Abstract
Systemic inflammation is a common complication that contributes to the abysmal outcomes in patients with advanced chronic kidney disease (CKD). It is caused by a range of factors associated with loss of kidney function, such as immune dysfunction, oxidative stress, and retention of uremic toxins, and it is worsened by genetic predisposition, inappropriate diets and lifestyles, common comorbidities such as cardiovascular disease and subclinical infections, and factors linked to the dialysis procedure. It is essential to monitor biomarkers of inflammation such as C-reactive protein in patients with CKD so that potentially modifiable causes of inflammation can be identified and treated. In this chapter, various aspects of this intriguing and challenging complication of CKD and its implications for the prognosis and management of patients with CKD are reviewed.
Keywords
cardiovascular disease, C-reactive protein (CRP), dialysis, mortality, end-stage renal disease, inflammation, interleukin-6 (IL-6), oxidative stress, protein-energy wasting
Outline
Chronic Inflammation: A Maladaptive Response in a Particular Setting, 208
Chronic Persistent Inflammation in Chronic Kidney Disease, 209
Consequences of Inflammation in Chronic Kidney Disease, 214
Mortality, 215
Development and Progression of Chronic Kidney Disease, 215
Protein-Energy Wasting, 215
Vascular Calcification, 215
Anemia and Erythropoiesis-Stimulating Agent Resistance, 216
Depression and Cognitive Impairment, 216
Endocrine Disorders, 216
Insulin Resistance, 217
Premature Senescence, 217
Quality of Life, 218
Measuring Inflammation in Chronic Kidney Disease Patients, 218
Managing the Inflamed Chronic Kidney Disease Patient, 220
Approaching a Patient With Inflammation, 220
Therapeutic Strategies in Inflamed Chronic Kidney Disease Patients, 221
Conclusions, 223
Conflict of Interest, 223
Among the singular features exhibited by patients with chronic kidney disease (CKD), the high prevalence of persistent systemic inflammation and its association with poor outcomes stands out. It is now well established that systemic inflammation is a prominent, perhaps even inherent, feature of advanced CKD and a major culprit for the abysmal outcomes observed in this population as regards progression of cardiovascular disease (CVD), poor quality of life, and premature mortality. Although the pathophysiology associated with systemic inflammation in CKD is not fully understood, there is a broad understanding of many of the factors prompting it and also of the consequences associated with it. In this chapter, we review various aspects of this intriguing and challenging complication of CKD and its implications for the prognosis and management of patients with CKD.
Chronic Inflammation: A Maladaptive Response in a Particular Setting
Inflammation (Latin, inflammatio, to set on fire) is a complex process with a crucial role in mammalian physiology. The purpose of the inflammatory process is to provide host defence against infection, tissue-repair response, adaptation to stress and restoration of a homeostatic state. A generic inflammatory pathway consists of inducers, sensors, mediators, and effectors, with each component determining the type of inflammatory response. After the stimuli, the immune system prompts a complex orchestration of cells, cytokines, and other molecules that act in a paracrine, autocrine, or endocrine manner to protect the human organism; a successful acute inflammatory response should result in the elimination of the infectious agents (or other harmful factors) followed by a resolution and repair phase. Although the inflammatory process should be regarded as a protective mechanism, inflammation underlies a wide variety of not only physiological but also pathological processes.
It is generally understood that a controlled inflammatory response is favorable to remove injurious stimuli and to initiate the healing process for the tissue, but it can become detrimental if deregulated. Actually the pathological potential of inflammation is unprecedented for a physiological process, being associated with atherosclerosis, diabetes, and cancer, among others.
Contrary to what happens with acute inflammation, the mechanisms leading to systemic chronic inflammatory states are in general poorly understood. It has been proposed that in certain conditions—other than infection and tissue damage—inflammation might presumably act as an adaptive response to tissue malfunction or homeostatic imbalance in order to restore homeostasis. In this sense, an adaptive change often provides short-term benefits; however, in a chronic phase, it can become maladaptive, as exemplified by a sustained decline in insulin sensitivity of the skeletal muscle or by squamous metaplasia of the respiratory epithelium that may be consequences of sustained inflammation. Indeed, inducible adaptive changes generally occur at the expense of many other physiological processes and therefore cannot be sustained without adverse side effects caused by the decline in the affected functions.
CKD is a disease where a sustained chronic inflammatory state is often observed, and it has been associated with poor outcomes in this group of patients, being described as one of the most important causes of cardiovascular events, poor quality of life, and even all-cause mortality. Despite considerable progress in the understanding of the consequences of chronic inflammation in patients with CKD, more research is needed to fully recognise the cellular, molecular, and clinical events associated with this pathological process.
Chronic Persistent Inflammation in Chronic Kidney Disease
Description of the Problem
Inflammation is a major feature in CKD patients, with increasing prevalence as renal function declines. In a report from the Chronic Renal Insufficiency Cohort (CRIC) including 3939 patients with different stages of CKD, all of the inflammatory biomarkers determined, such as C-reactive protein (CRP), interleukin (IL)-6, and tumor necrosis factor (TNF)-α, were inversely associated with measures of kidney function and positively associated with albuminuria. Moreover, according to data from the National Health and Nutrition Examination Survey III (NHANES III; n = 15,594), approximately 54% of the patients with glomerular filtration rate (GFR) between 15 and 60 mL/min had some degree of inflammation (CRP > 2.1 mg/L). In addition, the age-adjusted probability of having elevated CRP (>2.1 mg/L) rose from 44% to 69% when estimated GFR (eGFR) was <60 and <30 mL/min, respectively. Along the same lines, in a survey including 663 patients with CKD stage 5 (160 patients evaluated close to the start of dialysis and 503 patients on dialysis) from three different countries—Sweden, Germany, and Italy—approximately two-thirds of the patients showed CRP levels above 3.4 mg/L.
The reasons behind the increased incidence of systemic inflammation in end-stage renal disease (ESRD) patients are complex and multifactorial and include a range of nondialysis and dialysis-related factors. The combination of an impaired immune response coupled with persistent immune stimulation may have a role in the low-grade chronic systemic inflammation and altered cytokine balance that characterize the uremic state and that may translate into increased morbidity and mortality. The close association between low-grade inflammation, comorbidities, and poor outcome highlights the importance of searching for strategies to ameliorate and avoid such conditions.
Etiology of Inflammation in Chronic Kidney Disease
The pathophysiology involved in the development of chronic inflammation in CKD has not yet been elucidated completely; however, it has been described as being the consequence of a multifactorial etiology with interactions with a number of factors, some of which are shown in Fig. 14.1 . The described factors include not only the decline of GFR and the noxious effect of uremic toxins but also several aspects usually present in this group of patients, such as comorbidities, superimposed illnesses, genetic predisposition, and therapeutic interventions, including the dialysis procedure itself. In this section we briefly describe the main etiological factors of inflammation in CKD.
Immune Dysfunction Proper of Chronic Kidney Disease
The uremic state in patients with kidney failure affects general immunity, with the two major branches—innate and acquired immunity—being altered in CKD patients. These alterations lead to a state of persistent systemic inflammation and acquired immunosuppression, circumstances that are manifested not only by increased risk for infections, which in turn associate with inflammation, but also by impaired response to vaccination and higher incidence of cancer.
In the uremic milieu, disturbances in the number and function of basically all immune cells have been described. Observed functional alterations include spontaneous activation, decreased phagocytic capacity, and increased apoptosis, altogether leading to increased generation and release of cytokines and reactive oxygen species (ROS), as well as diminished defensive capacity. The main alterations of immunocompetent cells associated with the uremic state are listed in Table 14.1 .
| Immunocompetent Cell | Alterations in Uremic State |
|---|---|
| Monocytes and macrophages | General expansion of circulating monocytes Spontaneous activation and decreased phagocyte capacity Overproduction of IL-1β, IL-6, IL-12, and TNF-α |
| Polymorphonuclear leukocytes (PMNL) | Spontaneous activation Decreased phagocytic capacity and reduced bactericidal capabilities Altered apoptosis |
| Lymphocytes B | B-cell lymphopenia Impaired differentiation and maturation Increased apoptosis |
| Lymphocytes T | Decreased proliferation Depletion of naive and central memory T cells Increased apoptosis Reduced ratio of CD4/CD8 257 Increased ratio of Th1/Th2 |
| Dendritic cells | Depletion of cells Decreased antigen presentation capabilities |
As a consequence of the altered function of the immune cells, a state of hypercytokinemia and elevated acute phase proteins are characteristic features of the uremic milieu, where the delicate equilibrium between proinflammatory cytokines and their inhibitors is clearly deregulated. This topic will be discussed more in detail later.
Oxidative Stress
A disturbed balance between increased production of ROS and a decrease in antioxidant defenses is a hallmark of CKD. Specifically, a multifactorial increase in the production of ROS is not counterbalanced by a corresponding increase of the antioxidant capacity, which is profoundly disrupted in CKD. Oxidative stress and inflammation are manifested conjointly in uraemia, creating a vicious circle in which redox-sensitive transcription factors (e.g., nuclear factor kappa B [NF-κB]) mediate the activation of proinflammatory cytokines, chemokines, and adhesive molecules responsible for the excessive formation of ROS. For instance, NF-κB triggers a cascade of inflammatory reactions, activated directly but also indirectly as a consequence of enhanced formation of advanced glycation end products (AGEs). The production of AGEs is increased in CKD, and AGEs are additionally retained due to the compromised kidney function. AGEs act via receptors for advanced glycation end products (RAGE), amplifying inflammation. This close relationship is reflected by the association between inflammation and oxidative stress markers in CKD.
Increased markers of oxidative stress appear early in the course of CKD and remain uncorrected, or even deteriorate with dialysis initiation. Moreover, inflammation and oxidative stress contribute to CKD progression.
The key role of NF-κB in the translation of the oxidative stress into inflammatory response explains the high potential of its antagonist, the nuclear factor erythroid 2-related factor (Nrf2), in enhancing the antioxidant barrier and protecting cells against inflammation. Besides suppressing NF-κB, Nrf2 activates the transcription of several antioxidant enzymes such as superoxide dismutase, UDP-glucuronosyltransferase, NAD(P)H:quinoneoxidoreductase-1, heme oxygenase-1, glutathione S-transferase, and others. Accordingly, in experimental studies performed in Nrf2 knockout mice, the inflammatory response associated with NF-κB activation is much more pronounced compared with the wild-type mice. Moreover, ablation of Nrf2 genes leads to the development of lupus-like nephritis and aggravates inflammation in diabetic kidney disease, highlighting the nephroprotective role of Nrf2. Physiologically, Nrf2 expression is induced by oxidative stress and inflammation to counterbalance their potentially deleterious effects and to prevent cellular injury. However, in uremia this mechanism is altered, and, paradoxically, severe oxidative stress and enhanced inflammation are accompanied by reduced activity of Nrf2. Consequently, downregulation of Nrf2 may play a role in the progression of CKD, and strategies to restore its activity seem to yield promising results in experimental models and may have an effect on retarding the progression of CKD in humans. A phase III clinical trial on bardoxolone methyl (an Nrf2-activating and nuclear factor-κn-inhibiting semisynthetic oleanane triterpenoid compound) in patients with type 2 diabetes mellitus and CKD stage 4 (BEACON) was terminated because of an increase in heart failure events in the bardoxolone methyl group. However, the potential of gaining benefits from this treatment in patients with Alport syndrome is currently (2017) being investigated in a phase II/III trial (CARDINAL; https://clinicaltrials.gov/ct2/show/NCT03019185 ).
Fluid Overload and Sodium Expansion
Volume expansion is an important risk factor for increased mortality in CKD patients, especially in ESRD patients undergoing dialysis. There is evidence linking volume expansion with the so-called malnutrition, inflammation, atherosclerosis (MIA) syndrome. Interstitial accumulation of fluid in the lung, known as pulmonary congestion , is much more prevalent in patients with ESRD, than is generally presumed based solely on clinical observations. Also, pulmonary congestion is associated with adverse outcomes and increased markers of systemic inflammation. The mechanisms underlying this association have yet to be elucidated; however, one of the apparent results of vascular congestion is increased permeability of membranes in the gastrointestinal tract and leakage of endotoxins into the vascular system, triggering systemic inflammation as described elsewhere in this chapter. Otherwise, the presence of the inflammatory status, resulting in increased vessel permeability and hypoalbuminemia, is sufficient to drive fluid shifts that result in volume derangements and congestion.
In addition, volume expansion could be a consequence of increased sodium loading. High sodium intake, sometimes enhanced by high dialysate sodium concentration in the dialysate targeted at reducing intradialytic complications, is commonly found in hemodialysis (HD) patients. The adverse effects of sodium are not limited to volume expansion; sodium excess is also responsible for oxidative stress and could be linked to systemic inflammation reflected by increased serum CRP in population-based studies.
Metabolic Acidosis
Chronic positive acid balance contributes to numerous deleterious outcomes, including progression of CKD and increased mortality. There are several lines of evidence supporting a strong link between acidosis and inflammation, although the molecular mechanism underlying this relationship is not fully understood. The NF-κB transcription factor is likely to play a central role in this process because it regulates the transcription of genes encoding inflammatory cytokines, such as interferon (IFN)-β, TNF-α, and IL-1, IL-2, IL-6, and IL-8, and it could be influenced by low environmental pH. Accordingly, exposure of canine kidney cells to pH stress resulted in at least threefold upregulation of proinflammatory genes. In another experimental study, environmental acidosis potentiated TNF-α synthesis and upregulated inducible nitric oxide (iNOS) activity in peritoneal macrophages. In dialysis patients, correction of metabolic acidosis by the administration of sodium bicarbonate resulted in a decrease of TNF-α expression in peritoneal dialysis (PD) patients and changes in IL-10 secretion in patients treated with HD. Moreover, correction of acidosis in patients treated with HD or PD resulted in a reduction of protein degradation because acidosis and inflammation are counterparts in the etiology of protein-energy wasting (PEW).
Nevertheless some controversy exists around the real benefit of bicarbonate correction in these patients. Contrary to the results presented previously, in a cross-sectional study of HD patients, no significant difference in the inflammatory indices (CRP, IL-6) was observed among the different groups of serum bicarbonate concentrations. In addition, results from the CRIC study showed that bicarbonate level was a better predictor of renal outcomes in participants with better preserved kidney function compared with those with more advanced CKD. Moreover, the same study showed that increase in serum bicarbonate level above 24 mmol/L increased the risk of heart failure even in those without preexisting CVD.
Thus although correction of metabolic acidosis in CKD patients may be an effective tool to slow the progression of kidney disease or reduce muscle wasting, there is not enough evidence supporting its general use in order to control inflammatory status, especially in view of possible adverse consequences of overcorrection of the serum bicarbonate level.
Comorbidities
Oral health and periodontal disease
Periodontitis is a dysbiotic inflammatory disease that is highly prevalent in patients with CKD. The risk of periodontitis was found to be several fold higher in the Renal Impairment In Secondary Care (RIISC) cohort study (odds ratio [OR], 3.8; 95% confidence interval [CI], 2.5 to 5.6) compared with a geographically matched control population. In the mentioned study, the severe form of periodontitis—allegedly responsible for the systemic complications—affected about 50% of the RIISC cohort, whereas its prevalence in a general population reached only 1% to 1.5%. A plausible explanation for the high prevalence of periodontitis in this group of patients is the observed changes in the oral cavity accompanying CKD, usually described as xerostomia, and modifications in the microbial community. These conditions, together with impaired immunity, poor oral hygiene, and malnutrition, enhance the manifestations of poor oral health in CKD patients.
In the course of periodontal disease, the presence of pathogens and bacterial lipopolysaccharides (LPS) activates the defenses of the oral mucosa, resulting in the release of proinflammatory cytokines and chemokines that attract inflammatory cells to the site of infection. Activated neutrophils are a source of ROS, contributing to local tissue destruction and allowing the translocation of proinflammatory factors, such as endotoxins (e.g., LPS) and ROS end products, to the blood circulation, resulting in systemic inflammation. In fact, several studies have shown that patients with periodontitis have elevated levels of CRP and exert an adverse effect on general health. Furthermore, an increasing body of evidence supports the hypothesis that poor oral health may constitute a risk factor for CKD progression through endothelial injury. It remains to be determined whether more successful management of poor oral health and periodontitis will reduce the risk of inflammation, infection, PEW, and atherosclerotic complications in CKD patients.
Bowel bacteria overgrowth and altered gut microbiota
The gut microbial community (microbiota) has an important role in the renal pathophysiology, with profound alterations of the gut microbial flora (called dysbiosis ) typically found in CKD patients. The mutual interplay between microbiota and kidney has been acknowledged, and the term gut-kidney axis has been coined. The high ammonia concentration responsible for lowering the pH in the gastrointestinal tract, the prolonged colonic transit, the dietary restrictions leading to decreased fiber intake, the fluid overload, and medication (phosphate binders, iron, antibiotics) are only a few of the numerous factors underlying the altered composition of the intestinal microbiota in uremic patients. There is a convincing body of findings in the literature demonstrating associations between systemic inflammation and gut dysbiosis in CKD. Dysbiosis disrupts the intestinal epithelial barrier structure and leads to increased gut permeability (leaky gut). This process allows the translocation of bacteria or bacterial LPS into the inner environment, as well as provokes the leak of gut-derived uremic toxins and their toxic derivatives into the systemic circulation. As a consequence, a cascade of proinflammatory response, targeted to eliminate the translocated pathogens, is activated with the enrollment of the innate immune system. Intestinal epithelial cells secrete IL-1, IL-6, and IL-18, and macrophages stimulate the development of effector CD4 + T cells TH1 and TH17. In addition, adaptive immunity might have a role in this process by changing the balance between Treg and T-effector cells.
It is thought that gut-derived toxins might also play a role in the progression of kidney disease and in the pathogenesis of CKD-associated complications, including accelerated CVD. In past years, it has been described that toxins generated by colonic bacteria, including p-cresyl sulfate, trimethylamine- N -oxide, alfa-phenylacetyl-l-glutamine, and indoxyl sulfate, may accumulate in renal tubular cells, provoking their injury by production of inflammatory cytokines and profibrotic factors.
In this sense, the use of pre- and probiotics (living microorganisms that confer benefit to the host) to attenuate the imbalance in the microbiota has been proposed as a promising intervention to minimize inflammation that potentially could delay CKD progression in this group of patients.
Nutritional problems
Cachexia and obesity represent divergent manifestations of nutritional problems associated with CKD, and both may contribute to enhanced inflammatory activity. Obesity, a common finding in CKD patients—especially among those undergoing PD—is associated with a state of chronic, low-grade inflammation and insulin resistance (IR). Adipose tissue regulates food intake and energy expenditure and controls inflammation by secretion of adipokines and proinflammatory cytokines. Considering the important effect that loss of renal function has on the clearance of these substances, the systemic effects of adipokines in patients with CKD appear to be greater than in the general population. Also, it has been found that the production of proinflammatory molecules in the abdominal subcutaneous fat is increased in patients with CKD compared with patients who do not have the disease. Moreover, the distribution of fat is important in the pathogenesis of inflammation; visceral fat appears to produce adipokines more actively than subcutaneous adipose tissue. Visceral abdominal fat is a main producer of IL-6. In a group of patients with CKD evaluated shortly before the start of renal replacement therapy (RRT), there was a significant association between serum IL-6 and truncal fat but not between IL-6 and nontruncal fat. This association has also been seen in nondialyzed patients with CKD stage 3 to 5 and in PD patients where an increased amount of visceral fat correlated with higher levels of inflammatory parameters and increased coronary artery calcification. Interestingly, an abnormal abdominal fat mass deposition in HD patients may associate not only with a higher prevalence of inflammation and increased mortality risk but also with PEW. This circumstance may be mediated by the increased levels of TNF-α, one of the key mediators in a crosstalk between visceral fat and inflammatory response, which may originate in significant amounts from macrophages resident in the adipose tissue, having in fact an anorexic effect that is likely to induce cachexia.
Sleep apnea and hypoxia
The lung-kidney link has received very limited attention to date, although the mutual interrelations between these two apparently distant organs are close and visible from the earliest stages of the human life. However, it is now accepted that CKD, hypertension, and obstructive sleep apnea (OSA) constitute a triad of mutually interrelated comorbidities with a deleterious effect on individuals’ quality of life and mortality. Chronic obstructive pulmonary disease (COPD) and OSA are manifested frequently in patients with CKD. COPD and OSA have a wide range of systemic effects, with inflammation, oxidative stress, and IR being some of their consequences. These findings support the role of hypoxia as a causative factor involved in the proinflammatory status of CKD individuals.
It has been described that in hypoxic conditions an oxygen-dependent cell signaling reaction is activated, influencing the expression of adaptive, inflammatory, and apoptotic genes that lead to hypoxic inflammation. The central role in this orchestration of transcription factors is played by the hypoxia-inducible factor (HIF). HIF, targeted to restore optimal cell oxygenation, manifests proinflammatory properties by promoting glycolysis in the inflammatory cells and affecting the expression of cytokines, adhesion molecules, and proinflammatory enzymes via NF-κB related pathways. In parallel to its direct role in eliciting and maintaining inflammation, hypoxia influences the immune response by the activation of the sympathetic system, which promotes inflammation and oxidative stress and results in a vicious circle of mutually enhancing proinflammatory reactions. High sympathetic activity may also explain an association between hypoxia, proteinuria, and progression of CKD observed in some pulmonary diseases, but also in people who live at high altitudes.
Interestingly, in a recent study prolonged intradialytic hypoxemia was associated with high levels of laboratory indicators of inflammation and increased all-cause hospitalization and mortality. Alarmingly, prolonged intradialytic hypoxemia was experienced by 10.2% of 983 individuals in a cohort of chronic HD patients, and its prevalence may be even further enhanced in PD due to increase of intraabdominal pressure.
Vitamin D deficiency
Intact kidney function plays a central role in vitamin D physiology, and low levels of 25-OHD and 1,25 dihydroxyvitamin D (1,25-OHD) accompany kidney disease from its earliest stages. Vitamin D acts as a key regulator of the immune system, and its deficiency has been proposed as a contributor to defective inflammatory response in CKD. There are several mechanisms by which vitamin D deficiency may promote inflammation, including NF-κB signaling, renal expression of the regulated on activation, normal T-cell expressed and secreted (RANTES) cytokine, activation of the renin-angiotensin-aldosterone system (RAAS), and activation of the TNF-α converting enzyme (TACE) with subsequent release of TNF-α, intercellular adhesion molecule (ICAM)-1, and vascular cell adhesion protein 1 (VCAM-1) into the circulation.
Emerging evidence of the antiinflammatory effect of vitamin D suggest that vitamin D could be a promising tool for therapeutic purposes far beyond its established role in the treatment of secondary hyperparathyroidism.
Lifestyle Factors
Many lifestyle factors may potentiate the inflammatory status in CKD patients. Among them, diet may play a central role in the regulation of chronic inflammation because it is a source of both anti- and proinflammatory constituents. In a recent study analyzing two community-based cohorts of elderly individuals, it was demonstrated that a proinflammatory diet is associated with systemic inflammation as well as reduced kidney function. In this study, the proinflammatory effect of dietary constituents such as sugar and saturated fatty acids appeared to be mediated by CRP and linked to worsening kidney function. In this regard, dietary habits and food patterns are probably targets of major relevance in modifying systemic inflammation.
On the other hand, physical inactivity, a common finding in CKD patients, has been found to be associated with higher values of inflammatory markers. Moreover, physical exercise has been proposed as a valid antiinflammatory strategy. Among the several pathways and mechanisms by which physical activity may attenuate chronic inflammation, we can mention secretion of antiinflammatory cytokines, lowering of CD14 + CD16 + monocytes, reduction in the expression of TLR4, and reduction of oxidative stress.
Likewise, cigarette smoking is regarded as one of most important modifiable lifestyle factors affecting general health. Cigarette smoke has been shown to alter both innate and immune systems, augmenting the production of numerous proinflammatory cytokines (such as TNF-α, IL-1, IL-6, IL-8, and granulocyte-macrophage colony-stimulating factor [GM-CSF]) and decreasing the levels of antiinflammatory cytokines (such as IL-10). Moreover, cigarette smoking has been shown to enhance oxidative stress mediated by RAGE and its signaling pathways. Among other known deleterious consequences to general health, tobacco consumption has been associated with increased incidence of kidney disease. The first report of a high risk for ESRD associated with smoking was reported in the Multiple Risk Factor Intervention Trial (MRFIT), and consecutive studies supported this finding. , Interestingly, smoking cessation has not been proven to have an important effect on the risk of CKD; the risk of developing renal failure in smoking men remained significantly enhanced even after 9 years of cessation, although the risk significantly decreased as the years elapsed. Nevertheless, the effect of cigarette smoking on relevant clinical outcomes warrants focus on prevention of smoking initiation, as well as smoking cessation among current smokers in CKD individuals and in the general population.
Genetic Predisposition
Recent studies show that the inflamed uremic phenotype is also the result of genetic factors. The observation that Asian dialysis patients treated in the United States have a markedly lower adjusted relative risk of mortality than Caucasians supports this statement. Indeed, a substantial heritability (35% to 40%) has been found for CRP and IL-10 production, and many studies demonstrate a significant effect of genetic variations on the uremic inflammatory response.
Our understanding of the genetic predisposition to inflammation in patients with CKD has increased enormously in the last couple of years, and undoubtedly variations in the genome play an important role for the development of specific phenotypes in CKD. For instance, it is now known that the IL-6 gene has functional variants that affect inflammation and risk for CVD among dialysis patients, supporting a causal role for IL-6 in CVD. Interestingly, the IL-6 gene variants, together with those from the lymphotoxin-α gene, independently predicted risk for CVD in a cohort of dialysis patients. Moreover, genetic variations in the IL-6 gene seem to influence inflammatory and peritoneal transport parameters, thereby contributing to the interpatient variability in small solute transport rate at the start of PD. Also, genetically determined interindividual differences in TNF-α, IL-10, myeloperoxidase, and peroxisome proliferator-activated receptor (PPAR)-γ release have been associated with the prevalence of inflammation, CVD, and survival in CKD
On the other hand, external and internal environmental stresses may affect the phenotype via changes in the epigenome. Aberrant DNA methylation—related to uremic dysmetabolism—may have complex interactions for the development of premature CVD. Shortening of telomeres (nucleoprotein complexes protecting the chromosome ends that are involved in chromosome stability and repair) has been associated with an inflammatory phenotype and increased mortality in dialysis patients.
Dialysis Technique
In addition to the intrinsic inflammation accompanying CKD, patients undergoing dialysis are subjected to additional potential inflammation-activating factors. Indeed, there are several factors specific of the dialysis technique that are widely accepted as contributors to inflammation. Those factors include bioincompatible membranes, dialysate back-flow, clotted access grafts, and catheter infections in HD, and peritonitis and bioincompatible glucose-based solutions in PD, as well as failed kidney transplant, and, in all types of techniques, endotoxemia and fluid overload.
Factors associated with hemodialysis technique
A number of factors related to the HD procedure promote inflammation. Although interestingly, dialysis-related inflammation seems to be associated with a specific genomic pattern, several in vivo studies suggest that the membrane composition, the type and quality of dialysis, and the type of vascular access may contribute to inflammatory processes. It has been demonstrated in a randomized study that HD patients treated with polyamide membranes presented lower CRP levels compared with those exposed to cuprophane or polycarbonate membranes. Accordingly, a significant relationship exists between membrane bioincompatibility and circulating levels of CRP, IL-6, and albumin. Likewise, it has been proposed that the amount of convective transport and the frequency of dialysis might have an influence on inflammation. The combination of better hemodynamic stability, high-flux synthetic and biocompatible membranes with ultrapure dialysis fluid that is offered by hemodiafiltration seems to reduce inflammatory activity and the production of proinflammatory cytokines compared with conventional HD. , Also, short daily HD (six sessions per week of 3 hours each) was associated with a reduction in left-ventricular hypertrophy and inflammatory mediators compared with conventional HD (three sessions per week of 4 hours each).
It has been also postulated that exposure of blood to dialysate may contribute to chronic inflammation in addition to effects related to the immunogenic properties of the dialysis membrane. Cytokine production may be triggered by substances present in the dialysis fluid, which may penetrate intact dialyzer membranes. Indeed, small bacterial DNA fragments in dialysis fluid can pass through dialyzer membranes and enter circulation. In accordance, a switch from conventional to online-produced ultrapure dialysate resulted in lower bacterial contamination with a significant decrease in CRP and IL-6 blood levels in several studies. Apart from reducing levels of proinflammatory cytokines, changing from conventional to ultrapure dialysate improved nutritional status, decreased the need of erythropoiesis-stimulating agents (ESAs), and reduced the incidence of carpal tunnel syndrome.
Furthermore, vascular access is a fundamental aspect in HD patients, and its dysfunction, and especially the use of central venous catheters, is among the most important sources of morbidity and mortality among HD patients. The type of vascular access has an important effect on inflammatory status and further outcomes. In a study comparing arteriovenous fistulas with arteriovenous grafts versus central venous catheters, inflammatory mediators (IL-1, IL-1 β, and CRP, but not TNF-α) were significantly higher in the latter. Clotted access grafts and access infections are both significant contributors to the inflammatory process in HD patients. It has also been speculated that biofilm formation is a cause of inflammation in this patient group.
Thus although adequate dialysis treatment to some extent can ameliorate some uremia-related proinflammatory factors, there are many dialysis-related factors that can contribute to the inflammatory state observed in CKD patients.
Factors associated with peritoneal dialysis technique
The PD procedure per se may exacerbate systemic inflammation, although to a lesser extent than the HD procedure. Moreover, inflammation promotes angiogenesis, an important constituent of morphological alterations in the peritoneal membrane that contribute to discontinuation of PD therapy. This relation is illustrated by the fact that alleviating inflammation reduces peritoneal angiogenesis. Also, the most threatening complication of PD, encapsulating peritoneal sclerosis (EPS), is preceded by peritoneal inflammation, and patients who subsequently develop EPS have higher levels of inflammatory cytokines during PD.
Conventional bioincompatible glucose-based PD solutions are important sources of inflammation, especially due to the content of glucose degradation products (GDPs) generated during heat sterilization. GPDs have shown to induce peritoneal inflammation and the formation of AGEs. In addition, glucose-based solutions lead to a substantial uptake of glucose, which may be associated with the induction of oxidative stress, dyslipidemia, and malnutrition, all of which are potent causes of inflammation.
Peritoneal transport status is another important fact to consider in PD patients. Patients presenting high peritoneal solute transport rates have worse clinical conditions, characterized by worse nutritional status and enhanced inflammation. Although adverse clinical outcomes in the group of high transporters may be closely related to volume overload, analysis of the cytokine profile in effluent may predict altered transport characteristics. Also, alterations of macrophage heterogeneity in peritoneal effluent were associated with different PD outcomes. In addition, high incidence of overt peritonitis and exit-site and tunnel infections are important contributors to increased inflammatory response in patients undergoing PD. Moreover, an indwelling catheter potentiates inflammatory response through a foreign body reaction.
Consequences of Inflammation in Chronic Kidney Disease
During the last few years and supported by a vast scientific basis, chronic inflammation has been acknowledged as a major culprit in a whole spectrum of pathological results associated with CKD. Poor outcomes, including CVD, diabetes, obesity, malnutrition, endocrine disruptions, and even premature aging, have been directly associated with a persistent inflammatory state. The specific pathways by which this association is mediated are still not fully understood; however, it is thought that inflammation, beside its own direct effect, potentiates other known risk factors, such as oxidative stress, IR, endothelial dysfunction, vascular calcification, bone mineral disorders, and depression, some of them previously described in this chapter. In this section we provide a brief review of some of the better-understood consequences of persistent inflammation in CKD patients.
Mortality
The association between inflammation and mortality in patients with CKD has been observed even in early stages of the disease. In patients with CKD stages 2 to 4, the multivariable-adjusted hazard ratio of death among those in the highest quartiles of IL-6 or CRP (and fibroblast growth factor-23 [FGF-23]) was several times higher than those in the lowest quartiles. Both IL-6 and CRP, the most frequently used inflammatory biomarkers, have demonstrated high predictive value in the assessment of cardiovascular risk and all-cause mortality in this group of patients. Moreover, other inflammatory markers, such as IL-1, IL-18, TNF-α, and P-selectin, have also shown mortality predictive value, although these markers have limited application in clinical practice. However, it has been proposed that the use of combined inflammatory biomarkers (CRP, IL-1β, IL-6, IL-18, and TNF-α) could be a valid strategy to improve predictive accuracy.
The link between inflammation and mortality is easily understood through the numerous complications associated with elevated inflammatory markers; nevertheless, it is interesting to highlight that the concurrent presence of inflammation with other pathologies such as PEW or CVD potentiates the mortality rates in ESRD patients. Namely, in a population of patients initiating dialysis (NECOSAD II study), the combination of inflammation, PEW, and CVD increased the mortality risk more than expected from the addition of their solo effects, implying additive interaction.
Development and Progression of Chronic Kidney Disease
Multiple lines of evidence support the contribution of inflammation to the onset and progression of CKD. Epidemiological studies report association between the decline in eGFR and proinflammatory markers in both the general population and patients with CKD. Several mechanisms may interact in the development of renal injury in the inflammatory milieu, and CKD itself contributes to enhanced inflammation. Inflammation has been shown to aggravate glomerular injury; IL-6 regulates mesangial cell growth and TNF-α contributes to endothelial damage in various experimental models. Also, in transgenic mice, interstitial matrix synthesis was regulated by TNF-β, and overexpression of IL-6 caused glomerulosclerosis and tubular damage. Recently the associations between plasma levels of proinflammatory cytokines (IL-1, IL-1 receptor antagonist, IL-6, TNF-α, TGF-β, high-sensitivity CRP, fibrinogen) and the progression of CKD were investigated in 3430 participants of the CRIC study. In this study, levels of fibrinogen, TNF-α, and albumin showed an association with rapid loss of kidney function, reaffirming the previous findings from experimental models.
Protein-Energy Wasting
PEW, a state of nutritional and metabolic derangements characterized by loss of muscle mass and fuel reserves of the body, is a common finding in patients with CKD. Unequivocal evidence supports the causative relation between inflammation and PEW, with inflammation encompassed into the etiological model of PEW.
Inflammation contributes to PEW by different mechanisms, among which anorexia is one of the better understood. Thus high serum concentrations of different inflammatory markers, including CRP and proinflammatory cytokines (IL-6 and TNF-α), have been associated with anorexia in HD patients. Inflammatory cytokines have the capacity to regulate appetite through disturbing specific brain areas related to appetite regulation. Specific cytokines access the brain and act directly on hypothalamic neurons and/or generate mediators, targeting both peripheral and/or brain target sites, and, as a consequence, influence the size, duration, and frequency of meals.
On the other hand, inflammation in CKD patients paradoxically increases resting energy expenditure (REE), contributing to muscle loss. In an animal model, the infusion of inflammatory cytokines (TNF-α, IL-1, and IL-6) resulted in increase in the rate of muscle protein breakdown ; in humans, inflammatory markers are related to markers of muscle mass, indicating an important role of cytokines in the development of PEW and muscle catabolism.
In addition, other features enhanced by inflammation, such as increased IR and activation of the adenosine triphosphate ubiquitin-proteolytic system, have proved to have a role in the pathogenesis of PEW.
Vascular Calcification
Large artery calcification (assessed by computer tomography or chest x-ray) is highly prevalent from the earliest stages of CKD, reaching a prevalence as high as 70% in individuals with ESRD and even affecting as many as 15% of uremic pediatric patients. Vascular calcification has catastrophic effects on the prognosis, raising cardiovascular mortality disproportionally and increasing the risk of sudden death. Of note, elevated markers of inflammation (CRP and IL-6) in the presence of vascular calcification predict an even worse prognosis.
In CKD, both intima and media arterial calcification are markedly increased. Whereas intima and media calcification differ morphologically and their clinical consequences are also different, both are associated with local and systemic inflammation.
There is a substantial body of evidence supporting the involvement of the inflammatory response in the pathogenesis of vascular calcification. In a recent study in early CKD patients, biopsy-verified media calcification was associated with local upregulation of proinflammatory molecules but not with increased serum levels of phosphorous, calcium, or calcium x phosphorous index. Also, in advanced CKD, only sclerostin—but not other circulating markers of bone metabolism—predicted biopsy-verified vascular calcification.
Among the different inflammatory biomarkers in CKD patients, TNF-α and IL-1 promote the progression of vascular calcification, whereas several other inflammatory markers, including IL-8 and IL-18, are associated with its occurrence. In vitro studies have shown that TNF-α induces calcification of vascular smooth muscle cells (VSMCs), and coculture of these cells with monocyte/macrophages can accelerate this process. Moreover, two regulating molecules that belong to the TNF receptor superfamily, osteoprotegerin (OPG) and receptor activator of NF-κB ligand (RANKL), are probably key players mediating calcification of VSMCs. RANKL is one of the major osteoclast maturation factors, whereas OPG functions as a soluble decoy receptor for RANKL and inhibits its effects. This finely regulated balance is profoundly disrupted in the presence of ESRD because OPG accumulates in such a setting, causing a decrease in circulating RANKL, which in turn constitutes a risk factor for unfavorable vascular outcomes. This would explain the detrimental effects on survival of both increased inflammation and OPG levels observed in HD patients.
Another important player involved in the process of extrabone calcification is fetuin-A. This inhibitor of ossification, which acts as a negative acute-phase reactant, has been shown to decrease with progression of CKD. Mice lacking the gene encoding for fetuin-A rapidly develop ectopic soft tissue ossification and die at an early age. In CKD, low levels of circulating fetuin-A are associated with increased cardiovascular burden and mortality.
Taken together, these data provide evidence for the existence of an active interplay between vascular calcification and inflammation and underscore its importance as a nonclassical cardiovascular risk factor in CKD.
Anemia and Erythropoiesis-Stimulating Agent Resistance
The pathogenesis of anemia in CKD is multifactorial, and resistance to ESA—a consequence of inflammation—is a key contributor. For instance, HD patients with increased CRP levels require as much as 80% higher doses of ESA compared with those with CRP below 20 ng/mL.
Although the link between inflammation and ESA resistance and anemia is undoubted, the exact pathophysiological mechanism underlying this phenomenon is not fully understood. Some possible mechanisms have been proposed, including reduced iron availability and deterioration in the red cell line.
Erythroid progenitors require coordination of iron acquisition and cell proliferation in the consecutive stages of maturation. In a pathway controlled by hepcidin, inflammation compromises the availability of iron for heme synthesis and, in consequence, impairs erythropoiesis. Hepcidin is a peptide hormone that regulates iron absorption and its release from the macrophages. Due to increased hepcidin synthesis, iron is retained in enterocytes and macrophages, leading to functional iron deficiency. Hepcidin is known to increase in CKD, also independently of inflammation, and a decrease of hepcidin levels has been observed in response to administration of ESA. There is also some evidence that ESA may decrease levels of inflammatory markers, especially IL-6, which in turn may explain its influence on hepcidin levels
Recently a regulatory role of HIFs in hepcidin synthesis and iron metabolism has emerged. This may constitute a new link in a relationship between inflammation and anemia, perhaps opening new perspectives for therapeutic use of HIF stabilizers in management of CKD.
On the other hand, inflammatory cytokines, TNF-α and IL-1, inhibit erythropoietin production, whereas TNF-α, INF-γ and IL-6 may directly suppress the erythroid progenitor cell proliferation. Moreover, inflammatory cytokines (mostly TNF-α) are responsible for the shorter life span of mature erythrocytes because they provoke changes in erythrocyte membrane and enhance erythrophagocytosis.
Depression and Cognitive Impairment
Depression is highly prevalent in CKD, with some reports describing the presence of depressive symptoms in up to 70% of HD patients. Depression is closely and directly associated with inflammation. Although this association may be a result of confounding, a causative relation is supported by the effect of antiinflammatory drugs on the course of depression and by a decrease of IL-6 levels in response to antidepressant treatment. High serum levels of proinflammatory cytokines (IL-1, IL-6, TNF-α) and cell-mediated immune activation, typically found in CKD, led to increased activity of the hypothalamic–pituitary–adrenal (HPA) axis and decreased availability of tryptophan for serotonin and melatonin synthesis. All of these immune-inflammatory crossed pathways contribute to depression and also to chronic fatigue, which may be a constituent of—or associated with—depressive disorders. Not unexpectedly, poor health-related quality of life (HRQoL) associates with proinflammatory cytokines and their regulation in HD patients. Interestingly, despite the strong association between IL-6 and fatigue in HD patients, no such a relationship has been found in healthy individuals.
On the other hand, in a cognitive study of the CRIC cohort, higher markers of inflammation (hs-CRP, fibrinogen, IL-1β) were associated with an increased risk of impairment in attention but not with other cognitive functions. Depression, fatigue, and impaired cognition may undeniably contribute to a vicious circle of anorexia, physical inactivity, inflammation, and PEW.
Endocrine Disorders
The uremic state is associated with abnormalities of the endocrine system, affecting hormone production, metabolism, feedback regulation, and altered target tissue sensitivity. Evidence suggests that the observed hormonal dysmetabolism may be aggravated by persistent inflammation. Several hormonal axes are usually found to be altered in CKD patients, although the total implications of the disturbances in terms of outcomes are not completely clear.
Thyroid hormones
Inadequate kidney function, in addition to other factors accompanying CKD such as inflammation, exerts an influence on the thyroidal status of renal patients in whom alterations in thyroid hormones are observed in the absence of an underlying intrinsic thyroid disorder. The most common observed thyroid disorder in this group of patients is the so-called low T 3 syndrome, characterized by a decrease in total (T 3 ) and free (fT 3 ) triiodothyronine plasma concentration, whereas thyroid-stimulating hormone (TSH) levels are usually normal. This condition, present in approximately one-fourth of patients with CKD, is discussed in an ongoing debate regarding whether the observed thyroid alterations, like other hormonal alterations in CKD, represent a pathological condition or a physiological, adaptive, and protective mechanism against the disease. This question does not yet have a clear answer; however, some studies have reported worse outcomes associated to these hormonal derangements, suggesting that it is not a completely physiological process. In fact, previous studies have connected the state of subclinical hypothyroidism with low-grade persistent inflammation. Indeed, IL-6 signaling has been reported to downregulate the peripheral conversion of total thyroxine (T 4 ) into T 3 in both experimental and clinical studies.
Sex hormones
Testosterone deficiency or hypogonadism is a common finding in men with CKD. Although the high prevalence of hypogonadism observed in this group of patients to a great extent might be a consequence of the failing kidney per se , inflammation might have an important role in its physiopathology because it is known that inflammatory cytokines have a suppressive effect on the hypothalamic–pituitary–testicular axis. In fact, several studies depict a strong inverse association between endogenous testosterone and surrogates of inflammation in various CKD populations. Important to note is that testosterone also has immune-modulatory actions per se , as suggested by the suppression of cytokine production in hypogonadal men with diabetes, coronary heart disease, and metabolic syndrome after supplementation with testosterone. The relationship between inflammatory markers and other sex hormones such as estrogen or prolactin in CKD patients has been less studied.
Hypothalamic–pituitary–adrenal
The HPA axis is a finely tuned neuroendocrine system that plays a main role in processes such as stress-response, energy balance, circadian rhythm, immunity, reproduction, emotions, and cognition. The HPA axis has been little studied in CKD. However, derangements in the functioning of HPA could be expected, as for other endocrine axes, and there is some evidence that deregulation of HPA function may be present in earlier stages of CKD (e.g., CKD stage 2). The link between inflammation and alterations in the HPA axis is not well defined; however, it is known that increased levels of cortisol observed in CKD are related in part to decreased activity of the 11β-hydroxysteroid dehydrogenase type 2 (11b HSD2), which is responsible for the conversion of cortisol into cortisone, its inactive metabolite. The activity of this hormone decreases in presence of inflammation.
Other hormones
The inflammatory response inhibits the action of growth hormone (GH), as supported by forearm perfusion studies demonstrating that resistance to pharmacological doses of GH is not related to uremia per se but rather to an increased inflammatory state.
Insulin Resistance
IR, defined as a reduced sensitivity of the liver, skeletal muscles, and adipose tissue to insulin action, is a frequent finding in CKD from the earliest stages of the disease. In ESRD, IR is believed to be present in virtually all patients, although a direct correlation between insulin sensitivity and degree of decline of eGFR is weak or absent. Inflammation, especially potentiated by oxidative stress, is one of the established causative factors predisposing CKD patients to IR. Proinflammatory cytokines may promote IR by inhibiting insulin-signaling pathways on the post receptor level in skeletal muscles (TNF-α) and liver (IL-6), whereas adiponectin mediates insulin-sensitizing effects. Interestingly, immune cells do not develop IR. This protective mechanism, ensuring a sustained energy flow in acute inflammation, becomes debilitating in chronic and persistent conditions of immune stimulation. In cross-sectional studies, the homeostatic model assessment of insulin resistance (HOMA-IR) was positively associated with CRP, IL-6, fibrinogen, and TNF-α in different cohorts of dialysis patients. Many factors typically coexisting with inflammation and CKD, such as physical inactivity, anemia, vitamin D deficiency, leaky gut, acidosis, and others, also contribute to IR and constitute a vicious circle of mutual interplay. IR profoundly influences outcomes because it is associated with cardiovascular events, mortality, and CKD progression. On the other hand, IR is a modifiable risk factor, and administration of angiotensin receptor blockade has shown promising results ameliorating IR and decreasing inflammatory markers in patients with CKD stages 2 to 4.
Premature Senescence
Inflammation may determine the pace of aging; and their mutual relationship is reflected in the term inflammaging . The attrition of telomeres—the repetitive regions located at the end of chromosomes—is postulated to be a surrogate marker of cell senescence. There is considerable evidence linking the shortening of the length of telomeres to inflammatory response; it is observed in circulating macrophages exposed to enhanced expression of proinflammatory cytokines (IL-6, TNF-α, IFN-γ) and hyperactivity of NF-κB.
CKD shares similarities with senescence and predisposes to premature aging via a variety of mechanisms. Oxidative stress is likely to play a prominent role in this process. On the other hand, stress-induced premature senescence contributes reciprocally to the chronic inflammatory state of advanced CKD. Senescence-associated secretory phenotype (SASP), understood as a proinflammatory drift resulting in release of a variety of proinflammatory cytokines, chemokines, and growth factors, may develop as a response to oxidative stress triggered by uremia. The SASP contributes to reinforcement of senescence and can transmit the senescence to neighbor cells in a paracrine manner.
Quality of Life
HRQoL is a subjective assessment of the effect of a disease on the physical, psychological, and social functioning dimensions of the patient’s life. HRQoL is included in the World Health Organization’s definition of health and is a strong, independent predictor of hospitalization and mortality in ESRD. In a study including patients with CKD stages 2 to 5 from the Stockholm region in Sweden, HRQoL scores deteriorated progressively with the loss of renal function and were the lowest in CKD stage 5. However, CRP and not GFR had stronger predictive capacity of impaired HRQoL in this study. The results indicated that even a relatively moderate increase in CRP may affect HRQoL. The relationship between proinflammatory cytokines and self-rated health and well-being was also reported in a cohort of women visiting a primary care unit in Sweden. Poor self-rated health was associated mostly with higher levels of IL-6 and TNF-α, whereas impaired general well-being was related to increased TNF-α and IL-1β. Further studies are needed to determine whether and to what extent treatments targeted toward decrease of inflammatory markers may affect HRQoL.
Measuring Inflammation in Chronic Kidney Disease Patients
With the diagnosis of inflammation, as with the diagnosis of other medical conditions, finding the right biomarker for this purpose is of utmost interest. The search for the perfect marker is complicated in patients with kidney disease, in whom some basic inflammatory markers (e.g., erythrocyte sedimentation rate, ESR) are affected by the decline of renal function.
In recent years, several novel inflammatory biomarkers have been described and tested; embracing a wide catalog of possibilities that go from plasma proteins, such as CRP; classical cytokines, such as IL-6, IL-10, and TNF; and cytokine receptors, such as TNF-receptors, to attractive new molecules such as pentraxin 3 (PTX3) or soluble TNF-like weak inducer of apoptosis (sTWEAK).
It has been advocated, often not based on conclusive evidence proving their superiority, that these more specific biomarkers are better determinants of inflammatory status and represent more independent indicators of disease severity. What is true is that despite many markers having been studied and suggested, only very few of them meet requirements that make them appropriate for use in clinical practice. In this section, we discuss some of the most studied inflammatory markers in CKD ( Table 14.2 ).
| CRP | IL-6 | IL-10 | TNF-α | ESR | PTX-3 | |
|---|---|---|---|---|---|---|
| Availability | High | Intermediate | Low | Low | High | Low |
| Cost | Low | Intermediate | High | High | Low | High |
| Interpretation | Easy | Easy | Difficult | Intermediate | Easy b | Difficult |
| Reliability in CKD | High | High | Unknown | Unknown | Low? b | High |
| Usefulness in clinical practice | High | High a | Low | Low | Low? b | Low |
a Usefulness of IL-6 is potentially high, but IL-6 is still rarely used in clinical practice.
C-Reactive Protein
CRP is an acute-phase protein first described in relation to its capacity to bind pneumococcal capsular polysaccharide. It is produced by the liver under the stimulus of inflammatory cytokines, in particular IL-6. CRP levels rise in about 2 to 8 hours after the injury, reaching their peak around 48 hours later, with a half-life of 18 to 19 hours. Apart from liver failure, there are few known conditions that interfere with CRP production, and kidney failure seems not to have an influence.
CRP has a suitable profile (i.e., stability over time, lack of circadian variation, and no effect of food intake) that makes it especially useful in renal care, and it is by far the most measured inflammatory marker in patients with CKD. Thus CRP remains the prototypic marker of inflammation.
Unlike what happens in the general population, in whom consensus has been made in terms of the established cut-off values for diagnosis of inflammation (i.e., undetectable or very low levels are considered normal, values between 1 and 10 mg/L are considered moderately elevated, and values over 10 mg/L are considered markedly elevated), little agreement exists regarding the optimal cut-off point to diagnose clinically significant inflammation in CKD patients, and usually arbitrary and nonconsensual cut-off values have been used. Some years ago, based on published data from pooled European cohorts, a cut-off point of 10 mg/L was proposed to define uremia-related inflammation, and this value has become the most frequently used cut-off point in clinical research for the prediction of outcome. However, recent studies have shown a substantial increase in mortality risk associated with CRP concentrations >3 mg/L.
Even though further research is needed to establish optimal the cut-off point for diagnosis and initiation of treatment in this group, the wide availability of CRP determination, its relatively inexpensive price, and the facility of interpretation make CRP a very favored inflammatory marker to be used in clinical practice.
Interleukins
Interleukins are a group of secreted proteins and signal molecules with fundamental regulatory roles in the immune system.
IL-6
IL-6 (MW 22 to 27 kD) is produced by numerous types of immune cells, including monocytes, mesothelial cells, fibroblasts, adipocytes, and lymphocytes, usually in response to physiological stimuli such as infection, trauma, or other tissue damage.
Among many other properties, IL-6 induces acute-phase reaction, being one of the starters of the inflammatory response. However, what is more interesting about IL-6 is that it acts as both a proinflammatory and an antiinflammatory cytokine. In addition, whereas most other cytokines function via paracrine/autocrine mechanisms, the major effects of IL-6 are a consequence of its presence in the circulation, and its effects can be exerted at sites distinct and distant from its origin.
IL-6 can be detected in the 1 pg/mL range in healthy individuals and is elevated in most, but not all, ESRD patients. Although decreased elimination may be a major cause of elevated IL-6 levels in ESRD patients, increased cytokine generation also plays a role. For example, circulating concentrations of IL-6 increase after HD sessions, providing evidence of HD-induced delayed inflammatory response.
Plasma IL-6 levels correlate with increased mortality and poor outcome in ESRD patients, consistent with studies relating IL-6 to mortality in the elderly and in healthy men at risk for myocardial infarction.
IL-10
IL-10 (18 kD) is a product of immune-active cells, mainly monocytes and lymphocytes, and appears to have a critical role in suppressing the inflammatory response. Within the inflammatory response, the secretion of IL-10 is delayed and always follows that of proinflammatory factors with a latency of a few hours. IL-10 is mainly cleared through the kidneys, probably by glomerular filtration and tubular metabolism, so its plasma half-life is markedly increased in ESRD, leading to elevated plasma levels.
Although some groups have speculated that subjects with the highest IL-10 levels have better immune balance, a recent study reported that patients with higher levels of IL-10 (above the median of the population) exhibited increased risk of CVD events and overall mortality, probably as a reflection of an overall proinflammatory milieu. Although for the moment the lack of conclusive data about the role of IL-10 as a protective or risk marker does not make it a useful marker for the clinical practice, it is an interesting marker for research purposes.
TNF-
TNF, a proinflammatory cytokine (17 kD) originally associated with killing of tumoral cells, has a pivotal role in regulating both pro- and antiinflammatory mediators. It is produced mainly by activated macrophages, although many other cell types can produce it. TNF-α has been regarded as a key regulator of the cytokine cascade, providing a rapid form of host defence against infection; however, excessive TNF-α can be destructive. TNF influences lipid metabolism, coagulation, IR, and endothelial dysfunction. Interestingly, TNF-α levels have a rather weak correlation with CRP, suggesting that circulating levels of TNF may be influenced by a number of different factors,
Although a relationship between renal function and TNF-α and its soluble receptors has been demonstrated in patients with varying degrees of renal failure, it is not clear to what extent reduction in renal function is directly responsible for increased levels of TNF-α. In CKD patients, higher levels of TNF have been related to poor outcomes, especially nutritional outcomes.
Other cytokines
Other cytokines have also been found to be increased in patients with CKD. IL-12 and IL-18 are elevated during the earlier stages of CKD but are not associated with arterial stiffness. IL-18 is associated with GFR, suggesting that its elevation in CKD is largely dependent on renal clearance.
Other Biomarkers of Inflammation
A large number of other substances, including specific molecules involved in the inflammatory response, have been studied as potential inflammatory markers; however, due to less availability, difficulties in interpretation, and lack of additional predictive capacity over and above that of commonly used inflammatory markers, most of these are unsuitable at present to be used in clinical practice. Among commonly available biomarkers (which will not be further discussed here), white blood cell count and immunocompetent cells, including leukocytes, monocytes, and lymphocytes (see Table 14.1 ), ESR (see Table 14.2 ), and plasma albumin and fibrinogen levels are often used to assess presence and severity of inflammation.
Pentraxin 3
PTX3, a member of the pentraxin superfamily, is characterized by a cyclic multimeric structure with structural relation to other pentraxin molecules such as CRP. PTX3 is produced by a number of different cells, including endothelial cells, after primary inflammatory signals, activating the classic pathway of complement and having a dual role of protection against pathogens and control of autoimmunity.
However, it has been recognized that PTX3 has multifaceted properties extending beyond the fields of immunity and inflammation to CVD and other chronic conditions.
In normal conditions, pentraxin is detected in blood in very low levels, although it increases rapidly after noxious triggers.
In CKD patients, as in the general population, PTX3 represents a specific and sensitive marker connecting inflammation with CVD. These data and epidemiological and clinical data support PTX3 as a valid biomarker for atherosclerosis and other poor outcomes.
Other, less commonly used inflammatory biomarkers in research protocols include, among others: high-mobility group box 1 protein (HMGB1), sTWEAK, soluble TNF-related apoptosis-inducing ligand (sTRAIL), orosomucoid or α1-acid glycoprotein, soluble receptor for advanced glycation end-products (sRAGE), S100 calcium-binding protein A12 (S100A12; also called calgranulin C , or EN-RAGE ). These—and many other biomarkers related to inflammation—reflect various specific aspects of inflammation that are relevant to its pathophysiology and downstream consequences.
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