INTRODUCTION

Chronic kidney disease (CKD) is a prevalent disease associated with increased risk of mortality and cardiovascular morbidity.¹ A study published in 2004, with a cohort of 1,120,295 adults, demonstrated an increased risk of death, cardiovascular events and hospitalization in association with an estimated glomerular filtration rate (GFR) of less than 60 ml/min/1.73m2. The adjusted hazard ratio (HR) increased from 1.² (for CKD stage 3a) to 5.9 (for CKD stage 5), excluding renal transplanted patients and dialysis patients.

In addition to cardiovascular disease, infections are an importante cause of mortality, constituting the second most common cause of death among patients with end-stage renal disease (ESRD). Powe et al showed that sepsis is a frequent event in patients on hemodialysis (HD) or peritoneal dialysis (PD), occurring in 10% of patients over a 7 years follow-up period³. Also, mortality risk due to sepsis is about 100 to 300-fold higher in dialysis patients than the general population⁴

This increased risk of infection is due to an immune dysfunction that affects both the innate and adaptive immune systems of CKD patients, leading to a state of both immune activation and immunosuppression.

It occurs irrespective of the underlying disease and confers an increased predisposition to infections, virus-associated cancers and impaired response to vaccination⁵. Uremia, malnutrition, chronic inflammation, vitamin D deficiency and dialysis-related factors contribute to the impaired immune function that characterizes CKD. The aim of this review is to summarize the main causes and mechanisms leading to immune dysfunction in CKD.

INNATE IMMUNITY

The innate immune system is responsible for a rapid and nonspecific inflammatory response to tissue injury and infection. It envolves the recognition of specific pathogen-associated molecular patterns (PAMPs) by receptors expressed on effector cells of the innate imune system, such as dendritic cells, macrophages, monocytes, neutrophils, and epithelial cells. Once activated, these effector cells are triggered to perform their functions, including phagocytosis, induction of inflammatory

response and presentation of antigens⁶. Pattern-recognition receptors are divided into three classes and all of them are affected in ESRD:

• Secreted pattern-recognition receptors: this class is represented by mannose-binding lectin (MBL) family. These molecules bind to microbial cell walls and function as opsonins, allowing microbial recognition by the complement system and phagocytes. MBL levels are significantly elevated in patients on HD⁷, and this seems to have prognostic importance since high levels during the pretransplantation period are associated with worse patient and graft survival on patients undergoing kidney-pancreas transplantation⁸. However, another study showed that low levels of MBL are associated with increased all-cause mortality in HD patients⁹.

• Endocytic pattern-recognition receptors: localized on the surfasse of phagocytes, they are responsible for the uptake of pathogens into lysosomes. In ESRD patients, two major macrophage scavenger receptors, SR-A and CD36, are increased, which may represente a chronic stimulation of these receptors in an environment of inflammation and oxidative stress^10,11.

• Signaling pattern-recognition receptors: represented by the Toll-like receptor (TLR) family, these receptors recognize multiple pathogenic components and mediate the activation of numerous cytokines and the complement pathway^12,13. They also have a role in the maturation of dendritic cells - antigen-presenting cells (APCs) - by upregulating the expression of the costimulatory molecules CD80 and CD86¹⁴. Uremia is known to impair the antigen presentation capabilities of APCs and it seems to be caused by defects in TLR expression. In fact, TLR4 expression was found to be diminished in predialysis patients, mainly those predisposed to bacterial infections, and it is associated with reduced synthesis of cytokines in response to LPS stimulation¹⁵. Comparable results were found in HD patients, leading to the hypothesis that endotoxins contained in the dialysate may suppress TLR4 expression¹⁶.

In ESRD patients, there is spontaneous activation of monocytes, mainly of the CD14+CD16+ subset, which have a particularly high capacity to produce inflammatory cytokines and contribute to the oxidative stress and systemic inflammation that prevail in CKD¹⁷. In addition, the monocyte’s phagocytic capacity is also impaired, contributing to the increased susceptibility to infection of this population¹⁸.

Polymorphonuclear leucocytes’ phagocytic capacity is also compromised. Anding et al reported that the bactericidal function of neutrophils in HD patients was inferior to those in the control group¹⁹. Moreover, it is also known that uremic solutes modulate neutrophils’ apoptosis. Many studies showed accelerated apoptosis of normal neutrophils in the serum of ESRD patients^20,21. Enhanced apoptosis reduces the response against infections; however removal of activated imune cells is also important to ameliorate the inflammatory state of ESRD^22,23.

ADAPTIVE IMMUNITY

The adaptive immune system is responsible for an antigen-specific response and involves production of antibodies and a memory of immune responses that enables long-term protection to the host. T and B lymphocytes are the main agents of adaptive immunity²⁴. ESRD patients’ increased risk of infection, impaired response to vaccination and higher anergy rates to delayed-type hypersensitivity skin tests (e.g. tuberculin test) suggest a deficient adaptive immunity²⁵.

In vitro studies have shown decreased T-cell proliferation in the uremic milieu^26,27. ESRD patients were found to have significantly depleted naïve and central memory CD4+ and CD8+ T-cells and a reduced CD4+/CD8+ ratio. Also, a negative correlation was found between the number of naive T-cells and severity of azotemia, hyperphosphatemia and oxidative stress²⁸. This finding may be due to increased susceptibility of these cells to apoptosis, as was shown in multiple studies^29-31. In fact, Meier et al showed that activated T-cells undergo a process of accelerated apoptosis that is more pronounced in HD patients when compared to pre-dialysis patients and healthy controls. This may be explained by the persistent contact between blood and the synthetic HD membrane³⁰. Moreover, uremic patients also have a predominance of Th2 over Th1 lymphocytes, which is caused by an increased susceptibility to apoptosis of the latter. Since Th1 lymphocytes are the principal effectors of cell-mediated immunity, this imbalance may be responsible for the altered cellular immunity seen in uremic patients^31,32.

ESRD is also associated with B-cell lymphopenia, which may be mediated by increased apoptosis and impaired differentiation and maturation of transitional B-cells, through resistance to the biological actions of B-Cell Activating Factor of tumor necrosis family (BAFF)^33,34.

Impaired innate and adaptive immunity determines the poor response to vaccination seen in CKD patients. Reduced capacity of dendritic cells to present antigens and changes in T-cell subsets and functions compromise the ability of B-cells to produce long term memory⁵. Litjens et al evaluated the antigen-specific T-cell response after vaccination with hepatitis B surface antigen (HBsAg) of ESRD patients. They concluded that the production of antigen-specific effector memory CD4+ T-cells is severely reduced in ESRD patients, which was strongly associated with low serologic response to HBsAg³⁵.

Improving vaccination effectiveness is of upmost importance since ESRD patients, mainly those on HD, are particularly susceptible to infection with hepatitis B virus (HBV) due to increased exposure to blood products. Some studies showed improvement in vaccination response in ESRD patients with immune adjuvants, such as granulocyte macrophage colony stimulating factor (GM-CSF) and levamisole, agentes that promote antigen presenting cells’ function and maturation.^36-39

More recently, a CPG-oligodeoxynucleotide adjuvant, which stimulates TLR9, has successfully enhanced hepatitis B antigen-based vaccine seroprotection in CKD patients⁴⁰.

In addition to disturbance of the immune system, there are other factors that contribute to impaired immunity in CKD patients, including intestinal dysbiosis, oxidative stress and inflammation and other CKD complications such as mineral bone disease (MBD) and anemia.

INTESTINAL DYSBIOSIS

Numerous studies have shown that uremia profoundly alters the gut microbiota, leading to an increase in intestinal pathobions^41,42. Metabolic acidosis, retention of uremic toxins, volume overload with intestinal congestion, frequent use of antibiotics and iron that promote bacterial overgrowth and dietary restrictions are the main contributors to intestinal dysbiosis in CKD⁴³. Wang et al proved that uremia increases bacterial translocation, which is associated with increased levels of IL-6 and C-reactive protein, markers of systemic inflammation⁴².

Further, the work of McIntyre and colleagues showed increased levels of lipopolysaccharide (LPS), a component of the outer membrane of gram-negative bacteria, in patients on HD and PD⁴⁴. Thus, uremiarelated dysbiosis may be responsible for the persistent inflammatory state seen in CKD patients.

OXIDATIVE STRESS AND INFLAMMATION

Oxidative stress presents early during CKD progression and is inversely correlated with GFR⁴⁵. Increased reactive oxygen species in CKD are attributable to retention of uremic solutes and increased production of cytokines, as well as antioxidant depletion⁴⁶. In spite of being critical for the response against infection, it may cause deleterious effects, such as hypertension, endothelial dysfunction, atherosclerosis and inflammation if not properly regulated.

ERYTHROPOIETIN AND IRON

Erythropoietin (EPO) is a hormone secreted by the kidney in response to cellular hypoxia in order to stimulate the production of red blood cells in the bone marrow. CKD patients have lower levels of EPO when compared to healthy subjects and are consequently treated with recombinant human EPO. Iron deficiency, infection and proinflammatory cytokines such as IL-1, TNF-α and interferon-γ play a major role in causing a poor response to this therapy⁴⁷. In order to achieve increased responsiveness to erythropoiesis-stimulating agents, the majority of CKD patients need iron supplementation. However, iron therapy is associated with depressed polymorphonuclear killing capacity and it also affects T and B lymphocytes^48,49.

MINERAL BONE DISEASE

Vitamin D deficiency is a very prevalent finding in CKD patients. Traditionally, it has been associated with hyperparathyroidism and mineral bone disease; however, studies have demonstrated its pleiotropic effects, which include modulation of the immune system, regulation of inflammatory responses and suppression of the renin-angiotensin system, among others^50,51. In fact, vitamin D repletion was linked to a decreased Th1 mediated autoimmune diseases and na increased bactericidal activity⁵².

High levels of cytosolic calcium of polymorphonuclear leucocytes were found in HD patients, which were associated with decreased reactivity upon stimulation and consequently dysfunction⁵³. This sustained elevation of cytosolic calcium appears to be due to the effects of parathyroid hormone (PTH), the levels of which are increased in CKD patients, and may be prevented by calcium channel blockers^53,54.

Hyperparathyroidism also adversely affects the function of T and B lymphocytes, leading to impaired cellular and humoral immunity⁵⁵.

RENIN-ANGIOTENSIN SYSTEM

In addition to its pathogenic role in the regulation of blood pressure and the progression of CKD and cardiovascular diseases, angiotensin- II also has a major role in regulating inflammation. It is involved in cell proliferation and chemotaxis, as well as recruitment of proinflammatory cells to the site of injury^56,57. It was also demonstrated that inflammatory cells such as T-cells, NK-cells and monocytes contain renin-angiotensin system elements, suggesting that they are able to produce and deliver angiotensin-II to the damaged tissue, perpetuating the inflammatory process⁵⁸.

CONCLUSION

Uremia is associated with a state of both immune dysfunction (which affects the innate and adaptive system and is characterized with increased prevalence of infections), and immunoactivation caused by a persistent and systemic inflammatory state. This abnormality in the immune system seems to contribute to the cardiovascular and

infectious complications that constitute the most common causes of death among CKD patients.