Updated Pathways in Cardiorenal Continuum after Kidney Transplantation

Abstract

Cardiovascular disease (CVD) remains one of the leading causes for increased morbidity and mortality in chronic kidney disease (CKD). Kidney transplantation is the preferred treatment option for CKD G5. Improved perioperative and postoperative care, personalized immunosuppressive regimes, and refined matching procedures of kidney transplants improves cardiovascular health in the early posttransplant period. However, the long-term burden of CVD is considerable. Previously underrecognized, the role of the complement system alongside innate immunity, inflammaging, structural changes in the glomerular filtration barrier and early vascular ageing also seem to play an important role in the posttransplant management. This review provides up-to-date knowledge on these pathways that may influence the cardiovascular and renal continuum and identifies potential targets for future therapies. Arterial destiffening strategies and the applicability of sodium-glucose cotransporter 2 inhibitors and their role in cardiovascular health after kidney transplantation are also addressed.

1. Introduction

Cardiovascular disease (CVD) is one of the leading causes of morbidity and mortality in chronic kidney disease [1]. Uremic milieu induced immune [2] and inflammatory responses cause cardiovascular remodeling [3,4], resulting in advanced vascular ageing and vascular calcification. Kidney transplantation, the preferred treatment option for end stage kidney disease, particularly improves cardiovascular health in the early posttransplant period. However, in long-term follow up data on kidney transplant, recipients show a significantly greater burden of cardiovascular complications as compared to the general population [5].

The cardiorenal syndrome, an umbrella term that has been widely used to define the interaction between the kidney and heart functions [6], can also contribute to posttransplant complications [7,8]. Even so, data on cardiorenal syndrome after kidney transplantation are limited.

There is enough evidence supporting the fact that left ventricular function and left ventricular hypertrophy improves after kidney transplantation [9]. However, the impairment of the vascular structure after kidney transplantation remains unchanged and might even worsen [10,11]. The pathophysiological mechanisms behind vascular remodeling observed in the posttransplant period include atherosclerosis, arteriosclerosis, and cardiorenal syndrome. For example, activation of the complement system [12,13,14,15], low-grade inflammation [2], and structural and functional changes of the glomerular filtration barrier [3] (Figure 1) are the hallmarks of posttransplant changes in the cardiovascular and renal continuum. We aim to critically review pathways that might help to better understand pathophysiological mechanisms and develop new treatment options for improving cardiovascular health after kidney transplantation.

2. Ischemia Reperfusion Injury in Kidney Transplantation

2.1. The Role of Thrombo-Inflammation

Ischemia Reperfusion Injury (IRI) is an inevitable part of kidney transplantation. The processes of ischemia and reperfusion in transplanted kidneys are distinct and have different consequences on kidney cells and their environment [4].

The kidney graft is exposed to ischemia during organ preservation and transportation. Ischemia induced anaerobic metabolism causes intracellular accumulation of lactate and, therefore, a decrease in intracellular pH. Acidosis induces the release of lysosomal enzymes that damage intracellular structures. Simultaneously, the adenosine triphosphate (ATP) depletion leads to the Ca²⁺ ion overload in the cells, which increase protease activity and prepares for mitochondrial release of reactive oxygen species (ROS) after reperfusion [4,5].

Ischemia sets off a phenotype change in the endothelial cells (EC) and parenchymal cells. As EC starts to express heparan sulphate, hyaluronan, syndecan-1, CD44, and metalloproteinases, the uncontrollable shedding of the glycocalyx begins [6]. This change of phenotype is recognized by innate immunity response. The endothelium becomes vulnerable and unprotected from the attacks of the contact, the coagulation, and the complement systems [7].

The reperfusion phase restores normoxemia and pH. The normoxemia boosts oxidative stress by releasing and furtherly hyperproducing ROS. Meanwhile, normal pH drives the protease-induced damage of intracellular structures [8]. These changes in homeostasis are unfavorable for ischemia-altered endothelial, tubular, and perivascular cells and thereby launches cell death through necrosis and apoptosis [4]. As a result, the released cellular fragments behave as damage-associated molecular patterns (DAMPs), which are recognized by innate and adaptive immunity response [16], and this sets in motion the thrombo-inflammatory cascade [17]. Although the reperfusion process aims to heal the ischemia-damaged cells, the activation of DAMPs deepens kidney transplant injury and contributes to delayed graft function (DGF) [12].

Additionally, the recognition of phenotypically altered cells is controlled by mannose-binding lectin, collectin, ficolin, C1q receptors, and C3b [13]. During the ischemia, collectin-11 (CL-11) binds L-fucose that is largely expressed on renal tubular cells [14], and marginally expressed on epithelial cells and glomerular mesangium [15]. During the reperfusion, CL-11 forms a complex with mannan-binding lectin serine protease 1 (MASP1) and MASP2 and activates the lectin pathway of the complement system, consequently leading to production of membrane attack complex (MAC). Simultaneously, ficolin 2 and natural antibodies (IgM, IgG) contribute to the formation of leucocyte-platelet complexes. Activated neutrophiles fuel further tissue damage by migrating into parenchyma [17]. In the end, all the three pathways of the complement system—a part of the thrombo-inflammatory cascade—are activated.

2.2. Links to Cardiovascular Continuum

IRI in kidney allograft affects different kidney cells, including the renal endothelium, and may cause the dysfunction of endothelial cells in other organs such as the cardiovascular system.

The endothelium forms one of the biggest cellular layers in the human body [14]. The heterogeneity of the endothelial cell (EC) properties along the vascular tree is determined by the endothelial glycocalyx [16,18], which is responsible for vascular permeability and protection, aside from leukocyte and platelet adhesion to ECs [19]. The endothelium expresses a diversity in appearance ranging from continuous cell layers with dense glycocalyx in the brain and heart [20,21] to significantly less compact glycocalyx in lung capillaries [22]. The fenestrated glycocalyx has been found in the kidneys and endocrine glands, whilst sinusoid glycocalyx has been found in liver, spleen, and bone marrow [23]. Nevertheless, the endothelial glycocalyx has a role in the development of albuminuria, which is an early marker of cardiovascular disease both in diabetes [24] and in the general population, and it also provides evidence of microvascular dysfunction in the kidneys [25,26].

The importance of endothelial dysfunction, i.e., endotheliopathy [7], has recently been recognized in kidney transplantation. The ECs and endothelial glycocalyx are the targets of the thrombo-inflammation that engages the complement, the contact, and the coagulation systems [27]. It has been hypothesized that kidney transplantation itself can improve endothelial glycocalyx stability in the early posttransplant period by reducing vascular injury, modifying syndecan-1 concentration [28], and retaining EC function up to 24-month posttransplant [29]. However, the role of endothelial injury in a transplanted kidney caused by IRI and allograft rejections could further contribute to the unfavorable cardiorenal outcomes and needs further investigation.

Injured ECs release less nitric oxide (NO) that, under normal circumstances, inhibits thrombosis by reducing the expression of chemokine and P-selectin, in addition to regulating transcription of the adhesion molecules [17], e.g., the intracellular cell adhesion molecule (ICAM-1) and vascular cell adhesion molecule (VCAM-1). Enhanced expression of ICAM-1 and VCAM-1 have been linked to pretransplant atherosclerotic burden and posttransplant outcomes [30]. Moreover, VCAM-1 induces vascular smooth cell (VSMC) function switching to the synthetic phenotype [30], and thereby contributes to inflammation. As for ICAM-1, it may induce aortic valve calcification [31].

Beyond the effect on adhesion molecules, IRI activates matrix metalloproteinases (MMP). MMP-9 is activated through different pathogenetic pathways, which amplifies cardiovascular risk and upregulates transforming growth factor beta (TGF-β), leading to vascular remodeling and, thus, vascular calcification [32,33]. Unfortunately, vascular calcification is not reversible after kidney transplantation and tends to progress in the 5-year posttransplant period [28], further contributing to CVD. Moreover, increased urinary MMP-9 was correlated with tubular atrophy and interstitial fibrosis in kidney transplant and could predict early and long-term graft function [34].

Higher CV morbidity and mortality both in subjects on hemodialysis and peritoneal dialysis could be linked to stimulation of the complement system and low-grade systemic inflammation that damages ECs [35,36]. The uncontrolled activation of the complement system by the pretransplant kidney replacement modality, together with posttransplant IRI, contributes to the remaining increased risk for CVD. Activation of the lectin pathway might even lead to myocardial infarction (MI). The highest tertile of ficolin 2 concentration was associated with early MI (OR 1.55, p = 0.03) in the adults [37]. The same report showed that pentraxin-3 (PTX-3)—a possible histological marker of acute kidney transplant rejection [38] —in combination with MBL/ficolin/collectin-associated serine protease-3 (MASP-3) added credibility to the Framingham score in predicting MI. Notably, ficolin 2 rs7851696 gene polymorphism has been shown to be associated with DGF in deceased kidney transplant recipients [39].

The alternative pathway of the complement system, and thus dysregulation of factor B and C3bBbP, impacts adverse outcomes in heart failure [40]. However, the role of this pathway in kidney transplant has been regarded as borderline significant [7]. Altered expression of some other complement components, including factor B, C1, C3, C4, and C5a, are involved in coronary artery disease as compared to healthy controls. Moreover, circulating C1q levels were associated with arterial stiffness in the middle-aged and older healthy subjects [41]. Although the inhibition of C5 in animals can preserve endothelial glycocalyx and kidney graft function [42], the applicability of these findings in humans is debatable.

Finally, thrombo-inflammation is the driving force for chronic allograft rejection that histologically is defined by interstitial fibrosis and tubular atrophy (IFTA) [43] and remains one of the challenges in transplantology. Despite improvement in immunosuppressive treatment, better posttransplant care, and lower rate of acute rejection, IFTA is one of the leading causes of allograft loss in kidney transplants [44]. Supposedly, the same signaling pathways drive the progress of atherosclerosis and arteriosclerosis in the posttransplant period [45]. Additionally, deteriorating kidney transplant function worsens molecule permeability in glomeruli and exposes the cardiovascular system to uremic toxins. Hence, chronic allograft rejection plays an important role here by contributing to the uremic milieu and inflammaging.

3. Shrunken Pore Syndrome

The recently recognized GFR-marker cystatin C shows higher correlation to cardiovascular disease than creatinine [46]. There are several explanatory models to this. One is the Shrunken pore syndrome (SPS), which is the condition where cystatin C based kidney function estimation disagrees with creatinine-based kidney function in the absence of non-renal influence on the levels of cystatin C and creatinine [46]. Recently, the National Kidney Foundation (NKF) and the American Society of Nephrology (ASN) task force recognized the importance of cystatin C use in evaluating kidney function [47]. A thirty percent lower eGFR for cystatin C compared to eGFR for creatinine defines the syndrome. The concept of SPS explains different glomerular filtration properties of middle-sized and large molecules (Figure 2) and is related to accumulation of atherosclerosis promoting proteins [48], contributing to cardiorenal syndrome and increased mortality [49,50,51,52].

It is believed that the driving histologic change in SPS is the “shrinking” of pores [46]. Thickening of the glomerular basement membrane in diabetic kidney disease was correlated to the presence of SPS compared to absence of SPS in patients with normal thickness of the glomerular membrane in a recent study [53]. Increased cystatin C in obesity might be explained by the same structural changes in the kidney [54,55]. Interestingly, the duplication of the glomerular basement membrane is a hallmark of both progressing diabetic nephropathy [56] and transplant glomerulopathy [3], and thus might affect the reduced permeability of middle-sized molecules in glomeruli.

The role of SPS in kidney transplant has not been studied except for by Zhou et al. [57], who reported that kidney transplant recipients with SPS had higher risk for delayed graft function and worse 1-year kidney graft function, though the role of immunosuppression was not well attributed. The limitation of cystatin C use after kidney transplantation is based on its possible interference with high doses of glucocorticoids that may increase production of cystatin C [58]. Even though it did not affect the diagnosis of acute kidney injury in glucocorticoid recipients in the cohort of critically ill patients [59], the interrelationship between cystatin C and glucocorticoids seems to be dose dependent.

The evidence of changes in levels of various molecules, like those observed in SPS, are limited. Lower cystatin C and beta2-microglobulin based eGFR´s are shown to be superior to creatinine-based kidney function in predicting cardiovascular events and delayed graft function [60], as reported by the large Folic Acid for Vascular Outcome Reduction in Transplantation (FAVORIT) study. Additionally, increased concentration of gelatinases (MMP-2 or gelatinase A), stromelysin (MMP-3), and osteoprotegrin has been linked to both chronic transplant nephropathy [61] and SPS [48], even in the absence of kidney failure [52].

Indeed, SPS with normal measured glomerular filtration [52] does not ameliorate cardiovascular risk. Both living and deceased kidney donors with preexisting SPS could supposedly contribute to CVD and CV risk after kidney transplantation [62]. Hence, the use of cystatin C in renal transplantation needs further studies [58], especially if tubular injury co-occurs [63].

4. Arterial Stiffness

Early vascular ageing (EVA) has been increasingly recognized as an impelling cause of high cardiovascular morbidity and mortality in CKD [64]. Patients with CKD undergo advanced vascular structural and functional remodeling driven by uremic toxins including advanced glycation products (AGE) [65]. Endothelial senescence [66], inflammaging [67], oxidative stress, bone-vascular cross talk [68], and gut dysbiosis [69] are the main hallmarks of EVA in kidney failure. In addition, low donor birth weight might also contribute to increased cardiovascular risk after kidney transplantation [70].

Despite the growing knowledge about the molecular and pathophysiological pathways of vascular ageing, the available therapeutic options for arterial stiffening are scarce. Although, kidney transplantation could be considered as one treatment strategy since some reports show that kidney transplant recipients have significantly lower carotid-femoral pulse wave velocity (cfPWV) as compared to matched CKD [71]. However, diverging results emerge from studies with both shorter [72,73] and longer follow-up [74]. Disagreement among studies could depend on separate pretransplant cardiovascular profile, diverse organ retraction, and transplantation techniques, as well as deviating posttransplant care.

Targeting inflammageing via the gut miocrobiome is another possible destiffening strategy. Gut associated lymphoid tissue is the storage of innate immunity cells (dendritic cells, macrophages, T cells, M cells, IgA producing B cells, etc.) [75]. Thereby, the leaky gut syndrome, related to uremia and medications, activates these local storages, and causes dislocation of the microbiota. This results in a systemic inflammatory response that worsens kidney graft outcome [76] and, consequently, cardiovascular homeostasis. The same concerns have been suggested at disturbances of oral and urinary microbiota [77]. The European Society of Hypertension working group on vascular structure and function and the ARTERY Society have currently published a review that addresses aspects of interrelation between inflammaging and arterial stiffness [78]. Some of them could be applicable for kidney transplant population.

Vitamin K plays an important role in cardiovascular health [79]. It is mainly derived from butter, egg yolks, animal products, and fermented food [80]. Smaller amounts are found in vegetables and synthetized by gut microbiota [80]. Conversely, the benefits of Vitamin K2 supplementation in thea CKD population are matter of debate [81]. The KING [82] trial reported 14.2% reduction in carotid-femoral pulse wave velocity, a proxy of arterial stiffness, after 8 weeks intake of menaquinone-7 (vitamin K2). Meanwhile, a Belgian study [83] showed that those kidney transplant recipients who had a higher cumulative mycophenolate mofetil dose accompanied by the absence of anti-vitamin K medication had a lower coronary artery calcification score (CAC). Besides vitamin K2 supplement, the peroral correction of hypomagnesemia (a possible side effect of calcineurin inhibitors) [84] and the intake of dietary polyphenols, such as soy isoflavones [85,86], and curcumin [87,88], have been shown to improve arterial stiffness. Furthermore, isoflavones may alleviate IRI related complications and IFTA [88].

Senolytics, the compounds whose main task is to eliminate senescent cells [89], could become a game changer in inhibiting early vascular ageing in CKD. Endothelial cell, endothelial progenitor cells, and VSMCs undergo senescence that is responsible for atherosclerosis, arteriosclerosis, and vascular calcification [90]. The accumulation of cell-free mitochondrial DNA is one of the hallmarks of senescence, as well as of IRI. Promising results originating from animal studies show that Dasatinib, a potent BCR-ABL-kinase inhibitor used in leukemia, in combination with the bioflavonoide Quercetin can improve cardiac allograft outcome [91]. However, the lack of selectivity and potential pharmacologic interactions of these drugs limits their use in kidney transplant so far [92]. Of note, the previously mentioned curcumin has also been attributed to the first generation of senolytics [89] and could eventually be considered as a supplement for kidney transplant patients.

Arterial remodeling can be modified by targeting immune checkpoints [93]. CD40-CD40L, CD28, CD80/CD86, and CD30 determine atherosclerosis [93] and are affected both by posttransplant immune modulating therapies and kidney allograft rejection treatment [94,95,96]. Out of immunosuppressive therapies, belatacept blocks CD26-CD80/CD86 on antigen-presenting cells [94] and might reduce atherosclerosis similarly to abatacept, as shown in a mice model [97]. Despite contradictive results in human studies [98,99], belatacept could possibly be considered as a treatment of choice in high CV risk transplant patients.

Recently, the monotherapy of sodium-glucose cotransporter 2 inhibitor (SGLT-2i) empaglifozin, or in combination with glucagon-like peptide-1 receptor agonist (GLP-1 agonist) liraglutide, showed superiority over monotherapy of insulin or liraglutide in destiffening arteries in diabetes mellitus type 2 [100]. Additionally, a favorable effect on endothelial glycocalyx thickness measured in sublingual microvessels was reported [100] after 12 months of follow-up. GLP1-agoinsts have shown cardioprotective properties in several recent studies and are recommended as a first choice therapy in obese diabatic patients with cardiovascular morbidities in KDIGOs diabetes in the aCKD guidelines from 2019 [101]. As SGLT-2i also might be a promising treatment in the kidney transplant population, we discuss its applicability in the subsequent section.

5. Sodium-Glucose Cotransporter 2 Inhibitors

SGLT2i:s have multiple cardiorenal protective effects, ranging from metabolic to vascular, as well as hemodynamic and antifibrotic mechanisms. SGLT2-i:s have been shown to improve blood pressure, reduce weight, diminish blood sugar, and lessen inflammation and fibrosis. A decrease in blood glucose is mediated by inhibiting the uptake of filtered glucose in the proximal renal tubular segment, resulting in the increased excretion of glucose and sodium that leads to a higher urine output. The increased sodium load in the tubule activates the tubuloglomerular feedback response [102], which results in a constriction of the afferent arteriole and, hence, diminishes albuminuria. Emerging evidence shows that SGLT2i reduce vascular remodeling [103] and have anti-inflammatory effects [104]. Canagliflozin (CANA), as compared to dapagliflozin (DAPA) and empagliflozin (EMPA), decreased IL-6 levels and downregulated glycolytic enzyme hexokinase II (HKII) in lipopolysaccharide stimulated human coronary artery endothelial cells (HCAECs) [104]. CANA, DAPA, and EMPA have been able to restore VE-cadherin loss in HCAECs exposed to 10% stretch [105] and inhibit reactive oxygen species (EMPA, DAPA) [105,106]. Targeting oxidative stress can prevent progress of renal fibrosis [107].

It is known that SGLT2i might improve endothelial function, regulate vascular repair, and affect VSMC function [103]. The SGLT2i:s role on microvascular rarefaction should be studied more in detail, as this phenomenon has been linked to both CVD and CKD [108,109,110].

The introduction of SGLT-2i has resulted in a dramatic decrease of kidney and cardiovascular adverse outcomes in both diabetic and non-diabetic patients [111]. This class of drugs diminishes albuminuria and has an additive protective effect on ACE-inhibitors/ARB. There is a great interest to use these drugs in patients with kidney transplants, as many of these patients demonstrate albuminuria and a slow decline in kidney function, which is most evident in those with chronic allograft rejection. Similar to patients with native kidney disease, CV mortality is the leading cause of death in transplant patients. However, these patients have often been excluded from randomized clinical trials (RCT) [112,113], but there are many of the above-mentioned factors that tentatively favor the use of SGLT-2i in kidney transplant patients [114], and an alteration in energy metabolism will most likely improve CV and kidney outcomes [115].

Thus far, there are no studies large enough studying renal transplant patients on SGLT-2i. There is, to date, one small RCT study by Halden et al. [116], who enrolled 24 patients on EMPA in renal transplant recipients with posttransplant diabetes mellitus and had 25 in the control group. This study showed that the SGLT-2i after 6 months was safe and improved glycemic control. There are also a few small case-control studies not revealing any harmful infections or adverse effects [117,118].

Before we have large RCT:s, it is important to show some caution. It seems probable that similar beneficial effects will be seen in the transplant patients as has been seen in the nontransplant patients. Transplant nephrologists should be vigilant in patients with recurrent urinary tract infections and genital infections; however, with prophylactic measures and high hygienic standards, infection risks are most likely slim. Diuretics may be reduced prior to initiation to avoid hypotension and patients need to be well informed about suspending SGLT2-i intake at fever, fluid loss, starvation, and in conjunction with surgery. While waiting for more studies, the initiation of SGLT-2i should preferably be avoided during the first 6–12 months post-transplantation when immunosuppression exposure is profound.

6. Conclusions

Cardiovascular disease is the leading cause for mortality in kidney transplant recipients. Neither improved immunosuppression strategies nor better peri- and postoperative care have reduced the CVD burden. There are several scientific gaps that should be covered for better understanding of cardiovascular and renal interrelationships after kidney transplantation.

Foremost, the activation of the complement system that occurs immediately after donor kidney extraction and the related endotheliopathy should be considered as a target in developing novel drugs. New endothelium-preserving therapies could potentially reduce posttransplant cardiovascular risk and help to maintain graft function.

Secondly, the ischemia reperfusion syndrome alongside to recipient’s uremic milieu might contribute to the decreased permeability of middle-sized molecules throughout the glomerular filtration barrier in the transplanted kidney. Furthermore, the introduction of cystatin C and the identification of Shrunken Pore Syndrome may add important information about cardiovascular disease and long-term results after kidney transplantation.

Thirdly, there is an urgent need of studies for validating the efficacy of arterial destiffening strategies in kidney transplant recipients, e.g., senolytics. SGLT-2i show promising cardiorenal risk reduction in chronic kidney disease population and, moreover, have beneficial effects on endothelial function and arterial stiffness. Although SGLT-2i use in kidney transplant recipients needs larger scale trials that take into account functional and structural posttransplant changes in the cardiovascular and renal continuum, they show promising effects in patients with kidney diseases.