Analysis of Pathogenic Autoreactive T cells in Patients with Antiphospholipid Syndrome and its Application for Developing Specific Immune Therapies

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Antiphospholipid syndrome (APS) is characterized by recurrent thrombosis and intrauterine fetal loss in association with antiphospholipid antibodies. β_2-glycoprotein I (β_2GPI), a plasma glycoprotein that binds various kinds of negatively charged substances including phospholipids, is known to be the most common antigenic target for antiphopholipid antibodies associated with the clinical features of APS. Accumulating evidences in APS patients as well as in experimental APS indicate an important role of β_2GPI-specific CD4^+ T cells in anti-β_2GPI antibody production. In this research project, we have identified and characterized autoreactive CD4^+ T cells to β2GPI that promote antiphospholipid antibody production in APS patients. β_2GPI-specific CD4^+ T cells preferentially recognize the antigenic peptide containing the major phospholipid-binding site in the context of DRB4^*0103 (DR53). T-cell receptor β chains of β_2GPI-specific T cells are highly restricted and mainly utilize rearranged Vβ_7 or Vβ_8 gene segments. T-cell helper activity that stimulates B cells to produce anti-β_2GPI antibodies is mediated through IL-6 and CD40-CD40 ligand engagement. β_2GPI-specific T cells respond to reduced β_2GPI and recombinant β_2GPI fragments produced in bacteria, but not to native β_2GPI, indicating that the epitopes recognized by β_2GPI-specific T cells are apparently cryptic. Activation of β2GPI-specific T cells resulting in production of pathogenic anti-β2GPI antibodies can be induced by the exposure to cryptic peptides of β_2GPI. Finally, β_2GPI-specific T cell is a reasonable target of potential therapeutic strategies that selectively suppress pathogenic antiphospholipid antibody production in APS patients.