Genetic analysis of autoantibodies in idiopathic thrombocytopenic purpura reveals evidence of clonal expansion and somatic mutation

[Pathophysiology of immune thrombocytopenia]

Immune thrombocytopenia is an autoimmune disease characterized by a peripheral destruction of platelets. B lymphocytes play a key role but pathogenesis is more complex, involving humoral and cellular immunity associated with an inappropriate platelet production. In this article, we review the different pathogenic pathways, leading to new therapeutic strategies.

Update on the cloning of monoclonal anti-desmoglein antibodies from human pemphigus patients: implications for targeted therapy

Autoantibodies in pemphigus foliaceus (PF) and vulgaris (PV) bind to desmoglein (Dsg) 1 and 3, respectively, and cause loss of keratinocyte adhesion. To characterize the pathogenicity and genetics of such antibodies we have used phage display to isolate monoclonal antibodies (mAbs) from patients. PCR is used to clone the heavy and light chain variable region of the peripheral B cells into a vector that creates a phage particle with the antibody expressed on its surface and the cDNA encoding that antibody inside. The library of phage produced from a PF or PV patient are then panned on a plate containing Dsg1 or Dsg3 to isolate clones. The cDNA of each clone is sequenced to characterize the genetics of the expressed mAb. The mAb from each unique clone is tested for pathogenicity either by injecting into normal human skin organ culture or into neonatal mice. Pathogenic antibodies cause typical pemphigus blisters. In both PV and PF patients the heavy chain (VH) genes used for Dsg-binding antibodies are severely restricted. PV and PF patients have both pathogenic and non-pathogenic mAbs. The immunochemical characteristics of the antibodies (including pathogenicity) sort with the VH, not the VL, gene. These monoclonal pathogenic antibodies can be used to screen peptide libraries to find short peptides that block antibody binding. In summary, the antibody response is restricted and, therefore, it may be feasible to target the specific pathogenic antibodies for therapy.

Characteristics of immune thrombocytopenic purpura: a guide for clinical practice

Immune (idiopathic) thrombocytopenic purpura (ITP) is an autoantibody-mediated condition characterized by an abnormally low number of platelets in the circulating blood. Originally, the cause of ITP was attributed to accelerated antibody-mediated platelet destruction where the rate of thrombopoiesis was inadequate to offset the increased rate of platelet destruction. However, new evidence has indicated that insufficient or inadequate platelet production is also responsible for low platelet counts, and research has focused on the development of treatments that increase platelet production. ITP affects both children and adults and can be either acute or chronic. To manage and treat ITP effectively, an exhaustive assessment of the signs and symptoms must be undertaken because the clinical manifestation of ITP can be highly variable among patients. At the moment, the diagnostic approach in ITP is based largely on a process of exclusion due to the lack of available data regarding clinical and laboratory parameters. The diagnostic procedures used in children and adults are similar and involve collecting the patient's history and performing a physical examination. Laboratory investigations are kept simple in patients with suspected ITP and include a full blood count and peripheral blood smear. A number of specialized laboratory assays have been developed with varying degrees of success. There remains scope for improving and simplifying the diagnostic process to identify ITP.

Charges drive selection of specific antibodies by phage display

Phage display technology has emerged as a leading approach to select proteins with improved properties for many different types of applications. The selection typically selects not only for improved binding properties but also for other factors such as efficiency of protein production and folding in Escherichia coli, the host in which the proteins and the phage are produced. Furthermore, the selection methodology is likely to influence the character of retrieved variants. We have now defined the extent whereby the charge of the displayed proteins influence the selection process, resulting in an increased average positive charge among selected proteins in comparison to the proteins that are harbored in the library before selection. Implications of and possible routes to minimize this effect are discussed.

The geoepidemiology of immune thrombocytopenic purpura

First described in 1735 (Watson-Williams et al., 1958), immune-mediated platelet destruction is a phenomenon of protean associations that has historically varied in its definition. Recently, consensus guidelines were proposed for a standardized system of nomenclature that preserves the acronym "ITP" but encompasses a number of causes of immune-mediated thrombocytopenias, including both primary immune thrombocytopenia as well as such entities as thrombocytopenia associated with connective tissue diseases or cancer. In this paper, we will focus on current aspects of geoepidemiology, pathophysiology, diagnosis and management of adult and pediatric primary immune thrombocytopenia. It is clear that both genetic and extrinsic factors exist for ITP and are likely different between children and adults. Immune thrombocytopenia remains a challenging problem but our understanding of its pathophysiology has greatly improved.

Analysis of B- and T-cell clonality and HLA class II alleles in patients with idiopathic thrombocytopenic purpura: Correlation withHelicobacter pyloriinfection and response to eradication treatment

Many authors have recently found a positive correlation between Helicobacter pylori infection and idiopathic thrombocytopenic purpura (ITP), the most common autoimmune hematological disorder. In order to clarify the pathogenic mechanism of H. pylori-associated ITP, we have investigated 52 consecutive ITP adult patients for Helicobacter pylori infection, B- and T-cell clonality and HLA class II alleles. Thirty-four ITP patients (65.4%) were infected by H. pylori and bacterium eradication was accompanied by a long-term platelet response in 17 (53.1%) of them. A B-cell clonality was found in three patients (5.8%, two patients H. pylori-negative and one patient H. pylori-positive). The ITP patients with H. pylori infection showed a HLA-DRB1*11, *14 and -DQB1*03 frequencies significantly higher and a -DRB1*03 frequency significantly lower than in H. pylori-negative patients. Moreover, an HLA-DQB1*03 pattern was associated with a higher probability of platelet response to eradication treatment. If our study documents the efficacy of eradication treatment in H. pylori-infected ITP patients, it may also help to identify different subgroups of ITP patients with probably different pathogeneses of thrombocytopenia and, finally, different responses to eradication treatment.

Pathobiology of secondary immune thrombocytopenia

Primary immune thrombocytopenic purpura (ITP) remains a diagnosis of exclusion both from nonimmune causes of thrombocytopenia and immune thrombocytopenia that develops in the context of other disorders (secondary immune thrombocytopenia). The pathobiology, natural history, and response to therapy of the diverse causes of secondary ITP differ from each other and from primary ITP, so accurate diagnosis is essential. Immune thrombocytopenia can be secondary to medications or to a concurrent disease, such as an autoimmune condition (eg, systemic lupus erythematosus [SLE], antiphospholipid antibody syndrome [APS], immune thyroid disease, or Evans syndrome), a lymphoproliferative disease (eg, chronic lymphocytic leukemia or large granular T-lymphocyte lymphocytic leukemia), or chronic infection, eg, with Helicobacter pylori, human immunodeficiency virus (HIV), or hepatitis C virus (HCV). Response to infection may generate antibodies that cross-react with platelet antigens (HIV, H pylori) or immune complexes that bind to platelet Fcγ receptors (HCV), and platelet production may be impaired by infection of megakaryocyte (MK) bone marrow–dependent progenitor cells (HCV and HIV), decreased production of thrombopoietin (TPO), and splenic sequestration of platelets secondary to portal hypertension (HCV). Sudden and severe onset of thrombocytopenia has been observed in children after vaccination for measles, mumps, and rubella or natural viral infections, including Epstein-Barr virus, cytomegalovirus, and varicella zoster virus. This thrombocytopenia may be caused by cross-reacting antibodies and closely mimics acute ITP of childhood. Proper diagnosis and treatment of the underlying disorder, where necessary, play an important role in patient management.

The ITP syndrome: pathogenic and clinical diversity

Immune thrombocytopenia (ITP) is mediated by platelet autoantibodies that accelerate platelet destruction and inhibit their production. Most cases are considered idiopathic, whereas others are secondary to coexisting conditions. Insights from secondary forms suggest that the proclivity to develop platelet-reactive antibodies arises through diverse mechanisms. Variability in natural history and response to therapy suggests that primary ITP is also heterogeneous. Certain cases may be secondary to persistent, sometimes inapparent, infections, accompanied by coexisting antibodies that influence outcome. Alternatively, underlying immune deficiencies may emerge. In addition, environmental and genetic factors may impact platelet turnover, propensity to bleed, and response to ITP-directed therapy. We review the pathophysiology of several common secondary forms of ITP. We suggest that primary ITP is also best thought of as an autoimmune syndrome. Better understanding of pathogenesis and tolerance checkpoint defects leading to autoantibody formation may facilitate patient-specific approaches to diagnosis and management.

The broad antibacterial activity of the natural antibody repertoire is due to polyreactive antibodies

Polyreactive antibodies bind to a variety of structurally unrelated antigens. The function of these antibodies, however, has remained an enigma, and because of their low binding affinity their biological relevance has been questioned. Using a panel of monoclonal polyreactive antibodies, we showed that these antibodies can bind to both Gram-negative and Gram-positive bacteria and acting through the classical complement pathway can inhibit bacterial growth by lysis, generate anaphylatoxin C5a, enhance phagocytosis, and neutralize the functional activity of endotoxin. Polyreactive antibody-enriched, but not polyreactive antibody-reduced, IgM prepared from normal human serum displays antibacterial activity similar to that of monoclonal polyreactive IgM. We conclude that polyreactive antibodies are a major contributor to the broad antibacterial activity of the natural antibody repertoire.

Light-chain-restricted germinal centres in reactive lymphadenitis: report of eight cases

AIMS

Light-chain-restricted germinal centres are generally associated with the existence of a neoplastic lymphoproliferative disorder. The aim was to present a series of cases with persistent lymph node enlargement that featured some germinal centres showing light chain immunoglobulin restriction.

METHODS AND RESULTS

A series of six reactive lymphadenitis and two Castleman's disease cases was analysed by immunohistochemistry, IgH-polymerase chain reaction (PCR) and microdissected PCR. In all cases some germinal centres contained a population of plasma cells and plasmacytoid germinal centre cells showing light chain immunoglobulin restriction. In three cases the monotypic cells also showed distinct Bcl-2 expression. Two of the cases showed a predominant IgH rearrangement on a florid polyclonal background and one had an IgH monoclonal rearrangement, as revealed by PCR. Microdissected germinal centre PCR revealed a dominant repeated band in one of three cases and in another case a non-repeated clonal peak was observed. One of the patients developed a follicular lymphoma, which became evident from a subsequent biopsy.

CONCLUSIONS

These findings may be a manifestation of an underlying disorder in the regulation of the immune response, or an exaggeration of the germinal centre oligoclonal nature. This should be taken into account in the differential diagnosis of follicular hyperplasia.

The pathogenesis of chronic immune thrombocytopenic purpura

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder in which the patient's immune system reacts with a platelet autoantigen(s) resulting in thrombocytopenia due to immune-mediated platelet destruction and/or suppression of platelet production. Platelet membrane proteins, for reasons that are unclear, become antigenic and stimulate the immune system to produce autoantibodies and cytotoxic T cells. The initial antigenic response probably occurs in the spleen followed by stimulation of other antibody-producing tissues, particularly the bone marrow. Autoantibodies against platelet glycoprotein (GP) IIb-IIIa and/or GPIb-IX are produced by the majority of ITP patients and can be detected using antigen-specific assays. Many patients produce multiple antibodies; this has been attributed to the phenomenon of epitope spreading. Once produced, autoantibody may either bind to platelets, causing their destruction by either phagocytosis or possibly complement activation and lysis, or bind to megakaryocytes, resulting in decreased thrombopoiesis. Evidence for platelet destruction in ITP includes the following: (1) infusion of ITP blood or plasma into normal recipients may result in thrombocytopenia; (2) there is decreased intravascular survival of radiolabeled platelets in most ITP patients; (3) morphologic and in vitro evidence of platelet phagocytosis can be demonstrated; and (4) cytotoxic T cells can induce lysis of autologous platelets. Evidence for suppressed platelet production in ITP includes the following: (1) morphologic studies show megakaryocyte damage in most ITP patients; (2) there is normal or decreased platelet turnover in the majority of patients; (3) in vitro studies show antibody-induced inhibition of megakaryocyte production and maturation; and (4) an increase in the platelet count occurs in many ITP patients receiving treatment with thrombopoietin mimetics. In summary, activation of the immune system by platelet autoantigens in ITP may result in platelet destruction and/or inhibition of platelet production. The importance of each mechanism in the individual patient probably varies.

Pathogenesis of chronic immune thrombocytopenic purpura

PURPOSE OF REVIEW

This article summarizes recent insights into the pathophysiology of immune thrombocytopenic purpura, a disorder in which autoantibodies against cell-specific glycoproteins (GPIIb-IIIa, GPIb-IX and others) accelerate platelet destruction.

RECENT FINDINGS

Autoantibodies are produced by a limited number of B-cell clones. Platelet antibodies may also impair megakaryocyte development and platelet turnover, thromobopoietin levels are normal or only modestly increased and a compensatory increase in platelet production is not effective in many patients. Patients may show impaired immune regulation manifested by increased proliferation of helper T lymphocytes. Cytotoxic T lymphocytes from patients can lyse platelets in vitro. If cytotoxic T lymphocytes are also capable of perturbing megakaryocyte function, this mechanism may contribute to impaired platelet production. Polymorphisms in the Fcgamma-RIIIa gene may correlate with response to certain forms of therapy and similar genetic approaches may help to identify subsets of patients that differ in their natural history and response to various interventions.

SUMMARY

Better understanding of autoantibody development, inhibition of thrombopoiesis and Fcgamma receptor and other polymorphisms will assume increased importance in elucidating the pathogenesis and targeting treatment of chronic immune thrombocytopenic purpura.

Isolation of pathogenic monoclonal anti-desmoglein 1 human antibodies by phage display of pemphigus foliaceus autoantibodies

Pemphigus foliaceus (PF) is a blistering disease caused by autoantibodies to desmoglein 1 (Dsg1) that cause loss of epidermal cell adhesion. To better understand PF pathophysiology, we used phage display to isolate anti-Dsg1 mAbs as single-chain variable fragments (scFvs) from a PF patient. Initial panning of the library isolated only non-pathogenic scFvs. We then used these scFvs to block non-pathogenic epitopes and were able to isolate two unique scFvs, each of which caused typical PF blisters in mice or human epidermis models, showing that a single mAb can disrupt Dsg1 function to cause disease. Both pathogenic scFvs bound conformational epitopes in the N terminus of Dsg1. Other PF sera showed a major antibody response against the same or nearby epitopes defined by these pathogenic scFvs. Finally, we showed restriction of the heavy-chain gene usage of all anti-Dsg1 clones to only five genes, which determined their immunological properties despite promiscuous light-chain gene usage. These mAbs will be useful for studying Dsg1 function and mechanisms of blister formation in PF and for developing targeted therapies and tools to monitor disease activity.

Targeting Pemphigus Autoantibodies through their Heavy-Chain Variable Region Genes

Pemphigus vulgaris (PV) is a potentially fatal blistering disease characterized by autoantibodies against cell surface adhesion proteins desmoglein (Dsg) 3 and Dsg1. Previous studies using phage display to clone Dsg-reactive monoclonal antibodies from a PV patient demonstrated that a limited number of antibody variable region genes encode the autoantibody repertoire, with different genes for pathogenic and non-pathogenic mAbs. Here, we investigated the feasibility of specific autoantibody targeting in pemphigus. We produced rabbit anti-idiotypic antibodies against two pathogenic and two non-pathogenic PV mAbs. Antisera inhibited binding of the immunizing mAb to Dsgs by ELISA as well as pathogenicity against cultured human keratinocytes. Antisera also inhibited other mAbs using the same variable region heavy chain (V(H)) genes, despite different light chains or somatic mutations. Additionally, peptide phage display identified peptide sequences that bound PV mAbs in a V(H)-specific manner. To evaluate the therapeutic potential of V(H) gene-targeted reagents, preimmune sera and antisera were used to adsorb pathogenic antibodies from PV sera. Pooled antisera significantly reduced pathogenic activity from the original PV patient's serum and bound pathogenic antibodies from two other PV sera, suggesting shared autoantibody V(H) gene usage among PV patients. Together, these data suggest novel V(H) gene-targeted approaches toward PV treatment.

Molecular characterization of the variable domains of an ?IIb?3-specific immunoglobulin�M ? platelet cold agglutinin in a follicular lymphoma patient with treatment refractory autoimmune thrombocytopenia: idiotypic overlap between ?IIb?3 integrin antibodies

BACKGROUND

Cold hemagglutinins are generally immunoglobulin M (IgM) kappa antibodies reactive at temperatures below 37 degrees C and if of high titer may cause hemolysis. Platelet (PLT) cold agglutinins (CAs) are rare and poorly characterized. A detailed molecular characterization of the variable domains of a pathologic, PLT-reactive, CA is presented.

CASE REPORT

A 70-year-old woman was admitted with rectal bleeding accompanied by widespread petechiae, bruising, tongue and buccal mucosa bleeding, and epistaxes and proved refractory to HLA- and HPA-matched PLTs. Detailed investigation showed monoclonal heavy-chain gene rearrangement with an IgM paraprotein of 3.3 g per L and a trace of kappa Bence Jones protein in the urine, compatible with a diagnosis of secretory B-cell non-Hodgkin's lymphoma (B-NHL). PLT antibody (PAIg) investigations revealed a potent IgM kappa PLT CA. Sequencing of the rearranged variable domain genes of the malignant clone together with idiotype-specific antibodies obtained by DNA-based immunization of rabbits and matrix-assisted laser desorption/ionization-time-of-flight analysis of the PAIgM provided a irrefutable link between the thrombocytopenia, the IgM paraprotein, and the PAIgM against alphaIIbbeta3. The thrombocytopenia and bleeding were refractory to standard treatment and PLT transfusion, but treatment with rituximab resulted in a recovery of the PLT count and a complete remission of B-NHL.

CONCLUSION

The IgM kappa paraprotein derived from the malignant B-cell clone was a potent and clinically significant CA against alphaIIbbeta3. The testing for PLT CAs in patients with a paraprotein and refractory to matched PLTs may aid the selection of appropriate treatment.

B cells in autoimmune diseases: insights from analyses of immunoglobulin variable (Ig V) gene usage

The role of B cells in autoimmune diseases has not been fully elucidated. It is also unclear whether breaking of B cell tolerance in patients with autoimmune diseases is due to underlying defects in the molecular mechanisms involved in the arrangement of antibody genes or deficiencies in the subsequent selective influences that shape the antibody repertoire. Analysis of immunoglobulin (Ig) variable (V) gene usage is beginning to provide answers to some of these questions. Such analyses have identified some differences in the basic Ig V gene repertoire of patients with autoimmune diseases compared to healthy controls, even though none of these differences can be considered major. Defects in positive and negative selection, mutational targeting and, in some cases, receptor editing have also been detected. In addition, analysis of Ig V gene usage in target organs and tissues of patients with autoimmune diseases has clearly demonstrated that there is a highly compartmentalized clonal expansion of B cells driven by a limited number of antigens in these tissues. Great progress has been made in the structural and functional characterization of disease-associated antibodies, largely because of the development of the combinatorial library technique. Use of antibodies generated by this technique offers great promise in identifying B cell epitopes on known target antigens and in gaining greater insights into the pathogenic role of B cells in both B and T cell mediated autoimmune diseases.

Serum leptin levels in patients with childhood immune thrombocytopenic purpura

Acute immune thrombocytopenic purpura (ITP) induces thrombocytopenia by means of an autoimmune mechanism. Recent studies suggested that T helper immune response is responsible for the pathogenesis of chronic ITP. Despite several studies that were carried out, we do not have a clue as to what triggers the autoimmunity. Leptin is a 16-kd protein secreted from the adipose tissue. Leptin is structurally similar to interleukin (IL)-2, IL-6, and IL-15. The structural similarities between leptin receptor and hematopoietic cytokine receptors suggested that leptin could play a role in hematopoiesis and immune function. Recent studies suggested that leptin could play an important role in autoimmunity. We made a prospective analysis of a series of 39 newly diagnosed acute childhood ITP in a year period. Serum leptin levels were obtained after diagnosis and before treatment and all patients were followed up at least 6 months to designate acute or chronic event. We conclude that in childhood acute ITP, leptin did not play a role in the pathophysiology of the disease. Further investigations are needed to examine what triggers T cells and how the autoimmune disease became.

The analysis and quantification of a clonal B cell response in a hyperimmunized anti-D donor

Healthy volunteers are hyperimmunized with RhD-positive red cells in order to obtain plasma containing high titres of anti-D immunoglobulin, which is used for the prevention of haemolytic disease of the fetus and newborn. We analysed the anti-D immune response in a donor who had been hyperimmunized for 7 years and who showed declining anti-D titres despite re-immunization. A phage display library representing the complete immunorepertoire and a second library representing the IGHV3 superspecies family genes (IGHV3s) repertoire in the donor were constructed and analysed. A clonal Ig-gene rearrangement was quantified in the peripheral blood by limiting dilution polymerase chain reaction (PCR) All RhD-binding phages from both libraries, except one, had heavy chains with IGH-VDJ rearrangements of the same clonal origin, but with different patterns of somatic mutations and joined with different light chains. Limiting dilution PCR performed on mRNA and genomic DNA showed a frequency of 1 clonal B cell in 2000 IgG1/3-positive B cells. We show the presence of clonally related RhD-specific B cells in a hyperimmunized anti-D donor who had declining anti-D titres and who was unresponsive to re-immunization. Furthermore, we found a high frequency of clonal B cells. These results contribute to the understanding of the immune response against RhD in hyperimmunized anti-D donors.

Confounding B-cell defences: lessons from a staphylococcal superantigen

Studies of microbial superantigens that target large clonal sets of B cells through conserved antigen-receptor-variable-region sites are providing new insights into the mechanisms of B-cell activation-induced cell death. These investigations have shown differences between the clonal regulation of follicular B cells (B2 cells) and the innate-like marginal-zone B cells and B1 cells, and have also shown how B-cell superantigens can affect specialized host defences against infection. Agents designed to emulate the properties of B-cell superantigens might also provide new approaches for the treatment of B-cell-mediated autoimmune and neoplastic diseases.

The pathogenesis of immune thrombocytopaenic purpura

Immune thrombocytopaenic purpura (ITP) is an autoimmune bleeding disease that is rarely fatal. However, in many adults treatment is unsatisfactory, with as much morbidity from the immunosuppressive effects of treatment as from bleeding. Identifying the underlying disease process should help us to identify more targeted therapies and improve not only the treatment but also the quality of life of patients with this disorder.

Helicobacter pylori-associated immune thrombocytopenia

Idiopatic thrombocytopenic purpura (ITP), a disorder characterized by autoantibody-mediated platelet destruction, may be primary or secondary to various illnesses including lymphoproliferative, autoimmune, or infectious diseases. There are increasing data on the association between Helicobacter pylori infection and idiopathic thrombocytopenic purpura and the significant increase in platelet count after bacterial eradication. The aim of this review is to consider the studies so far published on Helicobacter pylori infection and idiopathic thrombocytopenic purpura in order to evaluate a possible pathogenic correlation between these two conditions. A review of the literature data show that Helicobacter pylori eradication in patients with idiopathic thrombocytopenic purpura is effective in increasing platelet count in approximately half of the cases. However, since the studies so far published are few, sometimes controversial and involve small series of patients, further controlled studies on larger numbers of patients with longer follow-up are needed to confirm these preliminary findings.

Immune thrombocytopenia following successful treatment of cancer in children

A predisposition to developing immune thrombocytopenia (ITP) has not been reported in survivors of childhood cancer. We report a case series of childhood cancer survivors who developed an isolated thrombocytopenia in the presence of a normocellular bone marrow. Five children, two with endodermal sinus tumors and three with acute lymphoblastic leukemia, developed ITP at a median of 4 years (range: 0.2-8 years) after completion of therapy. We suggest the association of ITP in survivors of childhood malignancy may not be co-incidental as chemotherapy may cause persistent immune dysfunction.

Immune Thrombocytopenic Purpura of Childhood

Immune mediated thrombocytopenia (ITP) is a common manifestation of autoimmune disease in children. Although patients often present with bruises, petechiae, and some mucosal bleeding, the incidence of life-threatening hemorrhage is rare (0.2–0.9%) but can be fatal when presenting in vital organs. A wide range of therapeutic regimens are currently in use, including observation alone, as the majority of children recover within 4–6 months regardless of treatment. A growing understanding of the pathophysiology of acute ITP in children has not impacted the controversy surrounding treatment, but has clarified the mechanism of action of the most frequently used agents in chronic ITP. Newer monoclonal antibodies such as Rituxan have proved very useful in chronic or refractory ITP and studies are ongoing to determine the best regimens using this form of immune modulation. Splenectomy and newer agents to boost platelet production are also under study in chronic ITP. Neonates may also have a form of immune thrombocytopenia with extensive bruising and thrombocytopenia called neonatal alloimmune thrombocytopenic purpura (NATP). Rather than autoantibodies, the platelet destruction is secondary to transplacental maternal IgG alloantibodies. During pregnancy mothers may become sensitized to platelet membrane antigens present on fetal platelets. These antibodies may result in serious bleeding, including intracranial hemorrhage in the perinatal period. Once identified, these mothers may require treatment during future pregnancies to minimize serious bleeding in the fetus and neonate. Treatment in utero and immediately following delivery is focused on restoring neonatal platelets to a safe level and preventing life-threatening bleeding.

Evidence that human autoimmune thrombocytopenia mediated by both immunoglobulin isotypes IgM and IgG is an independent disease entity

Autoimmune thrombocytopenic purpura (AITP) is a bleeding disorder caused by clonally restricted self-reactive antibodies with specificity for platelet glycoproteins. Anti-platelet autoantibodies in AITP mainly belong to the IgG class. The occurrence of anti-platelet autoantibodies of the IgM isotype has been reported, and AITP is partially mediated by antibodies of both isotypes, IgM and IgG. Using a technique of quantitative immunoblotting of immunoglobulins on self-tissues, followed by multiparametric statistical analysis of the data, we here demonstrate that patients with IgM- and IgG-mediated AITP are readily discriminated from patients with IgM-mediated AITP as well as from patients with IgG-mediated AITP at the basis of self-reactive antibody repertoires of isotypes IgM and IgG toward non-platelet antigens of human origin. Our data suggest that, in view of the important physiological functions of self-reactive antibody repertoires, human AITP mediated by both immunoglobulin isotypes IgG and IgM may be an independent disease entity. The role of autoantibody isotype for the pathophysiology of AITP might currently be underestimated, and diagnostic and therapeutic procedures in AITP might profit from considering autoantibody isotype more carefully.

B cell superantigens: a microbe's answer to innate-like B cells and natural antibodies

Marginal zone B cells and B-1 cells have been termed innate-like B cells as they express limited repertoires that play special roles in immune defenses against common infections. These B cells are the sources of natural antibodies and are capable of highly accelerated clonal responses that help counter blood-borne infections. We have characterized a class of microbial product with highly adapted binding interactions with host immunoglobulins/B cell receptors (BCRs), which enable the targeting of large supra-clonal sets of B cells for activation-associated apoptotic death. In recent studies, we have shown that all B cells with V region-targeted BCRs are susceptible. However, compared to follicular B cells, in vivo exposure preferentially causes innate-like B cells to undergo induced death with subsequent long-lasting supra-clonal depletion and immune tolerance. Based on these properties, it is likely that B cell superantigens influence the pathogenesis of some common infections, but also may provide novel therapeutic opportunities to treat B cell neoplastic and autoimmune diseases.

Autoimmune thrombocytopenia

Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder in which platelets coated with mainly antibodies against platelet GPIIb/IIIa and GPIb/IX are destroyed in the spleen. Recent evidence suggests that platelets are also destroyed by cytotoxic T cells. The diagnosis is made by exclusion for other causes of thrombocytopenia. As routine blood counts are becoming more available, many mild cases of ITP (platelets >30 x 10(9) L(-1)) are being diagnosed and they usually do not require treatment. In patients with platelet counts persistently <30 x 10(9) L(-1), treatment with corticosteroids, and/or intravenous immunoglobulin (IVIG) or anti-D may be required. The primary goal of treatment is to maintain the platelet count at a safe level with minimal side effects. After 3-6 months, if spontaneous remission has not occurred and if the side effects are significant, splenectomy is recommended. This is the single most effective treatment of ITP. The refractory patients who fails splenectomy and subsequently first- and second-line therapies, is a management dilemma. Therapeutic options are limited, available treatments potentially toxic and the chances of sustained response low. Observation with no active treatment is a reasonable option. With the increased availability of the thrombopoietic agents in the future, there may be a good prospect of keeping the platelet counts of these refractory patients at a safe long-term level with one of these drugs.

Genetic and functional characterization of human pemphigus vulgaris monoclonal autoantibodies isolated by phage display

Pemphigus is a life-threatening blistering disorder of the skin and mucous membranes caused by pathogenic autoantibodies to desmosomal adhesion proteins desmoglein 3 (Dsg3) and Dsg1. Mechanisms of antibody pathogenicity are difficult to characterize using polyclonal patient sera. Using antibody phage display, we have isolated repertoires of human anti-Dsg mAbs as single-chain variable-region fragments (scFvs) from a patient with active mucocutaneous pemphigus vulgaris. ScFv mAbs demonstrated binding to Dsg3 or Dsg1 alone, or both Dsg3 and Dsg1. Inhibition ELISA showed that the epitopes defined by these scFvs are blocked by autoantibodies from multiple pemphigus patients. Injection of scFvs into neonatal mice identified 2 pathogenic scFvs that caused blisters histologically similar to those observed in pemphigus patients. Similarly, these 2 scFvs, but not others, induced cell sheet dissociation of cultured human keratinocytes, indicating that both pathogenic and nonpathogenic antibodies were isolated. Genetic analysis of these mAbs showed restricted patterns of heavy and light chain gene usage, which were distinct for scFvs with different desmoglein-binding specificities. Detailed characterization of these pemphigus mAbs should lead to a better understanding of the immunopathogenesis of disease and to more specifically targeted therapeutic approaches.

Multi-dysfunctional pathophysiology in ITP

Idiopathic thrombocytopenic purpura (ITP) is an organ-specific autoimmune disorder characterized by a low platelet count and mucocutaneous bleeding. The decrease of platelets is caused by increased autoantibodies against self-antigens, particularly IgG antibodies against GPIIb/IIIa. The production of these autoantibodies by B cells depends on a number of cellular mechanisms that form a network of modulation, with T cells playing a pivotal role in pathophysiology. Delineation of the dysfunction of cellular immunity has recently been attempted. This review will focus on these recent advances applicable to ITP and to highlight how these may translate into novel approaches to treatment in the future. Multi-dysfunction in these networks may include a failure of self-antigen recognition and tolerance, involvement of abnormal cell surface molecules, altered Th1/Th2 cytokine profiles, impaired megakaryocytopoiesis and impaired cell-mediated cytotoxicity. In ITP, multi-step dysfunctions in these networks may take place that finally lead to the occurrence of the disease. Therefore, unveiling these dysfunctions is vital in understanding the pathophysiology of ITP and will finally lead to the development of new therapies to fight the disease.

Genetically recombinant antibodies: new therapeutics against candidiasis

Historically, the therapy of serious fungal infection has been dominated by monotherapy with the polyene antibiotic amphotericin B. Clinical failures, side effects, the lack of alternatives and the toxicity of this drug have heightened the need to produce alternative therapies, which have included fluconazole, voriconazole and caspofungin. The observation that recovery from disseminated candidiasis was associated with an antibody response to the 47 kDa Candida heat-shock protein (HSP)90 homologue, coupled with the ability to sequence all the antibodies from patients who have recovered from the infection and to re-express the dominant ones as fragments in Escherichia coli, has opened the possibility of immunotherapy. The first recombinant antibody fragment, Mycograb (Neu Tec Pharma plc), against Candida HSP90 is now in clinical trials in patients with disseminated candidiasis in Europe and the US. Laboratory and early clinical data support the concept of synergy between Mycograb and amphotericin B. This should improve outcome and diminish the risk of resistance occurring to either drug, without an increase in toxicity, as this should be minimal in a human antibody fragment representing the natural antibody that a patient produces on recovery.

The Role of Autoreactive T-Cells in the Pathogenesis of Idiopathic Thrombocytopenic Purpura

Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disease mediated by antiplatelet autoantibodies. The major target of these autoantibodies is a platelet membrane glycoprotein, GPIIb-IIIa, which is a receptor for fibrinogen and other ligands. We recently identified CD4+ T-cells autoreactive to GPIIb-IIIa in ITP patients. These T-cells are considered pathogenic because they help B-cells produce antibodies that bind to normal platelet surfaces. GPIIb-IIIa-reactive T-cells respond to chemically reduced and tryptic peptides of GPIIb-IIIa but not to native GPIIb-IIIa, indicating that the epitopes they recognize are "cryptic" determinants generated at a subthreshold level by the processing of native GPIIb-IIIa under normal circumstances. Although GPIIb-IIIa-reactive T-cells are also detected in healthy individuals, they are activated in vivo only in ITP patients. Activation of GPIIb-IIIa-specific T-cells and the subsequent production of pathogenic anti-GPIIb-IIIa antibodies can be induced by functional antigen-presenting cells in the spleen that present cryptic GPIIb-IIIa peptides to these T-cells. The pathogenic process of ITP can be explained as a continuous loop in which B-cells produce antiplatelet autoantibodies, splenic macrophages phagocytose antibody-coated platelets and present GPIIb-IIIa-derived cryptic peptides, and GPIIb-IIIa-reactive CD4+ T-cells exert their helper activity. Further studies examining the mechanisms that induce the processing and presentation of cryptic peptides derived from the platelet antigen at disease onset will clarify how the pathogenic autoantibody response in ITP is initiated.

Helicobacter pylori infection and immune thrombocytopenic purpura: an update

Data are accumulating on the association between Helicobacter pylori infection and idiopathic thrombocytopenic purpura (ITP) and the significant increase in platelet count after bacterial eradication. The aim of this review was to consider the studies so far published on H. pylori infection and ITP in order to evaluate a possible correlation between these two conditions. A review of the literature showed that 278 out of the 482 ITP patients investigated (58%) were positive for H. pylori infection and that the bacterium was eradicated in 88% of cases. Eradication therapy was accompanied by a complete or partial platelet response in approximately half the cases. Overall, these data show that H. pylori eradication in patients with ITP is effective in increasing platelet count. However, because the studies so far published are few, are sometimes controversial and involve small series of patients, further studies on larger numbers of patients with longer follow-up are needed to confirm these preliminary findings.

Pathogenic T-cell responses in patients with autoimmune thrombocytopenic purpura

Autoimmune thrombocytopenic purpura (AITP) is a bleeding disorder in which autoantibodies are directed against an individual's own platelets, leading to enhanced clearance through Fc receptor (R)-mediated phagocytosis by macrophages residing in the reticuloendothelial system, particularly in the spleen. This review surveys the recent current literature and updates our understanding of the cell-mediated immunology of AITP. It will focus on the relationship between T-cell reactivities and cytokine profiles in patients with AITP. Understanding these cellular immune aspects of AITP is vital for developing antigen-specific immunotherapies to treat the disease.

Mechanisms of action of therapeutics in idiopathic thrombocytopenic purpura

Idiopathic thrombocytopenic purpura (ITP) is a common immune disorder caused by platelet-reactive autoantibodies. Antibody-coated platelets are cleared more rapidly from the circulation, often in the spleen, than they can be replaced by compensatory stimulation of platelet production in the bone marrow. In some patients, platelet production is depressed as well. ITP in adults does not generally remit spontaneously, and most patients require treatment to prevent bleeding at one time or another. Therapy with corticosteroids, danazol, intravenous immune globulin, anti-D antibody, and several other agents inhibits clearance of the antibody-coated platelets but is rarely curative. Most patients will sustain a hemostatic response after splenectomy, although relapses may occur at any time. Patients may be more responsive to these same modalities after splenectomy, but treatment with an immunosuppressant that inhibits T- and B-cell function and cooperation, including azathioprine, cyclophosphamide, cyclosporine, mycophenolate mofetil, or anti-CD20, may be required. Antiviral therapy is useful in patients with HIV or hepatitis C infection, but no consensus has been reached as to the efficacy of antibiotics to eradicate Helicobacter pylori. Promising results have been seen in several patients treated with a modified thrombopoietin. It may be possible to design therapeutics that exploit the apparent restricted immunoglobulin gene usage by antiplatelet antibodies, perhaps in the form of engineered anti-idiotypic antibodies or other compounds that specifically target autoantibody-producing B cells. Rationale therapy awaits a more thorough understanding of autoantibody production.

Immunoglobulin gene rearrangement, repertoire diversity, and the allergic response

The immunoglobulin repertoire arises as a consequence of combinatorial diversity, junctional diversity, and the process of somatic point mutation. Each of these processes involves biases that limit and shape the available immunoglobulin repertoire. The expressed repertoire is further shaped by selection, to the extent that biased gene usage can become apparent in many disease states. The study of rearranged immunoglobulin genes therefore may not only provide insights into the molecular processes involved in the generation of antibody diversity but also inform us of pathogenic processes and perhaps identify particular lymphocyte clones as therapeutic targets. Partly as a consequence of the low numbers of circulating IgE-committed B-cells, studies of rearranged IgE genes in allergic individuals have commenced relatively recently. In this review, recent advances in our understanding of the processes of immunoglobulin gene rearrangement and somatic point mutation are described, and biases inherent to these processes are discussed. The evidence that some diseases may be associated with particular gene rearrangements is then considered, with a particular focus on allergic disease. Reviewed data suggest that an important contribution to the IgE response may come from cells that use relatively rare heavy chain V (V(H)) segment genes, which display little somatic point mutation. Some IgE antibodies also seem to display polyreactive binding. In other contexts, these 3 characteristics have been associated with antibodies of the B-1 B-cell subset, and the possibility that B-1 B-cells contribute to the allergic response is therefore considered.

Improving selection of αIIbβ3-binding phage antibodies with increased reactivity derived from immunized donors

Although many studies of the immune response in polytransfused Glanzmann thrombasthenia (GT) patients and in autoimmune thrombocytopenic purpura (AITP) have demonstrated the frequent development of Abs directed against the alphaIIbbeta3 integrin, little is known about the induced anti-alphaIIbbeta3 autoantibodies at the molecular level. Phage display is a powerful technology for selecting and engineering mAbs expressed on the surface of filamentous bacteriophage. Combinatorial libraries of single-chain IgG were constructed from splenocytes from two patients with AITP and one patient with GT. In a previous study, activated platelets or alphaIIbbeta3-expressing CHO cells selection was performed to isolate human IgG anti-alphaIIbbeta3 binding fragments using combinatorial libraries created from the B cells of a GT and an AITP patient. However, we have experienced practical problems such as enrichment of truncated antibodies during selection. We decided to test prolonged treatments with elution agents after screening on the purified form of the alphaIIbbeta3 integrin activated with the RGD peptide. We obtained a higher percentage of clones with full-size antibody fragments as well as an enrichment of more specific alphaIIbbeta3-binding phage-Abs. Some of them, recognizing the activated form of the integrin, would be interesting to further study as potential diagnostic or therapeutic agents in acute coronary syndromes. Sequencing of selected phage-Abs revealed that they used different VH and VL genes with, for the majority of them, a high level of extensive hypermutations in the complementarity determining regions, indicating the diversity of the antigen-driven immune response that occurred in GT and AITP patients.