A molecular framework for two-step T cell signaling: Lck Src homology 3 mutations discriminate distinctly regulated lipid raft reorganization events

Costimulation by CD28 or lipid-raft-associated CD48 potentiate TCR-induced signals, cytoskeletal reorganization, and IL-2 production. We and others ha ve proposed that costimulators function to construct a raft-based platform( s) especially suited for TCR engagement and sustained and processive signal transduction. Here, we characterize TCR/CD48 and TCR/CD28 costimulation in T cells expressing Lck Src homology 3 (SH3) mutants. We demonstrate that L ck SH3 functions after initiation of TCR-induced tyrosine phosphorylation a nd concentration of transducers within rafts, to regulate the costimulation -dependent migration of rafts to the TCR contact site. Expression of kinase -active/SH3-impaired Lck mutants disrupts costimulation-dependent raft recr uitment, sustained TCR protein tyrosine phosphorylation, and IL-2 productio n. However, TCR-induced apoptosis, shown only to require "partial" TCR sign als, is unaffected by expression of kinase-active/SH3-impaired Lck mutants. Therefore, two distinctly regulated raft reorganization events are require d for processive and sustained "complete" TCR signal transduction and T cel l activation. Together with recent characterization of CD28 and CD48 costim ulatory activities, these findings provide a molecular framework for two si gnal models of T cell activation.