Functional analysis of rabbit anti-peptide antibodies which mimic autoantibodies against the beta 1-adrenergic receptor in patients with idiopathic dilated cardiomyopathy

Functional autoantibodies targeting G protein-coupled receptors in rheumatic diseases

G protein-coupled receptors (GPCRs) comprise the largest and most diverse family of integral membrane proteins that participate in different physiological processes such as the regulation of the nervous and immune systems. Besides the endogenous ligands of GPCRs, functional autoantibodies are also able to bind GPCRs to trigger or block intracellular signalling pathways, resulting in agonistic or antagonistic effects, respectively. In this Review, the effects of functional GPCR-targeting autoantibodies on the pathogenesis of autoimmune diseases, including rheumatic diseases, are discussed. Autoantibodies targeting β1 and β2 adrenergic receptors, which are expressed by cardiac and airway smooth muscle cells, respectively, have an important role in the development of asthma and cardiovascular diseases. In addition, high levels of autoantibodies against the muscarinic acetylcholine receptor M3 as well as those targeting endothelin receptor type A and type 1 angiotensin II receptor have several implications in the pathogenesis of rheumatic diseases such as Sjögren syndrome and systemic sclerosis. Expanding the knowledge of the pathophysiological roles of autoantibodies against GPCRs will shed light on the biology of these receptors and open avenues for new therapeutic approaches.

Transfer of immune components from rabbit autoimmune cardiomyopathy into severe combined immunodeficiency (SCID) mice induces cardiomyopathic changes

BACKGROUND

Growing evidence suggests that autoimmune mechanism plays an important role in the pathogenesis of cardiomyopathy. The purpose of this study was to investigate whether passive transfer of IgG and/or lymphocytes from rabbits with autoimmune cardiomyopathy is able to reproduce cardiomyopathic changes in severe combined immunodeficiency (SCID) mice.

METHODS AND RESULTS

SCID mice were injected intraperitoneally with IgG and/or peripheral blood lymphocytes (PBL) from either rabbits immunized with both beta1-adrenoceptor peptide and M2-muscarinic receptor peptide (beta1+M2 group) or rabbits with adjuvant (N group). Thirty five SCID mice were divided into seven groups; N-IgG, N-PBL, N-IgG & PBL, (beta1+M2)-IgG, (beta1+M2)-PBL, (beta1+M2)-IgG & PBL and control groups. Heart weight in three (beta1+M2) groups were significantly increased. All mice in three (beta1+M2) groups showed high titer of rabbit anti-beta1 adrenoceptor autoantibodies, and 4 mice in the (beta1+M2)-PBL group and 3 mice in the (beta1+M2)-IgG & PBL group showed a significant increase in titer of rabbit anti-M2-muscarinic receptor autoantibodies. Focal infiltration of inflammatory cells in the myocardium was observed in the (beta1+M2)-IgG & PBL group. In the (beta1+M2)-PBL group and (beta1+M2)-IgG & PBL group, cardiomyocyte diameters were significantly increased. Some myocytes of the (beta1+M2)-IgG & PBL group exhibited intracellular edema, clumps of Z-band and increased numbers of mitochondria by using electron microscopy.

CONCLUSION

Transfer of IgG and PBL from rabbits immunized with combined beta1 and M2 peptides was able to reproduce the early stage of cardiomyopathic changes in SCID mice.

Antibodies as molecular mimics of biomolecules: roles in understanding physiological functions and mechanisms

Physiologists have routinely used understanding of the immune system to generate antibodies against regulatory molecules, growth factors, plasma membrane receptors, and other mammalian molecules in the development of analytical tools and assays. In taking this notion further, antibodies have been used in vivo to modulate physiological systems and to improve our understanding of their molecular interactions. To develop antibodies with physiological activity (efficacy), physiologists have worked with immunologists in developing interdisciplinary insights, requiring basic knowledge of immune system function in designing strategies to generate antibodies that interact with endogenous molecules of physiological interest, in vivo. Antibodies in different physiological systems have been shown to enhance or inhibit endogenous molecular functions. Two approaches have been used: passive and active immunization. Antibodies in these contexts have provided tools to develop further insights into molecular physiological mechanisms. Perhaps surprisingly, enhancing antibodies have been developed against a diverse set of target molecules including several members of the growth hormone/insulin-like growth factor-I axes and those of the beta(2)-adrenoceptor axis. Antibodies that inhibit the actions of somatostatin have also been developed. A further novel approach has been the development of antibodies that interact with adipose cells in vivo. These have the potential to be used in therapeutic antiobesity approaches. Antibodies with efficacy in vivo have provided new insights into molecular physiological mechanisms, enhancing our understanding of these complex processes.

Antibodies Raised Against the Second Extracellular Loop of the Human Muscarinic M3 Receptor Mimic Functional Autoantibodies in Sjogren's Syndrome

Functional antimuscarinic M3 receptor (M3R) autoantibodies have been shown to inhibit cholinergic neurotransmission at the postsynaptic level and appear to mediate parasympathetic dysfunction, including sicca symptoms in Sjögren's syndrome (SS). The precise epitope(s) involved in the inhibition of M3R-mediated cholinergic neurotransmission has not been defined. In this study, an active immunization approach to raise antibodies with functional activity against the second extracellular loop of the M3R was used and their functional properties were compared with those of human autoantibodies. Peptides corresponding to the second extracellular loop of the M3R were used as immunogens in rabbits, and antisera were tested for inhibition of carbachol-evoked colon smooth muscle contraction in parallel with immunoglobulin G from a patient with SS. Anti-M3R antibodies were affinity purified on a peptide representing a dominant functional epitope at the COOH terminus of the second extracellular loop of the M3R and tested for concentration-dependent inhibition. Experimentally raised anti-M3R antibodies, like the human autoantibodies, showed concentration-dependent and noncompetitive inhibition of carbachol-evoked colon contractions. Inhibitory activity was detected by functional assays at concentrations as low as 3 ng/ml, which was below the threshold of detection of antibody by peptide enzyme-linked immunosorbent assay. It is concluded that the experimentally raised anti-M3R antibodies share the functional properties of autoantibodies in patients with SS.

Transfer of Rabbit Autoimmune Cardiomyopathy into Severe Combined Immunodeficiency Mice

Growing evidence suggests that the autoimmune mechanism plays an important role in the pathogenesis of dilated cardiomyopathy. The purpose of this study was to evaluate the effect on the cardiac structure and function by the transfer of immunoglobulin G (IgG) and/or lymphocytes from rabbits immunized with a synthetic peptide corresponding to the sequence of the second extracellular loop of beta1-adrenoceptor (beta peptide) into severe combined immunodeficiency (SCID) mice. CB-17 SCID mice were injected intraperitoneally with 2 mg of IgG and/or 1 x 10(7) peripheral blood lymphocytes (PBL) from either rabbits immunized with both beta1 peptide and adjuvant (beta group), and adjuvant or rabbits with adjuvant only (N group). Thirty-five SCID mice were divided into seven groups: (1) N-IgG group; (2) N-PBL group; (3) N-IgG+PBL group; (4) beta-IgG group; (5) beta-PBL group; (6) beta-IgG+PBL group; and (7) control group. Morphological, serological and endocrinological studies were performed 70 days after the transfer. Results showed that heart weight and heart weight/body weight ratio in the beta-IgG+PBL group tended to be increased as compared with those in other groups. All mice in the beta-IgG group, two in the beta-PBL group and four in the beta-IgG+PBL group showed high titer of rabbit anti-beta1-adrenoceptor antibodies. Brain natriuretic peptide in the beta-IgG+PBL group showed a significant increase as compared with those in the control group and N-IgG+PBL. Pathohistologically, focal infiltration of inflammatory cells in the myocardium was observed in one mouse of the beta-IgG+PBL group. Rabbit CD3-positive T-lymphocytes in the myocardium were observed in two mice of the beta group. In conclusion, transfer of IgG and PBL from rabbits immunized with beta1 peptide was able to induce the early stages of myocardial damage in SCID mice. These data provide direct evidence that the autoimmune mechanism is important in the pathogenesis of dilated cardiomyopathy.

Modulation of intracellular Ca2+ via L-type calcium channels in heart cells by the autoantibody directed against the second extracellular loop of the alpha1-adrenoceptors

The effects of methoxamine, a selective alpha1-adrenergic receptor agonist, and the autoantibody directed against the second extracellular loop of alpha1-adrenoceptors were studied on intracellular free Ca2+ levels using confocal microscopy and ionic currents using the whole-cell patch clamp technique in single cells of 10-day-old embryonic chick and 20-week-old fetal human hearts. We observed that like methoxamine, the autoantibody directed against the second extracellular loop of alpha1-adrenoreceptors significantly increased the L-type calcium current (I(Ca(L))) but had no effect on the T-type calcium current (I(Ca(T))), the delayed outward potassium current, or the fast sodium current. This effect of the autoantibody was prevented by a prestimulation of the receptors with methoxamine and vice versa. Moreover, treating the cells with prazosin, a selective alpha1-adrenergic receptor antagonist blocked the methoxamine and the autoantibody-induced increase in I(Ca(L)), respectively. In absence of prazosin, both methoxamine and the autoantibody showed a substantial enhancement in the frequency of cell contraction and that of the concomitant cytosolic and nuclear free Ca2+ variations. The subsequent addition of nifedipine, a specific L-type Ca2+ channel blocker, reversed not only the methoxamine or the autoantibody-induced effect but also completely abolished cell contraction. These results demonstrated that functional alpha1-adrenoceptors exist in both 10-day-old embryonic chick and 20-week-old human fetal hearts and that the autoantibody directed against the second extracellular loop of this type of receptors plays an important role in stimulating their activity via activation of L-type calcium channels. This loop seems to have a functional significance by being the target of alpha1-receptor agonists like methoxamine.

Pathophysiology of viral myocarditis: the role of humoral immune response

The pathophysiology of viral myocarditis is still a matter of debate. Humoral autoimmunity in postviral heart disease remains an attractive but complex hypothesis. Antigenic mimicry with or without cytolytic antibody properties has been shown to play a role in the immunopathogenesis of myocarditis with respect to sarcolemmal/myolemmal epitopes (including the beta-receptor), myosin and some mitochondrial proteins including the antinucleotide translocator (ANT)-carrier and dihydrolipoamid dehydrogenase. Today, refined two-dimensional Western blots are able to identify receptors and enzymes that are target of a humoral immune response or the consequence of an "immunization process." A humoral immune response to an invading agent will most likely lead to immunodestruction first. After conversion to IgG, the continuing antibody response may indicate the healing or healed process and last for many years or life-long. This paper reviews our present knowledge on the humoral immune response in myocarditis and its interplay with the viral agents and the other components of the immune system.

Beneficial effect of angiotensin-converting enzyme inhibitor on dilated cardiomyopathy induced by autoimmune mechanism against beta1-adrenoceptor

We have shown that a peptide corresponding to the sequence of the second extracellular loop of the human beta1-adrenoceptor (beta1-peptide) was able to induce an autoimmune cardiomyopathy in rabbits. In this study, we examined the effect of angiotensin-converting enzyme inhibitor (ACEI) on beta1-peptide-induced cardiomyopathy. Rabbits were divided into four groups: (1) control group (n= 6) receiving saline injection; (2) beta1-peptide group (n = 8) immunized with beta1-peptide; (3) ACEI group (n = 6), lisinopril (3 mg/day) given orally and receiving saline injection; and (4) ACEI + beta1-peptide group (n = 7), lisinopril (3 mg/day) given orally and immunized with beta1-peptide. Our results showed that, after 1 year, all rabbits in the beta1-peptide group had an increase in heart weight, wall thinning and dilatations of both ventricles as compared with rabbits in the ACEI + beta1-peptide group that had normal heart weight and shape. All rabbits in the beta1-peptide group exhibited multifocal degeneration and necrosis of myocardial cells with moderate infiltration of inflammatory cells. In the ACEI + beta1-peptide group, three rabbits showed focal degeneration and necrosis of myocardial cells accompanied by mononuclear cells. The lesions in this group were apparently less marked than those in the beta1-peptide group. In conclusion, ACEI protects the myocardium from injury induced by an autoimmune mechanism against beta1-adrenoceptor.

Active immunization of combined beta1-adrenoceptor and M2-muscarinic receptor peptides induces cardiac hypertrophy in rabbits

BACKGROUND

The high prevalence of patients with dilated cardiomyopathy (DCM) with anti-beta1-adrenoceptor and/or anti-M2-muscarinic receptor autoantibodies in their sera has been observed. However, the pathophysiological role of these autoantibodies in the development of cardiomyopathy is unknown. We previously reported an experimental model of early-stage DCM-like cardiomyopathy induced by immunizing rabbits for 1 year with synthetic peptides corresponding to the sequence of the second extracellular loop of either beta1-adrenoceptor or M2-muscarinic receptor. Because approximately half the sera of patients with DCM that recognize one of the two receptor sequences also recognize the second sequence, a model was created in rabbits simultaneously immunized with the synthetic peptides corresponding to the second extracellular loop of the beta1-adrenoceptor and M2-muscarinic receptor.

METHODS AND RESULTS

All rabbits (n = 8) immunized with both peptides had a high titer of both anti-beta1-adrenoceptor and anti-M2-muscarinic receptor autoantibodies in their sera, whereas none of the sera from control rabbits injected with saline (n = 9) was positive. No significant cross-reaction with peptides other than those used for immunization was found. The weight of the hearts of immunized rabbits increased significantly. The hearts of immunized rabbits showed marked concentric left ventricular hypertrophy with mild inflammatory cell infiltration. In these rabbits, mild or moderate interstitial fibrosis was also observed. In electron micrographs, immunized rabbits showed focal myofibrillar lysis, loss of myofilament, and a marked increase in the number of mitochondria and deposition of dense granules in both sarcoplasm and myofibrils. Conversely, one of the control rabbits showed scant mononuclear cell infiltration. However, in this control rabbit, no significant alteration was found by electron microscopy.

CONCLUSION

Our results showed the coexistence of both anti-beta1-adrenoceptor and anti-M2-muscarinic receptor autoantibodies in the sera has pathophysiological importance, shown by their ability to induce cardiac hypertrophy in rabbits.

The effect of amiodarone on the beta-adrenergic receptor is due to a downregulation of receptor protein and not to a receptor-ligand interaction

Downregulation of beta adrenergic receptors (beta-AR) by amiodarone (Am) have been reported in several studies both in vivo and in vitro. The mechanism underlying the antiadrenergic effect of Am is, however, still unclear. The aim of this study was to characterize whether the antiadrenergic effect of amiodarone is due to binding to the beta-AR or to downregulation of the beta-AR receptor protein. All experiments were performed on confluent mouse AT-1 cardiomyocytes cultured for 6 days. In acute experiments, equilibrium binding with [3H]-CGP-12177 to beta-AR was not directly inhibited by Am and the equilibrium binding constant did not change during prolonged exposure up to 72 hours. After Am exposure for 48 hours beta-AR density was decreased by 26% (p<0.005). T3 partially prevented the downregulation elicited by Am (p<0.05). A Western blot analysis with beta1-AR antibodies revealed a decreased signal intensity in cells treated with Am for 48 h as compared to control (p<0.05). Isoproterenol-provoked cAMP response did not change after acute exposure to Am. After incubation for 48 hours with Am there was, however, a 20% decrease in cAMP response as compared to control (p<0.05). This study shows that the effect of Am on beta-AR is due to a downregulation of the beta-AR protein and not to a competitive or non-competitive receptor-ligand interaction. This indicates a new pharmacological mechanism for modulation of beta-AR, which probably is transcriptionally regulated.

Antibodies against beta1 and beta2 adrenergic receptors in myasthenia gravis

Patients with myasthenia gravis have antibodies and T cells that react with the beta1- and beta2-adrenergic receptors. These receptors, as well as other auto-antigens, are present on cardiomyocytes, skeletal muscle cells and lymphocytes and are of importance for the regulation of the functions of these organs. Antibodies against the beta1-adrenergic receptor have been implicated in dilated cardiomyopathies. Myasthenia gravis (MG) patients have been suggested to have a higher than normal prevalence of heart disease. We have analysed the isotypes, subclasses, and binding sites of the beta-adrenergic receptors antibodies in both MG patients and healthy individuals and the correlation between beta-adrenergic receptors antibodies and heart disease in MG patients. The patients have IgG antibodies that react with both beta1- and beta2-adrenergic receptors. The subclasses were predominantly IgG2 and IgG4. By using synthesised overlapping peptides representing the immunodominant regions on the receptors, it was shown that the antibodies bound to partially overlapping sites on both beta1- and beta2-adrenergic receptors, but not to peptides from the acetylcholine receptor. beta-adrenergic receptor antibodies were found in 34/125 MG patients. Seven out of these 34 patients had symptomatic heart disease, all seven were over 70 years of age and had arteriosclerotic heart disease. There was no difference in the prevalence of clinical heart disease in patients with and without beta-adrenergic receptor antibodies. However, patients with heart disease had significantly higher levels of antibodies than healthy individuals and other patients. Antibodies against beta-adrenergic receptors in patients with myasthenia gravis binds to both beta1- and beta2-adrenergic receptors and might be implicated in the few patients with myasthenia gravis who have heart disease.

Beta 1-adrenoceptor autoimmunity in cardiomyopathy

A growing body of studies have confirmed that autoantibodies against beta 1-adrenoceptors are present in different types of cardiomyopathy. This suggests that they play a role in the pathophysiology of the disease. This article will review the data indicating the presence of anti-beta 1-adrenoceptor autoantibodies in cardiomyopathy. It will focus upon their structural and functional properties which could explain their possible role in the induction and development of cardiomyopathic diseases.

Characterization of anti-peptide antibodies directed against an extracellular immunogenic epitope on the human alpha 1-adrenergic receptor

A synthetic peptide corresponding to amino acids 192-218 of the second extracellular loop of the human alpha 1A-adrenergic receptor was used to raise antibodies in rabbits. Affinity-purified antibodies specifically recognized main bands with a molecular weight of about 68, 40 and 37 kD on the electrotransferred membrane proteins of rat ventricle membranes. The incubation of these antibodies with rat myocardial membranes resulted in a decrease in the number of binding sites for the specific radiolabelled alpha 1-antagonist prazosin. These antibodies were also able to mimic the effects of agonist stimulation as demonstrated by a positive chronotropic effect on cultured cardiomyocytes. These results constitute the first immunochemical evidence of the presence of both the A and B subtypes of the alpha 1-adrenergic receptor in the heart. They also confirm that the second extracellular loop of the alpha 1-adrenergic receptors is an immunologically and functionally important domain.

Autoimmunity in idiopathic dilated cardiomyopathy. Characterization of antibodies against the beta 1-adrenoceptor with positive chronotropic effect

BACKGROUND

Autoantibodies against the beta 1-adrenoceptor have been detected in the sera of patients with idiopathic dilated cardiomyopathy (DCM). The mechanisms by which these autoantibodies can alter normal receptor function are investigated, and the results are interpreted in the light of the beneficial effects of beta 1-blockade in some of these patients.

METHODS AND RESULTS

Autoantibodies against the beta 1-adrenoceptor, affinity purified from sera of patients with idiopathic DCM, were analyzed in a functional test system of spontaneously beating neonatal rat heart myocytes. Antibodies from rabbits immunized with peptides derived from the amino acid sequence of this receptor were also analyzed. Autoantibodies, against the second extracellular loop increased the beating frequency of isolated myocytes in a concentration-dependent manner, to approximately 80% of maximal isoproterenol stimulation. Rabbit anti-peptide antibodies against the second extracellular loop increased the beating frequency correspondingly. Autoantibodies and rabbit anti-peptide antibodies against the second extracellular loop were able to immunoprecipitate the unliganded receptor but not the antagonist-occupied receptor. In contrast, rabbit antibodies against the extracellular N-terminal sequence 34-57 of the beta 1-adrenoceptor were able to immunoprecipitate both the unliganded and the antagonist-occupied receptor although with no effect on the beating frequency of myocytes. The positive chronotropic effect of the antibodies was completely neutralized both by the addition of increasing concentrations of the beta 1-selective antagonist bisoprolol and by preincubation with the peptide corresponding to the second extracellular loop. The antibody-induced increase in beating frequency remained unchanged for more than 6 hours. This should be compared with the isoproterenol-stimulated beating frequency, which undergoes desensitization within 60 minutes. Addition of isoproterenol to autoantibody-stimulated myocytes resulted in only a small increase in beating frequency and did not cause desensitization. Antibodies had only a marginal effect on cyclic AMP production of stimulated cardiomyocytes compared with the 10-fold increase obtained after stimulation with isoproterenol.

CONCLUSIONS

The second extracellular loop of the beta 1-adrenoceptor is a specific target for antibodies with stimulatory activity detected in patients with idiopathic DCM. The antibodies have a positive chronotropic effect on isolated rat heart myocytes. Autoantibody stimulation does not cause the normal agonist-induced desensitization phenomena of the effector system. These findings could contribute to our understanding of the pathophysiological mechanisms of the autoantibodies and of the beneficial effect of beta 1-blocking agents in the treatment of patients with idiopathic DCM.

Antibodies to beta-adrenergic receptors disclosing agonist-like properties in idiopathic dilated cardiomyopathy and Chagas' heart disease

Recent studies confirm the existence of antibodies (Abs) to beta-adrenoceptors in patients with idiopathic dilated cardiomyopathy and Chagas' heart disease. These Abs can be shown to exert both stimulatory and inhibitory effects, which may play a role in the development of the cardiac abnormalities known to occur in these diseases, including advanced heart failure. The hypothesis is advanced that Chagas' heart disease and some forms of idiopathic dilated cardiomyopathy may represent, at least partially, a form of "adrenergic cardiomyopathy."

Production of anti-peptide antibodies directed against the first and the second extracellular loop of the human serotonin 5-HT1A receptor

The second extracellular loop of the beta-adrenergic and muscarinic acetylcholine receptors was shown to be an autoimmune target for antibodies in several autoimmune diseases. These autoantibodies and the antibodies induced against synthetic peptides corresponding to this loop have pharmacological and physiological properties upon receptor recognition which could explain their pathophysiological role. We here describe the immune properties of the first and second extracellular loops of another G protein-coupled receptor, the serotonin 5-HT1A receptor. The injection in rabbits of the free peptides Y16L and G21G corresponding to the first and second extracellular loops respectively induced anti-peptide antibodies with high titer, demonstrating the presence of a T-cell epitope on each peptide. Interestingly, in contrast to the G21G peptide that induced only anti-G21G antibodies (Ab-2 antibodies), the Y16L peptide induced two populations of antibodies. One recognized only the Y16L peptide (Ab-1 antibodies), the other recognized both peptides (Ab-12 antibodies). This reflects the presence on the two peptides of at least two B-cell epitopes. The fact that the G21G peptide induces only one antibody population might indicate that it possesses one immunodominant epitope involved in the Ab-2 antibody production and one cryptic epitope involved in the cross-reaction with the anti-Y16L antibodies. But only Ab-2 antibodies were able to recognize specifically the human protein receptor expressed in E coli in immunoblot.

Epitope analysis of T- and B-cell response against the human beta 1-adrenoceptor

Several reports have recently raised the possible significance of the presence of autoantibodies against the beta 1-adrenoceptor in patients with idiopathic dilated cardiomyopathy. An investigation was thus initiated to study the immune response against this receptor at the T-cell and the B-cell level. Using membranes of E coli transfected with the human beta 1-adrenoceptor gene as immunogen, T-helper cells of the immunized mice were stimulated with synthetic peptides derived from the receptor and predicted to be immunogenic to assess the T-cell immunodominant regions of the receptor. Three peptides derived from the second transmembrane region, from the second extracellular loop and from the C-terminal domain were shown to be stimulatory. Synthetic peptides, derived from two domains of the receptor which could be potential targets for autoantibodies, yielded an antibody response after immunization with the free peptides. The peptide derived from the N-terminal region yielded antibodies which recognized the receptor in immunoblot and by immunoprecipitation but they had no functional effect on the receptor. The peptide derived from the second extracellular loop yielded antibodies which recognized the receptor in immunoblot and by immunoprecipitation of the free receptor and which had a pharmacological effect on the receptor. The second extracellular loop thus contains T- and B-cell epitopes which could be involved in the autoimmune process.

Localization of a functional autoimmune epitope on the muscarinic acetylcholine receptor-2 in patients with idiopathic dilated cardiomyopathy

A peptide corresponding to the sequence 169-193 of the second extracellular loop of the human muscarinic acetylcholine receptor-2 was used as an antigen to screen sera from patients with idiopathic dilated cardiomyopathy (DCM, n = 36) and healthy blood donors (HBD, n = 40). The sera from 14 patients with DCM (38.8%) and 3 HBD (7.5%) recognized the muscarinic receptor peptide at dilutions varying from 1:20 to 1:160 in ELISA. A highly significant correlation (P = 0.006) was found between the presence of antimuscarinic receptor-2 autoantibodies and anti-beta-adrenoceptor-1 autoantibodies in the patients' sera. Affinity-purified autoantibodies from positive sera of patients with DCM recognized on the electrotransferred protein of rat ventricular membrane a major band of about 80 kD. Incubation of autoantibodies with membrane resulted not only in a decrease in the maximal binding sites (Bmax) but also in an increase in Kd of radioligand binding in a concentration-dependent manner. This suggests a mixed-type of inhibition. Moreover, preincubation with atropine abolished the inhibitory effect of autoantibodies on the receptor binding whereas carbachol appeared to have no effect on the activity of the autoantibodies. These data define a subgroup of patients with idiopathic DCM who have in their sera functionally active autoantibodies against muscarinic receptor-2.

Induction of a pharmacologically active clonotypic B cell response directed to an immunogenic region of the human beta 2-adrenergic receptor

It has been reported that autoantibodies against the beta 2-adrenergic receptors are involved in the pathology of allergic disorders and of Chagas' disease. Therefore, the immune response against a peptide (H26Q) corresponding to the putative second extracellular loop of the human beta 2-adrenergic receptor, which could be a target for autoantibody attack, was analysed in view of its possible immunogenicity. The free peptide induced a T cell-mediated humoral response in the context of three different murine MHC haplotypes. The T cell epitope was found to be localized in the N-terminal region of the peptide. Highly specific T helper cells were capable of stimulating B cells with the potential to generate a large antibody repertoire reactive with the loop peptide. MoAbs were screened to analyse this B cell response for antibodies potentially interfering with receptor function and a MoAb was found that impaired ligand binding to the receptor.