Autoreactive CD4⁺ T-cell clones to β₂-glycoprotein I in patients with antiphospholipid syndrome: Preferential recognition of the major phospholipid-binding site

Autoreactive CD4⁺ T cells to β₂-glycoprotein I (β₂GPI) that promote antiphospholipid antibody production were recently identified in patients with antiphospholipid syndrome (APS). To further examine antigen recognition profiles and T-cell helper activity in β₂GPI-reactive T cells, 14 CD4⁺ T-cell clones specific to β₂GPI were generated from 3 patients with APS by repeated stimulation of peripheral blood T cells with recombinant β₂GPI. At least 4 distinct T-cell epitopes were identified, but the majority of the β₂GPI-specific T-cell clones responded to a peptide encompassing amino acid residues 276 to 290 of β₂GPI (KVSFFCKNKEKKCSY; single-letter amino acid codes) that contains the major phospholipid-binding site in the context of the DRB4*0103 allele. Ten of 12 β₂GPI-specific T-cell clones were able to stimulate autologous peripheral blood B cells to promote anti-β₂GPI antibody production in the presence of recombinant β₂GPI. T-cell helper activity was exclusively found in T-cell clones capable of producing interleukin 6 (IL-6). In vitro anti-β₂GPI antibody production induced by T-cell clones was inhibited by anti-IL-6 or anti-CD40 ligand monoclonal antibody. In addition, exogenous IL-6 augmented anti-β₂GPI antibody production in cultures of the T-cell clone lacking IL-6 expression. These results indicate that β₂GPI-specific CD4⁺ T cells in patients with APS preferentially recognize the antigenic peptide containing the major phospholipid-binding site and have the capacity to stimulate B cells to produce antiβ₂GPI antibodies through IL-6 expression and CD40-CD40 ligand engagement. These findings are potentially useful for clarifying the pathogenesis of APS and for developing therapeutic strategies that suppress pathogenic antiphospholipid antibody production in these patients.