CD28 co-stimulates TCR/CD3-induced phosphoinositide turnover in human T lymphocytes

Upon engagement of TCR with peptide-MHC complexes displayed on the surface of antigen-presenting cells, T lymphocytes undergo a sustained elevation of intracellular Ca2+ concentration ([Ca2+](i)), which is required for cytoki ne production. In the present work, we investigate how inositol lipid metab olism can be activated for a prolonged time to ensure a sustained link betw een receptor triggering and downstream signaling effectors. Four lines of e vidence indicate that an extensive phosphoinositide turnover induced by TCR and CD28 engagement allows this task to be accomplished: (i) continuous ph osphoinositide breakdown is required for a sustained [Ca2+](i) increase in antigen-stimulated T cells; (ii) TCR triggering results in a continuous rel ease of inositol phosphates from the cell membrane paralleled by a massive and sustained phosphoinositide re-synthesis due to free inositol reincorpor ation; (iii) TCR-induced phosphoinositide turnover is strongly increased by CD28 ligation; and (iv) CD28 engagement augments and sustains the TCR-indu ced [Ca2+](i) increase. Our results show that the T cell pool of phosphoino sitides is continuously reformed during T cell-APC cognate interaction, the reby explaining how sustained receptor triggering can transduce an equally sustained [Ca2+](i) increase. Importantly, our data identify a novel step i n the signaling cascade where co-stimulation converges with TCR-generated s ignals to sustain and amplify the activation process.