Stefan Heidenreich

How to assess glomerular function and damage in humans

In human subjects, the assessment of renal function and of its changes by interventions is limited to the measurement of glomerular filtration rate (GFR), renal blood flow and the estimation of proteinuria. In humans, GFR can be determined exactly by measuring the clearance of an ideal filtration marker, such as inulin. The classic method of measuring inulin clearance in humans includes constant intravenous infusion of the compound and timed collections of urine. In order to avoid the need for timed urine collections, a number of alternative procedures have been devised. All these methods only use determinations of inulin in plasma or serum. From these, the total body inulin clearance is obtained using pharmacokinetic calculations. In order to measure total body clearance, usually called plasma clearance, inulin is either given as a constant intravenous infusion or as a bolus infusion. Both procedures overestimate GFR because of incomplete distribution of inulin during the study periods. The error may be minimized by using model-independent pharmacokinetic calculations. Unlike inulin, creatinine is not a perfect filtration marker. This is because the substance is not only eliminated by glomerular filtration but also by tubular secretion. The extent of tubular creatinine secretion is not constant in various individuals. Serum creatinine concentration is a commonly used measure of renal function in clinical practice. This parameter is determined both by the renal elimination and by the production of the compound. Differences in creatinine production among subjects and over time in a single individual may occur because of changes in muscle mass. Radioisotopic filtration markers can easily and accurately be measured in plasma and serum. Using this method, the plasma concentration-time curve of these compounds can easily be studied after intravenous bolus injection. From the plasma concentration-time curves obtained, the total body clearance (plasma clearance) of the substances can be calculated using pharmacokinetic models. Most frequently, 125l-iothalamate, 99mTc-diethylenethiaminepenta-acetic acid and 51Cr-ethylenediaminetetra-acetic acid are used for the estimation of GFR in humans. The total body clearance of all these filtration markers overestimates GFR. The error induced by this phenomenon is particularly relevant at low levels of GFR. In recent years, iohexol has been used as a filtration marker. The substance can be measured in plasma, serum and urine using high-performance liquid chromatography. So far, good agreement has been shown for GFR determined by the classic inulin clearance and by the iohexol plasma clearance. Screening for proteinuria is commonly performed using reagent test strips. Quantitative measurements of marker proteins can be used to estimate the extent and the site of damage in the nephron. These measurements may be used to estimate the progression of renal disease and the response to therapeutic interventions. Of particular interest is the degree of albuminuria which indicates nephropathy in diabetic patients and end-organ damage in patients with hypertension.

ACE Inhibitor Versus β-Blocker for the Treatment of Hypertension in Renal Allograft Recipients

Angiotensin-converting enzyme (ACE) inhibitors have been shown to slow the progression of chronic renal failure. However, the value of ACE inhibitors for the treatment of hypertension in renal allograft recipients has not been established. ACE inhibitors dilate the efferent glomerular arteriole, an effect that may aggravate the decrease in glomerular filtration rate resulting from cyclosporine-induced vasoconstriction at the afferent glomerular arteriole. Therefore, the goal of this double-blind, randomized study was to compare the antihypertensive and renal effects of the ACE inhibitor quinapril with those of the beta-blocker atenolol in renal allograft recipients in whom hypertension developed 6 to 12 weeks after transplantation. All patients received cyclosporine as an immunosuppressant and had stable graft function (serum creatinine concentration, <220 micromol/L) at entry into the study. Twenty-nine patients who received quinapril (daily dose titrated between 2.5 and 20 mg) and 30 patients who received atenolol (daily dose titrated between 12.5 and 100 mg) completed the 24-month study. The two groups did not differ in age, sex ratio, height, and weight before entry into the study. Quinapril decreased diastolic blood pressure from 96+/-1 to 84+/-1 mm Hg (average throughout treatment period), and atenolol decreased diastolic blood pressure from 96+/-1 to 83+/-1 mm Hg. The serum creatinine concentration did not change significantly in either group after 24 months (129+/-8 micromol/L at entry and 148+/-19 micromol/L after 24 months in the quinapril group and 131+/-6 micromol/L at entry and 152+/-15 micromol/L after 24 months in the atenolol group; P=NS for both groups). After 24 months, the change in urinary albumin excretion from baseline was -10+/-15 mg/d in the quinapril group and 52+/-32 mg/d in the atenolol group (P=0.03). These results show that quinapril and atenolol are effective antihypertensive drugs when used after renal transplantation. Moreover, compared with atenolol, quinapril has no adverse effects on graft function. The relative reduction in albuminuria observed with quinapril as compared with atenolol could indicate a beneficial effect of quinapril on long-term graft function.

Monocyte CD14: a multifunctional receptor engaged in apoptosis from both sides.

Like all other immune system cells, monocytes and macrophages may undergo apoptotic cell death in response to specific triggers and mediators or as a consequence of aging. However, factors inducing apoptosis and the involved cellular and molecular mechanisms are much better investigated and understood for lymphocytes. Th2 cell‐derived cytokines such as interleukin‐4 (IL‐4) are able to induce monocyte apoptosis most effectively. This process is preceded by down‐regulation of the CD14 surface receptor. Mediators such as lipopolysaccharide (LPS) suppress and postpone apoptosis in parallel with up‐regulation of CD14. Macrophages are rather resistant against apoptotic damage, and factors able to evoke apoptosis in monocytes are often ineffective in macrophages. Resistance of macrophages against apoptotic triggers may be beneficial for inflammatory processes where macrophages are engaged and needed as phagocytes for ingestion and removal of moribund cells. The multifunctional CD14 receptor of monocytes/macrophages is supposed to be involved in the apoptotic network on both sides: as a surface molecule of monocytes that can promote survival and antagonize apoptosis and as a recognition receptor of macrophages that enables or supports interaction with apoptotic cells. J. Leukoc. Biol. 65: 737–743; 1999.

Outcome of Kidney Transplantation in Patients with Inherited Thrombophilia: Data of a Prospective Study

Inherited prothrombotic risk factors predispose patients to thromboembolic events. In kidney transplant recipients, thrombophilia may manifest itself with venous thrombosis, microvascular occlusion, or acute rejection with major consequences for allograft survival. This is a prospective study on 165 renal allograft recipients to evaluate the contribution of genetic thrombophilic risk factors to transplant outcome. Besides antithrombin, protein C, and protein S deficiencies, none of which was found in our patient group, factor V G1691A (FV G1691A), prothrombin G20210A (PT G20210A) mutations, and methylenetetrahydrofolate reductase (MTHFR) gene C677T polymorphisms were studied. The primary endpoint of the study was occurrence of an acute rejection within the first 90 d and transplant loss within 1 yr. Heterozygous FV G1691A and PT G20210A mutations and the MTHFR T677T variant were significantly associated with acute rejections with rejection rates of 68%, 67%, and 71%, respectively, as compared with 35% in patients not carrying these genotypes. Many rejections that were histologically proven were acute vascular ones. Transplant loss was significantly associated exclusively with the PT G20210A group (50% 1-yr graft survival; odds ratio, 10.0; 95% confidence interval, 1.8 to 56.1). PT G20210A patients exerted the highest prothrombotic activity pretransplant, as determined by prothrombin 1.2 fragments (PT F1.2), which may be the background for minor outcome. In conclusion, common prothrombotic mutations are significantly associated with acute rejections, especially vascular rejections, and for PT G20210A also with early transplant failure. Screening for hypercoagulable states pretransplant is recommended to intensify anticoagulatory treatment posttransplant.

Subtotal Ablation of Parietal Epithelial Cells Induces Crescent Formation

Parietal epithelial cells (PECs) of the renal glomerulus contribute to the formation of both cellular crescents in rapidly progressive GN and sclerotic lesions in FSGS. Subtotal transgenic ablation of podocytes induces FSGS but the effect of specific ablation of PECs is unknown. Here, we established an inducible transgenic mouse to allow subtotal ablation of PECs. Proteinuria developed during doxycycline-induced cellular ablation but fully reversed 26 days after termination of doxycycline administration. The ablation of PECs was focal, with only 30% of glomeruli exhibiting histologic changes; however, the number of PECs was reduced up to 90% within affected glomeruli. Ultrastructural analysis revealed disruption of PEC plasma membranes with cytoplasm shedding into Bowmans space. Podocytes showed focal foot process effacement, which was the most likely cause for transient proteinuria. After >9 days of cellular ablation, the remaining PECs formed cellular extensions to cover the denuded Bowmans capsule and expressed the activation marker CD44 de novo. The induced proliferation of PECs persisted throughout the observation period, resulting in the formation of typical cellular crescents with periglomerular infiltrate, albeit without accompanying proteinuria. In summary, subtotal ablation of PECs leads the remaining PECs to react with cellular activation and proliferation, which ultimately forms cellular crescents.

Down-Regulation of Monocyte Apoptosis by Phagocytosis of Platelets: Involvement of a Caspase-9, Caspase-3, and Heat Shock Protein 70-Dependent Pathway

Monocytes interact and cross-talk with platelets in many settings including inflammation, hemostasis, or vascular disorders. During inflammatory diseases, there is a rapid targeting of monocytes and platelets to points of inflammation and endothelial injury, where they lie side-by-side. In this in vitro study, we investigated different interactions between monocytes and platelets and elucidated whether platelets might affect monocyte apoptosis. Freshly isolated human monocytes were rendered apoptotic by serum deprivation or CD95 ligation and cocultured with platelets. Monocyte apoptosis was determined by flow cytometry, TUNEL staining, DNA electrophoresis, and transmission electron microscopy imaging. We could show that monocyte apoptosis was highly suppressed when platelets were added to the cultures. Transmission electron microscopy depicted that monocytes completely ingested thrombocytes by phagocytosis. Blocking thrombocyte uptake by the phagocytosis inhibitor cytochalasin D abrogated the enhanced monocyte survival and led to high apoptosis levels. Monocyte survival was paralleled by down-regulation of caspase-9 and -3 and up-regulation of heat shock protein 70 during uptake of platelets. Platelet supernatants and contents of platelet granules were ineffective in altering monocyte senescence. Also, ingestion of latex beads or zymosan by monocytes was ineffective to mimic platelet-dependent rescue from apoptosis. In conclusion, this study shows that platelets can suppress apoptosis of monocytes by a specific phagocytosis-dependent process with further consequences for atherosclerotic or inflammatory conditions.

Monocyte activation for enhanced tumour necrosis factor-α and interleukin 6 production during chronic renal allograft rejection

To evaluate the contribution of immune mechanisms in the initiation and progression of chronic renal allograft rejection we investigated monocyte-derived cytokine synthesis in vitro in 16 patients with histologically proven chronic rejection; 22 transplant patients with stable function served as controls. Basal tumour necrosis factor-alpha (TNF-alpha) production, measured by L 929 bioassay, was low and not significantly different in both groups. By triggering TNF-alpha formation in vitro by lipopolysaccharide (LPS), high concentrations of TNF-alpha (155.1 +/- 243.0 ng/ml) were measured in monocyte cultures from chronic rejection patients which greatly exceeded the TNF-alpha levels of 11.5 +/- 14.5 ng/ml in the control group. Measurement of interleukin 6 (IL-6) levels by enzyme immunoassay gave similar results, with significantly higher IL-6 concentrations in LPS-triggered monocyte cultures from chronic rejection compared with stable function patients. Additional stimulation of LPS-treated monocytes with interferon-gamma (IFN-gamma) as a priming agent enhanced TNF-alpha formation in stable function patients and, in contrast, slightly reduced monokine formation in chronic rejection patients, which suggests that in this group a high activation level of monocytes has already been reached in vivo by T cell factors such as IFN-gamma. Treatment of monocyte cultures in vitro with prednisolone reduced TNF-alpha formation differently in most but not all cultures from chronic rejection and stable function patients; thus this in vitro test system might be helpful in predicting the benefit of an intensified immunosuppressive regimen for chronic rejection patients.(ABSTRACT TRUNCATED AT 250 WORDS)

INFECTION BY CANDIDA ALBICANS INHIBITS APOPTOSIS OF HUMAN MONOCYTES AND MONOCYTIC U937 CELLS

Infectious microorganisms can differently induce or Inhibit apoptosis of immunocompetent effector and host cells. In this study we examined the influence of an infection by Candida albicans (C. albicans) on programmed cell death of monocytic U937 cells and human monocytes. Basal and tumor necrosis factor α (TNF‐α)‐induced DNA fragmentation of U937 cells was significantly inhibited by an infection with C. albicans. Enhanced apoptosis of U937 cells, induced by TNF‐α, caused a diminished candidacidal activity of the effector cells, whereas inhibition of apoptosis by granulocyte‐macrophage colony‐stimulating factor (GM‐CSF) was paralleled by an intensified host defense. Pretreatment of U937 cells or monocytes with the cyclooxygenase blocker indomethacin completely abolished the reduction of DNA fragmentation induced by the yeast. Studying the underlying mechanisms we found that C. albicans induced formation of prostaglandin E2 (PGE2) by U937. Exogenous administration of PGE2 down‐regulated apoptosis of U937 or human monocytes to a similar extent as did fungal infection. Activation of protein kinase A by the cAMP analogue 8‐bromo‐cAMP inhibited U937 apoptosis, as did PGE2 On the other hand, rp‐cAMP, a blocker of the cAMP‐dependent signal transduction, restored and elevated DNA fragmentation levels down‐regulated by C. albicans. U937 cells expressed the bcl‐2 protein but the infection with fungi or PGE2 treatment did not increase proto‐oncogene expression. Monocytic effector cells may therefore strengthen the defense against C. albicans by an autocrine feedback regulation via a PGE2‐dependent, cAMP‐transduced inhibition of apoptosis. J. Leukoc. Biol. 60: 737–743; 1996.

Prognostic value of lymphocyte apoptosis in acute rejection of renal allografts.

BACKGROUND Apoptosis is a cellular phenomenon generally found within rejecting transplants. It may play a role in physiological or therapy-associated deletion of infiltrating lymphocytes or in graft cell destruction. Our study focuses on apoptosis of infiltrating lymphocytes during acute kidney rejection after the initial steroid pulse therapy and on possible prognostic implications. METHODS Renal biopsy specimens of 23 transplant recipients with acute tubulo-interstitial rejection were examined for appearance of apoptosis and compared with 11 transplant biopsies with unspecific organ injury accompanied by lymphocyte infiltration. In all patients, biopsies were performed after steroid pulse therapy, and, after confirmation of rejection, antilymphocytic antibody treatment was carried out. Apoptosis was determined via terminal deoxynucleotidyltransferase-mediated dUTP-digoxigenin nick end labeling analysis and confirmed by electron microscopy. RESULTS Apoptosis of lymphocytes or of tubular epithelium was detected in 11 cases of acute rejection (48%), respectively. Four biopsies showed lymphocytic as well as tubular apoptosis, whereas five sections showed no signs of programmed cell death. In biopsies revealing unspecific injury, tubular cell apoptosis was more frequently found (73%) compared with lymphocyte apoptosis (27%, P<0.05). Most interestingly, patients with a beneficial recovery from acute rejection had a higher proportion of lymphocyte apoptosis compared with patients with poor rejection outcome. The Bcl-2 oncoprotein was widely found within infiltrating lymphocytes without counter-regulating apoptosis. CONCLUSIONS Lymphocyte apoptosis is found as frequently as tubular cell apoptosis in rejecting renal grafts after steroid pulse therapy and might have prognostic value for rejection outcome.

Influence of Heme Oxygenase-1 on Microcirculation After Kidney Transplantation

BACKGROUND Cytoprotective proteins, such as heme oxygenase-1 (HO-1), play a decisive role in ischemia-reperfusion injury during kidney transplantation. The aim of this study was to investigate the impact of heme oxygenase-1 on microcirculation and on ischemia-reperfusion injury in an isogenic kidney transplantation rat model. MATERIALS AND METHODS Seventy male Lewis rats were distributed into three groups. In Group 1(control), the kidneys were only mobilized. In Groups 2 and 3, bilateral nephrectomy was performed, and a kidney from another Lewis rat was orthotopically transplanted on the left side. The donor animals in Group 3 received preconditioning with the HO-1 inductor hemin. 24 h after reperfusion graft function and morphology were examined. Microcirculation was investigated by in vivo microscopy of the renal surface 1 h after reperfusion. RESULTS HO-1 preconditioning led to significantly lower serum creatinine and serum urea, as well as less histological damage and inducible nitric oxide synthase expression. Microcirculation was improved by a significant enlargement of the vascular diameter and an increase of the capillary flow. CONCLUSIONS Treatment with hemin improves microcirculation by induction of HO-1 and reduces ischemia-reperfusion injury after kidney transplantation. HO-1 induction was shown to be a promising approach in the preconditioning of donor kidneys.