Recruitment and activation of PTP1C in negative regulation of antigen receptor signaling by Fc gamma RIIB1

Negative Regulation of Myeloid Cell Proliferation and Function by the SH2 Domain-Containing Tyrosine Phosphatase-1

The SH2 domain containing tyrosine phosphatase SHP-1 has been implicated in the regulation of a multiplicity of signaling pathways involved in hemopoietic cell growth, differentiation, and activation. A pivotal contribution of SHP-1 in the modulation of myeloid cell signaling cascades has been revealed by the demonstration that SHP-1 gene mutation is responsible for the overexpansion and inappropriate activation of myelomonocytic populations in motheaten mice. To investigate the role of SHP-1 in regulation of myeloid leukocytes, an HA epitope-tagged dominant negative (interfering) SHP-1 (SHP-1C453S) was expressed in the myelo-monocytic cell line U937 using the pcDNA3 vector. Overexpression of this protein in SHP-1C453S transfectants was demonstrated by Western blot analysis and by detection of decreased specific activity. Growth, proliferation, and IL-3-induced proliferative responses were substantially increased in the SHP-1C453S-overexpressing cells relative to those in control cells. The results of cell cycle analysis also revealed that the proportion of cells overexpressing SHP-1C453S in S phase was greater than that of control cells. The SHP-1C453S-expressing cells also displayed diminished rates of apoptosis as detected by flow cytometric analysis of propidium iodide-stained cells and terminal deoxynucleotidyltransferase-mediated fluorescein-dUTP nick end-labeling assay. While motility and phagocytosis were not affected by SHP-1C453S overexpression, adhesion and the oxidative burst in response to PMA were enhanced in the SHP-1C453S compared with those in the vector alone transfectants. Taken together, these results suggest that SHP-1 exerts an important negative regulatory influence on cell proliferation and activation while promoting spontaneous cell death in myeloid cells.

Cloning and Characterization of a Lymphoid-Specific, Inducible Human Protein Tyrosine Phosphatase, Lyp

Protein tyrosine phosphatases act in conjunction with protein kinases to regulate the tyrosine phosphorylation events that control cell activation and differentiation. We have isolated a previously undescribed human phosphatase, Lyp, that encodes an intracellular 105-kD protein containing a single tyrosine phosphatase catalytic domain. The noncatalytic domain contains four proline-rich potential SH3 domain binding sites and an NXXY motif that, if phosphorylated, may be recognized by phosphotyrosine binding (PTB) domains. Comparison of the Lyp amino acid sequence with other known proteins shows 70% identity with the murine phosphatase PEP. The human Lyp gene was localized to chromosome 1p13 by fluorescence in situ hybridization analysis. We also identified an alternative spliced form of Lyp RNA, Lyp2. This isoform encodes a smaller 85-kD protein with an alternative C-terminus. The lyp phosphatases are predominantly expressed in lymphoid tissues and cells, with Lyp1 being highly expressed in thymocytes and both mature B and T cells. Increased Lyp1 expression can be induced by activation of resting peripheral T lymphocytes with phytohemagglutinin or anti-CD3. Lyp1 was found to be constitutively associated with the proto-oncogene c-Cbl in thymocytes and T cells. Overexpression of lyp1 reduces Cbl tyrosine phosphorylation, suggesting that it may be a substrate of the phosphatase. Thus, Lyp may play a role in regulating the function of Cbl and its associated protein kinases.

Gab-Family Adapter Proteins Act Downstream of Cytokine and Growth Factor Receptors and T- and B-Cell Antigen Receptors

We previously found that the adapter protein Gab1 (110 kD) is tyrosine-phosphorylated and forms a complex with SHP-2 and PI-3 kinase upon stimulation through either the interleukin-3 receptor (IL-3R) or gp130, the common receptor subunit of IL-6–family cytokines. In this report, we identified another adapter molecule (100 kD) interacting with SHP-2 and PI-3 kinase in response to various stimuli. The molecule displays striking homology to Gab1 at the amino acid level; thus, we named it Gab2. It contains a PH domain, proline-rich sequences, and tyrosine residues that bind to SH2 domains when they are phosphorylated. Gab1 is phosphorylated on tyrosine upon stimulation through the thrombopoietin receptor (TPOR), stem cell factor receptor (SCFR), and T-cell and B-cell antigen receptors (TCR and BCR, respectively), in addition to IL-3R and gp130. Tyrosine phosphorylation of Gab2 was induced by stimulation through gp130, IL-2R, IL-3R, TPOR, SCFR, and TCR. Gab1 and Gab2 were shown to be substrates for SHP-2 in vitro. Overexpression of Gab2 enhanced the gp130 or Src-related kinases–mediated ERK2 activation as that of Gab1 did. These data indicate that Gab-family molecules act as adapters for transmitting various signals.

Regulation of B Cell Receptor-Mediated MHC Class II Antigen Processing by FcγRIIB1

The processing and presentation of Ag by Ag-specific B cells is highly efficient due to the dual function of the B cell Ag receptor (BCR) in both signaling for enhanced processing and endocytosing bound Ag. The BCR for IgG (FcγRIIB1) is a potent negative coreceptor of the BCR that blocks Ag-induced B cell proliferation. Here we investigate the influence of the FcγRIIB1 on BCR-mediated Ag processing and show that coligating the FcγRIIB1 and the BCR negatively regulates both BCR signaling for enhanced Ag processing and BCR-mediated Ag internalization. Treatment of splenic B cells with F(ab′)2 anti-Ig significantly enhances APC function compared with the effect of whole anti-Ig; however, whole anti-Ig treatment is effective when binding to the FcγRIIB1 was blocked by a FcγRII-specific mAb. Processing and presentation of Ag covalently coupled to anti-Ig were significantly decreased compared with Ag coupled to F(ab′)2anti-Ig; however, the processing of the two Ag-Ab conjugates was similar in cells that did not express FcγRIIB1 and in splenic B cells treated with a FcγRII-specific mAb to block Fc binding. Internalization of monovalent Ag by B cells was reduced in the presence of whole anti-Ig as compared with F(ab′)2 anti-Ig, but the internalized Ag was correctly targeted to the class II peptide loading compartment. Taken together, these results indicate that the FcγRIIB1 is a negative regulator of the BCR-mediated Ag-processing function.

Expression of Dominant-Negative Src-Homology Domain 2-Containing Protein Tyrosine Phosphatase-1 Results in Increased Syk Tyrosine Kinase Activity and B Cell Activation

The Src-homology domain 2 (SH2)-containing cytoplasmic tyrosine phosphatase, SHP-1 (SH2-containing protein tyrosine phosphatase-1), interacts with several B cell surface and intracellular signal transduction molecules through its SH2 domains. Mice with the motheaten and viable motheaten mutations are deficient in SHP-1 and lack most mature B cells. To define the role of SHP-1 in mature B cells, we expressed phosphatase-inactive SHP-1 (C453S) in a mature B cell lymphoma line. SHP-1 (C453S) retains the ability to bind to both substrates and appropriate tyrosine-phosphorylated proteins and therefore can compete with the endogenous wild-type enzyme. We found that B cells expressing SHP-1 (C453S) demonstrated enhanced and prolonged tyrosine phosphorylation of proteins with molecular masses of 110, 70, and 55–60 kDa after stimulation with anti-mouse IgG. The tyrosine kinase Syk was hyperphosphorylated and hyperactive in B cells expressing SHP-1 (C453S). SHP-1 and Syk were coimmunoprecipitated from wild-type K46 cells, K46 SHP-1 (C453S) cells, and splenic B cells, and SHP-1 dephosphorylated Syk. Cells expressing SHP-1 (C453S) showed increased Ca2+ mobilization, extracellular signal-regulated kinase activation, and homotypic adhesion after B cell Ag receptor engagement. Thus, SHP-1 regulates multiple early and late events in B lymphocyte activation.

Alloreactivity and association of human natural killer cells with the major histocompatibility complex

All NK cells potentially lytic for autologous cells but not expressing self-major histocompatibility complex (MHC)-reactive receptors could be eliminated by a negative selection mechanism during ontogeny. This idea is based on the existence of a NK cell subset expressing a specific inhibitory receptor for allogeneic MHC alleles. As ancestral haplotypes of the MHC appear to define identical MHC haplotypes in unrelated individuals, unrelated individuals having the same ancestral haplotype should also have the same NK-defined allospecificities that have been shown to map to the human MHC. To test this prediction, multiple cell lines from unrelated individuals having the same ancestral haplotypes were tested for the NK-defined allospecificities. It was found that cells having the same ancestral haplotypes do have the same NK-defined specificities. Furthermore, the NK-defined phenotype of cells that possess two different ancestral haplotypes can be predicted from the NK-defined phenotypes of unrelated cells that are homozygous for the ancestral haplotypes concerned. Although the group 1 and 2 NK-defined allospecificities can be explained to some extent by HLA-C alleles, evidence is presented that additional genes may modify the phenotype conferred by HLA-C.

Requirement of PEST domain tyrosine phosphatase PEP in B cell antigen receptor-induced growth arrest and apoptosis

Signaling events leading to B cell growth or apoptosis are beginning to be unravelled, but detailed information is still lacking. To identify signaling molecules involved in B cell antigen receptor (BCR)-initiated pathways, we used the immature B cell line, WEHI-231, to investigate protein tyrosine phosphatases (PTP) whose expression was modulated by BCR ligation. Among the PTP cloned by reverse transcription-PCR, mRNA expression of the proline-, glutamic acid-, serine- and threonine-rich (PEST) domain phosphatase (PEP) was selectively elevated 3.1-fold within 3 h after anti-IgM antibody stimulation. In contrast, expression of another PEST domain phosphatase, PTP-PEST, was unaffected. Western blot analysis revealed that 71% of PEP was located in the cytosolic fraction, while 29% was in the membrane fraction. To examine the direct contribution made by PEP to BCR-initiated signal transduction, we transfected an antisense PEP cDNA into WEHI-231 cells. Two stable clones were established in which PEP expression was reduced by 34% and 47%, respectively. Strikingly, BCR-mediated inhibition of DNA synthesis was significantly rescued in the clones, and G1 phase cell cycle arrest and apoptosis were almost completely ablated. Considered collectively, these results indicate that PEP is a positive, crucial regulator in determining B cell fate triggered by BCR engagement.

Positive Recognition of MHC Class I Molecules by the Ly49D Receptor of Murine NK Cells

Members of the murine Ly49 family of receptors have been shown to inhibit and activate NK cell function. Subsets of Ly49-expressing NK cells mediate the rejection of bone marrow cell allografts and the lysis of allogeneic lymphoblasts. In this report we have studied Ly49-mediated positive and negative signaling in an in vitro cytotoxicity assay using sorted NK cell subsets as effectors and a panel of 51Cr-labeled Con A lymphoblasts as targets in the presence or the absence of Abs to Ly49 and/or class I molecules. Our results demonstrate that the activating receptor Ly49D delivers stimulatory signals for target cell lysis upon interacting with H2-Dd, Dr, and Dsp2, but not H2b or H2k class I Ags. On the other hand, the inhibitory receptor Ly49G2 delivers negative signals for target cell lysis upon interacting with Dd, Dr, and H2k, but not H2b or Dsp2, class I Ags. Furthermore, Ly49-mediated negative signaling dominates Ly49D-mediated positive signaling. Thus, lysis of class I MHC-bearing targets by NK cells is not merely the consequence of the absence of an Ly49-mediated negative signal, but also requires positive recognition of class I molecules by certain Ly49 receptors. Activation of NK cells by nonself class I molecules was not predicted by the missing self hypothesis.

Specificity of the SH2 Domains of SHP-1 in the Interaction with the Immunoreceptor Tyrosine-Based Inhibitory Motif-Bearing Receptor gp49B

Inhibitory receptors on hemopoietic cells critically regulate cellular function. Despite their expression on a variety of cell types, these inhibitory receptors signal through a common mechanism involving tyrosine phosphorylation of the immunoreceptor tyrosine-based inhibitory motif (ITIM), which engages Src homology 2 (SH2) domain-containing cytoplasmic tyrosine or inositol phosphatases. In this study, we have investigated the proximal signal-transduction pathway of an ITIM-bearing receptor, gp49B, a member of a newly described family of murine NK and mast cell receptors. We demonstrate that the tyrosine residues within the ITIMs are phosphorylated and serve for the association and activation of the cytoplasmic tyrosine phosphatase SHP-1. Furthermore, we demonstrate a physiologic association between gp49B and SHP-1 by coimmunoprecipitation studies from NK cells. To address the mechanism of binding between gp49B and SHP-1, binding studies involving glutathione S-transferase SHP-1 mutants were performed. Utilizing the tandem SH2 domains of SHP-1, we show that either SH2 domain can interact with phosphorylated gp49B. Full-length SHP-1, with an inactivated amino SH2 domain, also retained gp49B binding. However, binding to gp49B was disrupted by inactivation of the carboxyl SH2 domain of full-length SHP-1, suggesting that in the presence of the phosphatase domain, the carboxyl SH2 domain is required for the recruitment of phosphorylated gp49B. Thus, gp49B signaling involves SHP-1, and this association is dependent on tyrosine phosphorylation of the gp49B ITIMs, and an intact SHP-1 carboxyl SH2 domain.

Protein Interactions of Src Homology 2 (SH2) Domain-Containing Inositol Phosphatase (SHIP): Association with Shc Displaces SHIP from FcγRIIb in B Cells

Our recent studies revealed that the inositol phosphatase Src homology 2 (SH2) domain-containing inositol phosphatase (SHIP) is phosphorylated and associated with Shc exclusively under negative signaling conditions in B cells, which is due to recruitment of the SHIP SH2 domain to the FcγRIIb. In addition, we reported that SHIP-Shc interaction involves both SHIP SH2 and Shc phosphotyrosine binding domains. These findings reveal a paradox in which the single SH2 domain of SHIP is simultaneously engaged to two different proteins: Shc and FcγRIIb. To resolve this paradox, we examined the protein interactions of SHIP. Our results demonstrated that isolated FcγRIIb contains SHIP but not Shc; likewise, Shc isolates contain SHIP but not FcγRIIb. In contrast, SHIP isolates contain both proteins, revealing two separate pools of SHIP: one bound to FcγRIIb and one bound to Shc. Kinetic studies reveal rapid SHIP association with FcγRIIb but slower and more transient association with Shc. Affinity measurements using a recombinant SHIP SH2 domain and phosphopeptides derived from FcγRIIb (corresponding to Y273) and Shc (corresponding to Y317) revealed an approximately equal rate of binding but a 10-fold faster dissociation rate for FcγRIIb compared with Shc phosphopeptide and yielding in an affinity of 2.1 μM for FcγRIIb and 0.26 μM for Shc. These findings are consistent with a model in which SHIP transiently associates with FcγRIIb to promote SHIP phosphorylation, whereupon SHIP binds to Shc and dissociates from FcγRIIb.

Effector and predisposing genes in murine lupus

This Publication is No. 11789-IMM from the Department of Immunology, The Scripps Research Institute, 10550 North Torrey Pines Road/IMM3, La Jolla, CA 92037. The work of the author reported herein was supported, in part, by NIH grants AR39555, AR31203, and AG15061.

Kinetic comparison of the catalytic domains of SHP-1 and SHP-2

The phosphatase activity of SH2-containing protein tyrosine phosphatase (SHP) is inhibited by its SH2 domains and C-terminal tail. In order to determine the inhibitory effects of the SH2 domains and C-terminal tail, we have expressed and purified the catalytic domains of SHP-1 and SHP-2, and the SH2 domain truncated SHP-1 and SHP-2. We have then measured their kinetic parameters using p-nitrophenyl phosphate (p-NPP) and phosphotyrosine (pY) as substrates under the same experimental conditions. The results indicate that the pH-dependent profiles of SHP-1 and SHP-2 are mainly determined by their catalytic domains. Both enzymes have maximum activity at pH 5.0. In addition, the phosphatase activity of different forms of SHP-1 and SHP-2 decreases as the salt concentration increases. Without SH2 domains, both SHP-1 and SHP-2 are no longer inhibited by their C-terminal tails. However, the C-terminal tail of SHP-1 can further prevent the salt inhibition of the phosphatase activity. Under the same experimental conditions, the catalytic domain of SHP-1 is two times more active than the catalytic domain of SHP-2.

The carboxyl-terminal region of biliary glycoprotein controls its tyrosine phosphorylation and association with protein-tyrosine phosphatases SHP-1 and SHP-2 in epithelial cells

Biliary glycoprotein (Bgp, C-CAM, or CD66a) is an immunoglobulin-like cell adhesion molecule and functions as a tumor suppressor protein. We have previously shown that the Bgp1 isoform responsible for inhibition of colonic, liver, prostate, and breast tumor cell growth contains within its cytoplasmic domain two tyrosine residues positioned in immunoreceptor tyrosine-based inhibition motif (ITIM) consensus sequences. Moreover, we determined that these residues, upon phosphorylation, associate with the protein-tyrosine phosphatase SHP-1. In this report, we have further evaluated the structural bases of the association of Bgp1 with Tyr phosphatases. First, we demonstrate that Bgp1 also associates with the SHP-2 Tyr phosphatase, but not with an unrelated Tyr phosphatase, PTP-PEST. Association of Bgp1 and SHP-2 involves the Tyr residues within the Bgp1 ITIM sequences, Val at position +3 relative to the second Tyr (Tyr-515), and the SHP-2 N-terminal SH2 domain. In addition, our results indicate that residues +4, +5, and +6 relative to Tyr-515 in the Bgp1 cytoplasmic domain play a significant role in these interactions, as their deletion reduced Bgp1 Tyr phosphorylation and association with SHP-1 and SHP-2 by as much as 80%. Together, these results indicate that both SHP-1 and SHP-2 interact with the Bgp1 cytoplasmic domain via ITIM-like sequences. Furthermore, they reveal that the C-terminal amino acids of Bgp1 are critical for these interactions.

Effects of maternal antibodies on protection and development of antibody responses to human rotavirus in gnotobiotic pigs

Although maternal antibodies can protect against infectious disease in infancy, they can also suppress active immune responses. The effects of circulating maternal antibodies, with and without colostrum and milk antibodies, on passive protection and active immunity to human rotavirus (HRV) were examined in gnotobiotic pigs. Pigs received intraperitoneal injections of high-titer serum (immune pigs [groups 1 and 2]) from immunized sows, low-titer serum from naturally infected sows (control pigs [groups 3 and 4]), or no serum (group 5). Immune or control colostrum and milk were added to the diet of groups 2 and 4, respectively. After inoculation (3 to 5 days of age) and challenge (postinoculation day [PID] 21) with virulent HRV, the effects of maternal antibodies on protection (from diarrhea and virus shedding), and on active antibody responses (measured by quantitation of antibody-secreting cells [ASC] in intestinal and systemic lymphoid tissues by ELISPOT) were evaluated. Groups 1 and 2 had significantly less diarrhea and virus shedding after inoculation but higher rates of diarrhea and virus shedding after challenge than did groups 3 and 5. Group 1 and 2 pigs had significantly fewer immunoglobulin A (IgA) ASC in intestinal tissues at PID 21 and at postchallenge day (PCD) 7 compared to group 5. Significantly fewer IgG ASC were present in the intestines of group 2 pigs at PID 21 and PCD 7 compared to group 5. There was a trend towards fewer ASC in intestinal tissues of group 2 than group 1, from PID 21 on, with significantly fewer IgA ASC at PCD 7. IgG ASC in the duodenum and mesenteric lymph nodes of group 3 and 4 pigs were significantly fewer than in group 5 at PCD 7. These decreases in ASC emphasize the role of passive antibodies in impairing induction of ASC rather than in merely suppressing the function of differentiated B cells. To be successful, vaccines intended for populations with high titers of maternal antibodies (infants in developing countries) may require higher titers of virus, multiple doses, or improved delivery systems, such as the use of microencapsulation or immune stimulating complexes, to overcome the suppressive effects of maternal antibodies.

Evidence for an Upper Affinity Threshold for Anti-IgM–Induced Apoptosis in a Human B-Cell Lymphoma

The influence of ligand:receptor affinity on B-cell antigen receptor (BCR)-induced apoptosis in the IgM+ Burkitt lymphoma line, Ramos, was evaluated with a group of affinity-diverse murine monoclonal antibodies (MoAbs) specific for human B-cell IgM. The studies showed not only a minimal affinity threshold for the induction of apoptosis, but, interestingly, also a maximal affinity threshold above which increases in affinity were associated with diminished apoptosis. The lesser capacity of high-affinity MoAb to induce apoptosis was paralleled by a lesser capacity to induce receptor cross-linking. At high ligand concentration, high MoAb affinity was also associated with a diminished capacity to induce early protein tyrosine phosphorylation. The compromised capacity of two high-affinity MoAbs to trigger apoptosis may be, at least in part, explained by two separate phenomena that can impair the formation of mIgM cross-links: (1) more stable univalent binding and (2) a tendency for monogamous binding of both MoAb Fab to two Fab epitopes on mIgM. These in vitro studies suggest that the use of the highest affinity MoAbs for antireceptor immunotherapies that depend on receptor cross-linking might, on occasion, be contraindicated.

Evidence for an Upper Affinity Threshold for Anti-IgM–Induced Apoptosis in a Human B-Cell Lymphoma

The influence of ligand:receptor affinity on B-cell antigen receptor (BCR)-induced apoptosis in the IgM+ Burkitt lymphoma line, Ramos, was evaluated with a group of affinity-diverse murine monoclonal antibodies (MoAbs) specific for human B-cell IgM. The studies showed not only a minimal affinity threshold for the induction of apoptosis, but, interestingly, also a maximal affinity threshold above which increases in affinity were associated with diminished apoptosis. The lesser capacity of high-affinity MoAb to induce apoptosis was paralleled by a lesser capacity to induce receptor cross-linking. At high ligand concentration, high MoAb affinity was also associated with a diminished capacity to induce early protein tyrosine phosphorylation. The compromised capacity of two high-affinity MoAbs to trigger apoptosis may be, at least in part, explained by two separate phenomena that can impair the formation of mIgM cross-links: (1) more stable univalent binding and (2) a tendency for monogamous binding of both MoAb Fab to two Fab epitopes on mIgM. These in vitro studies suggest that the use of the highest affinity MoAbs for antireceptor immunotherapies that depend on receptor cross-linking might, on occasion, be contraindicated.

Tyrosine unphosphorylated platelet SHP-1 is a substrate for calpain

The platelet phosphotyrosine phosphatase (PTP) SHP-1 is tyrosine phosphorylated during thrombin-induced activation. Stimulation of platelets by the ionophore A23187 in the presence of CaCl2 induced a calpain dependent cleavage of SHP-1. SHP-1 proteolysis was undetectable during thrombin-induced stimulation. When SHP-1 was tyrosine phosphorylated by thrombin, further addition of A23187 failed to induce its cleavage. In the presence of tyrphostin to inhibit thrombin-induced SHP-1 tyrosine phosphorylation, SHP-1 was cleaved. Thus, only the tyrosine unphosphorylated form of SHP-1 was a substrate for calpain. A23187 induced the disappearance of all platelet phosphotyrosine proteins and a two-fold increase in PTP activity, both inhibited by pervanadate, a PTP inhibitor, but unaffected by calpeptin, a calpain inhibitor. The data show that SHP-1 is either tyrosine phosphorylated or cleaved by calpain, and suggest that SHP-1 cleavage does not contribute to A23187-induced PTP activity.

The MHC class I binding proteins LIR-1 and LIR-2 inhibit Fc receptor-mediated signaling in monocytes

The MHC class I binding proteins leukocyte immunoglobulin-like receptor (LIR)-1 and -2 recognize a similar broad spectrum of HLA-A, -B and -C alleles but are differentially expressed in lymphocytes, monocytes, and dendritic cells. In monocytes, phosphorylation of LIR-1 and LIR-2 results in the binding of the tyrosine phosphatase SHP-1. Coligation of either LIR with Fcgamma receptor I (CD64) inhibits tyrosine phosphorylation of the associated Fc receptor gamma chain and Syk molecules, as well as intracellular calcium mobilization. These findings suggest that LIR-1 and LIR-2 function as unique MHC class I receptors involved in the inhibition or down-modulation of monocyte activation signals, particularly those mediated through the receptors for IgG, IgE and IgA.

Crystal structure of the catalytic domain of protein-tyrosine phosphatase SHP-1

The crystal structures of the protein-tyrosine phosphatase SHP-1 catalytic domain and the complex it forms with the substrate analogue tungstate have been determined and refined to crystallographic R values of 0.209 at 2.5 A resolution and 0.207 at 2.8 A resolution, respectively. Despite low sequence similarity, the catalytic domain of SHP-1 shows high similarity in secondary and tertiary structures with other protein-tyrosine phosphatases (PTPs). In contrast to the conformational changes observed in the crystal structures of PTP1B and Yersinia PTP, the WPD loop (Trp419-Pro428) in the catalytic domain of SHP-1 moves away from the substrate binding pocket after binding the tungstate ion. Sequence alignment and structural analysis suggest that the residues in the WPD loop, especially the amino acid following Asp421, are critical for the movement of WPD loop on binding substrates and the specific activity of protein-tyrosine phosphatases. Our mutagenesis and kinetic measurements have supported this hypothesis.

Recruitment and activation of SHP-1 protein-tyrosine phosphatase by human platelet endothelial cell adhesion molecule-1 (PECAM-1). Identification of immunoreceptor tyrosine-based inhibitory motif-like binding motifs and substrates

Stimulation of platelet aggregation leads to tyrosine phosphorylation of a number of receptors and signaling molecules including platelet endothelial cell adhesion molecule-1 (PECAM-1). In this report, we demonstrate that both protein-tyrosine phosphatases SHP-1 and SHP-2 physically associate with different kinetics of assembly with tyrosine-phosphorylated human PECAM-1 during integrin alphaIIbbeta3-mediated platelet aggregation. Peptido-precipitation analysis revealed that tyrosine-phosphorylated peptides encompassing residues 658-668 and 681-691 of PECAM-1 bound specifically to both protein-tyrosine phosphatases SHP-1 and SHP-2. We further show that the association of SHP-1 with PECAM-1 occurs through the direct interaction of the src homology region 2 domains of SHP-1 with two highly conserved phosphotyrosine binding motifs within PECAM-1 having the sequences NSDVQpY663TEVQV and DTETVpY686SEVRK (where pY represents phosphotyrosine). In vitro dephosphorylation experiments using phosphotyrosyl PECAM-1 peptides encompassing either Tyr-663 or Tyr-686 revealed induction of SHP-1 catalytic activity, suggesting that PECAM-1 serves as a SHP-1 substrate. Surface plasmon resonance studies reveal that recombinant SHP-2 binds PECAM-1 phosphopeptides with 5-fold higher affinity than recombinant SHP-1. These data suggest that in hematopoietic cells such as platelets, PECAM-1 cellular signaling is regulated by the selective recruitment and activation of two distinct protein-tyrosine phosphatases, SHP-1 and SHP-2, via a common immunoreceptor tyrosine-based inhibitory-like motif.

The inositol polyphosphate 5-phosphatase ship is a crucial negative regulator of B cell antigen receptor signaling

Ship is an Src homology 2 domain containing inositol polyphosphate 5-phosphatase which has been implicated as an important signaling molecule in hematopoietic cells. In B cells, Ship becomes associated with Fcgamma receptor IIB (FcgammaRIIB), a low affinity receptor for the Fc portion of immunoglobulin (Ig)G, and is rapidly tyrosine phosphorylated upon B cell antigen receptor (BCR)-FcgammaRIIB coligation. The function of Ship in lymphocytes was investigated in Ship-/- recombination-activating gene (Rag)-/- chimeric mice generated from gene-targeted Ship-/- embryonic stem cells. Ship-/-Rag-/- chimeras showed reduced numbers of B cells and an overall increase in basal serum Ig. Ship-/- splenic B cells displayed prolonged Ca2+ influx, increased proliferation in vitro, and enhanced mitogen-activated protein kinase (MAPK) activation in response to BCR-FcgammaRIIB coligation. These results demonstrate that Ship plays an essential role in FcgammaRIIB-mediated inhibition of BCR signaling, and that Ship is a crucial negative regulator of Ca2+ flux and MAPK activation.

Aiolos regulates B cell activation and maturation to effector state

Aiolos encodes a zinc finger DNA-binding protein that is highly expressed in mature B cells and is homologous to Ikaros. In the periphery of mice homozygous for an Aiolos-null mutation, B cells exhibit an activated cell surface phenotype and undergo augmented antigen receptor (BCR)-mediated in vitro proliferative responses, even at limiting amounts of stimulant. In vivo, T cell-dependent B cell responses, including the formation of germinal centers and elevated serum IgG and IgE, are detected in Aiolos-deficient mice in the absence of immunization. Auto-antibodies and development of B cell lymphomas are frequently seen among aging Aiolos mutants. In sharp contrast to conventional B cells, B cells of the peritoneum, of the marginal zone, and the recirculating bone marrow population are greatly reduced.

Tyrosine 112 of latent membrane protein 2A is essential for protein tyrosine kinase loading and regulation of Epstein-Barr virus latency

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) is expressed on the plasma membrane of B lymphocytes latently infected with EBV and blocks B-cell receptor (BCR) signal transduction in EBV-immortalized B cells in vitro. The LMP2A amino-terminal domain that is essential for the LMP2A-mediated block on BCR signal transduction contains eight tyrosine residues. Association of Syk protein tyrosine kinase (PTK) with LMP2A occurs at the two tyrosines of the LMP2A immunoreceptor tyrosine-based activation motif, and it is hypothesized that Lyn PTK associates with the YEEA amino acid motif at LMP2A tyrosine 112 (Y112). To examine the specific association of Lyn PTK to LMP2A, a panel of LMP2A cDNA expression vectors containing LMP2A mutations were transfected into an EBV-negative B-cell line and analyzed for Lyn and LMP2A coimmunoprecipitation. Lyn associates with wild-type LMP2A and other LMP2A mutant constructs, but Lyn association is lost in the LMP2A construct containing a tyrosine (Y)-to-phenylalanine (F) mutation at LMP2A residue Y112 (LMP2AY112F). Next, the LMP2AY112F mutation was recombined into the EBV genome to generate stable lymphoblastoid cell lines (LCLs) transformed with the LMP2AY112F mutant virus. Analysis of BCR-mediated signal transduction in the LMP2AY112F LCLs revealed loss of the LMP2A-mediated block in BCR signal transduction. In addition, LMP2A was not tyrosine phosphorylated in LMP2AY112F LCLs. Together these data indicate the importance of the LMP2A Y112 residue in the ability of LMP2A to block BCR-mediated signal transduction and place the role of this residue and its interaction with Lyn PTK as essential to LMP2A phosphorylation, PTK loading, and down-modulation of PTKs involved in BCR-mediated signal transduction.

Superclustering of B Cell Receptor and FcγRIIB1 Activates Src Homology 2-Containing Protein Tyrosine Phosphatase-1

FcγRIIB1 (CD32) is a receptor that binds the Fc domain of Ag-complexed IgG. Coaggregation of B cell receptor (BCR) and FcγRIIB1 generates a dominant negative signal that inhibits B cell activation. In Ag-specific Id-positive B cells, the co-cross-linking of BCR and FcγRIIB1 by anti-Id Ab resulted in the association of both Src homology 2-containing protein tyrosine phosphatase (SHP-1) and Src homology 2-containing inositol phosphatase (SHIP) with the FcγRIIB1; however, only SHIP activity was detected. “Superclustering” of the BCR and FcγRIIB1 complex induced by stimulation with anti-Id Ab plus polyvalent Ag synergistically activated SHP-1. The degree of co-cross-linking between BCR and FcγRIIB1 may determine the activation status of SHP-1 and SHIP.

Membrane IgM-stimulated human B lymphocytes succumb to activation-related apoptosis at a G1-->S transition: influence of ligand affinity and valency

Culture of human B lymphocytes with polyclonally activating surrogates for type II T-cell-independent antigen, i.e., anti-IgM mAb and anti-IgM:dextran, resulted in both membrane IgM (mIgM)-triggered S/G2/M entry and apoptosis. Although high ligand valency could compensate for low affinity, and high affinity could compensate for low valency, in achieving mIgM-triggered apoptosis, the phenomenon was most pronounced when the soluble "antigen" had both high binding site affinity and valency. Most of the mIgM-triggered apoptosis may represent B cells which progress into G1 but fail to receive a sufficient level of continuous mIgM-mediated signaling during G1 for passage through a G1 --> S phase restriction point(s). This was supported by the findings that (a) a lesser proportion of mIg-triggered cells enter S phase than G1; (b) maximal mIgM-triggered apoptosis was noted at 48-72 h of culture and surrounding activated cell clusters; (c) mIgM-triggered apoptosis was not inhibited by pharmacologic blockers of S phase; and (d) a high proportion of viable mIgM-triggered B cell blasts in G1 succumb to apoptosis rather than enter S phase, if high-affinity multivalent ligand is washed from the cultures. In addition to quantitative aspects of initial receptor engagement, the potential for a protracted period of recurrent mIgM signaling events may influence whether apoptosis or cell cycle progression is the functional outcome of B cell encounter with a multivalent antigen.

Intravenous immunoglobulin inhibits IgE production in human B lymphocytes

BACKGROUND

Intravenous immunoglobulin (IVIG) is commonly used as both an immune-enhancing and immune-modulating agent. Treatment with high doses of IVIG diminishes IgE secretion in patients with severe steroid-dependent asthma.

OBJECTIVE

We studied the action of IVIG on IgE production in highly purified B lymphocytes stimulated without additional T cells to determine the action of IVIG on B lymphocytes.

METHODS

Human B cells were purified from tonsils, and T lymphocytes were removed by E-rosetting. B cells were cultured with IL-4 (400 U/mL) and anti-CD40 antibodies (1 microg/mL¿, with or without additional IVIG. Cell proliferation was determined by 3[H]-thymidine uptake, and supernatant IgE was determined by ELISA. Cell cycle analysis was performed by flow cytometry, and IgE transcripts were measured by in situ hybridization.

RESULTS

IVIG (5 mg/mL) decreased B-cell proliferation in IL4/anti-CD40-stimulated B cells by an average of 74% (+/-6%). Addition of IVIG up to 48 hours after initiation of cell culture led to significant diminution of cell proliferation at 96 to 120 hours. This effect was dose dependent, with 10 mg/mL being the most effective and doses under 0.1 mg/mL having minimal effect. IVIG diminished the number of stimulated cells progressing in the cell cycle by 30%, and there was no difference in cell viability between IVIG-treated and IVIG-untreated cells. The production of IgE in culture by anti-CD40/IL4-stimulated B lymphocytes was curtailed by greater than 80% after addition of 5 mg/mL IVIG. This was associated with a decrease in IgE (epsilon) transcripts in IVIG-treated cultures.

CONCLUSION

These data indicate that diminution of IgE production in anti-CD40/IL-4-stimulated B cells by IVIG is due to inhibition of early events related to proliferation and progression in the cell cycle.

Establishment and characterization of pro-B cell lines from motheaten mutant mouse defective in SHP-1 protein tyrosine phosphatase

Mice homozygous for the motheaten (Hcph(me)) mutation lack a functional SHP-1 protein tyrosine phosphatase, show severe immunologic dysregulation and die at an early age. Severe pneumonitis in me/me mice is associated with abnormal proliferation of macrophages and granulocytes. Overgrowth of macrophages in long term cultures of me/me bone marrow has prevented analyses of lymphopoiesis in vitro. To establish hematopoietic cell lines from me/me mice, we cultured me/me bone marrow with the PA6 stromal cell line in the presence of antagonistic antibody against the receptor (c-Fms) for macrophage colony stimulating factor (M-CSF). In these cultures, overgrowth of M-CSF-dependent macrophages was suppressed by the antagonistic antibody and other hemopoietic cell lineages were generated efficiently from me/me bone marrow. By using this culture system, we established me/me pro-B cell clones (MEBs) with rearranged DH-JH but not VH-DJH. The growth of MEB clones required IL-7 and c-Kit ligand, corresponding to normal pro-B cells which express SHP-1. MEB cells were sensitive to starvation by either IL-7 or c-Kit ligand, resulting in apoptotic death. The present culture system, which supports hematopoiesis of me/me bone marrow, provides useful tools for the determination of the role of SHP-1 in signal transduction of B lymphopoiesis.

Cutting Edge: FcγRII-B1 Regulates the Presentation of B Cell Receptor-Bound Antigens

Fcγ receptors (FcγRII) on B lymphocytes negatively regulate B cell receptor (BCR)-dependent activation upon cross-linking of the two receptors. The mechanism reflects the ability of the FcγRII cytoplasmic tail to recruit specific phosphatases that inactivate elements of the BCR-signaling cascade. We now show that cross-linking also blocks the processing and presentation of BCR-bound Ag. This occurs because the FcγRII isoform typically expressed by B cells (FcγRII-B1) is incompetent for endocytosis. When cross-linked, FcγRII-B1 acts as a dominant negative inhibitor of BCR endocytosis. In contrast, cross-linking of endocytosis-competent FcγRII isoforms did not inhibit endocytosis or processing of BCR-bound Ag. Thus, FcγRII-B1 acts not only to prevent B cell activation under conditions of Ab excess, but also to prevent clonotypic T cell activation by inhibiting the ability of B cells to generate specific MHC class II-bound TCR ligands.

High expression of inhibitory receptor SHPS-1 and its association with protein-tyrosine phosphatase SHP-1 in macrophages

SHPS-1 (or SIRP) is a member of the immunoglobulin (Ig) superfamily abundantly expressed in neurons and other cell types. Within its cytoplasmic domain, it possesses at least two immunoreceptor tyrosine-based inhibitory motifs, which are targets for tyrosine phosphorylation and mediate the recruitment of SHP-2, an Src homology 2 (SH2) domain-containing protein-tyrosine phosphatase. Since other immunoreceptor tyrosine-based inhibitory motifs-containing receptors have critical roles in the negative regulation of hemopoietic cell functions, we wanted to examine the expression of SHPS-1 in cells of hematological lineages. By analyzing a panel of hemopoietic cell lines, evidence was provided that SHPS-1 is abundantly expressed in macrophages and, to a lesser extent, in myeloid cells. No expression was detected in T-cell or B-cell lines. Expression of SHPS-1 could also be documented in normal ex vivo peritoneal macrophages. Further studies showed that SHPS-1 was an efficient tyrosine phosphorylation substrate in macrophages. However, unlike in non-hemopoietic cells, tyrosine-phosphorylated SHPS-1 in macrophages associated primarily with SHP-1 and not SHP-2. Finally, our analyses allowed us to identify several isoforms of SHPS-1 in mouse cells. In part, this heterogeneity was due to differential glycosylation of SHPS-1. Additionally, it was caused by the production of at least two distinct shps-1 transcripts, coding for SHPS-1 polypeptides having different numbers of Ig-like domains in the extracellular region. Taken together, these findings indicate that SHPS-1 is likely to play a significant role in macrophages, at least partially as a consequence of its capacity to recruit SHP-1.

Functional involvement of PTP-U2L in apoptosis subsequent to terminal differentiation of monoblastoid leukemia cells

A large family of protein tyrosine phosphatases (PTPs) bidirectionally regulate intracellular signaling pathways by reversing agonistic or antagonistic phosphorylation events derived from the action of protein tyrosine kinases. Receptor-like PTP PTP-U2 is expressed during phorbol ester-induced differentiation of monoblastoid leukemia U937 cells. We found that the shorter isoform, PTP-U2S, was expressed at an earlier phase in the course of differentiation and the longer isoform, PTP-U2L, was induced at a later phase. In the presence of 12-O-tetradecanoylphorbol-13-acetate, ectopic expression of PTP-U2L in U937 cells enhanced several characteristics of terminally differentiated cells. Most striking was that PTP-U2L enhanced apoptosis of the differentiated cells, which was only partially inhibited by caspase inhibitor Z-Asp-CH2-DCB. The catalytically inactive mutant PTP-U2L(C --> S) still retained the ability to enhance the differentiation but retained the ability to enhance the following apoptosis of the cells to a lesser extent. These data indicate a functional involvement of PTP-U2L in apoptosis subsequent to terminal differentiation of U937 cells. Since terminally differentiated blood cells often undergo apoptosis, the data also suggest that PTP-U2L might be involved in physiological turnover of hematopoietic cells in vivo.

Regulation of casein kinase 2 by direct interaction with cell surface receptor CD5

The transmembrane protein CD5, expressed on all T cells and the B1 subset of B cells, modulates antigen receptor-mediated activation. We used the yeast two-hybrid system to identify proteins that interact with its cytoplasmic domain and play a role in CD5 proximal signaling events. We found that the beta subunit of the serine/threonine kinase casein kinase 2 (CK2) interacts specifically with the cytoplasmic domain of CD5. Co-immunoprecipitation experiments showed activation-independent association of CK2 with CD5 in human and murine B and T cell lines and murine splenocytes. The interaction of CK2 holoenzyme with CD5 is mediated by the amino terminus of the regulatory subunit beta. CK2 binds and phosphorylates CD5 at the CK2 motifs flanked by Ser459 and Ser461. Cross-linking of CD5 leads to the activation of CD5-associated CK2 in a murine B-lymphoma cell line and a human T-leukemia cell line and is independent of net recruitment of CK2 to CD5. In contrast, CK2 is not activated following cross-linking of the B cell receptor complex or the T cell receptor complex. This direct regulation of CK2 by a cell surface receptor provides a novel pathway for control of cell activation that could play a significant role in regulation of CD5-dependent antigen receptor activation in T and B cells.

B Cell Antigen Receptor (BCR)-Mediated Formation of a SHP-2-pp120 Complex and Its Inhibition by FeγRIIB1-BCR Coligation

Accumulating evidence indicates that the Src homology 2-containing tyrosine phosphatase 2 (SHP-2) plays an important role in signal transduction through receptor tyrosine kinase and cytokine receptors. In most models, SHP-2 appears to be a positive mediator of signaling. However, coligation of FcγRIIB1 with B cell Ag receptors (BCR) inhibits BCR-mediated signaling by a mechanism that may involve recruitment of phosphatases SHP-1, SHP-2, and the SH2 containing inositol 5′phosphatase (SHIP) to the phosphorylated FcγRIIB1 immunoreceptor tyrosine-based inhibitory motif. The role of SHP-2 in BCR-mediated cell activation and in FcγRIIB1-mediated inhibitory signaling is unclear. In this study we assessed the association of SHP-2 with phosphotyrosine-containing cellular protein(s) before and after stimulation through these receptors. BCR stimulation induced the association of SHP-2 with a single major tyrosyl-phosphorylated molecule (pp120) that had an apparent molecular mass of 120 kDa. Coligation of FcγRIIB1 with BCR led to a rapid decrease in SHP-2 association with pp120. Analysis of the subcellular localization of pp120 showed that the complex of SHP-2 and tyrosyl-phosphorylated p120 occurs predominantly in the cytosol. Furthermore, the binding of the two molecules was mediated by the interaction of tyrosyl-phosphorylated p120 with the SHP-2 N-terminal SH2 domain. These findings indicate that SHP-2 and pp120 function in BCR signaling, and this function may be inhibited by FcγRIIB1 signaling.

Regulation of Apoptosis by Tyrosine-Containing Domains of IL-4Rα: Y497 and Y713, But Not the STAT6-Docking Tyrosines, Signal Protection from Apoptosis

IL-4 is a cytokine with important antiapoptotic activity. We have analyzed the role that tyrosine-containing domains within the cytoplasmic tail of IL-4Rα play in IL-4-mediated protection from apoptosis. 32D cells expressing a wt huIL-4Rα or one truncated at aa 557 were protected by huIL-4 from apoptosis while cells expressing a receptor truncated at aa 657 were not, suggesting that the carboxyl-terminal domain signals protection from apoptosis. However, changing Y713 within this region to phenylalanine had no effect. To analyze the contribution of tyrosine-containing domains independently, we transplanted regions of the huIL-4Rα to a truncated form of the huIL-2Rβ that could not signal protection from apoptosis. Transplantation of the huIL-4Rα domains containing Y497 or Y713 partially prevented cell death and together signaled protection from apoptosis in response to IL-2 as well as the wt IL-2Rβ. Mutation of Y497 and Y713 to phenylalanine inhibited protection. In contrast, transplantation of the domain containing the potential STAT6-docking tyrosines alone had no effect, yet it inhibited the protection mediated by the other domains. Although IL-4Rα signals Shc and SH2-containing inositol phosphatase (SHIP) phosphorylation, we could not establish an association between their activation and protection from apoptosis. Taken together, this study suggests that the domains of the huIL-4Rα containing Y497 and Y713 positively regulate protection from apoptosis while the domain containing the STAT6 docking sites suppresses this protection, and that additional signaling molecules other than insulin receptor substrate-1 (IRS1), Shc, or SHIP may be involved in antiapoptotic signaling.

Structural bases of Fc gamma R functions

This review describes structures which determine the biological activities triggered by Fc gamma R and account for the cell-mediated functions of IgG antibodies in physiology and pathology. The binding specificity and affinity of Fc gamma R depend primarily on IgG-binding structures, in their immunoglobulin-like extracellular domains. Binding is however also influenced by subunits that associate to multichain Fc gamma R. Effector and regulatory intracytoplasmic sequences that are unique to molecules of the Fc gamma RIIB family determine the internalization properties of these receptors. Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) are intracytoplasmic effector sequences shared by Fc gamma R and other receptors involved in the recognition of antigen, which trigger cell activation and internalization. Immunoreceptor Tyrosine-based Inhibition Motifs (ITIMs) are intracytoplasmic sequences, shared by Fc gamma RIIB and a growing number of negative coreceptors which negatively regulate cell activation via ITAM-bearing receptors. Altogether, these structures enable IgG antibodies to exert a variety of finely tuned biological effects during the immune response.

Human IgG Fc receptors

Human IgG receptors constitute a family of glycoprotein complexes consisting of ligand-binding, and associated signaling chains. Three leukocyte classes (Fc gamma RI, II, and III) and one separate endothelial Fc gamma R class (FcRB) are defined which are expressed on hematopoietic and endothelial cells. Upon interaction with IgG, Fc gamma R initiate a plethora of signaling cascades involving receptor signaling motifs, and protein tyrosine kinases and phosphatases. These cascades ultimately culminate in activation or deactivation of effector cells, resulting in initiation or down-modulation of cellular processes. Recent evidence points to a crucial in vivo role of Fc gamma R in both initiation and regulation of inflammatory and cytotoxic responses. These Fc gamma R-mediated immune responses can be exploited to develop novel immunotherapies.

Regulation of mouse PECAM-1 tyrosine phosphorylation by the Src and Csk families of protein-tyrosine kinases

PECAM-1 is an adhesion molecule expressed on hemopoietic and endothelial cells. Recently, it was observed that PECAM-1 becomes tyrosine-phosphorylated in response to a variety of physiological stimuli. Furthermore, tyrosine-phosphorylated PECAM-1 was shown to associate with SHP-2, a Src homology 2 (SH2) domain-containing protein-tyrosine phosphatase expressed ubiquitously. In light of the significance of tyrosine protein phosphorylation as a regulatory mechanism, we wished to understand better the nature and impact of the protein-tyrosine kinases (PTKs) mediating PECAM-1 tyrosine phosphorylation. Through reconstitution experiments in COS-1 cells, we determined that mouse PECAM-1 could be tyrosine-phosphorylated by Src-related PTKs and Csk-related PTKs, but not by other kinases such as Syk, Itk, and Pyk2. Using site-directed mutagenesis and peptide phosphorylation studies, we found that these PTKs were efficient at phosphorylating Tyr-686, but not Tyr-663, of PECAM-1. Src-related enzymes also phosphorylated mouse PECAM-1 at one or more yet to be identified sites. In other studies, we demonstrated that phosphorylation of PECAM-1 by Src or Csk family kinases was sufficient to trigger its association with SHP-2. Moreover, it was able to promote binding of PECAM-1 to SHP-1, a SHP-2-related protein-tyrosine phosphatase expressed in hemopoietic cells. Taken together, these findings indicated that the Src and Csk families of kinases are strong candidates for mediating tyrosine phosphorylation of PECAM-1 and triggering its association with SH2 domain-containing phosphatases under physiological circumstances.

Inhibitory signaling by B cell Fc gamma RIIb

The fact that B cells undergo feedback suppression, or negative signaling, through the interaction of secreted antibody with specific antigen has been extensively documented but the mechanisms involved in the process have been elusive. Experiments over the past year using B cell deletion mutants and dominant-negative enzymes have firmly established an important role for SH2-domain-containing inositol 5-phosphatase (SHIP) in negative signaling. Negative signaling through SHIP appears to inhibit the Ras pathway through SH2 domain competition with Grb2 and Shc and may involve consumption of intracellular lipid mediators that act as allosteric enzyme activators or that promote entry of extracellular Ca2+.

Enhancing leptin response by preventing SH2-containing phosphatase 2 interaction with Ob receptor

Leptin is an adipocyte-derived cytokine that regulates food intake and body weight via interaction with its Ob receptor (ObR). Serum leptin levels are chronically elevated in obese humans, suggesting that obesity may be associated with leptin resistance and the inability to generate an adequate ObR response. Evidence suggests that transcriptional activation of target genes by STAT3 (signal transducer and activator of transcription) in the hypothalamus is a critical pathway that mediates leptin's action. Herein we report that activation of ObR induces the tyrosine phosphorylation of the tyrosine phosphatase SH2-containing phosphatase 2 (SHP-2) and demonstrate that Tyr986 within the ObR cytoplasmic domain is essential to mediate phosphorylation of SHP-2 and binding of SHP-2 to ObR. Surprisingly, mutation of Tyr986 to Phe, which abrogates SHP-2 phosphorylation and binding to the receptor, dramatically increases gene induction mediated by STAT3. Our findings indicate that SHP-2 is a negative regulator of STAT3-mediated gene induction after activation of ObR and raise the possibility that blocking the interaction of SHP-2 with ObR could overcome leptin resistance by boosting leptin's weight-reducing effects in obese individuals.

Positive versus negative signaling by lymphocyte antigen receptors

Antigen receptors on lymphocytes play a central role in immune regulation by transmitting signals that positively or negatively regulate lymphocyte survival, migration, growth, and differentiation. This review focuses on how opposing positive or negative cellular responses are brought about by antigen receptor signaling. Four types of extracellular inputs shape the response to antigen: (a) the concentration of antigen; (b) the avidity with which antigen is bound; (c) the timing and duration of antigen encounter; and (d) the association of antigen with costimuli from pathogens, the innate immune system, or other lymphocytes. Intracellular signaling by antigen receptors is not an all-or-none event, and these external variables alter both the quantity and quality of signaling. Recent findings in B lymphocytes have clearly illustrated that these external inputs affect the magnitude and duration of the intracellular calcium response, which in turn contributes to differential triggering of the transcriptional regulators NF kappa B, JNK, NFAT, and ERK. The regulation of calcium responses involves a network of tyrosine kinases (e.g. lyn, syk), tyrosine or lipid phosphatases (CD45, SHP-1, SHIP), and accessory molecules (CD21/CD19, CD22, FcR gamma 2b). Understanding the biochemistry and logic behind these integrative processes will allow development of more selective and efficient pharmaceuticals that suppress, modify, or augment immune responses in autoimmunity, transplantation, allergy, vaccines, and cancer.

Genetic susceptibility to systemic lupus erythematosus

Considerable evidence suggests that the development of systemic lupus erythematosus (SLE) has a strong genetic basis. Recent studies have emphasized that this disease, like other autoimmune diseases, is a complex genetic trait with contributions from major histocompatibility complex (MHC) genes and multiple non-MHC genes. Etiologic genes in these disorders determine susceptibility, and no particular gene is necessary or sufficient for disease expression. Studies of murine models of lupus have provided important insight into the immunopathogenesis of IgG autoantibody production and lupus nephritis, and genetic analyses of these mice overcome certain obstacles encountered when studying patients. Genome-wide linkage studies of different crosses have mapped the position of at least 12 non-MHC disease-susceptibility loci in the New Zealand hybrid model of lupus. Although the identity of the actual genes is currently unknown, recent studies have begun to characterize how these genetic contributions may function in the autoimmune process, especially in terms of their role in autoantibody production. Studies of MHC gene contributions in New Zealand mice have shown that heterozygosity for particular haplotypes greatly increases pathogenic autoantibody production and the incidence of severe nephritis. The mechanism for this effect appears to be genetically complex. Studies in human SLE have mostly focused on the association of disease with alleles of immunologically relevant genes, especially in the MHC. Associations with various complement component deficiencies and an allele of a particular Fc gamma receptor gene (FCGR2A) also have been described. In a diversion from previous association studies, a recent directed linkage analysis of sibpairs with SLE was based on mapping studies in murine lupus and may be an important step toward identifying a new disease-susceptibility gene in patients. Since the genes that predispose to autoimmunity are probably related to key events in pathogenesis, their identification in patients and murine models will almost certainly provide important insight into the breakdown of immunological self-tolerance and the cause of autoimmune disease.

Natural and therapeutically-induced antibodies to cytokines

Serum samples obtained from non-immunocompromised patients treated therapeutically with recombinant cytokines (e.g. Il-1alpha; Il-1beta; Il-2 to Il-18; IFNalpha; GM-CSF; G-CSF; etc.) often reveal the presence of high affinity anti-cytokine antibodies. Antibody Fab binding in a saturable manner by ELISA and RIA or western immunoblotting prove their specificity. Antibody level often increases in these patients with repeated cytokine administration, suggesting boosts of antigen stimulation. However, the appearance in circulation of auto-antibodies to exogenous cytokine is not always associated with a decreased clinical response to therapy. The demonstration that non-neutralizing auto-antibodies to several natural cytokines can be found even in sera of normal healthy individuals never treated before with cytokines and particularly during the last trimester of pregnancy and in cord-blood, suggests that these naturally- occurring and therapeutically-induced auto-antibodies may exert different functions, not only as inhibitors or antagonists but also as beneficial physiological cytokine carriers or regulators of their activity.