A SNP in the gene PTPN22 is associated with type 1 diabetes, rheumatoid arthritis, lupus, Graves thyroiditis, Addison disease and other autoimmune disorders. T cells from carriers of the predisposing allele produce less interleukin-2 upon TCR stimulation, and the encoded phosphatase has higher catalytic activity and is a more potent negative regulator of T lymphocyte activation. We conclude that the autoimmune-predisposing allele is a gain-of-function mutant.

In T cells, cAMP-dependent protein kinase (PKA) type I colocalizes with the T cell receptor–CD3 complex (TCR/CD3) and inhibits T cell function via a previously unknown proximal target. Here we examine the mechanism for this PKA-mediated immunomodulation. cAMP treatment of Jurkat and normal T cells reduces Lck-mediated tyrosine phosphorylation of the TCR/CD3 ζ chain after T cell activation, and decreases Lck activity. Phosphorylation of residue Y505 in Lck by COOH-terminal Src kinase (Csk), which negatively regulates Lck, is essential for the inhibitory effect of cAMP on ζ chain phosphorylation. PKA phosphorylates Csk at S364 in vitro and in vivo leading to a two- to fourfold increase in Csk activity that is necessary for cAMP-mediated inhibition of TCR-induced interleukin 2 secretion. Both PKA type I and Csk are targeted to lipid rafts where proximal T cell activation occurs, and phosphorylation of raft-associated Lck by Csk is increased in cells treated with forskolin. We propose a mechanism whereby PKA through activation of Csk intersects signaling by Src kinases and inhibits T cell activation.

Reversible tyrosine phosphorylation of proteins is a key regulatory mechanism for numerous important aspects of eukaryotic physiology and is catalysed by kinases and phosphatases. Together, cells of the immune system express at least half of the 107 protein tyrosine phosphatase (PTP) genes in the human genome, most of which encode multidomain proteins that contain protein- and phospholipid-interaction domains. Here, we discuss the diverse but specific, and important, roles that PTPs have in immune cells, focusing mainly on T and B cells, and we highlight recent evidence that even subtle alterations in PTPs can cause immune dysfunction and human disease.

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In resting peripheral T cells, Csk is constitutively present in lipid rafts through an interaction with the Csk SH2-binding protein, PAG, also known as Cbp. Upon triggering of the T cell antigen receptor (TCR), PAG/Cbp is rapidly dephosphorylated leading to dissociation of Csk from lipid rafts. However, tyrosine phosphorylation of PAG/Cbp resumes after 3-5 min, at which time Csk reassociates with the rafts. Cells overexpressing a mutant Csk that lacks the catalytic domain, but displaces endogenous Csk from lipid rafts, have elevated basal levels of TCR--chain phosphorylation and spontaneous activation of an NFAT-AP1 reporter from the proximal interleukin-2 promoter as well as stronger and more sustained responses to TCR triggering than controls. We suggest that a transient release from Csk-mediated inhibition by displacement of Csk from lipid rafts is important for normal T cell activation.Activation of the Src family kinases Lck and Fyn after engagement of the T cell antigen receptor is an initiating event in T cell activation and leads to phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) 1 within the TCR complex (1). The subsequent recruitment of the tandem SH2 domain containing tyrosine kinase ZAP-70 to phosphorylated ITAMs generates an activated immune receptor signaling complex that is able to initiate downstream events leading to a functional T cell response (2, 3). The control and fine-tuning of the proximal signaling is not only essential for an effective T cell response to antigen, but also for avoiding exaggerated T cell activation and autoimmunity. Thus a balance must be maintained to avoid hypo-as well as hyper-reactivity and immunopathology. The TCR signaling machinery appears to be controlled by setting a threshold for activation to avoid too easy triggering. Suppression of the catalytic activity of Lck and Fyn by phosphorylation of a C-terminal residue (Tyr 505 in Lck, Tyr 528 in Fyn T ) by the C-terminal Src kinase, Csk, appears to be an important means of negative regulation of TCR signaling (4 -6). Complexed with Csk via binding to its SH3 domain is also a protein tyrosine phosphatase, PEP, that dephosphorylates the activating phosphorylation site (Tyr 394 in Lck, Tyr 416 in Fyn T ) (7,8). Recent discoveries indicate that the assembly of TCR signaling complexes occurs in specific membrane subdomains with high cholesterol and glycolipid contents, called glycosphingolipid-enriched microdomains or lipid rafts (9 -11). Key components, including Lck and LAT, are targeted to rafts by virtue of their lipid modifications, whereas other proteins such as the -chain can localize via interaction with raft components only after TCR engagement (11-13). Lipid rafts serve to concentrate and promote specific protein-protein interactions and the tyrosine phosphorylation of signaling intermediates by the Src family kinases during the proximal phases of immunoreceptor signaling via the TCR as well as via the B cell and Fc receptors (14 -16).The ubiquitously expressed, cytosolic Csk ty...