Objective: To identify the T cells responsive to beta2-glycoprotein I (beta2GPI) that mediate antiphospholipid antibody production in patients with antiphospholipid syndrome (APS).
Methods: In vitro proliferative responses and anti-beta2GPI antibody production induced by beta2GPI were examined in peripheral blood mononuclear cell (PBMC) cultures from 12 APS patients, 13 systemic lupus erythematosus patients without APS, and 12 healthy donors.
Results: Peripheral blood T cells from all subjects failed to respond to beta2GPI in its native form. In contrast, reduced beta2GPI was able to stimulate T cells not only from all 12 patients with anti-beta2GPI antibodies, but also from 10 of 25 individuals without anti-beta2GPI antibodies. The specificity of the responses to beta2GPI was confirmed by activation of the reduced beta2GPI-primed T cells by recombinant beta2GPI in secondary cultures. Characterization of the T cell response induced by beta2GPI revealed that the response was associated with the presence of the DR53-associated alleles, the responding T cells were CD4+ and restricted by HLA class II, and antigenic peptides were located in domains IV and/or V. Anti-beta2GPI antibody production was induced specifically in anti-beta2GPI antibody-positive patients, in PBMC cultures with reduced beta2GPI. Anti-beta2GPI antibodies produced in vitro recognized beta2GPI immobilized with cardiolipin or beta2GPI coated on "high-binding" polystyrene plates.
Conclusion: These results strongly suggest that CD4+ and HLA class II-restricted T cells responsive to beta2GPI are involved in the production of antiphospholipid antibodies in APS patients.