Scintigraphic distribution of complexes of antiinsulin antibodies and 123I-insulin. In vivo studies in rats

Intrinsic Resistance of Hepatocytes to Complement-Mediated Injury

When activated on or in the vicinity of cells, complement usually causes loss of function and sometimes cell death. Yet the liver, which produces large amounts of complement proteins, clears activators of complement and activated complexes from portal blood without obvious injury or impaired function. We asked whether and to what extent hepatocytes resist injury and loss of function mediated by exposure to complement. Using cells isolated from porcine livers as a model system, we found that, in contrast to endothelial cells, hepatocytes profoundly resist complement-mediated lysis and exhibit normal synthetic and conjugative functions when complement is activated on their surface. The resistance of hepatocytes to complement-mediated injury was not a function of cell surface control of the complement cascade but rather an intrinsic resistance of the cells dependent on the PI3K/Akt pathway. The resistance of hepatocytes to complement might be exploited in developing approaches to the treatment of hepatic failure or more broadly to the treatment of complement-mediated disease.

Dynamic visualization of a joint-specific autoimmune response through positron emission tomography

In the K/BxN mouse model of rheumatoid arthritis, the transfer of autoantibodies specific for glucose-6-phosphate isomerase (GPI) into naïve mice rapidly induces joint-specific inflammation similar to that seen in human rheumatoid arthritis. The ubiquitous expression of GPI and the systemic circulation of anti-GPI immunoglobulin G (IgG) seem incongruous with the tissue specificity of this disease. By using PET (positron emission tomography), we show here that purified anti-GPI IgG localizes specifically to distal joints in the front and rear limbs within minutes of intravenous injection, reaches saturation by 20 min and remains localized for at least 24 h. In contrast, control IgG does not localize to joints or cause inflammation. The rapid kinetics of anti-GPI IgG joint localization supports a model in which autoantibodies bind directly to pre-existing extracellular GPI in normal healthy mouse joints.

In vivo imaging of insulin receptors by PET: preclinical evaluation of iodine-125 and iodine-124 labelled human insulin

[A(14)-*I]iodoinsulin was prepared for studies to assess the suitability of labeled iodoinsulin for positron emission tomography (PET). Iodine-125 was used to establish the methods and for preliminary studies in rats. Further studies and PET scanning in rats were carried out using iodine-124. Tissue and plasma radioactivity was measured as the uptake index (UI = [cpm x (g tissue)(-1)]/[cpm injected x (g body weight)(-1)]) at 1 to 40 min after intravenous injection of either [A(14)-(125)I]iodoinsulin or [A(14)-(124)I]iodoinsulin. For both radiotracers, initial clearance of radioactivity from plasma was rapid (T(1/2) approximately 1 min), reaching a plateau (UI = 2.8) at approximately 5 min which was maintained for 35 min. Tissue biodistributions of the two radiotracers were comparable; at 10 min after injection, UI for myocardium was 2.4, liver, 4.0, pancreas, 5.4, brain, 0.17, kidney, 22, lung, 2.3, muscle, 0.54 and fat, 0.28. Predosing rats with unlabelled insulin reduced the UI for myocardium (0.95), liver (1.8), pancreas (1.2) and brain (0.08), increased that for kidney (61) but had no effect on that for lung (2.5), muscle (0.50) or fat (0.34). Analysis of radioactivity in plasma demonstrated a decrease of [(125)I]iodoinsulin associated with the appearance of labeled metabolites; the percentage of plasma radioactivity due to [(125)I]iodoinsulin was 40% at 5 min and 10% at 10 min. The heart, liver and kidneys were visualized using [(124)I]iodoinsulin with PET.

Insulin receptor antibodies inhibit insulin uptake by the liver: in vivo 123I-insulin scintigraphic scanning and in vitro characterization in autoimmune hypoglycemia

BACKGROUND

Insulin receptor antibodies can induce severe hypoglycemia or insulin resistance in rare autoimmune syndromes. In vitro properties of these antibodies occasionally explain the clinical features of the syndrome, but direct evidence of their in vivo activity is poor. We studied a 58-year-old male with rheumatoid arthritis who presented with hypoglycemic coma.

METHODS AND RESULTS

Antibodies were detected by inhibition of 125I-insulin binding to human insulin receptor-3T3 cells by the patient's serum. By immunofluorescence, they were immunoglobulin G of all four subclasses, immunoprecipitated insulin receptors from biotin-labeled cells, and triggered phosphorylation of the beta subunit of the insulin receptor. Insulin binding on the patient's red blood cells was markedly reduced. A biodistribution study after intravenous 123I-Tyr A14 insulin showed a marked inhibition of tracer uptake by the liver, reaching 10% of the injected dose (controls, mean +/- SD, 21.1 +/- 1.7%; n = 10). Time activity curves generated on the liver and on the heart were parallel, with a T1/2 of 11.5 minutes for both, suggesting that no specific uptake occurred in the liver, where tracer activity represented only the blood pool. Clearance of insulin from the blood was indeed slower than in controls and mainly occurred through the kidneys. Analysis of plasma 123I-insulin immunoreactivity and trichloroacetic acid precipitate showed that insulin degradation did not occur as in normal controls.

CONCLUSIONS

In this patient with hypoglycemic syndrome, insulin receptor antibodies with in vitro insulin-like activity are capable of blocking in vivo the access of insulin to the liver receptor compartment, as directly demonstrated by the markedly altered biodistribution of intravenously injected 123I-insulin.

Insulin binding and degradation by rat liver Kupffer and endothelial cells

The liver is the major site of insulin metabolism. Previous studies have suggested that hepatocytes were chiefly responsible for this activity, while contributions of Kupffer and other nonparenchymal liver cells remained controversial. In this study, we compared 125I-insulin binding and degradation by rat hepatocytes with insulin binding and degradation by sinusoidal Kupffer and endothelial cells. Kupffer cells were separated from endothelial cells by centrifugal elutriation. Hepatocytes had approximately 3.5 times more insulin binding sites than Kupffer cells and approximately eight times more binding sites than endothelial cells. In addition, wheat germ agglutinin (WGA)-purified solubilized receptors from all three cell types bound insulin in proportions similar to whole cells. Moreover, all three cell types were shown with a ribonuclease (RNase) protection assay to express insulin receptor mRNA. Hepatocytes degraded approximately four times more insulin than Kupffer cells, while endothelial cells degraded only negligible amounts of insulin. Based on morphometric data available in the literature, we estimated that nonparenchymal cells could account for approximately 10% to 15% of hepatic insulin degradation. We concluded that rat hepatocytes, Kupffer cells, and endothelial cells all have specific insulin receptors, and that nonparenchymal cells play a small but significant role in insulin degradation.

Polymorphism of insulin antibodies in six patients with insulin-immune hypoglycaemic syndrome

Insulin antibodies in six patients with immune hypoglycaemic syndrome were studied. The antibodies displayed a higher affinity for bovine insulin in two patients, were specific for human insulin in one patient and non-species specific in the other three patients. The predominant IgG subclass of the insulin antibodies was IgG4 in two patients, IgG3 in two and IgG1 in two. In one of these, the other three subclasses were also detectable. Insulin autoantibodies of four patients were homogeneous with regard to light chains (kappa), and those of the other two contained both kappa and gamma light chains. Analysis of insulin immune complex size by fast protein liquid chromatography was possible in three patients and demonstrated immune complexes with elution profile close to that of IgG, although not exactly superimposable to the one obtained with a mouse monoclonal insulin antibody. In two patients, avidity was too low to permit chromatography of the immune complexes, and, moreover, in these two cases insulin antibodies were of the IgG3 isotype and spontaneously formed aggregates independently of insulin binding. We conclude that insulin antibodies of the insulin immune syndrome are polymorphic but different from those generated by insulin therapy.

Is quantitative assessment of insulin-antibodies and autoantibodies feasible?

Nine selected sera were studied using radioimmunoassay and enzyme linked immunosorbent assay; eight contained insulin antibodies and were from Type 1 (insulin-dependent) diabetic patients, one of whom had antibody-mediated insulin resistance, and one contained insulin-autoantibodies and was from an asymptomatic blood donor. Sera were assayed in serial dilution to assess their suitability for use as reference standards. Dilution curves were non-parallel in radioimmunoassay but were parallel in immunosorbent assay. In all sera, insulin antibodies were readily detected in both assays whereas the low avidity insulin autoantibodies were only detected by immunosorbent assay and not at all by radioimmunoassay, suggesting that the assays respond differently to antibodies of different avidity. Avidity was estimated in liquid phase from the dissociation rate of preformed complexes of antibody and 125-iodinated insulin. When high avidity antibodies are used as a reference in radioimmunoassay, lower avidity antibodies are underestimated and vice versa. In contrast, in immunosorbent assay, any serum could be used as a reference regardless of avidity; furthermore competition experiments comparing the highest avidity insulin antibodies, from the insulin-resistant patient, with the insulin autoantibodies from the asymptomatic blood donor yielded near-superimposable curves. We conclude that radioimmunoassay is selective for high avidity antibodies whereas enzyme linked immunosorbent assay is not; computer modelling of the two assays supports this conclusion. In practice immunosorbent assay can be standardized using a reference serum, whereas experimental findings and mathematical considerations preclude the use of a standard serum in radioimmunoassay.

Immunological aspects of diabetes mellitus: prospects for pharmacological modification

It is now well known that insulin-dependent diabetes is a chronic progressive autoimmune disease. The prolonged prediabetic phase of progressive beta-cell dysfunction is associated with immunological abnormalities. A prediabetic period is suggested by the appearance of islet cell antibodies, anti-insulin antibodies, and anti-insulin receptor antibodies. The existence of activated T lymphocytes and abnormal T cell subsets are also other markers. There is still no concensus about the use of the immunosuppression superimposed upon conventional insulin therapy in early diagnosed IDDM and the follow-up of the relatives of IDDM patients who share the genetic predisposition and serological markers for the risk of future onset of IDDM. Treatment in the prodromal period cannot be justified because a link between the disease and early markers such as ICA has not been established with certainty (Diabetes Research Program NIH, 1983). Many immunopharmacological manipulations were reported to be effective in animal models. However, most of them are not readily applied to human subjects. Moreover, IDDM patients are now believed to be heterogeneous, with a complex genetic background. HLA-DR, and more recently DQ, are closely related to the genetic predisposition to IDDM but those genes are not themselves diabetogenic. The contribution of autoimmunity does not appear to be uniform, and in some cases, the contribution of virus is considered more important. There is a lack of a marker for the future onset of IDDM. ICA and ICSA were found after mumps infection, but the existence of those autoantibodies and even the co-existence of HLA-DR3 do not always indicate the future trend to insulin dependency. More precise markers will be disclosed through the biochemical analysis of the target antigens on pancreatic beta-cell for islet antibodies and effector T cells. Much safer and more effective immunopharmacological treatment will be developed through animal experimentation using rat and mouse models. The recent development and interest in this field will further facilitate the attainment of the goal for the complete prevention of IDDM.

Glycerol-insulin raises circulating insulin antibodies in type I diabetic patients treated with permanently implanted devices

The instability of insulin in the reservoirs of implantable insulin delivery devices has been a major obstacle in implementing this form of therapy. To overcome the problem of precipitation, a glycerol-insulin preparation has been used in large-scale long-term clinical trials. The aim of this study was to evaluate the stability of the glycerol-insulin solution and its effects on circulating insulin antibodies in eight type I diabetic patients who were implanted with an Infusaid pump (Infusaid Corporation, Norwood, MA) and followed for 1 year or more. Total insulin requirement did not change throughout the observation period. Plasma free insulin was higher during treatment with glycerol-insulin than with the standard insulin treatment (P less than .02). Insulin antibodies increased in all patients (P less than .05). High-performance liquid HPLC analysis of insulin samples from the pump reservoirs showed the generation of insulin modification products at a daily rate of 1.84%, reaching 40% to 50% of the total reservoir content 3 weeks after refilling; among these products, high molecular weight species accounted for about 15%. It is concluded that glycerol-insulin is not an adequate insulin preparation for use in implanted devices. Insulin deteriorated in the pump reservoirs, and insulin antibody concentration increased in the treated patients. It is believed that this antibody production is favored by circulating insulin fragments and polymers of insulin generated inside the pump reservoirs.