lifespan is shorter compared with individuals with no HIV infection and they prematurely exhibit a multitude of chronic diseases typically associated with advanced age. It was hypothesized that immune system aging may correlate with, and provide useful biomarkers for, this premature loss of healthspan in HIV‐positive subjects. Here, we tested whether the immune correlates of aging, including cell numbers and phenotypes, inflammatory status, and control of human cytomegalovirus (hCMV) in HIV‐positive subjects on long‐term successful ART (HIV+) may reveal increased “immunological age” compared with HIV‐negative, age‐matched cohort (HIV−) in participants between 50 and 69 years of age. Specifically, we expected that younger HIV+ subjects may immunologically resemble older individuals without HIV. We found no evidence to support this hypothesis. While T cells from HIV+ participants displayed differential expression in several differentiation and/or inhibitory/exhaustion markers in different T cell subpopulations, aging by a decade did not pronounce these changes. Similarly, while the HIV+ participants exhibited higher T cell responses and elevated inflammatory marker levels in plasma, indicative of chronic inflammation, this trait was not age‐sensitive. We did find differences in immune control of hCMV, and, more importantly, a sustained elevation of sCD14 and of proinflammatory CD4 and CD8 T cell responses across age groups, pointing towards uncontrolled inflammation as a factor in reduced healthspan in successfully treated older HIV+ patients.     

Strategies for biomarker discovery in fibrotic disease

The discovery and development of biomarkers for fibrotic diseases have potential utility in clinical decision-making as well as in pharmaceutical research and development. This review describes strategies for identifying diagnostic, prognostic and theranostic biomarkers. A range of technologies and platforms for biomarker discovery are highlighted, including several with specific relevance for fibrosis. Some challenges specific to fibrotic diseases are outlined including; benchmarking biomarkers against imperfect clinical measures of fibrosis, the complexity resulting from diverse aetiologies and target organs, and the availability of samples (including biopsy) from well-characterised patients with fibrotic disease. To overcome these challenges collaboration amongst clinical specialities as well as between academia and industry is essential. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.     

The Utility of Cerebral Blood Flow as a Biomarker of Preclinical Alzheimer’s Disease

There is accumulating evidence suggesting that changes in brain perfusion are present long before the clinical symptoms of Alzheimer’s disease (AD), perhaps even before amyloid-β accumulation or brain atrophy. This evidence, consistent with the vascular hypothesis of AD, implicates cerebral blood flow (CBF) in the pathogenesis of AD and suggests its utility as a biomarker of preclinical AD. The extended preclinical phase of AD holds particular significance for disease modification, as treatment would likely be most effective in this early asymptomatic stage of disease. This highlights the importance of identifying reliable and accurate biomarkers of AD that can differentiate normal aging from preclinical AD prior to clinical symptom manifestation. Cerebral perfusion, as measured by arterial spin labeling magnetic resonance imaging (ASL-MRI), has been shown to distinguish between normal controls and adults with AD. In addition to demonstrating diagnostic utility, CBF has shown usefulness as a tool for identifying those who are at risk for AD and for predicting subtle cognitive decline and conversion to mild cognitive impairment and AD. Taken together, this evidence not only implicates CBF as a useful biomarker for tracking disease severity and progression, but also suggests that ASL-measured CBF may be useful for identifying candidates for future AD treatment trials, especially in the preclinical, asymptomatic phases of the disease.     

Proteomic analysis of HBV-associated HCC: Insights on mechanisms of disease onset and biomarker discovery

The development of hepatocellular carcinoma (HCC) can be considered as an end-stage outcome of chronic hepatitis B virus (HBV) infection. Early prognostic markers are needed to allow effective treatments and prevent HCC from developing. Proteomics analysis has been used to identify markers from clinical samples from HCC patients. This approach can be further improved by identifying early biomarkers before the onset of HCC. One way would be to use the cell-based HBV replication system, which is reflective of the early stage of virus infection and thus secreted proteins identified at this stage may have relevance in HCC prognosis. In this review, we focus the discussion on the current status of proteomics analysis of cellular proteins and HCC biomarker identification, with a special highlight on the potential of the cell-based HBV replication system for the identification of prognostic HCC biomarkers.     

Lifespan‐extending caloric restriction or mTOR inhibition impair adaptive immunity of old mice by distinct mechanisms

Aging of the world population and a concomitant increase in age‐related diseases and disabilities mandates the search for strategies to increase healthspan, the length of time an individual lives healthy and productively. Due to the age‐related decline of the immune system, infectious diseases remain among the top 5–10 causes of mortality and morbidity in the elderly, and improving immune function during aging remains an important aspect of healthspan extension. Calorie restriction (CR) and more recently rapamycin (rapa) feeding have both been used to extend lifespan in mice. Preciously few studies have actually investigated the impact of each of these interventions upon in vivo immune defense against relevant microbial challenge in old organisms. We tested how rapa and CR each impacted the immune system in adult and old mice. We report that each intervention differentially altered T‐cell development in the thymus, peripheral T‐cell maintenance, T‐cell function and host survival after West Nile virus infection, inducing distinct but deleterious consequences to the aging immune system. We conclude that neither rapa feeding nor CR, in the current form/administration regimen, may be optimal strategies for extending healthy immune function and, with it, lifespan.     

Cancer,aging and immunotherapy: lessons learned from animal models

Aging of the immune system is associated with a dramatic reduction in responsiveness as well as functional dysregulation. This deterioration of immune function with advancing age is associated with an increased incidence of cancer. Although there is a plethora of reports evaluating the effect of immunotherapy in stimulating antitumor immune responses, the majority of these studies do not pay attention to the effect aging has on the immune system. Studies from our group and others indicate that immunotherapies could be effective in the young, are not necessarily effective in the old. To optimally stimulate an antitumor immune response in the old, it is necessary to (1) identify and understand the intrinsic defects of the old immune system and (2) use relevant models that closely reflect those of cancer patients, where self-tolerance and aging are present simultaneously. The present review summarizes some defects found in the old immune system affecting the activation of antitumor immune responses, the strategies used to activate stronger antitumor immune response in the old and the use of a tolerant animal tumor model to target a self-tumor antigen for the optimization of immunotherapeutic interventions in the old.     

Aging Biomarkers: From Functional Tests to Multi‐Omics Approaches

Aging results in various deleterious changes in the human body that may lead to loss of function and the manifestation of chronic diseases. While diseases can generally be reliably diagnosed, the aging process itself requires more sophisticated approaches to evaluate its progression. Numerous attempts have been made to establish biomarkers to quantify human aging at the cellular, tissue, and organismal level. Here, an up‐to‐date overview of biomarkers related to human aging with an emphasis on biomarkers that take into account different mechanisms of aging between individuals is provided. Classical discrete molecular and non‐molecular biomarkers handpicked by researches on the base of their strong correlation with age, as well as emerging omics‐based biomarkers, are discussed and potential future directions and developments in the field of aging assessment are outlined.