Binding of ligand or antibody to certain cell-surface proteins that are anchored to the membrane by glycophosphatidylinositol (GPI) can cause activation of leukocytes. However, it is not known how these molecules, which lack intracellular domains, can transduce signals. The GPI-linked human molecules CD59, CD55, CD48, CD24, and CD14 as well as the mouse molecules Thy-1 and Ly-6 were found to associate with protein tyrosine kinases, key regulators of cell activation and signal transduction. A protein tyrosine kinase associated with the GPI-linked proteins CD59, CD55, and CD48 in human T cells, and with Thy-1 in mouse T cells was identified as p56lck, a protein tyrosine kinase related to Src. This interaction of GPI-linked molecules with protein tyrosine kinases suggests a potential mechanism of signal transduction in cells.

Functional discrimination between structurally similar self and foreign antigens is a main attribute of adaptive immunity. Here we describe two feedback mechanisms in T lymphocytes that together sharpen and amplify initial signaling differences related to the quality of T cell receptor (TCR) engagement. Weakly binding ligands predominantly trigger a negative feedback loop leading to rapid recruitment of the tyrosine phosphatase SHP-1, followed by receptor desensitization through inactivation of Lck kinase. In contrast, strongly binding ligands efficiently activate a positive feedback circuit involving Lck modification by ERK, preventing SHP-1 recruitment and allowing the long-lasting signaling necessary for gene activation. The characteristics of these pathways suggest that they constitute an important part of the mechanism allowing T cells to discriminate between self and foreign ligands.

A 72 kb region of BAC259.12D that encompasses the HAR1 locus was sequenced in its entirety using a DNA Sequencing Kit (PE Applied Biosystems) with an automated DNA sequencer (ABI PRISM 3100; PE Applied Biosystems). HAR1 complementary DNA was cloned from a cDNA library of L. japonicus shoots using DNA fragments of the first exon of the HAR1 gene obtained by polymerase chain reaction (PCR) from BAC259-12D. After sequencing the cDNA clone, the full-length cDNA sequence was determined by 5 0 rapid amplification of cloned ends (RACE) on L. japonicus shoot messenger RNA. The soybean gene showing the highest similarity with HAR1, G. max CLV1B, was amplified from the genomic DNA of a soybean cultivar Enrei and of a hypernodulating mutant, En6500, by PCR with a specific primer set designed from the DNA Data Bank of Japan (DDBJ) database. They were sequenced using a DNA Sequencing Kit (PE Applied Biosystems) with an automated DNA sequencer. Complementation experimentsA SalI-PvuII fragment (12.5 kb) containing the entire HAR1 gene was excised from a BAC clone, 259-12D, and ligated into the SalI-SmaI site of binary vector pCAMBIA1301 (Cambia). It was transformed into A. tumefaciens AGL1 (provided by R. A. Ludwig) and used for transformation of har1-4 and har1-5. Transgenic plants were produced by the hypocotyl transformation procedure 30 , with hygromycin B as a selective marker. Seeds from transgenic lines were germinated in vermiculite pots, inoculated with M. loti strain TONO, and examined for plant growth and nodulation behaviour 24 days after inoculation.

T cells constantly sample their environment using receptors (TCR) that possess both a germline-encoded low affinity for major histocompatibility complex (MHC) molecules and a highly diverse set of CDR3 regions contributing to a range of affinities for specific peptides bound to these MHC molecules. The decision of a T cell "to sense and to respond" with proliferation and effector activity rather than "to sense, live on, but not respond" is dependent on TCR interaction with a low number of specific foreign peptide:MHC molecule complexes recognized simultaneously with abundant self peptide-containing complexes. Interaction with self-complexes alone, on the other hand, generates a signal for survival without a full activation response. Current models for how this distinction is achieved are largely based on translating differences in receptor affinity for foreign versus self ligands into intracellular signals that differ in quality, intensity, and/or duration. A variety of rate-dependent mechanisms involving assembly of molecular oligomers and enzymatic modification of proteins underlie this differential signaling. Recent advances have been made in measuring TCR:ligand interactions, in understanding the biochemical origin of distinct proximal and distal signaling events resulting from TCR binding to various ligands, and in appreciating the role of feedback pathways. This new information can be synthesized into a model of how self and foreign ligand recognition each evoke the proper responses from T cells, how these two classes of signaling events interact, and how pathologic responses may arise as a result of the underlying properties of the system. The principles of signal spreading and stochastic resonance incorporated into this model reveal a striking similarity in mechanisms of decision-making among T cells, neurons, and bacteria.