Protein-Bound Uremic Toxins and Immunity

Characteristics of T cell premature senescence in maintenance hemodialysis patients

BACKGROUND

Uremia-associated immunodeficiency, mainly characterized by T cell dysfunction, exists in patients on maintenance hemodialysis (MHD) and promotes systemic inflammation. However, T cell senescence, one of the causes of T cell dysfunction, has not been clearly revealed yet. In this cross-sectional research, we aimed to study the manifestation of T cell premature senescence in MHD patients and further investigate the associated clinical factors.

METHODS

76 MHD patients including 33 patients with cardiovascular diseases (CVD) and 28 patients with arteriovenous fistula (AVF) event history were enrolled in this study. Complementarity determining region 3 (CDR3) of T cell receptor (TCR) was analyzed by immune repertoire sequencing (IR-Seq). CD28- T cell subsets and expression of senescence marker p16 and p21 genes were detected by multicolor flow cytometry and RT-qPCR, respectively.

RESULTS

MHD patients had significantly decreased TCR diversity (P < 0.001), increased CDR3 clone proliferation (P = 0.001) and a left-skewed CDR3 length distribution. The proportion of CD4 + CD28- T cells increased in MHD patients (P = 0.014) and showed a negative correlation with TCR diversity (P = 0.001). p16 but not p21 expression in T cells was up-regulated in MHD patients (P = 0.039). Patients with CVD exhibited increased expression of p16 and p21 genes (P = 0.010 and 0.004, respectively), and patients with AVF events showed further TCR diversity and evenness reduction (P = 0.002 and 0.017, respectively) compared to patients without the comorbidities. Moreover, age, average convection volume, total cholesterol, high-density lipoprotein cholesterol and transferrin saturation were associated with TCR diversity or CD4 + CD28- T cell proportion (P < 0.05).

CONCLUSIONS

MHD patients undergo T cell premature senescence characterized by significant TCR diversity reduction and repertoire skew, as well as accumulation of the CD4 + CD28- subset and up-regulation of p16 gene. Patients with CVD or AVF events show higher level of immunosenescence. Furthermore, T cell senescence in MHD patients is associated with blood cholesterol and uremic toxin retention, suggesting potential intervention strategies in the future.

Protein-Bound Uremic Toxins and Inflammation Process in Hemodialysis Patients: Is There a Role for Adsorption Hemodiafiltration?

INTRODUCTION

Despite major advances in the field of dialysis, there are still some unmet needs such as reducing inflammation through adequate depuration. It is well known that the wide spectrum of pro-inflammatory and pro-atherosclerotic uremic toxins are inefficiently removed by current dialysis techniques. Adsorption seems to be an extra tool to remove toxins, but its effect and optimization have not been widely studied. The aim of this report was to present preliminary results regarding the possibility of performing hemodiafiltration with a highly adsorptive polymethylmethacrylate membrane.

METHODS

The study was first conducted in 10 patients in which the safety and feasibility of hemodiafiltration with PMMA BG-U 2.1 membrane were tested through measurement of hemolysis indices, transmembrane pressures, and dialysis adequacy. Twenty patients were prospectively observed for 18-month period in which they consecutively underwent standard hemodialysis, standard post-dilution hemodiafiltration, and polymethylmethacrylate-based post-dilution hemodiafiltration. Protein-bound uremic toxins concentrations and inflammatory markers were measured throughout the observed period.

RESULTS

HDF-PMMA was inferior to HDF in convective volume, but KT/V was similar, and no differences were noted in operating pressures during the two treatments. During HDF-PMMA period of treatment, we observed a significant reduction of CPR levels, and HDF-PMMA was superior to all other treatments in hepcidin removal even if this did not significantly affect hemoglobin levels. HDF-PMMA could significantly reduce indoxyl sulfate (indoxyl) concentration over a period of 6 months but not for p-cresyl sulfate (p-cresyl).

CONCLUSION

PMMA BG-U 2.1 membrane can be safely and efficiently used in hemodiafiltration. Moreover, as these preliminary results show, adding adsorption properties to convection and diffusion enabled an increased removal of indoxyl uremic toxin associated to a reduction in inflammation markers as CRP and hepcidin without any negative impact on albumin levels.

Immunosenescence and Immune Exhaustion Are Associated with Levels of Protein-Bound Uremic Toxins in Patients on Hemodialysis

BACKGROUND

The accumulation of protein-bound uremic toxins (PBUTs) in chronic kidney disease may affect patients' immune status. The aim of the study was to evaluate their potential impacts on lymphocyte alterations in patients on hemodialysis (HD).

METHODS

The plasma levels of PBUTs were assessed in 54 patients on HD and 31 healthy individuals, using ultra-performance liquid chromatography. The results correlated with the senescent and exhausted status of lymphocytes, based on certain surface molecules, analyzed by flow cytometry.

RESULTS

The plasma levels of PBUTs were significantly increased in the patients on HD compared with the healthy controls. The patients with residual kidney function had reduced hippuric acid (HA) levels, total (p = 0.03) and free (p = 0.04), and free IxS levels (p = 0.02). The total and free HA levels correlated negatively with less differentiated subpopulations, CD4+CD45RA+CD31+ (p = 0.037 and p = 0.027), CD8+CD28+CD57- (p = 0.01, p = 0.01), and naïve B cells (CD19+IgD+CD27-) (p = 0.04, p = 0.03). Both the total and the free pCS levels correlated positively with exhausted CD4 cells, p = 0.02 and p = 0.01, respectively. A multivariate analysis showed that IxS and age were the main independent parameters implicated in the reduction intotal CD4 and B lymphocytes and their naïve and early differentiated subsets.

CONCLUSIONS

Increased PBUTs levels are associated with immune disturbances of patients on HD, HA, and IxS in the immunosenescent and pCS in the immunoexhaustion alterations.

Neutrophil:lymphocyte ratio correlates with the uremic toxin indoxyl sulfate and predicts the risk of death in patients on hemodialysis

BACKGROUND

Chronic kidney disease (CKD) is a major public health issue associated with increased cardiovascular, infectious and all-cause mortality. The neutrophil:lymphocyte ratio (NLR) is a predictive marker of the risk of death and cardiovascular events. Uremic toxins, notably indoxyl sulfate (IS), are involved in immune deficiency and cardiovascular complications associated with CKD. The aim of this study was to assess whether the NLR was related to uremic toxins and could predict clinical outcome in hemodialysis (HD) patients.

METHODS

We conducted a prospective cohort study of 183 patients on chronic HD. The main objective was to study the correlation between the NLR and uremic toxin serum levels. The secondary objective was to test if the NLR can predict the incidence of mortality, cardiovascular events and infectious events.

RESULTS

Patients were separated into two groups according to the NLR median value (3.49). The NLR at inclusion was correlated with the NLR at the 6-month (r = 0.55, P < 0.0001) and 12-month (r = 0.62, P < 0.0001) follow-up. Among uremic toxins, IS levels were higher in the group with high NLR (104 µmol/L versus 81 µmol/L; P = 0.004). In multivariate analysis, the NLR remained correlated with IS (P = 0.03). The incidence of death, cardiovascular events and severe infectious events was higher in the group with high NLR [respectively, 38% versus 18% (P = 0.004), 45% versus 26% (P = 0.01) and 33% versus 21% (P = 0.02)] than in the low NLR group. Multivariate analysis showed an independent association of the NLR with mortality (P = 0.02) and cardiovascular events (P = 0.03) but not with severe infectious events.

CONCLUSIONS

In HD patients, the NLR predicted mortality and cardiovascular events but not severe infections and correlated positively with the level of the uremic toxin IS. The NLR could be an interesting marker for monitoring the risk of clinical events in CKD patients.

COVID-19 Vaccination in Kidney Transplant Candidates and Recipients

Kidney transplant candidates and kidney transplant recipients (KTRs) are at particular risk of severe complications of COVID-19 disease. In Western countries, mortality in affected hospitalized KTRs ranges between 19% and 50%. COVID-19 vaccination remains the most important measure to prevent the severity of infection in candidates and recipients of kidney transplant. However, the uraemic condition may affect the vaccine-induced immunity in patients with advanced chronic kidney disease (CKD) and in KTRs. Retention of uraemic toxins, dysbiosis, dysmetabolism, and dialysis can diminish the normal response to vaccination, leading to dysfunction of inflammatory and immune cells. In KTRs the efficacy of vaccines may be reduced by the immunosuppressive medications, and more than half of kidney transplant recipients are unable to build an immune response even after four administrations of anti-COVID-19 vaccines. The lack of antibody response leaves these patients at high risk for SARS-CoV-2 infection and severe COVID-19 disease. The aim of the present review is to focus on the main reasons for the impaired immunological response among candidates and kidney transplant recipients and to highlight some of the present options available to solve the problem.

Homeostasis in the Gut Microbiota in Chronic Kidney Disease

The gut microbiota consists of trillions of microorganisms, fulfilling important roles in metabolism, nutritional intake, physiology and maturation of the immune system, but also aiding and abetting the progression of chronic kidney disease (CKD). The human gut microbiome consists of bacterial species from five major bacterial phyla, namely Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, and Verrucomicrobia. Alterations in the members of these phyla alter the total gut microbiota, with a decline in the number of symbiotic flora and an increase in the pathogenic bacteria, causing or aggravating CKD. In addition, CKD-associated alteration of this intestinal microbiome results in metabolic changes and the accumulation of amines, indoles and phenols, among other uremic metabolites, which have a feedforward adverse effect on CKD patients, inhibiting renal functions and increasing comorbidities such as atherosclerosis and cardiovascular diseases (CVD). A classification of uremic toxins according to the degree of known toxicity based on the experimental evidence of their toxicity (number of systems affected) and overall experimental and clinical evidence was selected to identify the representative uremic toxins from small water-soluble compounds, protein-bound compounds and middle molecules and their relation to the gut microbiota was summarized. Gut-derived uremic metabolites accumulating in CKD patients further exhibit cell-damaging properties, damage the intestinal epithelial cell wall, increase gut permeability and lead to the translocation of bacteria and endotoxins from the gut into the circulatory system. Elevated levels of endotoxins lead to endotoxemia and inflammation, further accelerating CKD progression. In recent years, the role of the gut microbiome in CKD pathophysiology has emerged as an important aspect of corrective treatment; however, the mechanisms by which the gut microbiota contributes to CKD progression are still not completely understood. Therefore, this review summarizes the current state of research regarding CKD and the gut microbiota, alterations in the microbiome, uremic toxin production, and gut epithelial barrier degradation.

Correlation of inflammatory biomarkers with the diversity of Bacteroidaceae, Bifidobacteriaceae, Prevotellaceae and Lactobacillaceae families in the intestinal microbiota of patients with end stage renal disease

PURPOSE

Serum levels of inflammatory cytokines and uremic toxins, and their inter-correlations with the diversity of Bacteroidaceae, Bifidobacteriaceae, Prevotellaceae and Lactobacillaceae families in intestinal microbiota were investigated in patients with end stage renal disease (ESRD).

METHODS

Stool and blood samples from 20 ESRD patients on maintenance hemodialysis were collected. DNA genome of the bacterial composition of the stool samples was extracted and evaluated by the sequencing analysis of 16S rRNA genes. Serum levels of inflammatory cytokines and uremic toxins were then analyzed.

RESULTS

The mean serum concentrations of TNF-α, IL-6, indoxyl sulfate (IS) and p-cresol (PC) were 305.99 ​± ​12.03 ​ng/L, 159.95 ​± ​64.22 ​ng/L, 36.76 ​± ​5.09 ​μg/mL and 0.39 ​± ​0.15 ​μg/mL, respectively. The most significant positive correlation was observed between Prevotellaceae family and total antioxidant capacity (TAC), Lactobacilli species and CRP and PC, as well as Scardovia wiggsiae and IS (p ​< ​0.001). A negative correlation was also found between Bacteroides clarus and PC. Patients with ESRD on maintenance hemodialysis had elevated levels of PC and IS and increased levels of the inflammatory markers. The most positive correlation was found between microbiota and CRP and PC, while the most negative one was between microbiota and IL-1 and TAC.

CONCLUSIONS

The abundance and diversity of Bacteroidaceae, Bifidobacteriaceae, Prevotellaceae and Lactobacillaceae families and their correlations with clinical parameters could provide benefits in the ESRD patients but they could not promote the symptoms.

Gut-derived bacterial toxins impair memory CD4+ T cell mitochondrial function in HIV-1 infection

People living with HIV (PLWH) who are immune nonresponders (INRs) are at greater risk of comorbidity and mortality than are immune responders (IRs) who restore their CD4+ T cell count after antiretroviral therapy (ART). INRs have low CD4+ T cell counts (<350 c/μL), heightened systemic inflammation, and increased CD4+ T cell cycling (Ki67+). Here, we report the findings that memory CD4+ T cells and plasma samples of INRs from several cohorts are enriched in gut-derived bacterial solutes p-cresol sulfate (PCS) and indoxyl sulfate (IS) that both negatively correlated with CD4+ T cell counts. In vitro PCS or IS blocked CD4+ T cell proliferation, induced apoptosis, and diminished the expression of mitochondrial proteins. Electron microscopy imaging revealed perturbations of mitochondrial networks similar to those found in INRs following incubation of healthy memory CD4+ T cells with PCS. Using bacterial 16S rDNA, INR stool samples were found enriched in proteolytic bacterial genera that metabolize tyrosine and phenylalanine to produce PCS. We propose that toxic solutes from the gut bacterial flora may impair CD4+ T cell recovery during ART and may contribute to CD4+ T cell lymphopenia characteristic of INRs.

Short-Chain Fatty Acids Alleviate Hepatocyte Apoptosis Induced by Gut-Derived Protein-Bound Uremic Toxins

Chronic kidney disease (CKD) is characterized with the influx of uremic toxins, which impairs the gut microbiome by decreasing beneficial bacteria that produce short-chain fatty acids (SCFAs) and increasing harmful bacteria that produce gut-derived protein-bound uremic toxins (PBUTs). This study aimed to assess the proapoptotic effects of three major gut-derived PBUTs in hepatocytes, and the effects of SCFAs on apoptosis phenotype in vitro. HepG2 (human liver carcinoma cells) and THLE-2 (immortalized human normal liver cells) cell line were incubated with 0, 2, 20, 200, 2000 μM p-cresol sulfate (PCS), indoxyl sulfate (IS), and hippuric acid (HA), respectively, for 24 h. Flow cytometry analysis indicated that three uremic toxins induced varying degrees of apoptosis in hepatocytes and HA represented the highest efficacy. These phenotypes were further confirmed by western blot of apoptosis protein expression [Caspase-3, Caspase-9, B-cell lymphoma 2 (Bcl-2), and Bcl-2-associated X protein (Bax)]. Human normal hepatocytes (THLE-2) are more sensitive to PBUTs-induced apoptosis compared with human hepatoma cells (HepG2). Mechanistically, extracellular HA could enter hepatocytes, increase reactive oxygen species (ROS) generation, and decrease mitochondrial membrane potential dose-dependently in THLE-2 cells. Notably, coculture with SCFAs (acetate, propionate, butyrate) for 24 h significantly improved HA-induced apoptosis in THLE-2 cells, and propionate (500 μM) represented the highest efficacy. Propionate reduction of apoptosis was associated with improving mitochondria dysfunction and oxidative stress in a manner involving reducing Caspase-3 expression, ROS production, and increasing the Bcl-2/Bax level. As such, our studies validated PBUTs accumulation might be an important cause of liver dysfunction in patients with CKD, and supplementation of SCFAs might be a viable way to protect the liver for patients with CKD.

The AST-120 Recovers Uremic Toxin-Induced Cognitive Deficit via NLRP3 Inflammasome Pathway in Astrocytes and Microglia

Chronic kidney disease (CKD) is characterized by the progressive loss of renal function; moreover, CKD progression commonly leads to multiple comorbidities, including neurological dysfunction and immune disorders. CKD-triggered neuroinflammation significantly contributes to cognitive impairment. This study aimed to investigate the contribution of uremic toxins to cognitive impairment. Serum creatinine, blood urea nitrogen (BUN), indoxyl sulfate (IS), and p-cresol sulfate (PCS) levels were measured using an enzyme-linked immunosorbent assay and high-performance liquid chromatography. The creatinine, BUN, IS, and PCS levels were increased from 4 weeks after 5/6-nephrectomy in mice, which suggested that 5/6-nephrectomy could yield a CKD animal model. Further, CKD mice showed significantly increased brain and serum indoxyl sulfate levels. Immunohistochemistry analysis revealed hippocampal inflammation and NLRP3-inflammasomes in astrocytes. Further, the Y-maze and Morris water maze tests revealed learning and memory defects in CKD mice. AST-120, which is also an IS absorbent, effectively reduced serum and hippocampal IS levels as well as reversed the cognitive impairment in CKD mice. Additionally, NLRP3-knockout mice that underwent 5/6-nephrectomy showed no change in cognitive function. These findings suggested that IS is an important uremic toxin that induces NLRP3 inflammasome-mediated not only in microglia, but it also occurred in astrocytic inflammation, which subsequently causes cognitive impairment.