Intimate association of Thy-1 and the T-cell antigen receptor with the CD45 tyrosine phosphatase

Synthesis and evaluation of Nα,Nε-diacetyl-l-lysine-inositol conjugates as cancer-selective probes for metabolic engineering of GPIs and GPI-anchored proteins

Two myo-inositol derivatives having an Nα,Nε-diacetyl-l-lysine (Ac2Lys) moiety linked to the inositol 1-O-position through a self-cleavable linker and a metabolically stable 2-azidoethyl group linked to the inositol 3-O- and 4-O-positions, respectively, were designed and synthesized. The Ac2Lys moiety blocking the inositol 1-O-position required for GPI biosynthesis was expected to be removable by a combination of two enzymes, histone deacetylase (HDAC) and cathepsin L (CTSL), abundantly expressed in cancer cells, but not in normal cells, to transform these inositol derivatives into biosynthetically useful products with a free 1-O-position. As a result, it was found that these inositol derivatives could be incorporated into the glycosylphosphatidylinositol (GPI) biosynthetic pathway by cancer cells, but not by normal cells, to express azide-labeled GPIs and GPI-anchored proteins on cell surfaces. Consequently, this study has established a novel strategy and new molecular tools for selective metabolic labeling of cancer cells, which should be useful for various biological studies and applications.

Opportunities and challenges for the development of covalent chemical immunomodulators

Compounds that react irreversibly with cysteines have reemerged as potent and selective tools for altering protein function, serving as chemical probes and even clinically approved drugs. The exquisite sensitivity of human immune cell signaling pathways to oxidative stress indicates the likely, yet still underexploited, general utility of covalent probes for selective chemical immunomodulation. Here, we provide an overview of immunomodulatory cysteines, including identification of electrophilic compounds available to label these residues. We focus our discussion on three protein classes essential for cell signaling, which span the 'druggability' spectrum from amenable to chemical probes (kinases), somewhat druggable (proteases), to inaccessible (phosphatases). Using existing inhibitors as a guide, we identify general strategies to guide the development of covalent probes for selected undruggable classes of proteins and propose the application of such compounds to alter immune cell functions.

CD45 in human physiology and clinical medicine

CD45 is an evolutionary highly conserved receptor protein tyrosine phosphatase exclusively expressed on all nucleated cells of the hematopoietic system. It is characterized by the expression of several isoforms, specific to a certain cell type and the developmental or activation status of the cell. CD45 is one of the key players in the initiation of T cell receptor signaling by controlling the activation of the Src family protein-tyrosine kinases Lck and Fyn. CD45 deficiency results in T- and B-lymphocyte dysfunction in the form of severe combined immune deficiency. It also plays a significant role in autoimmune diseases and cancer as well as in infectious diseases including fungal infections. The knowledge collected on CD45 biology is rather vast, but it remains unclear whether all findings in rodent immune cells also apply to human CD45. This review focuses on human CD45 expression and function and provides an overview on its ligands and role in human pathology.

Thy-1 stimulation of mouse T cells induces a delayed T cell receptor-like signal that results in Ca2+‑independent cytotoxicity

Antibody-mediated crosslinking of Thy-1 [also known as cluster of differentiation (CD)90], results in a T cell receptor (TcR)‑like signal; however, the impact of Thy‑1 stimulation in comparison to TcR stimulation on T cell activation and effector function has yet to be fully elucidated. In the present study, the outcome of Thy‑1‑ and TcR‑induced stimulation of T cells was investigated in mice, using fragment crystalizable (Fc) receptor‑bound antibodies and costimulatory signals provided by syngeneic lipopolysaccharide‑matured bone marrow‑derived dendritic cells. Compared with TcR signaling, Thy‑1 signaling initiated a less robust proliferative response in T cells, as determined by tritiated‑thymidine incorporation. In addition, enzyme‑linked immunosorbent assays revealed that interleukin‑2 production was reduced, and the expression of CD25 and cyclin D3 was weaker in Thy‑1‑stimulated cells, as determined by western blotting; however, the expression of cyclin‑dependent kinase 6 was similar to that in TcR‑induced T cells. Furthermore, western blotting demonstrated that the phosphorylation of ζ-chain‑associated protein kinase 70 and extracellular signal‑regulated kinase 1/2 was delayed following Thy‑1 stimulation. DNA fragmentation assays revealed that cytotoxic effector function was also slower to develop in Thy‑1‑stimulated T cells, required more time to be effective and was largely Ca2+‑independent; these findings suggested that Fas ligand rather than granule‑associated perforin was involved in T cell effector function. In conclusion, the present results suggested that Thy‑1 signaling may contribute to the regulation of T cell homeostasis and the development of non‑specific T cell‑mediated cytotoxicity. However, further studies are required to elucidate the exact physiological roles of TcR‑like signals that result from Thy‑1 crosslinking and to investigate the molecular mechanisms that are involved.

CD45-associated protein promotes the response of primary CD4 T cells to low-potency T-cell receptor (TCR) stimulation and facilitates CD45 association with CD3/TCR and lck

Although it is clear that the CD45 tyrosine phosphatase is required for efficient T-cell activation and T-cell development, the factors that regulate CD45 function remain uncertain. Previous data have indicated that there is an association of CD45 with CD4 and the T-cell receptor (TCR) complex controlled by the variable ectodomain of CD45 and, following activation, by high- and low-potency peptides. This suggests that controlling substrate access to CD45 may be an important regulatory mechanism during T-cell activation. In the present study we have examined the role of the transmembrane adapter-like molecule CD45-associated protein (CD45-AP) in regulating the association of CD45 with CD3/TCR and lck, and in regulating primary CD4(+) T-lymphocyte activation. In CD4(+) T cells from CD45-AP-deficient mice, coimmunoprecipitation of CD45 with the CD3/TCR complex, in addition to lck, is significantly reduced compared with wild-type T cells. Functionally, this correlates with a decreased proliferative response, a decrease in interleukin (IL)-2 production, and a decrease in calcium flux upon stimulation with a low-potency altered peptide ligand. However, the response of CD45-AP-deficient T cells to stimulation with a high-avidity agonist peptide was largely intact, except for a modest decrease in IL-2 production. These data suggest that CD45-AP promotes or stabilizes the association of CD45 with substrates and regulates the threshold of T-cell activation.

Non-apoptotic Phosphatidylserine Externalization Induced by Engagement of Glycosylphosphatidylinositol-anchored Proteins

The exposure of phosphatidylserine (PS) on the cell surface is a general marker of apoptotic cells. Non-apoptotic PS externalization is induced by several activation stimuli, including engagement of immunoreceptors. Immune cells can also be activated by aggregation of glycosylphosphatidylinositol-anchored proteins (GPI-APs). However, it is unknown whether cell triggering through these proteins, lacking transmembrane and cytoplasmic domains, also leads to PS externalization. Here we show that engagement of GPI-APs in rodent mast cells induces a rapid and reversible externalization of PS by a non-apoptotic mechanism. PS externalization triggered by GPI-AP-specific monoclonal antibodies was dependent on the activity of H(+)-ATP synthase and several other enzymes involved in mast cell signaling but was independent of cell degranulation, free cytoplasmic calcium up-regulation, and a decrease in lipid packing as determined by merocyanine 540 binding. Surprisingly, disruption of actin cytoskeleton by latrunculin B or plasma membrane integrity by methyl-beta-cyclodextrin had opposite effects on PS externalization triggered through GPI-AP or the high affinity IgE receptor. We further show that PS externalization mediated by GPI-APs was also observed in some other cells, and its extent varied with antibodies used. Interestingly, effects of different antibodies on PS externalization were additive, indicating that independent stimuli converge onto a signaling pathways leading to PS externalization. Our findings identify the cell surface PS exposure induced through GPI-AP as a distinct mechanism of cell signaling. Such a mechanism could contribute to "inside-out" signaling in response to pathogens and other external activators and/or to initiation of other functions associated with PS externalization.

Modulation of immunological synapse by membrane-bound and soluble ligands

An efficient adaptive immune response should prevent pathogen infections and tumor growth without causing significant damage to host constituents. A crucial event determining the balance between tolerance and immunity is antigen recognition by T cells on the surface of antigen presenting cells (APC). Several molecular contacts at the interface between T cells and APCs contribute to define the nature of the adaptive immune response against a particular antigen. Upon TCR engagement by a peptide-MHC complex (pMHC) on the surface of an APC, a specialized supra-molecular structure known as immunological synapse (IS) assembles at the interface between these two cells. This structure involves massive re-distribution of membrane proteins, including TCR and pMHC complexes, as well as co-stimulatory and adhesion molecules. Furthermore, IS assembly leads to several important intracellular events necessary for T cell activation, such as recruitment of signaling molecules and cytoskeleton rearrangements. Because IS assembly leads to major consequences on the function of T cells, several studies have attempted to identify both soluble and membrane-bound molecules that could contribute to modulate the IS function. Here we describe recent literature on the regulation of IS assembly and modulation by TCR/pMHC binding kinetics, chemokines and cytokines focusing on their role at controlling the balance between adaptive immunity and tolerance.

Structural basis for the function and regulation of the receptor protein tyrosine phosphatase CD45

CD45 is the prototypic member of transmembrane receptor-like protein tyrosine phosphatases (RPTPs) and has essential roles in immune functions. The cytoplasmic region of CD45, like many other RPTPs, contains two homologous protein tyrosine phosphatase domains, active domain 1 (D1) and catalytically impaired domain 2 (D2). Here, we report crystal structure of the cytoplasmic D1D2 segment of human CD45 in native and phosphotyrosyl peptide-bound forms. The tertiary structures of D1 and D2 are very similar, but doubly phosphorylated CD3ζ immunoreceptor tyrosine-based activation motif peptide binds only the D1 active site. The D2 “active site” deviates from the other active sites significantly to the extent that excludes any possibility of catalytic activity. The relative orientation of D1 and D2 is very similar to that observed in leukocyte common antigen–related protein with both active sites in an open conformation and is restrained through an extensive network of hydrophobic interactions, hydrogen bonds, and salt bridges. This crystal structure is incompatible with the wedge model previously suggested for CD45 regulation.

Role of Thy-1 in in vivo and in vitro neural development and regeneration of dorsal root ganglionic neurons

We have examined the expression of Thy-1, an abundant glycosylphosphatidylinositol (GPI)-anchored glycoprotein, in dorsal root ganglia (DRG) and associated nerve fascicles, during postnatal development and following a nerve crush. The expression levels of Thy-1 in DRG neurons, dorsal roots, and central processes in spinal cord were rather low at postnatal day 2, and gradually increased as DRG neurons matured. During early development, the expression of Thy-1 within DRG neurons was low and equally distributed between plasma membrane and cytosol. With maturation, the staining intensities of Thy-1 in both the plasma membrane and the cytosol of DRG neurons became increased. We also studied Thy-1 expression in the regeneration of mature DRG neurons following the crush injury of sciatic nerve. Two days after the crush injury, Thy-1 expression dramatically decreased in the DRG neurons on the lesion side. Between 4 and 7 days after the injury, the expression of Thy-1 gradually increased and returned to a normal level 1 week after the sciatic nerve crush. The time course of the up-regulation of Thy-1 expression during regeneration matched that of the recovery of sensory functions, such as pain withdraw reflex, placing reflex, and the score of Basso-Beattie-Bresnahan Locomotor Rating Scale. Taken together, our results suggest that Thy-1 expression is developmentally regulated and is closely associated with the functional maturation of DRG neurons during both postnatal development and nerve regeneration. Furthermore, perturbation of Thy-1 function with anti-Thy-1 antibodies promoted neurite outgrowth from primary cultured DRG neurons, again confirming the inhibitory role of Thy-1 on neurite outgrowth.

Enhanced responses of glycosylphosphatidylinositol anchor-deficient T lymphocytes

The functions of GPI-anchored proteins in T lymphocyte activation have been controversial. This issue was addressed by studying the responses of T lymphocytes from T lymphocyte-specific GPI anchor-deficient mice to different stimuli that normally allow coligation of TCR and GPI-anchored proteins. Stimulation of GPI anchor-deficient T lymphocytes with ConA induced 2-fold higher proliferative responses than did normal cells. In response to allogeneic stimulation, proliferation of GPI anchor-deficient T lymphocytes was enhanced 2- to 3-fold. The response to ConA of a GPI anchor-deficient anti-OVA T lymphocyte clone generated from these mice was approximately 3-fold higher than that of cells from the same clone in which GPI anchor expression was restored by retroviral transduction. The response of the GPI anchor-deficient cloned anti-OVA T lymphocytes to antigenic stimulation was similar to that of the retrovirally restored cells. These results indicate that coligation with GPI-anchored proteins counteracts the response to TCR stimulation by ConA or alloantigen but not protein Ag.

Thy-1: More than a Mouse Pan-T Cell Marker

Thy-1 (CD90) is a small GPI-anchored protein that is particularly abundant on the surface of mouse thymocytes and peripheral T cells. T cell proliferation and cytokine synthesis in response to Thy-1 cross-linking by specific mAb suggests a role for Thy-1 in mouse T lymphocyte activation. However, a physiological ligand or counterreceptor for murine Thy-1 in the lymphoid compartment has not yet been identified. Thy-1 cross-linking, in the context of strong costimulatory signaling through CD28, results in an activating signal that can at least partially substitute for TCR signaling during mouse T cell activation. Remarkably, Thy-1 cross-linking also results in the potent costimulation of T cells activated through the TCR. This novel dual signaling capacity suggests a possible role for Thy-1 in the maintenance of T cell homeostasis in the absence of TCR triggering, as well as potentiating Ag-induced T cell responses.

A role for interleukin-12 in the regulation of T cell plasma membrane compartmentation

The immunological synapse initiates the clustering and stabilization of the T cell receptor by the formation of a large lipid microdomain that accumulates (e.g. CD4/CD8) and segregates (e.g. CD45 and LFA-1) some proteins of the T cell plasma membrane. This work shows that a fraction of transmembrane glycoproteins CD26 and CD45 (the R0 isoform in particular) is present in the rafts of fresh and activated human T lymphocytes. CD26 is proposed as the costimulator of TCR-dependent activation, and CD45 is essential to the T cell activation process because it dephosphorylates at least the inhibitory site of Src kinases. These findings support a more complex model of compartmentation, depending on the stage of T cell maturation and post-transcriptional and post-translational regulation. In addition, interleukin 12 (IL-12; inducer of TH1 responses) drives CD26 and CD45R0 to particular microdomains, thereby involving interleukins in the rules governing raft inclusion or exclusion. The physical association of CD26 and CD45R0 has long been reported. The results presented in this work fit a model in which IL-12 up-regulates a certain type of CD26 expression that interacts on the cell surface with CD45R0, near but outside of the raft core. The use of antisense oligonucleotides for the CD26 mRNAs demonstrated that both events (enhanced by IL-12), CD26-CD45R0 association and membrane compartment redistribution, are related. Thus, CD26 could be part of a shuttling mechanism for CD45 that regulates membrane tyrosine-phosphatase activities, e.g. to control IL-12 receptor-dependent signal transduction.

Apoptosis mediated through CD45 is independent of its phosphatase activity and association with leukocyte phosphatase-associated phosphoprotein

Besides the well-recognized role of CD45 as a major player in TCR signaling, we and others have demonstrated that cross-linking of CD45 with mAbs can induce cell death in T lymphocytes. To investigate the role of CD45 phosphatase activity in apoptosis induction, we expressed either wild-type or phosphatase-dead CD45 molecules in a CD45-deficient BW5147 T cell line. We show here that the phosphatase activity of CD45 was not required for apoptosis triggering after cross-linking of the molecule. It is noteworthy that a revertant of the CD45-negative BW5147 cell line, expressing a truncated form of CD45 lacking most of the cytoplasmic domain, was also susceptible to CD45-mediated death. Moreover, we also demonstrate that leukocyte phosphatase-associated phosphoprotein expression is totally dispensable for CD45-mediated apoptosis to occur. Taken together, these results strongly suggest a role for the extracellular and/or the transmembrane portion of CD45 in apoptosis signaling, which contrasts with the previously reported functions for CD45 in T lymphocytes.

Distinct patterns of membrane microdomain partitioning in Th1 and th2 cells

Here we show that activated Th1 and Th2 cells have distinct patterns of membrane compartmentalization into lipid rafts. TCR complex members are recruited efficiently to rafts and aggregate with rafts at the site of MHC/peptide contact in Th1 cells but not Th2 cells. TCR/raft association in Th1 cells is deficient in the absence of CD4, suggesting that CD4 aids recruitment of the TCR to rafts. We show differential utilization of rafts in Th1 and Th2 cells by cholesterol depletion studies, which alters calcium signaling in Th1 but not Th2 cells. Furthermore, Th2 cells have a decreased ability to respond to low-affinity peptide stimulation. These studies indicate that components of membrane microdomains are differentially regulated in functionally distinct CD4 T cells.

Potent Reversible Inhibitors of the Protein Tyrosine Phosphatase CD45

The cytosolic portion of CD45, a major transmembrane glycoprotein found on nucleated hematopoietic cells, contains protein tyrosine phosphatase activity and is critical for T-cell receptor-mediated T-cell activation. CD45 inhibitors could have utility in the treatment of autoimmune disorders and organ graft rejection. A number of 9,10-phenanthrenediones were identified that reversibly inhibited CD45-mediated p-nitrophenyl phosphate (pNPP) hydrolysis. Chemistry efforts around the 9,10-phenanthrenedione core led to the most potent inhibitors known to date. In a functional assay, the compounds were also potent inhibitors of T-cell receptor-mediated proliferation, with activities in the low micromolar range paralleling their enzyme inhibition. It was also discovered that the nature of modification to the phenanthrenedione pharmacophore could affect selectivity for CD45 over PTP1B (protein tyrosine phosphatase 1B) or vice versa.

CD45: new jobs for an old acquaintance

Identified as the first and prototypic transmembrane protein tyrosine phosphatase (PTPase), CD45 has been extensively studied for over two decades and is thought to be important for positively regulating antigen-receptor signaling via the dephosphorylation of Src kinases. However, new evidence indicates that CD45 can function as a Janus kinase PTPase that negatively controls cytokine-receptor signaling. A point mutation in CD45, which appears to affect CD45 dimerization, and a genetic polymorphism that affects alternative CD45 splicing are implicated in autoimmunity in mice and multiple sclerosis in humans. CD45 is expressed in multiple isoforms and the modulation of specific CD45 splice variants with antibodies can prevent transplant rejections. In addition, loss of CD45 can affect microglia activation in a mouse model for Alzheimer's disease. Thus, CD45 is moving rapidly back into the spotlight as a drug target and central regulator involved in differentiation of multiple hematopoietic cell lineages, autoimmunity and antiviral immunity.

Changes in the T cell receptor macromolecular signaling complex and membrane microdomains during T cell development and activation

Initiation and propagation of T cell receptor signaling pathways involves the mobilization and aggregation of a variety of signaling intermediates with the T cell receptor and associated molecules into specialized signaling complexes. Accumulating evidence suggests that differential regulation of the formation and composition of the T cell receptor macromolecular signaling complex may affect the different biological consequences of T cell activation. The regulatory mechanisms involved in the assembly of these complexes remains poorly understood, but in part is affected by the avidity of the T cell receptor ligand, co-stimulatory signals, and by the differentiation state of the T cell.

The CD45 tyrosine phosphatase regulates Campath-1H (CD52)-induced TCR-dependent signal transduction in human T cells

Campath-1H, a humanized mAb undergoing clinical trials for treatment of leukemia, transplantation and autoimmune diseases, produces substantial lymphocyte depletion in vivo. The antibody binds to CD52, a highly glycosylated molecule attached to the membrane by a glycosylphosphatidylinositol anchor. Cross-linked Campath-1H is known to activate T cells in vitro. We have investigated the molecular basis for these effects by comparing the protein tyrosine phosphorylation signals induced by Campath-1H and the CD3 mAb OKT3 in primary T cells, and in CD45(+)TCR(+), CD45(-)TCR(+) and CD45(+)TCR(-) Jurkat subclones transfected with CD52. Our results show that Campath-1H triggers similar tyrosine phosphorylation events as OKT3 in both primary T cells and in the CD45(+)TCR(+) Jurkat sub-clone, albeit at quantitatively lower levels. However, no phospholipase C gamma 1 activation nor calcium signals were detected in response to CD52 ligation. The CD52-mediated induction of protein tyrosine phosphorylation was absolutely dependent upon the expression of both the TCR and the CD45 phosphotyrosine phosphatase at the cell surface. Cross-linking of Campath-1H was essential for signal transduction in all cells investigated. Fluorescence resonance energy transfer was used to demonstrate CD52 homo-association at the cell surface in Jurkat T cells in a TCR- and CD45-independent manner, and CD52-TCR association in CD45(+)TCR(+) cells. We propose a model to explain the activating effects of Campath-1H in which CD52 mAb cross-linking causes the trapping of TCR polypeptides within molecular complexes at the cell surface, thereby inducing signals via the TCR by a process which depends on the CD45-mediated regulation of the p56(lck) and p59(fyn) tyrosine kinases.

Peripheral CD4+ T Cell Maturation Recognized by Increased Expression of Thy-1/CD90 Bearing the 6C10 Carbohydrate Epitope

The SM6C10 IgM autoantibody recognizes a surface determinant, 6C10, that is highly expressed on all immature thymocytes. In contrast, its expression on peripheral T cells appears developmentally regulated, i.e., absent from most naive T cells in spleen of neonatal mice, but expressed on 40–80% of naive CD4+ T cells in adult. In this paper, we demonstrate that SM6C10 recognizes a carbohydrate epitope on the Thy-1 glycoprotein using immunoprecipitation analysis, by binding to affinity-purified Thy-1 in an ELISA, and by sensitivity to N-glycosidase-F treatment. Retroviral Thy-1 gene transduction experiments into Thy-1− variant T cell lines and a pro-B cell line provide evidence that 6C10 glycosylated Thy-1 expression is not restricted to T cells but depends on the recipient cell. Therefore, differences in 6C10 levels among Thy-1+ T cells in mice likely reflect developmental regulation of posttranslational modification of the Thy-1 glycoprotein. The ability of naive CD4+ T cells to respond to anti-Thy-1 stimulation increases from neonate to adult, and 6C10− naive cells from adult mice respond poorly compared with 6C10+ cells, similar to the cells in neonatal mice. These results suggest that there is functional maturation by peripheral CD4+ T cells that coincides with 6C10 glycosylated Thy-1 up-regulation, and natural autoantibody recognizes this 6C10 carbohydrate epitope.

Interactions of CD45-associated protein with the antigen receptor signaling machinery in T-lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase playing an essential role during T-cell activation. This function relates to the ability of CD45 to regulate p56(lck), a cytoplasmic protein tyrosine kinase necessary for T-cell antigen receptor (TCR) signaling. Previous studies have demonstrated that CD45 is constitutively associated in T-lymphocytes with a transmembrane molecule termed CD45-AP (or lymphocyte phosphatase-associated phosphoprotein). Even though the exact role of this polypeptide is unclear, recent analyses of mice lacking CD45-AP have indicated that its expression is also required for optimal T-cell activation. Herein, we wished to understand better the function of CD45-AP. The results of our studies showed that in T-cells, CD45-AP is part of a multimolecular complex that includes not only CD45, but also TCR, the CD4 and CD8 coreceptors, and p56(lck). The association of CD45-AP with TCR, CD4, and CD8 seemed to occur via the shared ability of these molecules to bind CD45. However, binding of CD45-AP to p56(lck) could take place in the absence of other lymphoid-specific components, suggesting that it can be direct. Structure-function analyses demonstrated that such an interaction was mediated by an acidic segment in the cytoplasmic region of CD45-AP and by the kinase domain of p56(lck). Interestingly, the ability of CD45-AP to interact with Lck in the absence of other lymphoid-specific molecules was proportional to the degree of catalytic activation of p56(lck). Together, these findings suggest that CD45-AP is an adaptor molecule involved in orchestrating interactions among components of the antigen receptor signaling machinery. Moreover, they raise the possibility that one of the functions of CD45-AP is to recognize activated Lck molecules and bring them into the vicinity of CD45.

Contribution of kinases and the CD45 phosphatase to the generation of tyrosine phosphorylation patterns in the T-cell receptor complex zeta chain

The zeta subunit of the T-cell receptor complex plays a crucial role in coupling the antigen binding alphabeta and gammadelta heterodimers to the downstream activation pathways. Three tandem amino acid sequence motifs containing pairs of exactly spaced Tyr-X-X-Leu/Ile sequences, designated as Immunoreceptor Tyrosine-based Activation Motifs (ITAMs), control this function. The phosphorylated forms of ITAMs serve as docking sites for several src homology 2 (SH2) domain containing signaling proteins. The composition of the assembled signaling complex and the outcome of cell activation depends on the tyrosine phosphorylation pattern of the zeta polypeptide. The mechanism that conducts the generation of various phosphorylated forms has not yet been well established. In this study we have analyzed the ability of src family tyrosine kinases and the CD45 tyrosine phosphatase in determining the phosphorylation state of the different ITAMs and the individual tyrosine residues of the TCR zeta chain. The intracellular part of the zeta chain was phosphorylated by src family tyrosine kinases, p56lck and p59fyn in vitro. Synthetic oligopeptides representing full-length or half-sized ITAMs with a single tyrosine residue were also phosphorylated by both p56lck and p59fyn. In contrast, an additional membrane proximal tyrosine residue in the human zeta chain, located outside of the ITAMs, was not phosphorylated. We also examined the activity of the CD45 phosphatase, using a panel of ITAM derivatives, in which one or both tyrosines were phosphorylated. The efficiency of ITAM dephosphorylation by CD45 was dependent on the primary sequence of the oligopeptides and the position of the phosphotyrosine residues. The in vitro data suggest that the CD45 phosphatase rather than the tyrosine kinase(s) may control the generation of specific phosphorylation patterns of the zeta chain during cell activation.

Spontaneous clustering of Thy-1 antigens on CD4+ CD8+ thymocytes lacking TCR engagement by MHC/peptide complexes

While much is known concerning CD4+ CD8+ thymocytes positively or negatively selected through interaction of their TCR with self peptides bound to self-MHC molecules, little is known of the majority of CD4+ CD8+ thymocytes lacking this interaction. We developed a monoclonal antibody (mAb) 1D11, the ligand of which (1D11-L) is expressed on 60-70% of CD4+ CD8+ thymocytes but not on other subsets of thymocytes and peripheral T cells. 1D11-L expression on CD4+ CD8+ thymocytes reversely correlates with their TCR engagement, in vitro and in vivo. In addition, 1D11-L+ CD4+ CD8+ thymocytes were more susceptible than 1D11-L- CD4+ CD8- thymocytes to apoptosis. We also found that T lineage cells other than CD4+ CD8+ thymocytes and a Thy-1-expressing fibroblast cell line became positive for 1D11-L by cross-linking their Thy-1 antigens with anti-Thy-1 mAb but not with their Fab fragment, suggesting that 1D11 recognizes multimerized Thy-1 antigens. Confocal laser scanning microscopy revealed that Thy-1 antigens as well as 1D11-L are clustered on some CD4+ CD8+ thymocytes but not on the other subsets of thymocytes. Taken together, we suggest that clustering of Thy-1 antigens spontaneously and specifically occurs on CD4+ CD8+ thymocytes lacking TCR engagement by MHC/peptide complexes.

The triggering signal dictates the effect of docosahexaenoic acid on lymphocyte function in vitro

Docosahexaenoic acid (DHA) is an n-3 fatty acid beneficial to several human conditions including inflammation and autoimmune disease. To better understand the effect of DHA on immunity, we monitored the rise in cytosolic free calcium, interleukin 2 receptor (IL2R) expression, and proliferation of splenic lymphocytes triggered with three different stimuli in the presence or absence of DHA. We found that 10 microg DHA/mL suppressed concanavalin A-induced mitogenesis and the mixed lymphocyte reaction while concurrently enhancing proliferation stimulated with anti-Thy-1 antibodies. Proliferation, as measured by [3H]thymidine incorporation after 2 to 5 d of culture, was affected by DHA, but earlier activation effects such as elevation of cytosolic free calcium and IL2R expression were not altered. These results imply that DHA incorporated into membrane phospholipids differentially affects the activity of distinct membrane-bound receptors and signaling molecules. This result suggests that DHA may be used to modulate immune responses selectively, e.g., to suppress undesired autoimmunity while maintaining protective immunity.

Signal transduction in leucocytes via GPI-anchored proteins: an experimental artefact or an aspect of immunoreceptor function?

Membrane proteins anchored in the membrane via a glycolipid glycosylphosphatidylinositol (GPI) as well as some glycolipids are able to transduce signals and induce diverse functional responses in cells upon their cross-linking via antibodies or natural ligands. In some cases this signaling capacity seems to be due to associations of these molecules with specific transmembrane proteins. GPI-anchored proteins are components of membrane microdomains enriched in glycosphingolipids and cholesterol and devoid of most transmembrane proteins. These membrane specializations are relatively resistant to solubilization in solutions of some mild detergents at low temperatures. These 'GPI-microdomains' contain also cytoplasmic signaling molecules such as Src-family protein tyrosine kinases and trimeric G-proteins. Thus, at least some signaling elicited upon cross-linking of GPI-anchored proteins and glycolipids may be due to perturbation of the signaling molecules associated with these microdomains. It is suggested that these specialized areas of the membrane rich in signaling molecules interact with immunoreceptors (TCR, BCR, Fc receptors) cross-linked upon their interactions with ligands and importantly contribute to initiation of proximal phases of their signaling pathways.

Functional heterogeneity of Thy-1 membrane microdomains in rat basophilic leukemia cells

Antibody-mediated cross-linking of Thy-1 glycoprotein on the surface of rat mast cells and rat basophilic leukemia (RBL) cells initiates biochemical events which culminate in secretion of allergy mediators. Thy-1, like some other glycosylphosphatidylinositol (GPI)-anchored proteins, forms detergent-insoluble complexes containing protein tyrosine kinases (PTK) and some other molecules which are implicated in the signaling pathway. On the surface of a rat mast cell there are more than 10(6) Thy-1 molecules; however, it is not known which fraction of them is involved in transmembrane signaling, and what exactly is the heterogeneity of Thy-1 complexes. Using sucrose density gradient ultracentrifugation of detergent-lysed RBL cells we found that the density of Thy-1 complexes depended on the detergent used and the lysis conditions employed. Sepharose 4B gel chromatography fractionation followed by density gradient ultracentrifugation revealed both size and density heterogeneity of Thy-1 and Lyn PTK complexes. Cross-linking of surface Thy-1 caused significant changes in the density of these complexes, and an increase in Lyn kinase activity in low/medium-density fractions. Thy-1 in low-density fractions was relatively resistant to cleavage with phosphatidylinositol-specific phospholipase C (PI-PLC). Interestingly, removal of only a small fraction of surface Thy-1 by PI-PLC abolished the cell activation as determined by tyrosine phosphorylation of certain proteins. When Triton X-100 lysates were fractionated at 12000 x g, about 50 % of Thy-1 remained associated with the nuclear/cytoskeleton pellet; this fraction of Thy-1 exhibited an increased sensitivity to PI-PLC. Confocal laser scanning microscopy on fixed cells revealed that the total Thy-1 was relatively homogeneously distributed over the plasma membrane, whereas the PI-PLC-resistant Thy-1 was found mostly in small clusters. The combined data suggest that specialized membrane microdomains enriched in Thy-1 with increased sensitivity to PI-PLC are directly involved in coupling Thy-1 aggregation to transmembrane signaling.

Physical and functional association between thymic shared antigen-1/stem cell antigen-2 and the T cell receptor complex

Thymic shared antigen-1 (TSA-1)/stem cell Ag-2 (Sca-2) is a glycosylphosphatidylinositol (GPI)-anchored antigen expressed on lymphocytes. We have previously demonstrated that a signal via TSA-1/Sca-2 inhibits T cell receptor (TCR)-mediated T cell activation and apoptosis. To elucidate a molecular mechanism for TSA-1-mediated modulation of the TCR-signaling pathway, we examined whether TSA-1 is physically coupled to the TCR in the present study. TSA-1 was clearly associated with CD3zeta chains in T cell hybridomas, activated T cells, and COS-7 cells transfected with TSA-1 and CD3zeta cDNA. The physical association was confirmed on the surface of T cells in immunoprecipitation and confocal microscopy. The analysis using stable and transient transfectants expressing a transmembrane form of TSA-1 revealed that the association of CD3zeta did not require the GPI anchor of TSA-1. Finally, tyrosine phosphorylation of CD3zeta chains was induced after stimulation with anti-TSA-1, suggesting that a functional association between these two molecules also exists. These results imply that the physical association to CD3zeta underlies a regulatory role of TSA-1/Sca-2 in the TCR-signaling pathway.

The structure and function of CD26 in the T-cell immune response

CD26 is a widely distributed 110 kD cell-surface glycoprotein with known dipeptidyl-peptidase IV (DPP-IV) activity in its extracellular domain. This ecto-enzyme is capable of cleaving amino terminal dipeptides from polypeptides with either L-proline or L-alanine in the penultimate position. On human T cells, CD26 expression appears late in thymic differentiation and is preferentially restricted to the CD4+ helper/memory population, and CD26 can deliver a potent co-stimulatory T-cell activation signal. The cDNA sequence of CD26 predicts a type II membrane protein with only 6 amino acids in its cytoplasmic region, suggesting that, in addition to DPP-IV enzyme activity, other signal-inducing molecules may be associated with CD26. Considerable evidence exists that CD26 interacts, presumably in its extracellular domain, with both CD45, a protein tyrosine phosphatase, and adenosine deaminase (ADA), each of which is capable of functioning in a signal transduction pathway. In addition, CD26 is the receptor for ADA, and ADA on the cell surface is involved in an important immunoregulatory mechanism by which released ADA binds to the cell-surface ADA. This multifunctional molecule may be involved in cell migration and the HIV-1-associated loss of CD4+ cells through the process of programmed cell death. Thus, CD26 appears to play a key role in a number of aspects of lymphocyte function.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

The role of CD45 and CD45-associated molecules in T cell activation

CD45 (lymphocyte common antigen) is a receptor-linked protein tyrosine phosphatase that is expressed on all leucocytes, and which plays a crucial role in the function of these cells. On T cells the extracellular domain of CD45 is expressed in several different isoforms, and the particular isoform(s) expressed depends on the particular subpopulation of cell, their state of maturation, and whether or not they have previously been exposed to antigen. It has been established that the expression of CD45 is essential for the activation of T cells via the TCR, and that different CD45 isoforms display a different ability to support T cell activation. Although the tyrosine phosphatase activity of the intracellular region of CD45 has been shown to be crucial for supporting signal transduction from the TCR, the nature of the ligands for the different isoforms of CD45 have been elusive. Moreover, the precise mechanism by which potential ligands may regulate CD45 function is unclear. Interestingly, in T cells CD45 has been shown to associate with numerous molecules, both membrane associated and intracellular; these include components of the TCR-CD3 complex and CD4/CD8. In addition, CD45 is reported to associate with several intracellular protein tyrosine kinases including p56lck and p59fyn of the src family, and ZAP-70 of the Syk family, and with numerous proteins of 29-34 kDa. These CD45-associated molecules may play an important role in regulating CD45 tyrosine phosphatase activity and function. However, although the role of some of the CD45-associated molecules (e.g. CD45-AP and LPAP) has become better understood in recent years, the role of others still remains obscure. This review aims to summarize recent findings on the role of CD45 and CD45-associated molecules in T cell activation, and to highlight issues that seem relevant to ongoing research in this area.

Distinct interactions among GPI-anchored, transmembrane and membrane associated intracellular proteins, and sphingolipids in lymphocyte and endothelial cell plasma membranes

Glycosylphosphatidylinositol (GPI)-anchored glycoproteins are enriched in sphingolipid-rich plasma membrane domains, which are often isolated as low-density membrane complexes. This association is believed to arise from the interactions between the GPI-acyl chains and sphingolipids, but is not fully understood. In this study, we compared the physical properties of GPI-anchored glycoproteins from a non-polarized (murine T-lymphocyte) and a polarized (human endothelial) cell by equilibrium density gradient centrifugation after extraction by detergents under identical conditions. Unlike those on epithelial cells, the GPI-anchored proteins of lymphocytes (Thy-1 and the heat stable antigen CD24) were enriched in the floating fractions after extraction over a wide range of octylglucoside concentrations. In contrast, the floatability of endothelial GPI-anchored CD59 was markedly diminished, not only by octylglucoside, but also by increasing concentrations of Triton X-100. Distribution of cholera toxin binding ganglioside GM1 in the sucrose gradient fractions closely followed that of the GPI-anchored proteins in both lymphocytes and endothelial cells under most extraction conditions. Analysis of the intracellular acylated molecules revealed that a significant amount of p56(lck) was always associated with the floating GPI-rich fractions of lymphocytes when extracted by Triton X-100 or octylglucoside at 4 degrees C, while the behaviour of endothelial cell caveolin was comparable to that of CD59. The transmembrane glycoproteins CD45 in lymphocytes and MHC class I antigen in endothelial cells interacted weakly with GPI domains, whereas endothelial CD44 and lymphocyte CD26 displayed a strong association. These results show that: (1) the physical properties of different GPI-anchored proteins may vary significantly; and (2) transmembrane and acylated intracellular proteins could be associated with GPI domains to a variable extent. These differences probably reflect cell type-specific interactions of GPI anchors with the sphingolipid framework of plasma membranes, as well as extracellular interactions of GPI-anchored glycoproteins with neighbouring cell surface molecules.

Thy-3, a developmentally regulated T-cell glycoprotein associated to Thy-1 in detergent-resistant membrane microdomains

The function of the Thy-1 molecule, a major T-cell glycoprotein, is still obscure. Its functional properties might be due to the anchoring via a glycosylphosphatidylinositol group which favors gathering of molecules in functional membrane subregions called microdomains. Using novel monoclonal antibodies, we describe a 53-kDa Thy-1-associated glycoprotein called Thy-3. Thy-3 expression is restricted to T lymphocytes, becomes detectable on double-positive thymocytes, and depends on that of Thy-1. Anti-Thy-3 antibodies immunoprecipitate Thy-1 and Thy-3 or Thy-3 alone in detergents which preserve or disrupt microdomains, respectively. These antibodies induce thymocyte aggregation and interfere with adhesion of thymocytes to a thymic epithelial cell line as previously shown with anti-Thy-1 antibodies. Thus, Thy-3 is a T lineage-specific glycoprotein associated to Thy-1 in membrane microdomains and might contribute to the function of Thy-1 in T-cell differentiation.

The phosphotyrosine phosphatase inhibitor-phenylarsine oxide restores defective phosphoinositide hydrolysis response in anergic C3H-gld/gld lymphocytes

Mice homozygous for the gld (generalized lymphoproliferative disease) mutation develop both lymphadenopathy and autoimmune disease. CD4-CD8- (double negative, DN) T cells comprise the major population of T cells in mature C3H-gld/gld peripheral lymphoid tissues. These DN T cells are unresponsive to many forms of stimuli and have previously been shown to exhibit abnormally elevated levels of membrane phosphotyrosine phosphatase (PTPase) activity. In the present study, we demonstrate that IP3 production in response to mitogenic stimulation with Con A or anti-CD3 mAb (145-2CII) is significantly diminished in C3H-gld/gld lymphocytes when compared to that in congenic C3H(-)+/+ cells. The capacity to produce this second-messenger can be restored by pretreating C3H-gld/gld cells with the PTPase inhibitor, phenylarsine oxide (PAO). Although the inhibition of PTPase activity by treatment with PAO did restore C3H-gld/gld cell ability to produce IP3, the signal did not lead to lymphocyte proliferation, but instead to cell death. Our results suggest that the altered phosphoinositide hydrolysis observed in the mutant cells is related to their elevated membrane PTPase activity and that the anergy in these cells is at least in part related to the abnormally high activity of endogenous PTPases.

The extracellular domain of CD45 controls association with the CD4-T cell receptor complex and the response to antigen-specific stimulation

The CD45 tyrosine phosphatase plays an important role in regulating T lymphocyte activation, but the function of the different isoforms of CD45 is not known. T cell transfectants have been prepared that express individual CD45 isoforms in cells with a well-defined T cell receptor (TCR) from the D10 T helper 2 clone. We find that cells bearing low molecular weight CD45 isoforms are far more efficient in responding to stimulation with peptide and antigen-presenting cells compared with cells bearing high molecular weight CD45 isoforms. One hypothesis for the preferential activation of cells that express low molecular weight CD45 isoforms is that they interact with other cell surface antigens important in TCR signaling, altering their phosphorylation status and affecting the character of the signal transduction pathway. In this report, using cells expressing single isoforms, we demonstrate that low molecular weight isoforms of CD45 preferentially associate with CD4 and the TCR complex compared with high molecular weight isoforms. The molecular basis for this interaction was further examined using a glycosyl phosphatidyl inositol (GPI)-linked form of CD45Null (lacking tyrosine phosphatase domains), which preferentially associated with CD4 compared with GPI-linked CD45ABC, and cytoplasmic tail mutants of CD4, which retained the ability to coassociate. Using this panel of transfectants, it is clear that the interaction between CD4 and CD45 does not require the cytoplasmic domains of CD45, but is dependent on the specific external domain of the various isoforms: low molecular weight species were more likely to associate with the CD4-TCR complex than the higher molecular weight isoforms, and their ability to coassociate correlated with the magnitude of the response to specific antigen.

Rapid induction of a novel costimulatory activity on B cells by CD40 ligand

BACKGROUND

T cells and B cells communicate by direct cell--cell interaction that is crucial to the functioning of the immune system. It is well established that the interaction between B-cell-expressed CD40 and T-cell-expressed CD40 ligand (CD40L) is critical for T-cell-dependent antibody responses, but the role of this interaction in T-cell responses is less clear. In this study, we have used mice with targeted mutations in the genes encoding CD40L or CD28 to investigate how the CD40-CD40L interaction induces on B cells a costimulatory activity that acts in addition to antigen to trigger T-cell growth.

RESULTS

We show that T cells from Cd40L-deficient mice induce a substantially reduced costimulatory activity on B cells compared to wild-type T cells, particularly at early time points. Surprisingly, T cells, from CD40L-deficient mice induce similar levels of B7-1 and B7-2 as do wild-type T cells. We further show that the CD40L-mediated induction of costimulatory activity precedes the induction of B7-1, B7-2 and the heat-stable antigen (HSA). CD4 T cells isolated from the CD28-deficient mice can receive costimulatory activity from CD40L-induced B cells, demonstrating that the induced molecules can costimulate T cells by a CD28-independent mechanism. We have generated a novel monoclonal antibody that inhibits the CD40L-induced costimulatory activity. Expression of the epitope detected by this monoclonal antibody correlates with the induction of the costimulatory activity, and the molecule recognized by the monoclonal antibody is a single chain of around 85 kDa, distinct from B7-1, B7-2, ICAM-1, ICAM-2, ICAM-3, HSA CD5, integrin and 4-1BB ligand.

CONCLUSIONS

Our results demonstrate that CD40L is both necessary and sufficient for rapid, T-cell-mediated induction of costimulatory activity on B cells. This costimulatory activity is distinct from B7-1 and B7-2, and is independent of CD28.

Characterization of the interaction between CD45 and CD45-AP

CD45, a leukocyte-specific transmembrane protein tyrosine phosphatase, is required for critical signal transduction pathways in immune responses. To elucidate the molecular interactions of CD45 with other proteins involved in CD45-mediated signal transduction pathways, we have recently cloned a 30-kDa phosphorylated protein, CD45-AP, which specifically associates with CD45. Binding analysis employing several deleted or chimeric forms of CD45-AP and CD45 demonstrated that the potential transmembrane segment of CD45-AP bound to the transmembrane portion of CD45. CD45-AP was found in particulate fractions of lymphocytes along with CD45, indicating that it is likely to be a transmembrane protein. In addition, CD45-AP was resistant to proteolysis by tosylphenylalanyl chloromethyl ketone-treated trypsin applied to intact cells. This is consistent with the most likely membrane orientation of CD45-AP predicted from the amino acid sequence, that is, only a short amino-terminal segment of CD45-AP is extracellular. We propose that CD45-AP interacts with CD45 at the plasma membrane and that the bulk of CD45-AP located in the cytoplasm act as an adapter which directs the interaction between CD45 and other molecules involved in CD45-mediated signal transduction pathways.

Influence of MHC class I molecules on T-cell proliferation induced by CD3 or Thy-1 stimulation

We have reported that class I- [and lymphocyte function-associated antigen-1 (LFA-1-)] specific monoclonal antibodies (mAb) inhibit anti-CD3-mediated activation of naive T cells. The present study investigated the mechanism of this inhibition. CD28-specific mAb augmented stimulation induced by soluble CD3 mAb, but this costimulation was also inhibited by anti-class I or anti-LFA-1 mAb. However, stimulation of T cells was not inhibited when activated B cells were present. Neither B7-1- nor B7-2-specific blocking mAb or soluble CTLA-4, CD40 or gp39 restored the inhibition. Thus, other molecules expressed on activated B cells are implicated for T-cell activation, which could compensate blockade of class I or LFA-1 molecules. Inhibition induced by class I-specific mAb could potentially be mediated through extracellular, transmembrane or cytoplasmic domains of the target molecules. These possibilities were evaluated by the use of mice transgenic for the Qa-2 molecule, selected for expression of Qa-2 at levels equivalent to classical class I molecules. Qa-2 is inserted in the membrane through phosphatidylinositol linkages. Antibodies directed to Qa-2 inhibited CD3-induced stimulation, demonstrating that cytoplasmic and transmembrane protein sequences of class I molecules are not necessary for the inhibitory effect. Inhibition thus presumably depends on extracellular domains. Finally, T cells from beta 2-microglobulin knock-out mice responded to CD3-specific mAb as well as their class I-positive littermates. Nevertheless, stimulation of T cells from these mice with mitogenic anti-Thy-1 mAb was markedly reduced. Signalling by Thy-1 and the CD3 complex may normally occur through pathways in which class I molecules are implicated.

5'-nucleotidase activates and an inhibitory antibody prevents neuritic differentiation of PC12 cells

Ecto-5'-nucleotidase catalyses the hydrolysis of AMP at the surface of a variety of cells whereas it is absent from others. In addition to its catalytic activity, a function in neural development and also its interaction with extracellular matrix proteins has been reported. In order to further elucidate the biological function of ecto-5'-nucleotidase we have investigated the effect of 5'-nucleotidase on nerve growth factor-induced differentiation of PC12 cells. Furthermore, we compared the effect of an inhibitory versus a non-inhibitory monospecific antibody against the enzyme on neuritic differentiation and survival of PC12 cells that constitutively express the enzyme. When coverslips are coated with the soluble form of ecto-5'-nucleotidase in addition to collagen, there is a considerable increase in nerve growth factor-induced neurite length during the first 24 h of culture. Addition of an antibody to a culture medium that inhibits 5'-nucleotidase activity to 33% of control values dramatically reduces the number of neurites per cell within 3 days of culture. The cells round up, cluster and eventually die. On the contrary, another antibody that had no significant effect on enzyme activity affected neither nerve growth factor-induced neurite formation nor survival of PC12 cells. Addition of adenosine (200 nM, 10 or 20 microM) to the culture medium did not influence PC12 cell differentiation. The effects induced by the inhibitory antibody could be only partially prevented by simultaneous application of adenosine. Our results suggest that 5'-nucleotidase is essential for nerve growth factor-induced neurite outgrowth and survival of PC12 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Flow cytometric monitoring of allergen induced basophil activation

Human basophils express many clustered differentiation antigens (CD), including CD45; however, none are specific for these cells. In a previous study, we described a two-color immunofluorescence procedure, employing antibodies to CD45 and IgE for the cytometric evaluation of basophils. In the present work, we show that when sensitized basophils are stimulated by allergenic preparations, they demonstrate an upregulation of CD45 as well as a decrease in anti-IgE binding. Since CD45 antigen modulation was observed with all aeroallergens tested and the decrease in IgE expression varied with allergenic preparations, the measurement of CD45 upregulation was used to evaluate basophil activation. Using this approach, reproducible results were observed when atopic patients were tested at different time intervals. In addition, we show that the upregulation of CD45 on allergen stimulated basophils is a very rapid phenomenon that is observed after a few minutes and that this rapid flow cytometric method can also be used for investigations on nonallergenic mediators of basophil stimulation such as calcium ionophores.

CD45 protein-tyrosine phosphatase is specifically associated with a 116-kDa tyrosine-phosphorylated glycoprotein

CD45 is a protein-tyrosine phosphatase expressed on all cells of hematopoietic origin. In an attempt to further characterize CD45 function, we set out to identify molecule(s) that specifically associate with CD45. A 116-kDa protein was detected in immunoprecipitates from CD45+ cells but not CD45- cells. The association between CD45 and this 116-kDa protein can be reconstituted by mixing lysates from CD45- cell lines with purified CD45. p116 appears to associate with CD45 through the external, transmembrane, or membrane-proximal region of CD45 since p116 is associated with a mutant form of CD45 possessing a truncated cytoplasmic domain. The association of p116 with CD45 is not isoform-specific as p116 associates equally well with various CD45 isoforms. We have determined that p116 is a tyrosine-phosphorylated glycoprotein and that it is associated with CD45 in all hematopoietic cells examined. Because of its broad distribution, it is possible that identification of p116 will provide additional insight into the function of CD45 in lymphoid as well as non-lymphoid hematopoietic cells.

Signal transduction by the antigen receptors of B and T lymphocytes

B and T lymphocytes of the immune system recognize and destroy invading microorganisms but are tolerant to the cells and tissues of one's own body. The basis for this self/non-self-discrimination is the clonal nature of the B and T cell antigen receptors. Each lymphocyte has antigen receptors with a single unique antigen specificity. Multiple mechanisms ensure that self-reactive lymphocytes are eliminated or silenced whereas lymphocytes directed against foreign antigens are activated only when the appropriate antigen is present. The key element in these processes is the ability of the antigen receptors to transmit signals to the interior of the lymphocyte when they bind the antigen for which they are specific. Whether these signals lead to activation, tolerance, or cell death is dependent on the maturation state of the lymphocytes as well as on signals from other receptors. We review the role of antigen receptor signaling in the development and activation of B and T lymphocytes and also describe the biochemical signaling mechanisms employed by these receptors. In addition, we discuss how signal transduction pathways activated by the antigen receptors may alter gene expression, regulate the cell cycle, and induce or prevent programmed cell death.

CD45 tyrosine phosphatase activity and membrane anchoring are required for T-cell antigen receptor signaling

T cells that lack the CD45 transmembrane tyrosine phosphatase have a variety of T-cell receptor (TCR) signaling defects that are corrected by reexpression of wild-type CD45 or its intracytoplasmic domains. In this study, a chimeric molecule containing the myristylation sequence of Src and the intracellular portion of CD45, previously shown to restore function in CD45- T cells, was mutagenized to determine if membrane-associated CD45 tyrosine phosphatase activity is required to restore TCR-mediated signaling in CD45- T cells. Abolition of enzymatic activity by substitution of a serine for a critical cysteine in the first catalytic domain resulted in failure of this molecule to restore TCR signaling. Another mutation, in which a single amino acid substitution destroyed the myristylation site, resulted in failure of the chimeric molecule to partition to the plasma membrane. Although expressed at high levels and enzymatically active, this form of intracellular CD45 also failed to restore normal signaling in CD45- T cells. These findings strongly suggest that CD45's function in TCR signaling requires its proximity to membrane-associated tyrosine phosphatase substrates.

CD45 tyrosine phosphatase activity and membrane anchoring are required for T-cell antigen receptor signaling

T cells that lack the CD45 transmembrane tyrosine phosphatase have a variety of T-cell receptor (TCR) signaling defects that are corrected by reexpression of wild-type CD45 or its intracytoplasmic domains. In this study, a chimeric molecule containing the myristylation sequence of Src and the intracellular portion of CD45, previously shown to restore function in CD45- T cells, was mutagenized to determine if membrane-associated CD45 tyrosine phosphatase activity is required to restore TCR-mediated signaling in CD45- T cells. Abolition of enzymatic activity by substitution of a serine for a critical cysteine in the first catalytic domain resulted in failure of this molecule to restore TCR signaling. Another mutation, in which a single amino acid substitution destroyed the myristylation site, resulted in failure of the chimeric molecule to partition to the plasma membrane. Although expressed at high levels and enzymatically active, this form of intracellular CD45 also failed to restore normal signaling in CD45- T cells. These findings strongly suggest that CD45's function in TCR signaling requires its proximity to membrane-associated tyrosine phosphatase substrates.

Evidence for an association of CD45 with 32,000-33,000 MW phosphoproteins on murine T and B lymphocytes

In human T cells CD45 is reported to associate with both cell surface and intracellular molecules including CD2, CD4/CD8, CD5, p56lck and p59fyn. In this study the association of molecules with CD45 in murine T lymphocytes was explored using biotinylation, chemical cross-linking, immunoprecipitation and 32P-labelling. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of CD45 monoclonal antibody (mAb) (S-450-15.2) immunoprecipitates from Triton X-100 lysates of murine thymocytes that were surface biotinylated and treated with the chemical cross-linker 3,3'-dithio-bis(sulpho-succinimidylpropionate) (DTSSP) showed that CD45 can be chemically linked to molecules of 25,000-32,000, 42,000 and 60,000-70,000 MW. The CD45 mAb also co-precipitated a prominent 32,000 MW molecule from digitonin lysates of surface biotinylated murine thymocytes, splenocytes and D10 cells, but a weaker association was also detected on splenic B cells and on the murine B-cell lymphoma line A20. The results suggest that in these cells CD45 is associated with a 32,000 MW molecule which is exposed extracellularly. Experiments in which thymocytes were biotinylated after permeabilization with lysolecithin showed that additional molecules of 33,000, 55,000, 60,000 and 90,000 MW, presumably localized intracellularly, also co-precipitated with CD45. Labelling of murine thymocytes or D10 cells with H3(32)PO4 in vivo, and of CD45 immunoprecipitates by in vitro kinase reaction, revealed that the 32,000-33,000 MW molecules are phosphoproteins. The relationship of these molecules with the 30,000-34,000 MW molecules previously reported to associate with CD45 in human T cells is not clear as a number of differences were observed. Firstly, the molecular weight of the CD45-associated 32,000-33,000 MW molecule(s) on murine T cells and B cells is slightly lower than that observed in the human T-cell line Jurkat (34,000 MW). Secondly, phosphoamino acid analysis after in vitro kinase labelling of CD45 immunoprecipitates showed that the murine 32,000-33,000 MW molecules are phosphorylated exclusively on serines. Thirdly, although in vitro phosphorylation of the 32,000-33,000 MW molecules was inhibited by preincubation with either GTP-gamma-S or GDP-beta-S, the 32,000-33,000 MW CD45-associated molecules did not bind 32P-GTP, GDP-agarose, or react with antisera to a consensus sequence of G proteins. The crucial role of CD45 for proper function of the T-cell receptor (TCR), suggests that the CD45-associated 32,000-33,000 MW molecules and kinases also may play a role in the signalling events leading to T-cell activation.