SumnlaryThe CD45 transmembrane glycoprotein has been shown to be a protein phosphotyrosine phosphatase and to be important in signal transduction in T and B lymphocytes. We have employed gene targeting to create a strain of transgenic mice that completely lacks expression of all isoforms of CD45. The spleens from CD45-null mice contain approximately twice the number of B cells and one fifth the number ofT cells found in normal controls. The increase in B cell numbers is due to the specific expansion of two B cell subpopulations that express high levels ofimmunoglobulin (IgM) staining. T cell development is significantly inhibited in CD45-null animals at two distinct stages. The efficiency of the development of CD4-CD8-thymocytes into CD4+CD8 § thymocytes is reduced by about twofold, subsequently the frequency of successful maturation of the double positive population into mature, single positive thymocytes is reduced by a further four-to fivefold. In addition, we demonstrate that CD45-null thymocytes are severely impaired in their apoptotic response to cross-linking signals via T cell receptor (TCtk) in fetal thymic organ culture. In contrast, apoptosis can be induced normally in CD45-null thymocytes by non-TC1L-mediated signals. Since both positive and negative selection require signals through the TCR complex, these findings suggest that CD45 is an important regulator of signal transduction via the TCIk complex at multiple stages of T cell development. CD45 is absolutely required for the transmission ofmitogenic signals via IgM and IgD. By contrast, CD45-null B cells proliferate as well as wild-type cells to CD40-mediated signals. The proliferation of B cells in response to CD38 cross-linking is significantly reduced but not abolished by the CD45-null mutation. We conclude that CD45 is not required at any stage during the generation of mature peripheral B cells, however its loss reveals a previously unrecognized role for CD45 in the regulation of certain subpopulations of B cells.

The molecular mechanisms whereby the CD45 tyrosine phosphatase (PTPase) regulates T cell receptor (TCR) signaling responses remain to be elucidated. To investigate this question, we have reconstituted CD45 (encoded by Ptprc)-deficient mice, which display severe defects in thymic development, with five different expression levels of transgenic CD45RO, or with mutant PTPase null or PTPase-low CD45R0. Whereas CD45 PTPase activity was absolutely required for the reconstitution of thymic development, only 3% of wild-type CD45 activity restored T cell numbers and normal cytotoxic T cell responses. Lowering the CD45 expression increased CD4 lineage commitment. Peripheral T cells with very low activity of CD45 phosphatase displayed reduced TCR signaling, whereas intermediate activity caused hyperactivation of CD4+ and CD8+ T cells. These results are explained by a rheostat mechanism whereby CD45 differentially regulates the negatively acting pTyr-505 and positively acting pTyr-394 p56(lck) tyrosine kinase phosphorylation sites. We propose that high wild-type CD45 expression is necessary to dephosphorylate p56(lck) pTyr-394, suppressing CD4 T+ cell lineage commitment and hyperactivity.

Phosphatidylserine (PS) exposure is normally associated with apoptosis and the removal of dying cells. We observed that PS is exposed constitutively at high levels on T lymphocytes that express low levels of the transmembrane tyrosine phosphatase CD45RB. CD45 was shown to be a negative regulator of PS translocation in response to various signals, including activation of the ATP receptor P2X(7). Changes in PS distribution were shown to modulate several membrane activities: Ca(2+) and Na(+) uptake through the P2X(7) cation channel itself; P2X(7)-stimulated shedding of the homing receptor CD62L; and reversal of activity of the multidrug transporter P-glycoprotein. The data identify a role for PS distribution changes in signal transduction, rapidly modulating the activities of several membrane proteins. This seems to be an all-or-none effect, coordinating the activity of most or all the molecules of a target protein in each cell. The data also suggest a new approach to circumventing multidrug resistance.

The role of the CD45 phosphotyrosine phosphatase in coupling the T cell antigen receptor complex (TCR) to intracellular signals was investigated. CD45‐ HPB‐ALL T cells were transfected with cDNA encoding the CD45RA+B+C‐ isoform. The tyrosine kinase activity of p59fyn was found to be 65% less in CD45‐ cells than in CD45+ cells, whereas p56lck kinase activity was comparable in both sub‐clones. In CD45‐ cells the TCR was uncoupled from protein tyrosine phosphorylation, phospholipase C gamma 1 regulation, inositol phosphate production, calcium signals, diacylglycerol production and protein kinase C activation. Restoration of TCR coupling to all these pathways correlated with the increased p59fyn activity observed in CD45‐transfected cells. Co‐aggregation of CD4‐ or CD8‐p56lck kinase with the TCR in CD45‐ cells restored TCR‐induced protein tyrosine phosphorylation, phospholipase C gamma 1 regulation and calcium signals. Receptor‐mediated calcium signals were largely due (60–90%) to Ca2+ influx, and only a minor component (10–40%) was caused by Ca2+ release from intracellular stores. Maximal CD3‐mediated Ca2+ influx occurred at CD3 mAb concentrations at which inositol phosphate production was non‐detectable. These results indicate that CD45‐regulated p59fyn plays a critical role in coupling the TCR to specific intracellular signalling pathways and that CD4‐ or CD8‐p56lck can only restore signal transduction coupling in CD45‐ cells when brought into close association with the TCR.

Src homology 2 (SH2) domain–containing phosphotyrosine phosphatases (SHPs) are increasingly being shown to play critical roles in protein tyrosine kinase–mediated signaling pathways. The role of SHP-1 as a negative regulator of T cell receptor (TCR) signaling has been established. To further explore the function of the other member of this family, SHP-2, in TCR-mediated events, a catalytically inactive mutant SHP-2 was expressed under an inducible promoter in Jurkat T cells. Expression of the mutant phosphatase significantly inhibited TCR-induced activation of the extracellular-regulated kinase (ERK)-2 member of the mitogen-activated protein kinase (MAPK) family, but had no effect on TCR-ζ chain tyrosine phosphorylation or TCR-elicited Ca2+ transients. Inactive SHP-2 was targeted to membranes resulting in the selective increase in tyrosine phosphorylation of three membrane-associated candidate SHP-2 substrates of 110 kD, 55-60 kD, and 36 kD, respectively. Analysis of immunoprecipitates containing inactive SHP-2 also indicated that the 110-kD and 36-kD Grb-2–associated proteins were putative substrates for SHP-2. TCR-stimulation of Jurkat T cells expressing wild-type SHP-2 resulted in the formation of a multimeric cytosolic complex composed of SHP-2, Grb-2, phosphatidylinositol (PI) 3′-kinase, and p110. A significant proportion of this complex was shown to be membrane associated, presumably as a result of translocation from the cytosol. Catalytically inactive SHP-2, rather than the wild-type PTPase, was preferentially localized in complex with Grb-2 and the p85 subunit of PI 3′-kinase, suggesting that the dephosphorylating actions of SHP-2 may regulate the association of these signaling molecules to the p110 complex. Our results show that SHP-2 plays a critical role in linking the TCR to the Ras/MAPK pathway in Jurkat T cells, and also provide some insight into the molecular interactions of SHP-2 that form the basis of this signal transduction process.

To date, immunoreceptor tyrosine-based inhibition motifs (ITIMs) have been shown to mediate inhibitory properties. We report a novel triggering receptor expressed on myeloid cells (TREM) family member, TREM-like transcript-1 (TLT1), which differs from the activating members because its cytoplasmic tail contains two ITIMs at Y245 and Y281. A TLT1 splice variant (TLT1sp) encodes a different cytoplasmic tail lacking ITIMs. Both isoforms are expressed in resting platelet α-granules, which are up-regulated to the cell surface following activation. TLT1 recruited Src homology 2 domain-containing tyrosine phosphatase (SHP)-2 to the “classical” ITIM (Y281) but not the “nonclassical” ITIM (Y245). In contrast to previously characterized ITIM receptors, TLT1 enhanced, rather than inhibited, FcεRI-mediated calcium signaling in rat basophilic leukemia cells, a property dependent on the SHP-2 recruiting classical Y281 ITIM. Therefore, TLT1 represents a new costimulatory ITIM immunoreceptor and is the second ITIM-bearing receptor to be identified in platelets after platelet endothelial cell adhesion molecule-1.

An investigation into the role of CD45 isoforms in T cell antigen receptor signal transduction was carried out by transfecting CD45-negative CD4؉ CD8 ؉ HPB-ALL T cells with the CD45R0, CD45RBC, and CD45RABC isoforms. Fluorescence resonance energy transfer analysis showed that the CD45R0 isoform, but not the CD45RBC or CD45RABC isoforms, was found as homodimers and also preferentially associated with CD4 and CD8 at the cell-surface. A comparison was therefore made of T cell antigen receptor signaling between sub-clones expressing either CD45R0 or CD45RBC. Under basal conditions CD4-associated p56 lck tyrosine kinase activity and cellular protein tyrosine phosphorylation levels were higher in the CD45R0 ؉ than in the CD45RBC ؉ sub-clones. The CD45 tyrosine phosphatase regulates the threshold of T cell antigen receptor (TCR) 1 signaling by modulating the actions of the p56 lck and p59 fyn tyrosine kinases (1-4). The p56 lck kinase associates with the CD4 and CD8 co-receptors and initiates TCR signaling cascades by phosphorylation of immunoreceptor tyrosine-based activation motifs located in the TCRand CD3-⑀ chains (5, 6). In CD45 Ϫ/Ϫ mice T cell development is severely affected (7,8), and elevated TCR signaling thresholds are marked by dysfunctional p56 lck , together with defective TCR-chain phosphorylation and ZAP-70 recruitment (9). CD45 exerts a positive effect on p56 lck actions by de-phosphorylating the inhibitory C-terminal pTyr-505 (10 -13), but can also negatively regulate the kinase by dephosphorylating the pTyr-394 autophosphorylation site (14, 15). The actions of CD45 on p56 lck in CD4 ϩ T cells are directed selectively at the CD4-associated pool of the kinase (16). The dominant effect of CD45 in the T-lineage appears to be at pTyr-505, since T cell development in CD45 Ϫ/Ϫ mice can be largely restored by backcrossing to mice expressing the mutant lck Y505F -active transgene (17,18).Alternative splicing generates up to eight different CD45 isoforms (19,20) of which five are expressed at significant levels in T cells (21). All T cells express more than one CD45 isoform, and differential isoform expression is tightly controlled during thymic development and the activation of mature T cells (reviewed in Ref.1). However, the molecular consequences of differential CD45 isoform expression for TCR signaling, if any, are not yet well understood. Reconstitution of a murine CD45 Ϫ cell line with CD45 isoforms suggested that the CD45R0 isoform (lacking the A, B, and C exon-encoded segments of the ectodomain) promoted greater interleukin-2 secretion upon engagement of the TCR with the cognate major histocompatibility complex-peptide as compared with other isoforms (22). Co-capping experiments in these cells revealed preferential CD4-CD45R0 association (23). However, more detailed capping and co-immunoprecipitation studies indicated a basal association of CD45R0 with the TCR independent of CD4 expression and suggested that co-capping of CD4 with CD45R0 was mediated by this prior CD45R0-TCR association (24). Neverthel...

BCR-ABL and mutant JAK2 inhibit the Bcl-x(L) deamidation pathway and the apoptotic response to DNA damage in primary cells from patients with CML or polycythemia vera.