Critical Role of T Cells in PF4/Heparin Antibody Production

Heparin-induced thrombocytopenia (HIT) is an antibody-mediated disorder that can cause arterial or venous thrombosis/thromboembolism, and platelet factor 4 (PF4)/ heparin-reactive antibodies are essential to the pathogenesis of HIT. Our recent studies have demonstrated that marginal zone (MZ) B cells play a major role in production of PF4/heparin-specific antibodies. However, the role of T cells in production of these pathogenic antibodies is not clear. Here we showed that PF4/heparin complex-induced production of PF4/heparin-specific antibodies was markedly impaired in mice, in which CD4 T cells were depleted by administration of GK1.5 anti-CD4 monoclonal antibody. As expected, the CD4 T cell-depleted mice responded normally to T cell-independent antigen TNP-Ficoll but not T cell-dependent antigen NP-CGG, in agreement with the lack of CD4 T cells in these GK1.5-treated mice. Further, following adoptive transfer of a mixture of wild-type splenic B cells and splenocytes from B cell-deficient μMT mice, T and B cell-deficient Rag1 knockout mice responded to PF4/heparin complex challenge to produce PF4/heparin-specific antibodies. In contrast, Rag1-deficient mice that received a mixture of wild-type splenic B cells and splenocytes from Rag1-deficient mice barely produced PF4/heparin-specific antibodies upon PF4/heparin complex challenge. These data suggest that T cells are required for production of PF4/heparin-specific antibodies. Consistent with this concept, mice with B cells lacking CD40 molecule, a B cell costimulatory molecule that helps T cell-dependent B cell responses, displayed a marked reduction of PF4/heparin-specific antibody production following PF4/heparin complex challenge. Also as expected, mice with CD40-deficient B cells were able to respond to T cell-independent antigen TNP-Ficoll but not T cell-dependent antigen NP-CGG, consistent with the lack of T-cell help in these mice. Taken together, these findings demonstrate that T cells play an essential role in production of PF4/heparin-specific antibodies by MZ B cells.