Johan Hoebeke

Mapping of a functional autoimmune epitope on the beta 1-adrenergic receptor in patients with idiopathic dilated cardiomyopathy.

The presence and properties of serum autoantibodies against beta-adrenergic receptors in patients with idiopathic dilated cardiomyopathy were studied using synthetic peptides derived from the predicted sequences of the human beta-adrenergic receptors. Peptides corresponding to the sequences of the second extracellular loop of the human beta 1- and beta 2-adrenergic receptors were used as antigens in an enzyme immunoassay to screen sera from patients with dilated cardiomyopathy (n = 42), ischemic heart disease (n = 17), or healthy blood donors (n = 34). The sera of thirteen dilated cardiomyopathy patients, none of the ischemic heart disease patients, and four of the healthy controls monospecifically recognized the beta 1-peptide. Only affinity-purified antibodies of these patients had a inhibitory effect on radioligand binding to the beta 1 receptor of C6 rat glioma cells. They recognized the receptor protein by immunoblot and bound in situ to human myocardial tissue. We conclude that a subgroup of patients with idiopathic dilated cardiomyopathy have in their sera autoantibodies specifically directed against the second extracellular loop of the beta 1-adrenergic receptor. These antibodies could serve as a marker of an autoimmune response with physiological and/or pathological implications.

Catalytic activity of antibodies against factor VIII in patients with hemophilia A.

Hemophilia A is an X chromosome-linked recessive disorder resulting in defective or deficient factor VIII (FVIII) molecules, which, in its severe form, is a life-threatening and crippling hemorrhagic disease. Infusion of homologous FVIII to patients with severe hemophilia A results, in 25% of patients, in the emergence of alloantibodies against FVIII (inhibitors)(ref. 1) that inhibit FVIII procoagulant activity by steric hindrance of the interaction of FVIII either with stabilizing molecules, with molecules essential for its activity or with activating molecules. Here, we report on the proteolysis of FVIII by alloantibodies of two patients with severe hemophilia A, demonstrating a previously unknown mechanism by which FVIII inhibitors may prevent the pro-coagulant function of FVIII. The kinetic parameters of FVIII hydrolysis indicate a functional role for the catalytic immune response in the inactivation of FVIII in vivo. The characterization of alloantibodies against FVIII as site-specific proteases may provide new approaches to the treatment of FVIII inhibitors.

Immunomodulatory properties of cystatins

Abstract. Cystatins are natural tight-binding reversible inhibitors of cysteine proteases. Because these cysteine proteases exist in all living organisms and because they are involved in various biological and pathological processes, the control of these protease functions by cystatins is of cardinal importance. Cystatins are found in mammals but cystatin-like molecules are also present in mammals and parasites. In the immune system, cystatins modulate cathepsin activities and antigen presentation. They also induce tumor necrosis factor α and interleukin 10 synthesis, and they stimulate nitric oxide production by interferon γ-activated murine macrophages. In turn, nitric oxide has inhibitory activity on cysteine proteases, especially those from parasitic protozoa. Cystatins isolated from parasitic nematodes also have immunomodulatory activities that are distinguishable from those induced by lipopolysacharide-like molecules from endosymbiotic bacteria. On the whole, cystatins and cystatin-like molecules belong to a new category of immunomodulatory molecules. Doubtless increasing data will improve our knowledge of this property, leading to practical applications in immunotherapy.

Anti-SSA/Ro52 autoantibodies blocking the cardiac 5-HT4 serotoninergic receptor could explain neonatal lupus congenital heart block

The 52‐kDa SSA/Ro (Ro52) ribonucleoprotein is an antigenic target strongly associated with the autoimmune response in mothers whose children develop neonatal lupus and congenital heart block. When sera from patients with systemic lupus erythematosus were used as autoimmune controls in an enzyme immunoassay to screen for antibodies against the human serotoninergic 5‐HT4‐receptor, a high correlation was found between the presence of anti‐Ro52 protein antibodies in such sera and antibodies reacting with a synthetic peptide, corresponding to the second extracellular loop of the human 5‐HT4 receptor (amino acid residues 165–185). Homology scanning between the 5‐HT4 peptide and the sequence of the Ro52 protein indicated two potential common epitopes located between residues 365 and 396 of the Ro52 protein. Cross‐reactivity was found between the peptide derived from the 5‐HT4 receptor, and a peptide corresponding to residues 365–382 of the Ro52 protein. Autoantibodies, affinity‐purified on the 5‐HT4 receptor peptide, specifically recognized both the Ro52 protein and the 5‐HT4 receptor protein in immunoblots. The affinity‐purified antibodies antagonized the serotonin‐induced L‐type Ca channel activation on human atrial cells. This effect could explain the electrophysiological abnormalities in neonatal lupus.

Mechanisms of genetically-based resistance to malaria

Malaria remains one of the most prevalent parasitoses worldwide. About 350 to 500 million febrile episodes are observed yearly in African children alone and more than 1 million people die because of malaria each year. Multiple factors have hampered the effective control of this disease, some of which include the complex biology of the Plasmodium parasites, their high polymorphism and their increasingly high resistance to antimalarial drugs, mainly in endemic regions. The ancient interaction between malarial parasites and humans has led to the fixation in the population of several inherited alterations conferring protection against malaria. Some of the mechanisms underlying protection against this disease are described in this review for hemoglobin-inherited disorders (thalassemia, sickle-cell trait, HbC and HbE), erythrocyte polymorphisms (ovalocytosis and Duffy blood group), enzymopathies (G6PD deficiency and PK deficiency) and immunogenetic variants (HLA alleles, complement receptor 1, NOS2, tumor necrosis factor-α promoter and chromosome 5q31-q33 polymorphisms).

Selective modulation of CD4+ T cells from lupus patients by a promiscuous, protective peptide analog.

A peptide encompassing residues 131–151 of the spliceosomal U1-70K protein and its analog phosphorylated at Ser140 were synthesized as potential candidates for the treatment of patients with lupus. Studies in the MRL/lpr and (NZB × NZW)F1 lupus models have demonstrated that these sequences contain a CD4+ T cell epitope but administration of the phosphorylated peptide only ameliorates the clinical manifestations of treated MRL/lpr mice. Binding assays with soluble HLA class II molecules and molecular modeling experiments indicate that both peptides behave as promiscuous epitopes and bind to a large panel of human DR molecules. In contrast to normal T cells and T cells from non-lupus autoimmune patients, we found that PBMCs from 40% of lupus patients selected randomly and CFSE-labeled CD4+ T cells proliferate in response to peptide 131–151. Remarkably, however, we observed that phosphorylation of Ser140 prevents CD4+ T cells proliferation but not secretion of regulatory cytokines, suggesting a striking immunomodulatory effect of phosphorylated analog on lupus CD4+ T cells that was unique to patients. The analog might act as an activator of regulatory T cells or as a partial agonist of TCR.

Isolation of the serotoninergic 5‐HT4(e) receptor from human heart and comparative analysis of its pharmacological profile in C6‐glial and CHO cell lines

RT–PCR technique was used to clone the human 5‐HT4(e) receptor (h5‐HT4(e)) from heart atrium. We showed that this h5‐HT4(e) receptor splice variant is restricted to brain and heart atrium. Recombinant h5‐HT4(e) receptor was stably expressed in CHO and C6‐glial cell lines at 347 and 88 fmol mg−1 protein, respectively. Expression of h5‐HT4(e) receptors at the cell membrane was confirmed by immunoblotting. The receptor binding profile, determined by competition with [3H]‐GR113808 of a number of 5‐HT4 ligands, was consistent with that previously reported for other 5‐HT4 receptor isoforms. Surprisingly, we found that the rank order of potencies (EC50) of 5‐HT4 agonists obtained from adenylyl cyclase functional assays was inversely correlated to their rank order of affinities (Ki) obtained from binding assays. Furthermore, EC50 values for 5‐HT, renzapride and cisapride were 2 fold lower in C6‐glial cells than in CHO cells. ML10302 and renzapride behaved like partial agonists on the h5‐HT4(e) receptor. These results are in agreement with the reported low efficacy of the these two compounds on L‐type Ca2+ currents and myocyte contractility in human atrium. A constitutive activity of the h5‐HT4(e) receptor was observed in CHO cells in the absence of any 5‐HT4 ligand and two 5‐HT4 antagonists, GR113808 and ML10375, behaved as inverse agonists. These data show that the h5‐HT4(e) receptor has a pharmacological profile which is close to the native h5‐HT4 receptor in human atrium with a functional potency which is dependent on the cellular context in which the receptor is expressed.

Induction of neonatal lupus in pups of mice immunized with synthetic peptides derived from amino acid sequences of the serotoninergic 5-HT4 receptor

We have previously suggested that the recognition of a cross‐reactive epitope on the 5‐HT4 receptor and the 52‐kDa SSA / Ro protein by serotonin‐antagonizing autoantibodies could explain the electrophysiological symptoms of congenital heart block in neonatal lupus. To confirm this hypothesis, we immunized female mice with four synthetic peptides corresponding to the recognized epitopes. All mice developed anti‐peptide antibodies, which cross‐reacted with the Ro52 and 5‐HT4 receptor peptides and recognized both cognate proteins. Peptide‐immune mice were mated. The pups from mice immunized with the Ro52 peptides had no symptoms of neonatal lupus apart from bradycardia. However, pups from mice immunized with the 5‐HT4 receptor peptides and bradycardia, atrioventricular block of type I or II, longer QT intervals, skin rashes and neuromotoric problems. The 5‐HT4 receptor was detectable in the different fetal tissues affected (heart, skin and brain) by immunohistochemistry. Hearts from diseased pups were less developed and showed disorganized myocardial hyperplasia, compared to the normal littermates. These results demonstrate that the serotoninergic 5‐HT4 receptor is the antigenic target of physiopathological autoantibodies in neonatal lupus.

Catalytic IgG from Patients with Hemophilia A Inactivate Therapeutic Factor VIII

Factor VIII (FVIII) inhibitors are anti-FVIII IgG that arise in up to 50% of the patients with hemophilia A, upon therapeutic administration of exogenous FVIII. Factor VIII inhibitors neutralize the activity of the administered FVIII by sterically hindering its interaction with molecules of the coagulation cascade, or by forming immune complexes with FVIII and accelerating its clearance from the circulation. We have shown previously that a subset of anti-factor VIII IgG hydrolyzes FVIII. FVIII-hydrolyzing IgG are detected in over 50% of inhibitor-positive patients with severe hemophilia A, and are not found in inhibitor-negative patients. Although human proficient catalytic Abs have been described in a number of inflammatory and autoimmune disorders, their pathological relevance remains elusive. We demonstrate here that the kinetics of FVIII degradation by FVIII-hydrolyzing IgG are compatible with a pathogenic role for IgG catalysts. We also report that FVIII-hydrolyzing IgG from each patient exhibit multiple cleavage sites on FVIII and that, while the specificity of cleavage varies from one patient to another, catalytic IgG preferentially hydrolyze peptide bonds containing basic amino acids.

52-kDa Ro/SSA Epitopes Preferentially Recognized by Antibodies from Mothers of Children with Neonatal Lupus and Congenital Heart Block

Neonatal lupus erythematosus is a rare disorder caused by the transplacental passage of maternal autoantibodies. The 52-kDa Ro/SSA antigen (Ro52) ribonucleoprotein represents an antigenic target strongly associated with the autoimmune response in mothers whose children have neonatal lupus and cardiac conduction disturbances, mainly congenital heart block. The objective of this study was to identify putative Ro52/60-kDa Ro/SSA antigen (Ro60) epitopes associated with neonatal lupus and congenital heart block. The reactivity of IgG antibodies present in the sera from mothers with systemic lupus erythematosus and Sjögrens syndrome and in the sera from asymptomatic mothers (a longitudinal study of 192 samples from 66 subjects) was investigated by ELISA using Ro52, Ro60 and 48-kDa La/SSB antigen proteins, as well as 45 synthetic peptides, 13–24 residues long, of Ro52/Ro60 proteins. One to 19 samples collected before, during and after pregnancy were available for each mother. Forty-three disease controls selected randomly and normal sera were tested in parallel. Although no differences were found between Sjögrens syndrome and asymptomatic mothers of group I, who had at least one infant with neonatal lupus, and of group II, who had healthy babies only, significant differences were observed between lupus mothers from both groups. In the former group of lupus mothers, a significantly higher frequency of antibodies to Ro52 peptides 107–122 and 277–292 was observed. Between 18 and 30 weeks of gestation, the period of risk, there was clearly an elevated level of antibodies reacting with Ro52 peptides 1–13, 277–292 and 365–382. Antibodies to Ro52 peptide 365–382 have been shown previously to cross-react with residues 165–185 of the heart 5-HT4 serotoninergic receptor, and might be pathologically important. The level of these Ro52 antibody subsets decreased at the end of pregnancy and after delivery. IgG antibodies to Ro52 peptides 1–13, 107–122, 277–292 and 365–382 may therefore represent important biomarkers to predict a complication in pregnant lupus women with Ro52 antibodies.