Immunobiology of T helper cells and antigen-presenting cells in autoimmune thrombocytopenic purpura (ITP)

Participation of B-cell-activating factor receptors in the pathogenesis of immune thrombocytopenia

ESSENTIALS: Dysfunctional B-cell-activating factor (BAFF) system is related to many autoimmune diseases. The regulatory functions of BAFF/BAFF receptors were investigated in an in vitro coculture system. Different regulatory roles of BAFF were investigated via different receptors in immune thrombocytopenia. The upregulated BAFF receptors on autoreactive lymphocytes lead to their hypersensitivity to BAFF.

SUMMARY

BACKGROUND

The pathogenesis of immune thrombocytopenia (ITP) remains enigmatic. B-cell-activating factor (BAFF) and its receptors (BAFF receptor [BAFF-R], transmembrane activator and calcium modulator and cyclophilin ligand interactor [TACI], and B-cell maturation antigen) play central roles in the integrated homeostatic regulation of lymphocytes.

OBJECTIVES

To investigate the pathologic roles of BAFF receptors in regulating the bioactivities of lymphocytes in ITP.

METHODS

An in vitro culture system was established by stimulating CD14(-) peripheral lymphocytes with platelet-preloaded dendritic cells in the presence of recombinant human BAFF (rhBAFF; 20 ng mL(-1)). The functions of BAFF receptors were specifically blocked with blocking antibodies.

RESULTS

BAFF-R, besides prolonging the survival of B cells in both patients and healthy controls, prominently promoted the survival of CD8(+) T cells and the proliferation of B cells in patients with ITP. TACI, as a positive regulator, not only promoted the proliferation of CD4(+) and CD8(+) T cells, but also significantly enhanced the secretion of interleukin-4 in patients with ITP, but not in controls. Besides revealing the pathologic roles of BAFF receptors, these results also indicate that lymphocytes of patients with ITP have enhanced antiapoptotic or proliferative capacity as compared with those from healthy controls when exposed under similar stimulation of rhBAFF. Further study demonstrated that activated autoreactive B cells and CD4(+) T cells from patients with ITP showed significantly higher expression of BAFF-R or TACI than those from healthy controls.

CONCLUSIONS

Both BAFF-R and TACI are pathogenic participants in ITP. Their dysregulated expression in patients with ITP may lead to hyperreactivity of activated autoreactive lymphocytes in response to rhBAFF, and thus is highly significant in the pathogenesis of ITP.

Absolute lymphocyte count and risk of short-term infection in patients with immune thrombocytopenia

Patients with immune thrombocytopenia (ITP) may be at increased risk of infection because of the steroids and other immunosuppressive agents used in its treatment. This study aimed to identify events that are associated with infection within 6 months of diagnosis and the impact that infection has on survival. We retrospectively evaluated 239 patients (107 men, 132 women; median age 61 years) diagnosed between January 1997 and August 2011. Every patient received steroid treatment according to the platelet count and the extent of bleeding. Logistic regression analysis was used to identify risk factors associated with the development of infection within 6 months of ITP being diagnosed. Sixty-two patients (25.9 %) developed an infection within 6 months of diagnosis. Multivariate analysis revealed that a lower absolute lymphocyte count (ALC) at diagnosis (<1 × 10(9)/l) was an independent risk factor for infection (P = 0.039; 95 % confidence interval, 1.033-3.599; odds ratio, 1.928). The time to infection event is significant shorter in those of low ALC, compared with those of higher ALC (P = 0.032). Furthermore, the 1-year mortality rate after ITP diagnosis was significantly higher in those patients who developed an infection (P = 0.001). ITP patients with a low absolute lymphocyte count at diagnosis have an increased risk of infection, and those who develop infections have lower 1-year survival.

Animal models of immune thrombocytopenia (ITP)

With regards to research animal models related to immune thrombocytopenia (ITP), there is an extensive literature of over 300 publications published since 1959. It appears that many of these models either confirm what has been found in human ITP or, in some instances, are the first to describe a phenomenon related to ITP that is still of relevance today in human medicine. These models will undoubtedly play a significant role in the future research of human ITP particularly related to understanding of the pathogenesis of the disorder and the development of novel therapeutics. This review will highlight some of the major animal models utilized for ITP research and will present a somewhat historical aspect of the subject.

A murine model of severe immune thrombocytopenia is induced by antibody- and CD8+ T cell–mediated responses that are differentially sensitive to therapy

Immune thrombocytopenia (ITP) is a bleeding disorder characterized by antibody-opsonized platelets being prematurely destroyed in the spleen, although some patients with ITP may have a cell-mediated form of thrombocytopenia. Although several animal models of ITP have been developed, few mimic primary chronic ITP nor have any shown cell-mediated platelet destruction. To create this type of model, splenocytes from CD61 knockout mice immunized against CD61+ platelets were transferred into severe combined immunodeficient (SCID) (CD61+) mouse recipients, and their platelet counts and phenotypes were observed. As few as 5 × 104 splenocytes induced a significant thrombocytopenia and bleeding mortality (80%) in recipients within 3 weeks after transfer. Depletion of lymphocyte subsets before transfer showed that the splenocyte's ability to induce thrombocytopenia and bleeding completely depended on CD4+ T helper cells and that both CD19+ B cell (antibody)– and CD8+ T cell (cell)–mediated effector mechanisms were responsible. Treatment of the SCID mouse recipients with intravenous γ-globulins raised platelet counts and completely prevented bleeding mortality induced by antibody-mediated effector mechanisms but did not affect cell-mediated disease. This novel model not only shows both antibody- and cell-mediated ITP and bleeding but also suggests that these 2 effector mechanisms have a differential response to therapy.

Infections, antigen-presenting cells, T cells, and immune tolerance: their role in the pathogenesis of immune thrombocytopenia

In the last 20 years, many publications have shed new light on the complex immunopathogenesis of immune thrombocytopenic purpura. They are associated with 3 interrelated areas of environmental autoimmunity, for example, infectious influences, antigen-presenting cell (APC) function, and T-cell abnormalities, particularly tolerance induction. This article highlights the recent literature and argues that infectious agents and platelets can significantly modulate APCs, which create an environment that dysregulates autoreactive T cells, leading to the production of autoantibodies.

MHC Class I–Associated Peptides Identified From Normal Platelets and From Individuals With Idiopathic Thrombocytopenic Purpura

Major histocompatibility complex (MHC) class I molecules bind and display peptide antigens on the cell surface. CD8(+) T lymphocytes recognize peptides in association with class I proteins to initiate a cytotoxic immune response. To understand the specificity of such immune responses and to facilitate the development of therapies for disease, it is important to identify MHC-presented peptides. In this study, platelets, easily obtainable and often associated with immune-mediated disease, were selected to identify MHC class I-associated peptides. MHC-associated peptides presented on platelets of normal individuals and individuals with idiopathic thrombocytopenic purpura (ITP) were characterized. ITP is characterized by the premature immune destruction of platelets. It is associated with the production of antiplatelet autoantibodies, most often targeting platelet membrane GPIIb/IIIa or GPIb/IX. In addition to characterizing five fully and several partially sequenced peptides from platelets, the peptide GPRGA(L/I)S(L/I)(L/I) was identified from four of the five ITP patients. The anchor motif of this peptide correlates with the presence of the HLA-B7 allele. A BLAST search identified this peptide as GPIb (4-12). In conclusion, platelets from normal and ITP individuals can present peptides from general cellular proteins and platelet specific proteins, such as GPIb, to the immune system via MHC class I.

Autoimmune thrombocytopenia in chronic myeloid leukemia treated with interferon-alpha: differential diagnosis and possible pathogenesis

Interferon-alpha (INF-alpha) is an effective anti-neoplastic and anti-viral drug. Treatment with INF-alpha is frequently complicated by adverse effects, which may rarely be immune mediated. We report 2 patients with Ph+ chronic myeloid leukemia (CML) who developed autoimmune thrombocytopenia while receiving months of treatment with INF-alpha. This complication responded well to discontinuation of interferon and administration of steroids treatment. Here, we also summarize the literature on INF-alpha induced autoimmune thrombocytopenia, and discuss differential diagnosis and possible mechanisms involved in the development of thrombocytopenia during therapy with INF-alpha.

Idiopathic Thrombocytopenic Purpura: Better Therapeutic Responses of Patients with B- or T-Cell Clonality than Patients without Clonality

Results of recent studies of the pathogenesis of idiopathic thrombocytopenic purpura (ITP) have suggested activated helper T-cells drive B-lymphocytes to produce antibodies. Twenty-eight children and 85 adults with ITP entered this study. We performed polymerase chain reaction (PCR) using framework III variable region (V(H) FRIII)- and joining region (J(H))-specific primers to analyze immunoglobulin heavy-chain gene rearrangement (IgH GR) for B-cell clonality. We used multiplex PCR to analyze T-cell receptor (TCR) gamma-chain gene rearrangement (TCRgamma GR) for T-cell clonality. We diagnosed 10 cases as acute ITP and 97 cases as chronic ITP. The IgH GR result was positive in 77.8% of the acute-form cases and in 58.8% of the chronic-form cases. The TCRgamma GR result was positive in 11.1% of the acute cases and in 10.6% of the chronic cases. There was no difference in frequency of clonality between the acute and chronic forms. After treatment the platelet count normalized in 81.8% (36/44) of the chronic ITP cases with B-cell clonality and in 88.9% (8/9) of the chronic ITP cases with T-cell clonality, compared with a normalized platelet count in 46.2% (12/26) of the chronic ITP cases without clonality. The patients with T- or B-cell clonality appeared to have better therapeutic responses than patients without clonality. In conclusion, T- and B-cell clonality may play a positive role in determining therapeutic response.

Complete recovery from refractory immune thrombocytopenic purpura in three patients treated with etanercept

Management of patients with immune thrombocytopenic purpura (ITP) who have persistent, severe, and symptomatic thrombocytopenia following splenectomy is difficult and empirical. No single agent or regimen provides long-term success for most patients, and for most treatments it is difficult to assess whether benefits outweigh risks. We report three consecutive patients with critical chronic refractory ITP, who responded promptly and completely following treatment with etanercept, an inhibitor of tumor necrosis factor-alpha. These patients had failed 6-11 previous treatments. In the first patient, etanercept was given for its approved indication: a flare of co-existing rheumatoid arthritis. The next two patients were treated with etanercept because of successful outcomes in the previous patients. Although etanercept appeared to be effective treatment for ITP in these 3 patients, the experimental nature of this treatment and the potential risks must be emphasized. On the basis of these case reports, a clinical trial has been initiated to systematically evaluate the efficacy and risks of etanercept in the management of children and adults with chronic ITP.

Idiopathic thrombocytopenic purpura of childhood: a problem-oriented review of the management

Idiopathic thrombocytopenic purpura (ITP) is an often, but not always, self-limited bleeding disorder of childhood of unknown etiology. Duration of more than 6 months defines the chronic form and this occurs in approximately 20% of children and is the common form of the disorder in adults. At initial diagnosis, neither the individual bleeding risk nor the form of the disease, acute or chronic, is predictable, which exposes doctors and parents to fear of life-threatening bleeding. Consequently a majority of children will receive platelet-enhancing therapy to prevent major bleeding manifestations. Prophylactic interventions have proven to be effective in raising the platelet count within a short time period in several prospective trials, however two questions may be raised, (1) whether a rapid platelet increase efficiently prevents serious bleeding and (2) whether such a prophylactic intervention is of clinical value. Furthermore, a small subgroup of patients with major bleeding manifestations seems to be treated inadequately, since patients of this subgroup may bleed in spite of platelet-enhancing therapy. Clinical trials with long-term follow-up are needed, using clinical parameters as study endpoints, to answer the question of which subgroup of children with ITP should be treated upfront with platelet-enhancing therapy.

Controversies in the treatment of pediatric immune thrombocytopenias

Although pediatric immune thrombocytopenia (ITP) is common, there is no consensus on the optimal approach to therapy. Childhood ITP differs in its clinical course and trigger from adult immune thrombocytopenic purpura. There appear to be two clinical phenotypes among children with ITP: children with polyclonal autoantibody production triggered by an external exposure such as infection, and children with coexistent immune deficiency or dysregulation on a congenital or acquired basis. Treatment implications exist for each group. The first may be best managed by observation and conservative measures; for the latter, treatment could include normal intravenous gammaglobulin concentrate, anti-blood group D antigen (anti-Rh factor), or steroids. Early recognition of the thrombocytopenic child with immune dysfunction versus the normal child with a polyclonal response to a particular environmental antigen will result in better prognosis for both by selecting the appropriate therapy and minimizing long-term side effects.

Childhood immune thrombocytopenic purpura

Childhood immune thrombocytopenic purpura (ITP) is acute and generally seasonal in nature, suggesting that infectious or environmental agents may trigger the immune response to produce platelet-reactive autoantibodies 4 to 8 weeks following an infection. In general, the patient is well apart from the diffuse bruising and petechiae indicative of a profound thrombocytopenia. Over a period of 6 months, the thrombocytopenia resolves in approximately 85% of children, while the remaining 15% with persistent platelet consumption are designated as chronic ITP patients. The peak age of acute ITP is 2 to 5 years of age, a period when children experience the greatest frequency of viral infections. Children with the chronic form of ITP mirror the adult phenotype, in that females predominate, and there is no seasonal fluctuation of the disease. Evidence from our laboratory suggests that the activated platelet itself may play a role in perpetuating autoantibody production and immune dysregulation associated with ITP. Current data on lymphocyte studies and cytokine alterations noted in response to the variety of regimens used in children with ITP suggest that acute ITP is accompanied by autoantibodies to GPIb and a cytokine profile that is proinflammatory in nature. Early recognition of the immune dysregulation driving acute versus chronic ITP will distinguish those children who might benefit from immunotherapy versus those who will recover without therapeutic intervention.

Immune pathophysiology of autoimmune thrombocytopenic purpura

Chronic autoimmune thrombocytopenic purpura (AITP) is an immune-mediated, bleeding disorder in which platelets are opsonized by autoantibodies and prematurely destroyed by phagocytic cells in the reticuloendothelial system. It is classed as an organ-specific autoimmune disease primarily mediated by immunoglobulin G (IgG) autoantibodies and its etiology appears to be similar to that observed for other organ-specific autoimmune diseases. Th1 cells are important in the process, and the costimulation of Th1 cells and B cells takes place in a cytokine milieu that is reminiscent of a proinflammatory process. Chronic AITP has classically been treated with nonspecific, immunosuppressive regimens (e.g., steroids). One of the most significant developments in the treatment of AITP in the last 20 years has been the use of intravenous immunoglobulin (IVIg) and anti-D preparations. These treatments confer benefit to patients with AITP by significantly raising platelet counts. Despite this, their exact mechanisms of action remain elusive. This review focuses on cell-mediated and cytokine abnormalities within AITP, and presents data related to the mechanism of action of anti-D.

Inflammatory bowel disease associated with immune thrombocytopenic purpura in children

OBJECTIVE

Previous reports suggest an association between inflammatory bowel disease (IBD) and immune thrombocytopenic purpura (ITP) in adults. To date, only five children with both diseases have been described. The aim of the study was to describe the characteristics of children with IBD and ITP.

METHODS

Cases were obtained from the pediatric gastroenterology community by means of the pediatric gastroenterology internet bulletin board in June 1999. Eight cases were submitted from seven medical centers. Medical records were reviewed by two pediatric gastroenterologists and a pediatric hematologist.

RESULTS

The age range of the patients was 2.1 to 16.5 years, with a mean age of 9.6 +/- 5.2 years. Four children had ulcerative colitis, three had Crohn disease, and one had indeterminate colitis. All had colonic involvement of IBD. Of eight patients, three had IBD first, three had ITP first, and two had both simultaneously. At ITP diagnosis, platelet count was less than 10,000/mL in five children, 17,000/mL in one child, and 50,000 to 60,000/mL in two children. Of the three children diagnosed with ITP first, two initially had rectal bleeding at the time of ITP diagnosis. Bone marrow evaluations, performed in six of eight children, were consistent with ITP. Six of the eight children had chronic ITP, including three children who were 5 years of age or younger. Therapy for ITP included steroids (n = 6), intravenous immunoglobulin (n = 6), Rh o (D) intravenous immunoglobulin (n = 2), and splenectomy (n = 1).

CONCLUSIONS

The authors describe the largest pediatric case series of children with IBD and ITP. More than 50% of the children had the chronic form of ITP. Most patients responded to conventional therapy for ITP and IBD.

Thrombosis and shock induced by activating antiplatelet antibodies in human FcγRIIA transgenic mice: the interplay among antibody, spleen, and Fc receptor

Transgenic mouse lines were created that express FcγRIIA on platelets and macrophages at human physiologic levels, and they were used to explore the consequences in vivo of activating antiplatelet antibodies. Anti-CD9 antibody activated platelets of FcγRIIA transgenic (tg) mice and, following injection in vivo, caused more rapid severe thrombocytopenia than nonactivating antiplatelet antibody. Anti-CD9 injected into FcγRIIA tg crossed with FcR γ-chain knockout (γ-KO) mice caused thrombosis and shock in all mice, and death in 16 of 18 mice. The shock depended on platelet Fc receptor density and antibody dose. On histologic examination, the lung vasculature of anti-CD9–treated FcγRIIA tg × γ-KO mice contained extensive platelet-fibrin thrombi. Thrombosis and shock in FcγRIIA tg mice in the context of the FcR γ-chain knockout suggested the importance of the interplay of intravascular platelet activation and splenic clearance. Reduction of splenic clearance surgically (splenectomy) or functionally (monoclonal antibody treatment) also facilitated anti-CD9–mediated shock in FcγRIIA tg mice. The spleen, which clears nonactivating antibody-coated platelets leading to thrombocytopenia, appears to play a protective role in the thrombosis and shock observed with activating antiplatelet antibody. The data indicate that antibodies, which activate platelets in an FcγRIIA-dependent manner, can lead to thrombosis, shock, and death. Furthermore, antibody titer, platelet Fc receptor density, and splenic clearance are likely important determinants of the outcome.

Thrombosis and shock induced by activating antiplatelet antibodies in human FcγRIIA transgenic mice: the interplay among antibody, spleen, and Fc receptor

Transgenic mouse lines were created that express FcγRIIA on platelets and macrophages at human physiologic levels, and they were used to explore the consequences in vivo of activating antiplatelet antibodies. Anti-CD9 antibody activated platelets of FcγRIIA transgenic (tg) mice and, following injection in vivo, caused more rapid severe thrombocytopenia than nonactivating antiplatelet antibody. Anti-CD9 injected into FcγRIIA tg crossed with FcR γ-chain knockout (γ-KO) mice caused thrombosis and shock in all mice, and death in 16 of 18 mice. The shock depended on platelet Fc receptor density and antibody dose. On histologic examination, the lung vasculature of anti-CD9–treated FcγRIIA tg × γ-KO mice contained extensive platelet-fibrin thrombi. Thrombosis and shock in FcγRIIA tg mice in the context of the FcR γ-chain knockout suggested the importance of the interplay of intravascular platelet activation and splenic clearance. Reduction of splenic clearance surgically (splenectomy) or functionally (monoclonal antibody treatment) also facilitated anti-CD9–mediated shock in FcγRIIA tg mice. The spleen, which clears nonactivating antibody-coated platelets leading to thrombocytopenia, appears to play a protective role in the thrombosis and shock observed with activating antiplatelet antibody. The data indicate that antibodies, which activate platelets in an FcγRIIA-dependent manner, can lead to thrombosis, shock, and death. Furthermore, antibody titer, platelet Fc receptor density, and splenic clearance are likely important determinants of the outcome.