Pleiotropic Contributions of Phospholipase C-γ1 (PLC-γ1) to T-Cell Antigen Receptor-Mediated Signaling: Reconstitution Studies of a PLC-γ1-Deficient Jurkat T-Cell Line

Abstract

Phospholipase C-γ1 (PLC-γ1) plays a crucial role in the coupling of T-cell antigen receptor (TCR) ligation to interleukin-2 (IL-2) gene expression in activated T lymphocytes. In this study, we have isolated and characterized two novel, PLC-γ1-deficient sublines derived from the Jurkat T-leukemic cell line. The P98 subline displays a >90% reduction in PLC-γ1 expression, while the J.gamma1 subline contains no detectable PLC-γ1 protein. The lack of PLC-γ1 expression in J.gamma1 cells caused profound defects in TCR-dependent Ca²⁺ mobilization and NFAT activation. In contrast, both of these responses occurred at normal levels in PLC-γ1-deficient P98 cells. Unexpectedly, the P98 cells displayed significant and selective defects in the activation of both the composite CD28 response element (RE/AP) and the full-length IL-2 promoter following costimulation with anti-TCR antibodies and phorbol ester. These transcriptional defects were reversed by transfection of P98 cells with a wild-type PLC-γ1 expression vector but not by expression of mutated PLC-γ1 constructs that lacked a functional, carboxyl-terminal SH2 [SH2(C)] domain or the major Tyr⁷⁸³ phosphorylation site. On the other hand, the amino-terminal SH2 [SH2(N)] domain was not essential for reconstitution of RE/AP- or IL-2 promoter-dependent transcription but was required for the association of PLC-γ1 with LAT, as well as the tyrosine phosphorylation of PLC-γ1 itself, in activated P98 cells. These studies demonstrate that the PLC-γ1 SH2(N) and SH2(C) domains play functionally distinct roles during TCR-mediated signaling and identify a non-Ca²⁺-related signaling function linked to the SH2(C) domain, which couples TCR plus phorbol ester-CD28 costimulation to the activation of the IL-2 promoter in T lymphocytes.