Enhancing Immune Protection in Hemodialysis Patients: Role of the Polymethyl Methacrylate Membrane

Abstract

End-stage renal disease (ESRD) is characterized by deep disorders in both innate and adaptive immune systems that imply unbalance deactivation and immunosuppression. The central, widely recognized factors responsible for this immune dysregulation are uremia, uremic toxin retention, hemodialysis membrane biocompatibility, and related cardiovascular complications. Recently, several studies strengthened the concept that dialysis membranes are not considered as a simple diffusive/adsorptive device but as a platform to personalize a dialysis approach to improve the quality of life of ESRD patients. Therefore, understanding of the molecules associated with altered immune response is crucial and could lead to therapeutically intervention or adaptation of the dialysis procedure itself for the management of immunological dysfunction of ESRD patients. The polymethyl methacrylate (PMMA)-based membrane is characterized by a symmetrical structure with large-sized pores, providing a better hydrophobic and cationic adsorption capacity compared to the other synthetic membranes. Together with hydrophobic interactions, the high adsorption rate of cytokines (i.e., IL-6) can also be enhanced by the size of nano-pores placed on the membrane surface. PMMA membranes exhibit adsorptive properties for a large amount of uremic toxins including p-cresol and indoxyl sulfate, as well as β2-microglobulin characterized by higher molecular weight, maintaining the diffusive clearance of small molecules like urea with a great biocompatibility. Besides exerting a strong anti-inflammatory effects in line with the improvement of immune responses in patients undergoing dialysis, PMMA also plays a role in modulating adaptive immune response, i.e., can clear blood from soluble CD40, a natural antagonist of the CD40/CD40L signaling that acts inhibiting immunoglobulin production by B cells. This review provides an overview of the main concepts and current understanding of immune dysfunction in hemodialysis and summarizes the recent findings regarding PMMA-based dialysis as potential strategy to restore immune balance in ESRD patients.

Introduction

Chronic kidney disease (CKD) is defined as abnormalities of kidney structure or function present for more than 3 months [1]. The gradual decline in kidney function leads to an accumulation of toxins normally cleared by the kidneys, resulting in uremia. In this condition, hemodialysis (HD), an extracorporeal depurative technique that relies on the exchange of water and solutes between the blood and the dialysate, represents a life-saving renal replacement therapy (RRT) for critical end-stage renal disease (ESRD) patients [2]. Despite RRT, the mortality rate of patients with kidney failure remains very high: for 65- to 74-year-old patients, life expectancy is only 5 years and the risk of cardiovascular mortality is 10–20 times higher compared to same-aged no-CKD patients. This cardiovascular mortality risk is almost 100 times higher in CKD patients younger than 45 years. This higher mortality rate is also confirmed for infections and malignancies when patients on RRT are compared to the general population [3]. These clinical features suggested the hypothesis of CKD as a model of “premature ageing” [4] and progressively included insights on chronic inflammation and its consequences on CKD immunological imbalances [5]. Nowadays, according to the original definition of inflammaging as a “global reduction in the capability to handle with a variety of stressors and a concomitant progressive increase in the pro-inflammatory status” [6], CKD, more than a simple organ failure, appears to be the clinical phenotype of a syndromic picture with specific immunological and cellular patterns.

As a consequence, RRTs are actually reconsidering their efficiency in volume overload and standard uremic toxin removal and their biocompatibility in order to reduce the chronic inflammatory stress due to complement and coagulation activation to deal, in a more comprehensive view, with traditional [3] and nontraditional risk factors related to dialysis [7]. In the recent years, several progresses have been made in development of new biocompatible HD membranes, in terms of composition (i.e., cellulosic or synthetic), structure, ultrafiltration coefficient (i.e., high or low flux), molecular-weight retention, biocompatibility, and solute removal mechanisms (i.e., diffusion, convection, or adsorption) [8]. However, even if dialysis greatly prolongs the survival of ESRD patients, it is unable to completely mitigate the uremic condition, and the so-called residual syndrome is still connected to patients’ immune dysregulation that persists afterward the treatment [9].

From an immunological point of view, it is well accepted that ESRD patients display deep disorders in both innate and adaptive immune system in a complex network that imply, at the same time, theco-occurrence of immune activation and immune suppression [10]. CKD is associated with chronic inflammation, immune system dysfunction, and cellular senescence, leading to a condition called “inflammaging,” characterized by premature renal aging. This particular phenotype is mainly induced by dysfunction of neutrophils, monocytes, and natural killer cells with signaling mediated by NOD-like receptor P3 (NLRP3) and Toll-like receptor 4 (TLR4); activation of the complement system mediated by dialysis membrane surfaces; and from adaptive response, by the imbalance in the CD4+/CD8+ T-cell ratio, a reduction of Th2 and regulatory T cells, together with an altered interaction with B lymphocytes by CD40/CD40L. The characterization of such HD-mediated immune phenotypes is crucial for the development of novel CKD therapies.

Interactions between blood and components of the extracorporeal circuit, during each HD session, according to the specific dialyzer physical nanostructure and biochemical properties, determine progressive activation of coagulation and complement and immune systems. Particularly, leukocyte and platelet adhesions are the first triggers for thrombin generation with not only progressive platelet activation but also complement activation. These cascades generate a pro-thrombotic milieu enhanced by nonselective protein adsorption onto the membrane surface. Particularly, complement activation through lectin and alternative pathways induces C3 and C5 activation with consequent neutrophil upregulation and activation which heighten leucocyte adhesion, coagulation activation, and consequent inflammatory response. Moreover, increased release of C5b enhances MAC formation and consequent endothelial damage [11]. In the last decades, several studies reinforced the idea of considering dialysis membranes not just as a diffusive device but as a tool to tailor dialysis procedure to improve the global quality of life of ESRD patients. This opens a wide area of investigation, notably for the management of immunological dysfunction of ESRD patients. The role of a polymethyl methacrylate (PMMA)-based filter for HD, an effective cytokine-adsorbing hemofilter with its potential role of modulating the immune system, will be discussed below.

Immunological Dysfunction in HD

The immunodeficiency in ESRD patients has been correlated with the incidence of complications as well as recurrent infections (bacterial or viral), high frequency of malignant tumors, sepsis-induced mortality, and poor response to thymo-dependent vaccination [8]. From the side of the coin, CKD patients exhibit a systemic, chronic low-grade inflammation characterized by high levels of circulating inflammatory proteins (CRP, IL6) and oxidative stress biomarkers that represent the major determinant of “dialysis syndrome” (including malnutrition, cachexia, and vasculopathy), leading to mortality in dialysis patients.

Such inflammation is generally arisen from immune system response to uremia and contact of innate immune mediators with dialysis tubes, and it has been demonstrated that in the long term, it leads to premature aging, called inflammaging. Therefore, the immune system is known as one of the most important therapeutic targets to reduce morbidity and mortality in uremic and dialysis patients. Here, we will discuss the central mechanisms observed in immune dysfunction in HD patients.

Uremic toxins or chronic exposure to artificial membranes leads to neutrophils and monocyte-impaired activation with the consequent release of chemokines and pro-inflammatory cytokines such as IL-1, IL-6, and TNF-α. Despite an initial phase of transient neutropenia, resulting from the adsorption of neutrophils and monocytes to HD membranes or from neutrophils adhesion to the endothelial walls of the pulmonary capillaries, the first vascular surface of contact after leaving the dialyzer [12], these cells increased in the following HD treatment’s and their dysfunctional activation affects their phagocytic activity, which remains strongly decreased during HD independent from the dialysis type or membrane [7]. Traditionally, ESRD results in the expansion of pro-inflammatory CD4+CD28− T-cell and CD14+CD16++ monocyte populations, which are considered to be novel, nontraditional cardiovascular risk factors [13].

Circulating monocytes of ESRD patients present enhanced expression of Toll-like receptors (TLR2 and TLR4), and this fosters the hypothesis of the spontaneous activation of monocytes and macrophages in HD patients [7, 14, 15]. However, several investigators reported the depletion and dysfunction of dendritic cells (DCs), especially plasmacytoid DCs in correlation with HD treatment itself [15].

DC dysfunction is one of the major causes of the immunologic disturbance in patients with ESRD. A large plethora of studies demonstrated that cultured monocyte-derived DCs from patients with ESRD on HD are less able to stimulate T cells than those from healthy controls, due to a decreased expression of the pivotal costimulatory molecules such as CD80 and CD86. Lim et al. [16, 17] showed that uremic monocyte-derived DCs demonstrated reduced endocytosis and a decreased expression of costimulatory molecules (CD40, CD80, and CD86) and maturation markers (CD83) compared with those of normal monocyte-derived DCs cultured in control medium (Fig. 1). These findings indicated that the terminal differentiation of monocyte-derived DCs (i.e., the transition from an immature to a mature stage) in patients with ESRD on HD was impaired. In accordance, the terminal differentiation of monocyte-derived DCs was impaired in patients with CKD stages IV to V who were not undergoing RRT. Similar results have been obtained for circulating myeloid and plasmacytoid DC precursors directly isolated from patients with ESRD on HD.

Fig. 1.

Molecular mechanisms of ESKD immunological dysfunction and role of the PMMA membrane in reducing immune impairment. CKD is associated with chronic inflammation, immune system dysfunction, and cellular senescence, leading to a condition called “inflammaging.” This particular phenotype is mainly induced by dysfunction of neutrophils, monocytes, and natural killer (NK) cells with signaling mediated by Toll-like receptor 4 (TLR4); activation of the complement system mediated by dialysis membrane surfaces; and from adaptive response, by the imbalance in the CD4+/CD8+ T-cell ratio, a reduction of Th2 and regulatory T cells, together with an altered interaction with B lymphocyte by CD40/CD40L. ESKD results in the expansion of pro-inflammatory CD4⁺CD28⁻ T-cell and CD14+CD16++ monocyte populations, which are considered to be novel, nontraditional cardiovascular risk factors. Furthermore, uremic monocyte-derived DCs showed reduced endocytosis and a decreased expression of costimulatory molecules (CD40, CD80, and CD86) and maturation markers (CD83) compared with those of normal monocyte-derived DCs. These findings indicated that the terminal differentiation of monocyte-derived DCs (i.e., the transition from an immature to a mature stage) in patients with ESKD on HD was impaired. PMMA membranes appear to be highly effective in reducing circulating amount of soluble molecule sCD40 and uremic toxins that represents the natural antagonist of CD40/CD40L signaling on DC surface. PMMA dialysis membranes are highly effective for the removal of pCS and IS compared to PS membranes enabling clearance of β2-microglobulin and small solutes. The PMMA membrane adsorbs several cytokines as IL-6, IL-1β, TNF-α, and IL-8, thereby modulating chronic inflammation in HD patients with higher efficiency compared to the PS membrane; the adsorption efficiency to the PMMA membrane depends on the molecular weight of the cytokines. pCS, p-cresyl sulfate; IS, indoxyl sulfate.

Over the past decades, various research groups have explored ways to stimulate DCs, such as high-efficiency dialysis, the use of erythropoietin, and administering vaccine adjuvants such as GM-CSF and levamisole [18]; however, we are far away from a clinical approval of therapies for DC modulation during HD treatments. In addition to dysregulated apoptosis and activation, HD patients show increased release of senescent neutrophils [9].

Complement System

The complement system exerts an essential role in inducing systemic inflammation, infection susceptibility, impaired coagulation, and oxidative stress during HD procedure [7, 19]. Moreover, complement activation has also been correlated with long-term chronic complications of HD as well as cardiovascular events and the establishment of inflammaging [19–22]. The adsorption-based extracorporeal technologies present a satisfactory uremic toxin removal and blood purification, but HD filters show adhesion of several components that promote dysregulated complement activation [23]. For instance, the classical pathway is activated through adhesion of C1q to membrane-adsorbed immunoglobulin IgG. The lectin pathway activation is due to adsorption of Ficolin-2, the specific pathogen recognition receptor of this pathway, by the binding of circulating mannose-binding lectin-associated serine proteases. Together with classical and lectin pathway dysregulation, the alternative pathway activation is induced by the interaction of albumin, complement system component’s C3b, and bacterial components (e.g., lipopolysaccharide) [7, 9]. Recent insights also suggest a role for petraxin-3 and complement factor D as possible new uremic toxins: responsible for the cross-talk between lectin and the classical pathway in complement activation and its amplification, respectively; they are large and middle molecules possibly removed by specific dialyzers [24].

Complement activation also results in increase of generation of C3a and C5a anaphylatoxins and opsonin (C3b, iC3b) that intensifies inflammation by cytokine production, cytotoxic induction, oxidative stress, and the release of granule enzymes such as elastase [25–27]. In HD patients, a result of complement activation is the induced expression of adhesion molecules on leukocytes, especially CD11b/CD18 also called CR3 [19]. This augmented expression and consecutive leukocyte activation lead to leukopenia because of the binding with the C3b fragment on dialyzer membranes. On the other hand, the complement system activates the endothelium through the overexpression of adhesion molecules such as E-selectin and von Willebrand factor that contribute to neutropenia and thrombi formation and coagulation [7, 23, 28, 29].

In relation to the complement system’s role in the leukopenia CR3-mediated, high-density lipoprotein’s (HDL) biological functions are well known including anti-inflammatory, anti-oxidative, anti-thrombotic, and immune regulation effects [30] and significantly reduced polymorphonuclear cell apoptosis [31]. PMNs have binding sites for HDL, and their major apolipoprotein constituent Apo A-I significantly attenuates CR3 surface expression on activated PMNs [18–20]. These functions also support the thesis of qualitative alteration of HDL, and its anti-inflammatory properties decline in inflammatory diseases such as CKD and diabetes and in HD.

Indoleamine 2,3-Dioxygenase-1 and Nitric Oxide

Increase of indoleamine 2,3-dioxygenase-1 (IDO1) activity and changes in the level of nitric oxide (NO) have been recognized as evidence of an innate immunity dysfunction in HD [32]. IDO1 is a cytosolic enzyme responsible for catalyzing the initial step of tryptophan catabolism. Tryptophan metabolism has been identified as a metabolic checkpoint of immunoregulation, modulating macrophage and natural killer cell behavior. Several data show that IDO1 activity is increased in patients during HD and IDO1 cell overexpression decreasing tryptophan levels may also lead to the production of toxic kynurenine metabolites, promoting the release of pro-inflammatory cytokines [32, 33].

Although NO is a small, easily diffusible molecule by the dialysis membrane, its levels significantly increase during HD [34]. High blood NO levels are associated with hyperactivation of macrophage activity by both uremia and HD procedure itself [32]. Elevated NO levels and dysregulated IDO1 activity modulate not only the innate immune response but also the adaptive immunity.

HD-induced inflammaging represents a systemic, low-grade inflammation associated with increased pro-inflammatory cytokines both in bloodstream and tissues. Inflammaging can be induced by a wide range of conditions such as diabetes, hypertension, uremic toxins, genetic factors, or dialysis. These mechanisms can be either exogenous (i.e., dialyzer biocompatibility or contamination of dialysis solutions and catheters) or endogenous (e.g., genetic susceptibility, chronic inflammation, oxidative stress, and cellular senescence). Moreover, inflammaging contributes to a persistent immune innate and acquired dysregulation being pivotal in HD patients [7, 21].

However, different HD treatments influenced IDO1 blood activity and NO levels. Treatment with vitamin E (VIT-E)-coated membrane dialyzers significantly reduced IDO1 activity and NO formation when compared to treatment without VIT-E-loaded membranes. This condition also suggests that VIT-E membrane dialyzers might lower HD-induced inflammaging.

Uremia and Uremic Toxins

CKD results in the retention of low-molecular-mass metabolites, such as phenylacetic acid, homocysteine, various sulfates, guanidine compounds, and many others [35–37]. These have effects on immune cell activation, promote leukocyte apoptosis, and induce the oxidative reactions in phagocytes. Chronic oxidative stress increases protein oxidation, which reduces the activity of enzymes, cytokines, and antibodies, contributing to both general inflammation and immune dysfunction in CKD. Under circumstances of oxidative stress, LDL may be modified to oxidized LDL (oxLDL), which is highly atherogenic [31, 38, 39]. The concentration of oxLDL is elevated in patients with CKD in part because of the inability of HDL to reduce oxLDL and in part because of the increased LDL lifespan due to decreased renal clearance [9, 30, 40, 41].

Uremia disturbs systemic immune system by causing intestinal dysbiosis and by destabilizing the gut barrier [42]. The metabolic consequences of uremia allow pathogen overgrowth, which can increase the production of uremic toxins inside the gut and can reduce the production of immunoregulatory short-chain fatty acids [43].

In the last decade, mounting studies corroborate the role of uremic toxicity in the progression of kidney failure [44]. The European Uremic Toxin Work Group grouped the uremic compounds in three main categories based on both molecular weight and hemodialytic removal properties. According to this classification, the small water-soluble compounds are included in the first class characterized by the molecular weight up to 500 Da (e.g., urea). Following, the second class of compounds includes medium- and large-sized molecules (MW >500 Da) such as β2-microglobulin (β2-m), complement factor D, α1-macroglobulin, and many pro-inflammatory cytokines [45]. Notably, an increasing number of studies reported that many middle-size compounds exert multi-organ toxicity by inducing systemic hyper-inflammation, resulting in tissue damage and cardiovascular diseases. Furthermore, these toxins are strongly involved in neutrophil and monocyte activation by promoting the expression of several pro-inflammatory cytokines and reactive oxygen species [46].

Finally, the third class includes the so-called protein-bound uremic toxins (PBUTs), mostly characterized by a molecular weight lower than 500 Da. Many of these compounds including the microbiota-derived uremic toxins such as indoxyl sulfate and p-cresyl sulfate exert a role in exacerbating the progression of CKD [47]. Moreover, in both AKI and CKD, the PBUT accumulation is strongly correlated with cardiovascular complications, oxidative stress, hyper-inflammation, and cellular senescence. Importantly PBUTs have emerged as important therapeutic targets for HD patients since the albumin-bound fraction of these retention solutes hide their removal by conventional dialysis [48, 49]. This has stimulated the development of more advanced methods to improve the reduction of PBUT levels such as the use of symbiotics, prebiotics, and fecal microbiota transplantation [47–49].

RRT allows the reduction of uremic toxicity and inflammation through the purification of the blood of multiple toxic and pro-inflammatory factors. Of note, the dialysis purification efficiency together with the clearance rate of the compound is influenced by both the dialysis parameters and the class of membranes, which differ in biocompatibility, molecular cutoff, and diffusion coefficient for each different substance. For instance, standard HD is the most used RRT in clinical practice and is able to remove only the small water-soluble molecules (<500 Da) such as urea and creatinine by a diffusion mechanism [50]. Nevertheless, medium and large solutes are not adequately eliminated by conventional treatments [51, 52]. On the other side, several data showed that the high-flux membranes coupled with standard HD slightly improved the elimination of medium molecules. Additionally, it was demonstrated that the use of medium cutoff membranes associated with high-dose HD resulted in the enhancement of both small and medium toxic solute clearance [53]. Besides HD, hemodiafiltration (HDF) and hemofiltration are the more effective therapies in removing medium- and large-sized molecules than HD, but the effect remains small for PBUTs [54]. Due to the association of diffusion and convection mechanism, HDF improves the clearance of toxic factors such as parathyroid hormone and pro-inflammatory cytokines (e.g., IL-6, IL-8, and IL-12) and reduces the concentration of several CKD-related uremic toxins including β2-m, ADMA, leptin, and tryptophan when compared to standard HD [2, 50, 54].

From an immunological point of view, uremic toxins and dialysis treatment cause disturbances in the acquired response, influencing lymphocyte as the main effector cells of adaptive immunity [37]. Indeed, HD patients show a decrease in the number of lymphoid cell lineages, such as T cells and B cells. These types of cells have an increased susceptibility to apoptosis that negatively affect the adaptive immune response [7, 13, 37]. Apoptosis alone is not sufficient to explain the decrease of naive T cells and the relatively normal memory T-cell populations. Derangement and dysregulation of thymus functions in ESRD patients could reflect the reduction of naive T cells [55]. Further, IDO1 activity and NO formation also contribute to T-cell immunosuppression and Treg activation [33, 34]. The pivot of uremic toxins in promoting endothelial dysfunction and thereby vascular complications has recently re-emerged. In a useful systematic review of the literature, Harlacher et al. [56] analyzed the results of 39 studies investigating the effect of uremic serum or uremic toxins and indoxyl sulfate, p-cresol and p-cresyl sulfate, modified LDL, and the advanced glycation end products N-carboxymethyl-lysine and N-carboxyethyl-lysine on vascular dysfunction with a special focus on the endothelium [1, 56]. Authors confirmed the activation of inflammation, oxidative stress, leukocyte migration and adhesion, cell death, and a thrombotic phenotype upon uremic conditions or uremic toxin treatment of endothelial cells with a prominent involvement of reactive oxygen species, MAPK/NF-κB, aryl-hydrocarbon receptor, and RAGE signaling.

PMMA Can Ameliorate Immunological Impairment during HD

In the last decades, several studies reinforced the concept of considering dialysis membranes not just as a simple diffusive device but as a tool to tailor a dialysis procedure to improve the global quality of life of ESRD patients [45]. This opens a wide area of investigation, notably for the management of immunological dysfunction of ESRD patients.

Traditionally, cellulose-based membranes were eligible in standard HD for the elimination of small water-soluble molecules, such as urea and creatinine; however, medium-molecule uremic toxins such as β2-m and PBUTs resulted in poor removal due to its low cutoff [57]. Importantly, many data described that the cellulose membrane is able to activate the complement pathway and pro-inflammatory mediators in the blood of HD patients. Contrastingly, in recent years, a large number of synthetic membranes such as polysulfone (PS), polyethersulfone, PMMA, polyacrylonitrile, and polyvinylidene fluoride were developed starting from several synthetic polymers. Synthetic membranes were originally advanced to address the bio-incompatibility issue and to improve the limited permeability of cellulosic membranes [50, 54, 58].

For this purpose, a large number of studies were conducted to evaluate the efficiency of different synthetic polymers on HD performance in the clearance rate of uremic toxin removal due to their medium cutoff. These studies showed the higher performance of these membranes in the removal of medium molecules such as β2-m and the improvement of biocompatibility when compared with a cellulose-based membrane. In addition, many studies indicated that besides the transmembrane removal of medium-sized solute, the adsorption-based clearance was predominant for some the PMMA dialyzers compared with the PS membrane (Table 1).

Table 1.

Major findings in the recent literature

Clinical/experimental application	Major findings	Reference	Year
Cohort of 54 patients with SAP. Of a total of 54 SAP patients, 26 patients progressively developed AKI and required continuous HD with a PMMA membrane hemofilter	The blood purification using a PMMA hemofilter is effective for the treatment of AKI and acute lung injury/acute respiratory distress syndrome in SAP patients	Kinjoh et al. [61]	2022
In vitro human plasma incubation with fragments of PMMA hollow fibers	The PMMA membrane adsorbs more cytokines than a PS membrane, and the adsorption efficiency to the PMMA membrane depends on the molecular weight of the cytokines	Kishikawa et al. [68]	2022
Swine model of LPS-induced AKI, analysis of 28 pigs randomized into four groups: n = 7 healthy pigs, n = 7 endotoxemic pigs, n = 7 PS continuous HF (CVVH)-treated endotoxemic pigs, and n = 7 PMMA-CVVH treated endotoxemic pigs. Genome-wide transcriptomic analysis of circulating polymorphonuclear cells	PMMA-based CVVH treatment reduced renal injury and early fibrosis, modulated complement activation and cytokine storm by preserving immunological homeostasis also at the transcriptomic level of polymorphonuclear cells	Stasi et al. [41]	2021
Thirty-five stable adult HD patients randomly completed a single study of 4 h OL-HDF with PMMA (BG2.1U, Toray®, Tokyo, Japan) and PS (TS2.1, Toray®) membranes	High-flux PMMA dialysis membranes are highly effective for the removal of pCS compared to PS membranes, enabling clearance of β2-microglobulin and small solutes	Molina et al. [70]	2021
In vitro adsorption study investigating performance of oxy-SNPs of urea	oxy-SNPs with a multimodal nanoparticle size distribution with active carbonyl presented significantly more active sites for urea attachment	Abidin et al. [76]	2018
Cohort study of 123 patients on various dialysis modalities. Of these, 56 were on HD and 67 on PD.	In patients with active cardiac remodeling, elevations in sRAGE may have a prognostic significance. Its quantifications seem to be as a useful supportive tool that could help to identify high-risk patients	Dozio; Ambrogi, et al.	2018
Cohort of 142,412 patients on maintenance HD for a year from 2008 to 2009. Patients are treated with seven types of high-flux dialyzer membranes at baseline, including CTA, EVAL, PAN, PEPA, PES, PMMA, and PS.	The use of different membrane types may affect mortality in HD patients and the use of the PMMA membrane can improve prognosis	Abe et al. [69]	2017
In vitro development and testing of a biocompatible PES HD membrane embedded with PCA-g-MWCNTs	PES/PCA-g-MWCNT nanocomposite membrane, a viable candidate to serve as the HD membrane	Abidin et al. [75]	2017
Cohort of 50 patients divided into the training group (n = 26; this population was used for the microarray analysis and to generate the initial hypothesis-generating genomic model) and a testing group (n = 24; this group of patients was used to confirm and validate results obtained by microarray)	HD patients present a different immune pattern compared to the un-dialyzed CKD patients characterized by increased biological elements involved in proliferation/activation of cytotoxic effector lymphocytes and in the immune-inflammatory cellular machinery	Zaza; Granata et al.	2013
A total of 67 patients with CAD. Patients were classified into acute coronary syndrome (n = 51) and SA (n = 16) subgroups	IL-6 neutralizing strategies could present novel therapeutic avenues in the treatment of ACS.	Ma; Yuan et al.	2013
The study was conducted using ascites fluid from 1 patient who regularly underwent cell-free and concentrated ascites reinfusion therapy at the department of blood purification in Japan	The PMMA hemofilter with a larger surface area showed the highest level of IL-6 clearance	Hirayama et al. [63]	2011
Cohort of 5 ESRD patients (mean age: 65 years) who had been on maintenance thrice-weekly dialysis for 42.4±15.8 months	PMMA membranes enhance immune response to anti-HBv in HD patients	Duranti and Duranti [72]	2011
A pilot study with 15 patients with renal failure in 2007 in the Department of Nephrology of Pellegrin Hospital and the Center for Treatment of Kidney Diseases, Bordeaux, France	PMMA BK-F membranes (Toray Medical Company, Japan) allow a dramatic diminution of the molecule sCD40, highlighting the potential role of the BK-F membrane in the improvement of hepatitis B vaccination of ESRD patients who failed to mount a protective immune response	Contin-Bordes et al. [71]	2010
A total of 42 uremic patients on MDH. Of the 42, 22 patients with a history of acute cardiovascular events served as the MHD1 group and 20 patients without acute cardiovascular events served as the MHD2 group. N = 30 patients with advanced CKD without acute cardiovascular events just before HD therapy served as the CKD control group and 30 healthy volunteers as the normal control group	The Treg/Th17 balance was disturbed by uremia, especially in patients with adverse cardiovascular events. This Th17/Treg imbalance might act synergistically with microinflammation on immune-mediated atherosclerosis and contribute to the high incidence of adverse cardiovascular events	Zhang; Hua	2010
Cohort study of 260 consecutive HD patients, comparing 435 PS patient-years and 85 PMMA patient-years. PMMA membranes (n = 33) were used for those patients who did not tolerate (e.g., for pruritus) PS membranes (n = 227). Low-flux dialyzers (n = 233) were compared with high-flux dialyzers (n = 37)	In PMMA patients, laboratory data with respect to inflammation, anemia, and nutrition were significantly improved compared with the PS group	Kreusser; Reiermann, et al.	2010
Reduction of amyloidogenic immunoglobulin light chain in 1 patient with primary amyloidosis suffering nephritic syndrome, who chose not to receive high-dose melphalan chemotherapy	Removal of the Bence Jones protein by the PMMA membrane serves as an alternate therapeutic method in the treatment of primary amyloidosis	Hata; Nishi, et al.	2009
Cohort of 43 patients with septic shock were assessed by monitoring of the blood IL-6 level with a rapid assay system and immediate initiation of critical care including PMMA-CHDF for cytokine removal	The clinical efficacy of PMMA-CHDF in consecutive patients with septic shock on ICU admission or who developed it early during their ICU stay was assessed by monitoring of the blood IL-6 level with a rapid assay system and immediate initiation of critical care including PMMA-CHDF for cytokine removal	Nakada et al. [62]	2008
C57BL/6 wild-type mice were obtained from CLEA (Tokyo, Japan), and IL-6 KO mice were provided by Dr. Manfred Kopf (Institute of Integrative Biology, Eidgenössische Technische Hochschule, Zurich, Switzerland)	Mechanisms other than IL-6-dependent pathways are involved in the induction of IL-17 in naive T cells in which Stat1 activation is maintained	Kimura; Naka	2007
Two matched groups of 13 standard 3 times/week HD patients were treated for 6 months with PMMA-based PLD and NPLD, respectively	The removal of large solutes by protein-leaking dialyzers reduces chronic inflammation (IL-6, IL-1β, TNF-α) in HD patients	Galli; Benedetti, et al.	2005
PMMA-based protein-leaking dialyzers analysis in 26 patients with hyperhomocysteinemia for 6 months	Reduction of the homocysteine level after PMMA treatment	Galli et al. [64]	2003
Cohort of 19 patients. HD procedures were performed using one of the following membranes: low-flux ultrafiltration rate MRC hollow-fiber in 5 patients, medium-flux CTA hollow-fiber in 6 patients, and high-flux PS hollow-fiber in 8 patients	PMMA membrane reduced renal itching within 3 months	Kato et al. [66]	2001
Cohort of randomized 84 patients from 13 Centers (n = 42 patients to the conventional treatment and n = 42 to the experimental treatment)	HD with BK-F PMMA, a large-pore and biocompatible membrane, improves the anemia	Locatelli et al. [65]	1999
Cohort of 46 patients (n = 6 in the group of patients not treated with EPO; n = 6 in the group of patients treated with EPO; n = 34 in the group of comparison between patients dialyzed with BK-F for 2 years and those with a conventional cellulosic membrane for the same period)	KR4-0, an erythropoietic inhibitor, showed a strong erythropoiesis-inhibiting effect ameliorating anemia among HD patients	Yamada et al. [59]	1999
Ten stable anuric RDT patients (53±13 years) were treated for 1 week with each membrane (PMMA BK-F, PMMA BK-P, CA) in a randomized sequence	Use of the BK-F membrane in HD could afford satisfactory removal of high-MW substances	Bonomini, Fiederling	1996
Cohort of ten long-term chronic HD patients undergoing 4-h maintenance HD periods 3 times each week	For PMMA membranes, complement activation is least of the new cuprophane and cellulose acetate membrane dialyzers	Hakim; Fearon et al.	1984

The PMMA membrane is highly effective for the removal of pCS and IS compared to PS membranes enabling clearance of β2-microglobulin and small solutes. Furthermore, the PMMA hemofilter with a larger surface area showed the highest level of IL-1β, TNF-α IL-8, and IL-6 clearance.

HF, hemofiltration; PES, polyethersulfone; PAN, polyacrylonitrile; anti-HBv, anti-hepatitis B vaccination; pCS, p-cresyl sulfate; IS, indoxyl sulfate; SAP, severe acute pancreatitis; oxy-SNPs, oxidized starch nanoparticles; PCA-g-MWCNTs, poly(citric acid)-grafted multi-walled carbon nanotubes; CAD, coronary artery disease; SA, stable angina; NPLD, non-protein-leaking dialyzers.

Notably, the PMMA membrane was developed to offer high biocompatibility and enhance the molecule clearance by coupling the adsorption to the diffusion and convection mechanisms of HDF [2]. Moreover, as it is now largely recognized, while diffusion and convection per se are able to remove the large part of small- and medium-sized molecules, the adsorption enables the clearance of small-, medium-, and high-molecular-weight compounds, including PBUTs [59, 60].

The PMMA-based membrane is characterized by a symmetrical structure with large-sized pores, providing a better hydrophobic and cationic adsorption capacity compared to the other synthetic membranes [61–63]. Together with hydrophobic interactions, the high adsorption rate of cytokines can also be enhanced by the size of nano-pores placed on the membrane surface. However, the accurate mechanism of cytokine removal by the PMMA membrane still remains unclear. Recent studies highlighted that PMMA membranes exhibit adsorptive properties for a large amount of uremic toxins including β2-m, IL-6, and albumin characterized by the high molecular weight (Fig. 1). In addition, few studies reported that the PMMA dialyzer exhibits strong anti-inflammatory effects along with the improvement of anemia, pruritus, and immune responses in patients undergoing dialysis [64, 65].

Interestingly, clinical reports on PMMA employment in ESKD patients showed a better prognosis in patients compared with other synthetic PS-based membranes (Table 1) [66]. For instance, it was reported that the PMMA membrane in HD patients affected the body weight compared to PS on the same patients [67]. Furthermore, it was reported an improvement in hemodynamics and a significant reduction in IL-6 levels in septic patients treated with RRT using a PMMA dialyzer [68]. The efficiency of the PMMA membrane was also described in an in vivo study conducted on pig models of septic AKI [52]. The dialytic treatment based on PMMA resulted to be more effective in recovering the renal function during septic-induced kidney injury compared to the PS hemofilter due to suppression of complement activation, reduction of renal fibrosis, and systemic inflammation. In addition, the modulation in PBMC gene expression was correlated with the lower activation of complement factors and preserved the immune system against the LPS-mediated response, limiting immunological dysfunction and renal damage [52]. Finally, the study conducted by the Japanese Nationwide Dialysis Registry describes that PMMA membranes can reduce mortality in HD patients by a strong reduction of inflammation and cardiovascular disease correlated with CKD [69].

In the last years, novel PMMA membranes have been improved and adapted for a new renal replacement modality such as online HDF. The new PMMA membranes (e.g., BG-U) appear to be highly effective in reducing p-cresyl sulfate levels when compared to PS membranes [70]. All these data suggest that the indication of PMMA dialyzers may represent a practical compromise in efficient dialysis treatment. However, further long-term prospective studies are needed to clarify these findings. Contini et al. [71] demonstrated that PMMA BK-F membranes (Toray Medical Co., Japan) allow an essential improvement of humoral immune response of ESRD patients. Most chronic uremic patients undergoing dialysis show a diminished antibody response to anti-hepatitis B vaccination, and in this regard, high sCD40 serum levels seem to inhibit the immunocompetent response. Duranti et al. [72] showed that PMMA dialyzers could enhance the long-term immune response to anti-hepatitis B vaccination in nonresponder ESRD patients.

Interactions between blood and components of the extracorporeal circuit, during each HD session, according to the specific dialyzer physical nanostructure and biochemical properties, determine progressive activation of coagulation and complement and immune systems. Particularly, leukocyte and platelet adhesions are the first triggers for thrombin generation not only with progressive platelet activation but also complement activation. These cascades generate a pro-thrombotic milieu enhanced by nonselective protein adsorption onto the membrane surface. Particularly, complement activation through lectin and alternative pathways induce C3 and C5 activation with consequent neutrophil upregulation and activation which heighten leucocyte adhesion, coagulation activation, and consequent inflammatory response. Moreover, increased release of C5b enhances MAC formation and consequent endothelial damage [11].

PMMA adsorptive properties could determine platelet adhesion to the surface of the conventional PMMA membranes. A new PMMA membrane dialyzer, NF, has been developed to solve this issue reducing the excessive negative charge on dialyzer surface and preventing alterations of the absorbed water on its surface, showing lower platelet adhesion and sufficient protein adsorption [73]. First developed for standard HD (NF-H), the NF series is actually designed also for HDF (NF-U), with optimal clinical results in terms of provided dialytic dose, albumin loss, hemocompatibility, and removal of middle molecules [74] when compared to HdX and standard HDF dialyzers.

Technology advances have been developed to address the growing need to remove uremic toxins during an HD session. Muhammad NidzhomZainol Abidin et al. [75–77] presented an attractive approach to collectively remove uremic toxins by combining membrane filtration and adsorption process. In this work, dual-layer hollow-fiber membranes consisting of a PS inner layer well attached to a PS/amino-silanized PMMA outer layer were prepared via a co-extrusion spinning process based on non-solvent-induced phase separation, obtaining significant urea removal (39.2%) and showed desired sieving characteristics for other solutes.

In conclusion, the PMMA membrane plays a crucial role in assuring efficient uremic toxin removal, lowering HD-induced inflammaging, being protective toward immune system function during HD inflammaging, and blood purification. PMMA removal and adsorptive high performances allow to reduce the concentration of circulating molecules like β2-microglobulin, activated complement factor C3, pro-inflammatory cytokines (e.g., TNFa, IL-1, IL-6, and IL-8), free molecule (κ type and λ) of immunoglobulin light chain (Bence Jones protein), and sCD40 (Fig. 1). All this leads to an overall positive effect on immune-inflammatory activation during HD treatment.

Conclusions

Several reports demonstrated that ESRD is accompanied by deep disturbance of the immune system, impacting both the innate and the adaptive arms. Indeed, ESRD patients display features of immune response deficiency such as recurrent infections (bacterial or viral), high frequency of malignant tumors, and poor response to thymo-dependent vaccination with increased morbidity and mortality. In the last years, a large number of studies reinforced the concept that dialysis membranes are not considered as a simple diffusive/adsorptive device but as a platform to personalize dialysis approach to improve the quality of life of ESRD patients. The technology improvements of the last years showed that PMMA membranes could be adapted for a new renal replacement modality based on adsorption. The new PMMA membranes (e.g., BG-U) appear to be highly effective in reducing circulating amount of larger molecules as β2-microglobulin as well as activated complement factor C3, pro-inflammatory cytokines (e.g., TNFa, IL-1, IL-6, and IL-8), free molecule (κ type and λ) of immunoglobulin light chain (Bence Jones protein), and, more importantly, the natural antagonist of CD40/CD40L signaling: the soluble molecule sCD40 and uremic toxins. The high retention of these pro-inflammatory mediators allows researchers and nephrologists to compare the survival rate of PMMA dialysis-treated patients with the conventional PS membrane, highlighting a significantly better prognosis after PMMA treatments based on molecular evidences (Table 1). Therefore, understanding of the molecules associated with altered immune response is crucial and could lead to therapeutically intervention and/or adaptation of the dialysis procedure itself for the management of immunological dysfunction of ESRD patients.

Conflict of Interest Statement

The authors have no conflicts of interest to declare.

Funding Sources

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Author Contributions

Rossana Franzin and Giuseppe Castellano contributed to conceptualization, writing, and editing of the manuscript. Alessandra Stasi and Gianvito Caggiano contributed to the conceptualization and drafting of the work. Elena Squiccimarro, Gianvito Caggiano, Vincenzo Losappio, Marco Fiorentino, and Carlo Alfieri contributed to the draft. Loreto Gesualdo and Giovanni Stallone critically revised the manuscript. Giuseppe Castellano contributed substantially to the work reported by critical revisions and draft editing. All the authors read and agreed to the published version of this manuscript.

Funding Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.