Idiopathic thrombocytopenic purpura pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

AITP is characterized by the production of autoreactive antibodies against one's own platelets, resulting in increased platelet destruction by RES phagocytes

Pathogenesis

Increased platelet destruction:

• Autoantibody‐induced platelet destruction:
  • phagocytosis in the reticuloendothelial system
  • Platelet phagocytosis by splenic macrophages and peripheral blood neutrophils
  • Abnormal IgG auto-antibody recognizes glycoprotein IIb/IIIa, glycoprotein Ib/IX complex, GP Ia/IIa, and GP VI etc
  • IgG auto-antibody binds to the circulating platelet membranes through glycoproteins
  • Autoantibody-coated platelets induce Fcg receptors and bind to antigen-presenting cells (Splenic macrophages or dendritic cells) where the platelets undergoes phagocytosis.
  • Antigen-presenting cells degrade glycoproteins and and not only degrade glycoprotein IIb/IIIa (light blue oval), thereby amplifying the initial immune response, but also may generate cryptic epitopes from other platelet glycoproteins
  • Activated antigen-presenting cells
  • express these novel peptides on the cell surface along with costimulatory help (represented in part by the interaction between CD154 and CD40) and the relevant cytokines that facilitate the proliferation of the initiating CD4-positive T-cell clones (T-cell clone 1) and those with additional specificities (T-cell clone 2)
  • B-cell immunoglobulin receptors that recognize additional platelet antigens (B-cell clone 2) are thereby also induced to proliferate and synthesize anti–glycoprotein Ib/IX antibodies (green) in addition to amplifying the production of anti–glycoprotein IIb/IIIa antibodies (orange) by B-cell clone 1
• Autoreactive T lymphocyte‐mediated platelet lysis
  • chronic AITP may be the result of an abnormal Th cell defect that could direct autoreactive B cells to differentiate and secrete IgG autoantibodies.
  • act upon megakaryocytes in the bone marrow
  • Antibody production in ITP appears to be driven by CD4-positive helper T cells reacting to platelet surface glycoproteins, possibly involving CD40:CD40L co-stimulation [32-34]. Splenic macrophages appear to be the major antigen-presenting cells [35]

immune‐mediated decreased platelet production:

• abnormal thrombopoiesis
• degenerative changes in both nuclei and cytoplasm
• reduced platelet turnover
• megakaryocyte damage
• autoantibody‐induced suppression of in vitro megakaryocytopoiesis

Acute AITP primarily affects children and often occurs after a viral or bacterial infection

• often follows an infectious event
• Antigenic mimicry (molecular structures on infectious or environmental agents that have similarities to host antigenic structures and stimulate an immune response that then cross-reacts with host antigens)
• T cells in this disorder are not necessarily directed at platelet antigens.
• antiplatelet antibodies are because of the antiviral antibodies cross-reacting with the patient's platelets.
• infectious agent is cleared and antibodies either disappear or affinity mature toward the infectious agent, the antiplatelet reactivity is lost

chronic AITP (platelet counts < 150 • 109]L for more than 6 months)) redominant in adults, with more women being affected than men

• true organ-specific autoimmune disorder
• no previous history of an infectious event and almost always requires some form of immunosuppressive therapy
• anti- 72 COOPAMAH ET AL body specificities are directed against several platelet glycoprotein epitopes
• associated with T-cellrelated and cytokine abnormalities
• Autoantibody‐induced platelet destruction:
  • immune-targeted tissue abnormally expresses selfantigens that are then recognized by autoreactive Th cells.

Associated Conditions

Conditions associated with

Gross Pathology

On gross pathology, characteristic findings of itp include:

• Acute
• Chronic

Microscopic Pathology

On microscopic histopathological analysis, characteristic findings of itp include:

• Acute
• Chronic