The conformation of the CD3 complex on T lymphocytes is modified by calcium ions

While calcium ions are known to play a prominent role in signal transduction in the activation of T lymphocytes, its mechanism of action, target and function have not been elucidated. One crucial event in the calcium-dependent process is the activation of the CD3 complex, and this, too, is not understood. While studying the release of Ca2+ from intracellular stores by several monoclonal antibodies (mAb) against CD3, we found that one of them, mAb 141, was ineffective unless free Ca2+ was present in the external medium. By flow cytometric analysis of the binding of this mAb to Jurkat T cells and peripheral blood lymphocytes we showed that 141 does not recognize the CD3 complex when external Ca2+ is chelated by EGTA. The binding was restored by addition of Ca2+ but not Mg2+. Finally at least one subunit of the CD3 complex displayed a modified electrophoretic migration rate when immunoprecipitated by Leu-4 in the absence of external free Ca2+. These results suggest that the conformation of the CD3 complex depends on Ca2+, the epitope recognized by 141 being concealed at low Ca2+ concentration.