Negative regulation of T-cell activation and autoimmunity by Mgat5 N-glycosylation

T-cell activation requires clustering of a threshold number of T-cell receptors (TCRs) at the site of antigen presentation, a number that is reduced by CD28 co-receptor recruitment of signalling proteins to TCRs. Here we demonstrate that a deficiency in beta1,6 N-acetylglucosaminyltransferase V (Mgat5), an enzyme in the N-glycosylation pathway, lowers T-cell activation thresholds by directly enhancing TCR clustering. Mgat5-deficient mice showed kidney autoimmune disease, enhanced delayed-type hypersensitivity, and increased susceptibility to experimental autoimmune encephalomyelitis. Recruitment of TCRs to agonist-coated beads, TCR signalling, actin microfilament re-organization, and agonist-induced proliferation were all enhanced in Mgat5-/- T cells. Mgat5 initiates GlcNAc beta1,6 branching on N-glycans, thereby increasing N-acetyllactosamine, the ligand for galectins, which are proteins known to modulate T-cell proliferation and apoptosis. Indeed, galectin-3 was associated with the TCR complex at the cell surface, an interaction dependent on Mgat5. Pre-treatment of wild-type T cells with lactose to compete for galectin binding produced a phenocopy of Mgat5-/- TCR clustering. These data indicate that a galectin-glycoprotein lattice strengthened by Mgat5-modified glycans restricts TCR recruitment to the site of antigen presentation. Dysregulation of Mgat5 in humans may increase susceptibility to autoimmune diseases, such as multiple sclerosis.

Thrombotic microangiopathy in renal transplant recipients treated with cyclosporin A

Thrombotic microangiopathy is an uncommon but well described complication of renal transplantation. This study is a review of the case records of 18 patients with biopsy proven post transplant thrombotic microangiopathy, without cellular rejection. There was no single characteristic underlying cause of renal failure in native kidneys. Although only two (11%) patients had undergone previous transplantation, 16 (89%) had panel reactive antibodies (PRA). All patients received prophylactic antilymphocyte globulin, a single patient had cyclosporin A (CSA) at the time of transplant and in 16 patients CSA was introduced when graft function was established. On this protocol 16 (89%) patients had early graft function. All patients developed acute renal failure and 16 (89%) required dialysis. Nine (50%) patients developed hematological abnormalities. All patients were treated aggressively with anti-rejection therapy, CSA was temporarily withdrawn, and 2 (11%) patients received plasmapheresis. Seven (39%) patients lost their grafts. Renal function in the remaining patients recovered to serum creatinine levels ranging from 104 mumol/l to 430 mumol/l (1.2 mg% to 4.8 mg%). All patients with surviving grafts had CSA successfully reintroduced. This study indicates that there is an association between patients who develop posttransplant thrombotic microangiopathy after CSA administration and high PRA levels. The condition appears to respond to anti-rejection therapy and stopping CSA in the majority of cases. The safe reintroduction of CSA suggests that endothelial cell damage in the posttransplant period may be multifactorial and not solely due to CSA therapy.

Disorders of potassium homeostasis: an approach based on pathophysiology

Disorders of potassium (K+) homeostasis are frequently encountered in clinical medicine and may have serious sequelae, particularly cardiac arrhythmias. Since long-term K+ balance depends on regulation of renal excretion of K+, the focus of this paper is to provide a novel way to analyze the K+ excretory process at the bedside in a noninvasive fashion. A fundamental aim was to incorporate recent new advances in K+ physiology to the clinical analysis of K+ disorders. In so doing, we have tried to replace eponyms and largely descriptive terms with more specific, but hypothetical pathophysiologic diagnoses. The approach we used focuses on an assessment of the components of K+ excretion in vivo. If the rate of excretion of K+ differs from the "expected" value for the stimulus of hypokalemia or hyperkalemia, one should determine whether the fault is with the flow rate and/or the [K+] in the terminal cortical collecting duct. The former is influenced primarily by the rate of excretion of osmoles when antidiuretic hormone acts, whereas the [K+] in the cortical collecting duct is determined by factors that modulate rate of electrogenic reabsorption of Na+ in that segment and its conductance for K+. By examining the extracellular fluid (ECF) volume status, the plasma renin activity, and the renal response to the induction of ECF volume contraction, we attempted to deduce whether the change in electrogenic reabsorption of Na+ was due to an altered Na+ transport or apparent permeability to chloride in the cortical collecting duct. We believe that an approach which draws heavily on pathophysiology can be of practical use at the bedside and, in addition, indicate areas in which more research could be fruitful. To illustrate these points, two clinical cases with hypokalemia and two with hyperkalemia were analyzed. Nevertheless, it is important to emphasize that the approach provided is speculative.

Studies to determine the basis for hyperkalemia in recipients of a renal transplant who are treated with cyclosporine

Hyperkalemia is commonly encountered in patients who receive a renal transplant and the immunosuppressive drug, cyclosporine. There is also a high incidence of hypertension (which is thought to be due to expansion of the extracellular fluid volume) and hyperchloremic metabolic acidosis in this group of patients. This constellation of findings led to the suspicion of the possibility that their basis might be type II hypoaldosteronism. To test this hypothesis, 12 patients with hyperkalemia (plasma K+, 5.1 +/- 0.2 mmol/L at the time of study) while receiving cyclosporine were studied. Patients who had diabetes mellitus, those receiving drugs known to cause hyperkalemia (e.g., beta blockers, angiotensin-converting enzyme inhibitors, K(+)-sparing diuretics), or those with a serum creatinine greater than 200 mumol/L were excluded. The renal response to hyperkalemia was inappropriate because the transtubular K+ concentration gradient (TTKG) was only 4.3 +/- 0.4 compared with a TTKG of 13 +/- 1, 2 h after 50 mmol of KCl was given to normal subjects. The TTKG, after administration of 200 micrograms of fludrocortisone, was still very low (5.6 +/- 0.6) in the patients compared with that of controls (12 +/- 1). After administration of 250 to 500 mg of acetazolamide to increase the delivery of bicarbonate to the distal nephron, the TTKG rose significantly to 11 +/- 1 in patients on cyclosporine, compared with 17 +/- 1 in the controls.(ABSTRACT TRUNCATED AT 250 WORDS)