Mercuric chloride (HgCl2) has been proposed to be a mitogen for human blood lymphocytes in vitro. In our previous study, we demonstrated that HgCl2 preferentially stimulates the CD4+ T cell subset to blast transformation and DNA synthesis and that the reaction is dependent on CD14+ accessory cells. In order to characterise the responding cells further and to elucidate the mechanism of T cell activation, the T cell receptor (TCR) Vβ chain expression of the blast–transformed cells was analysed by monoclonal antibodies and flow cytometry. The 22 TCR–Vβ–specific antibodies used were found to react with 55–80% of the naive CD4+ and CD8+ blood T cells from the different donors. Six to 18% of the lymphocytes, mainly CD4+ T cells, were blast transformed after addition of HgCl2. The distribution of the lymphoblasts carrying certain TCR Vβ chains were skewed, and 15–40% expressed the TCR Vβ2 chain. Furthermore, if cells were pretreated for 5 days with HgCl2, whereafter recombinant interleukin–2 in fresh medium was added, the TCR Vβ7+ T cell subset was also stimulated to blast transformation. The superantigen staphyloccal enterotoxin B, as a control, induced blast transformation in 10–26% of the lymphocytes, mainly CD4+ T cells, which were, as expected, positive for Vβ3, Vβ12, Vβ14 or Vβ17. We conclude that HgCl2 has characteristics of a superantigen, activating the human lymphocytes in a Vβ–chain–selective manner in vitro.

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