Dissection of thymocyte signaling pathways by in vivo expression of pertussis toxin ADP-ribosyltransferase

Stimulation of the T lymphocyte antigen receptor-CD3 complex (TCR-CD3) causes T cell activation by a process associated with increased phosphatidylinositol-specific phospholipase C (PI-PLC) activity. Evidence exists suggesting that GTP-binding (G) proteins, particularly the pertussis toxin (PT)-sensitive G_i proteins, participate in this signal transduction pathway. To clarify the role of G_i proteins in TCR-CD3 signaling, and to investigate other possible functions of G_i molecules in T cells, we expressed the S1 subunit of PT in the thymocytes of transgenic mice using the lymphocyte-specific lck promoter. Transgenic thymocytes contained S1 activity and exhibited profound depletion of G_i protein PT substrates in a manner suggesting their inactivation by S1 in vivo. Nevertheless, treatment of transgenic thymocytes with mitogenic stimuli provoked normal increases in intracellular free Ca²⁺ concentrations and IL-2 secretion, indicating that G_i proteins are not required for T cell activation. These normal signaling responses notwithstanding, mature thymocytes accumulated in lck-PT mice and did not appear in secondary lymphoid organs or in the circulation. Viewed in the context of the known features of Bordetella pertussis infection, our results suggest that a PT-sensitive signaling process, probably involving G_i proteins, regulates thymocyte emigration.