Requirement of SH2-containing protein tyrosine phosphatases SHP-1 and SHP-2 for paired immunoglobulin-like receptor B (PIR-B)-mediated inhibitory signal

Positive signaling through CD72 induces mitogen-activated protein kinase activation and synergizes with B cell receptor signals to induce X-linked immunodeficiency B cell proliferation

CD72 is a 45-kDa B cell transmembrane glycoprotein that has been shown to be important for B cell activation. However, whether CD72 ligation induces B cell activation by delivering positive signals or sequestering negative signals away from B cell receptor (BCR) signals remains unclear. Here, by comparing the late signaling events associated with the mitogen-activated protein kinase pathway, we identified many similarities and some differences between CD72 and BCR signaling. Thus, CD72 and BCR activated the extracellular signal-regulated kinase (ERK) and the c-Jun N-terminal kinase (JNK) but not p38 mitogen-activated protein kinase. Both CD72- and BCR-mediated ERK and JNK activation required protein kinase C activity, which was equally important for CD72- and BCR-induced B cell proliferation. However, CD72 induced stronger JNK activation compared with BCR. Surprisingly, the JNK activation induced by both BCR and CD72 is Btk independent. Although both CD72 and BCR induced Btk-dependent ERK activation, CD72-mediated proliferation is more resistant to blocking of ERK activity than that of BCR, as shown by the proliferation response of B cells treated with PD98059 and dibutyryl cAMP, agents that inhibit ERK activity. Most importantly, CD72 signaling compensated for defective BCR signaling in X-linked immunodeficiency B cells and partially restored the proliferation response of X-linked immunodeficiency B cells to anti-IgM ligation. These results suggest that CD72 signals B cells by inducing BCR-independent positive signaling pathways.

SHP-1 Requires Inhibitory Co-receptors to Down-modulate B Cell Antigen Receptor-mediated Phosphorylation of Cellular Substrates

Signaling through the B cell antigen receptor (BCR) is negatively regulated by the SH2 domain-containing protein-tyrosine phosphatase SHP-1, which requires association with tyrosine-phosphorylated proteins for activation. Upon BCR ligation, SHP-1 has been shown to associate with the BCR, the cytoplasmic protein-tyrosine kinases Lyn and Syk, and the inhibitory co-receptors CD22 and CD72. How SHP-1 is activated by BCR ligation and regulates BCR signaling is, however, not fully understood. Here we demonstrate that, in the BCR-expressing myeloma line J558L mu 3, CD72 expression reduces the BCR ligation-induced phosphorylation of the BCR component Ig alpha/Ig beta and its cytoplasmic effectors Syk and SLP-65. Substrate phosphorylation was restored by expression of dominant negative mutants of SHP-1, whereas the SHP-1 mutants failed to enhance phosphorylation of the cellular substrates in the absence of CD72. This indicates that SHP-1 is efficiently activated by CD72 but not by other pathways in J558L mu m3 cells and that inhibition of SHP-1 specifically activated by CD72 reverses CD72-induced dephosphorylation of cellular substrates in these cells. Taken together, BCR-induced SHP-1 activation is likely to require inhibitory co-receptors such as CD72, and SHP-1 appears to mediate the negative regulatory effect of CD72 on BCR signaling by dephosphorylating Ig alpha/Ig beta and its downstream signaling molecules Syk and SLP-65.

Molecular cloning of MIS, a myeloid inhibitory siglec, that binds protein-tyrosine phosphatases SHP-1 and SHP-2

We describe the molecular cloning and characterization of a novel myeloid inhibitory siglec, MIS, that belongs to the family of sialic acid-binding immunoglobulin-like lectins. A full-length MIS cDNA was obtained from murine bone marrow cells. MIS is predicted to contain an extracellular region comprising three immunoglobulin-like domains (V-set amino-terminal domain followed by two C-set domains), a transmembrane domain and a cytoplasmic tail with two immunoreceptor tyrosine-based inhibitory motif (ITIM)-like sequences. The closest relative of MIS in the siglec family is human siglec 8. Extracellular regions of these two siglecs share 47% identity at the amino acid level. Southern blot analysis suggests the presence of one MIS gene. MIS is expressed in the spleen, liver, heart, kidney, lung and testis tissues. Several isoforms of MIS protein exist due to the alternative splicing. In a human promonocyte cell line, MIS was able to bind Src homology 2-containing protein-tyrosine phosphatases, SHP-1 and SHP-2. This binding was mediated by the membrane-proximal ITIM of MIS. Moreover, MIS exerted an inhibitory effect on FcgammaRI receptor-induced calcium mobilization. These data suggest that MIS can play an inhibitory role through its ITIM sequences.

Inhibition of antigen-receptor signaling by Platelet Endothelial Cell Adhesion Molecule-1 (CD31) requires functional ITIMs, SHP-2, and p56(lck)

Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1, CD31) is a 130-kd member of the immunoglobulin gene superfamily that is expressed on the surface of platelets, endothelial cells, myeloid cells, and certain lymphocyte subsets. PECAM-1 has recently been shown to contain functional immunoreceptor tyrosine-based inhibitory motifs (ITIMs) within its cytoplasmic domain, and co-ligation of PECAM-1 with the T-cell antigen receptor (TCR) results in tyrosine phosphorylation of PECAM-1, recruitment of Src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2), and attenuation of TCR-mediated cellular signaling. To determine the molecular basis of PECAM-1 inhibitory signaling in lymphocytes, the study sought to (1) establish the importance of the PECAM-1 ITIMs for its inhibitory activity, (2) determine the relative importance of SHP-2 versus SHP-1 in mediating the inhibitory effect of PECAM-1, and (3) identify the protein tyrosine kinases required for PECAM-1 tyrosine phosphorylation in T cells. Co-ligation of wild-type PECAM-1 with the B-cell antigen receptor expressed on chicken DT40 B cells resulted in a marked reduction of calcium mobilization-similar to previous observations in T cells. In contrast, co-ligation of an ITIM-less form of PECAM-1 had no inhibitory effect. Furthermore, wild-type PECAM-1 was unable to attenuate calcium mobilization in SHP-2-deficient DT40 variants despite abundant levels of SHP-1 in these cells. Finally, PECAM-1 failed to become tyrosine phosphorylated in p56(lck)-deficient Jurkat T cells. Together, these data provide important insights into the molecular requirements for PECAM-1 regulation of antigen receptor signaling.

Src homology 2 domain-containing protein-tyrosine phosphatases, SHP-1 and SHP-2, are required for platelet endothelial cell adhesion molecule-1/CD31-mediated inhibitory signaling

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) is a newly assigned member of the Ig immunoreceptor tyrosine-based inhibitory motif superfamily, and its functional role is suggested to be an inhibitory receptor that modulates immunoreceptor tyrosine-based activation motif-dependent signaling cascades. To test whether PECAM-1 is capable of delivering inhibitory signals in B cells and the functional requirement of protein-tyrosine phosphatases (PTPs) for this inhibitory signaling, we generated chimeric Fc gamma RIIB1-PECAM-1 receptors containing the extracellular and transmembrane portions of murine Fc gamma RIIB1 and the cytoplasmic domain of human PECAM-1. These chimeric receptors were stably expressed in chicken DT40 B cells either as wild-type or mutant cells deficient in SHP-1(-/-), SHP-2(-/-), SHIP(-/-), or SHP-1/2(-/-) and then assessed for their ability to inhibit B cell Ag receptor (BCR) signaling. Coligation of wild-type Fc gamma RIIB1-PECAM-1 with BCR resulted in inhibition of intracellular calcium release, suggesting that the cytoplasmic domain of PECAM-1 is capable of delivering an inhibitory signal that blocks BCR-mediated activation. This PECAM-1-mediated inhibitory signaling correlated with tyrosine phosphorylation of the Fc gamma RIIB1-PECAM-1 chimera, recruitment of SHP-1 and SHP-2 PTPs by the phosphorylated chimera, and attenuation of calcium mobilization responses. Mutational analysis of the two tyrosine residues, 663 and 686, constituting the immunoreceptor tyrosine-based inhibitory motifs in PECAM-1 revealed that both tyrosine residues play a crucial role in the inhibitory signal. Functional analysis of various PTP-deficient DT40 B cell lines stably expressing wild-type chimeric Fc gamma RIIB1-PECAM-1 receptor indicated that cytoplasmic Src homology 2-domain-containing phosphatases, SHP-1 and SHP-2, were both necessary and sufficient to deliver inhibitory negative regulation upon coligation of BCR complex with inhibitory receptor.

The chicken B cell line DT40: a novel tool for gene disruption experiments

The use of the chicken DT40 B cell line is increasing in popularity due to the ease with which it can be manipulated genetically. It offers a targeted to random DNA integration ratio of more than 1:2, by far exceeding that of any mammalian cell line. The facility with which knockout cell lines can be generated, combined with a short generation time, makes the DT40 cell line attractive for phenotype analysis of single and multiple gene disruptions. Advantage has been taken of this to investigate such diverse fields as B cell antigen receptor (BCR) signaling, cell cycle regulation, gene conversion and apoptosis. In this review, we give a historical introduction and a practical guide to the use of the DT40 cell line, along with an overview of the main topics being researched using the DT40 cell line as a model system. These topics include B cell-specific subjects such as B cell signaling and Ig rearrangement, and subjects common to all cell types such as apoptosis, histones, mRNA modification, chromosomal maintenance and DNA repair. Attention is in each case brought to peculiarities of the DT40 cell line that are of relevance for the subject. Novel applications of the cell line, e.g., as a vector for gene targeting of human chromosomes, are also discussed in this review.

Requirement of Shp-2 tyrosine phosphatase in lymphoid and hematopoietic cell development

Shp-1 and Shp-2 are cytoplasmic phosphotyrosine phosphatases with similar structures. Mice deficient in Shp-2 die at midgestation with defects in mesodermal patterning, and a hypomorphic mutation at the Shp-1 locus results in the moth-eaten viable (me(v)) phenotype. Previously, a critical role of Shp-2 in mediating erythroid/myeloid cell development was demonstrated. By using the RAG-2-deficient blastocyst complementation, the role of Shp-2 in lymphopoiesis has been determined. Chimeric mice generated by injecting Shp-2(-/-) embryonic stem cells into Rag-2-deficient blastocysts had no detectable mature T and B cells, serum immunoglobulin M, or even Thy-1(+) and B220(+) precursor lymphocytes. Collectively, these results suggest a positive role of Shp-2 in the development of all blood cell lineages, in contrast to the negative effect of Shp-1 in this process. To determine whether Shp-1 and Shp-2 interact in hematopoiesis, Shp-2(-/-):me(v)/me(v) double-mutant embryos were generated and the hematopoietic cell development in the yolk sacs was examined. More hematopoietic stem/progenitor cells were detected in Shp-2(-/-):me(v)/me(v) embryos than in Shp-2(-/-) littermates. The partial rescue by Shp-1 deficiency of the defective hematopoiesis caused by the Shp-2 mutation suggests that Shp-1 and Shp-2 have antagonistic effects in hematopoiesis, possibly through a bidirectional modulation of the same signaling pathway(s).

mSiglec-E, a novel mouse CD33-related siglec (sialic acid-binding immunoglobulin-like lectin) that recruits Src homology 2 (SH2)-domain-containing protein tyrosine phosphatases SHP-1 and SHP-2

The sialic acid-binding immunoglobulin-like lectins (siglecs) represent a recently defined distinct subset of the immunoglobulin superfamily. By using the Src homology 2 (SH2)-domain-containing protein tyrosine phosphatase SHP-1 as bait in a yeast two-hybrid screen, we have identified a new member of the mouse siglec family, mSiglec-E. The mSiglec-E cDNA encodes a protein of 467 amino acids that contains three extracellular immunoglobulin-like domains, a transmembrane region and a cytoplasmic tail bearing two immunoreceptor tyrosine-based inhibitory motifs (ITIMs). mSiglec-E is highly expressed in mouse spleen, a tissue rich in leucocytes. The ITIMs of mSiglec-E can recruit SHP-1 and SHP-2, two inhibitory regulators of immunoreceptor signal transduction. This suggests that the function of mSiglec-E is probably an involvement in haematopoietic cells and the immune system as an inhibitory receptor. When expressed in COS-7 cells, mSiglec-E was able to mediate sialic acid-dependent binding to human red blood cells, suggesting that mSiglec-E may function through cell-cell interactions. In comparison with the known members of the siglec family, mSiglec-E exhibits a high degree of sequence similarity to both human siglec-7 and siglec-9. The gene encoding mSiglec-E is localized in the same chromosome as that encoding mouse CD33. Phylogenetic analysis reveals that neither mouse mSiglec-E nor CD33 shows a clear relationship with any human siglecs so far identified.

The SH2 domain containing tyrosine phosphatase-1 down-regulates activation of Lyn and Lyn-induced tyrosine phosphorylation of the CD19 receptor in B cells

SHP-1 is a cytosolic tyrosine phosphatase implicated in down-regulation of B cell antigen receptor signaling. SHP-1 effects on the antigen receptor reflect its capacity to dephosphorylate this receptor as well as several inhibitory comodulators. In view of our observation that antigen receptor-induced CD19 tyrosine phosphorylation is constitutively increased in B cells from SHP-l-deficient motheaten mice, we investigated the possibility that CD19, a positive modulator of antigen receptor signaling, represents another substrate for SHP-1. However, analysis of CD19 coimmunoprecipitable tyrosine phosphatase activity in CD19 immunoprecipitates from SHP-1-deficient and wild-type B cells revealed that SHP-1 accounts for only a minor portion of CD19-associated tyrosine phosphatase activity. As CD19 tyrosine phosphorylation is modulated by the Lyn protein-tyrosine kinase, Lyn activity was evaluated in wild-type and motheaten B cells. The results revealed both Lyn as well as CD19-associated Lyn kinase activity to be constitutively and inducibly increased in SHP-1-deficient compared with wild-type B cells. The data also demonstrated SHP-1 to be associated with Lyn in stimulated but not in resting B cells and indicated this interaction to be mediated via Lyn binding to the SHP-1 N-terminal SH2 domain. These findings, together with cyanogen bromide cleavage data revealing that SHP-1 dephosphorylates the Lyn autophosphorylation site, identify Lyn deactivation/dephosphorylation as a likely mechanism whereby SHP-1 exerts its influence on CD19 tyrosine phosphorylation and, by extension, its inhibitory effect on B cell antigen receptor signaling.

Stimulatory function of gp49A, a murine Ig-like receptor, in rat basophilic leukemia cells

Murine gp49, a 49-kDa type I transmembrane glycoprotein, is a member of the Ig-like receptors expressed on the surface of cells involved in natural immunity such as mast cells, NK cells, and macrophages. The two major subtypes, gp49A and gp49B, are encoded by two different genes adjacent to each other. gp49B contains an immunoreceptor tyrosine-based inhibitory motif in its cytoplasmic region and is known to function as an inhibitory molecule. In contrast, gp49A does not harbor any specific motif for signal transduction, nor has its physiological role been determined. Here we report on the stimulatory nature of gp49A by analyzing biochemical characteristics of chimeric molecules consisting of an ectodomain of Fc receptor and a C-terminal half of gp49A, namely the pretransmembrane, transmembrane, and cytoplasmic portions, expressed on the rat basophilic leukemia mast cell line. Cross-linking of the chimeric receptors evoked cytoplasmic calcium mobilization, PGD(2) release, and transcription of IL-3 and IL-4 genes, but did not elicit degranulation of the cells. The chimeric molecule could be expressed as a singlet and a homodimeric form on the cell surface. A pretransmembrane cysteine residue of gp49A was necessary for dimer formation. Dimerization was be necessary for their incorporation into glycolipid-enriched membrane fraction (GEM) upon cross-linking stimuli. The calcium mobilization response was inhibited by treatment of cells with methyl-beta-cyclodextrin, an inhibitor of GEM formation. Together with these results, it was strongly suggested that gp49A could be expressed as a homodimer and elicit activation signals that lead to calcium mobilization, eicosanoid production, and cytokine gene transcription through its incorporation into GEM.

Immune inhibitory receptors

With the detailed description and analysis of several inhibitory receptor systems on lymphoid and myeloid cells, a central paradigm has emerged in which the pairing of activation and inhibition is necessary to initiate, amplify, and then terminate immune responses. In some cases, the activating and inhibitory receptors recognize similar ligands, and the net outcome is determined by the relative strength of these opposing signals. The importance of this modulation is demonstrated by the sometimes fatal autoimmune disorders observed in mice with targeted disruption of inhibitory receptors. The significance of these receptors is further evidenced by the conservation of immunoreceptor tyrosine-based inhibitory motifs during their evolution.

Negative regulation of B cell receptor-mediated signaling in B-1 cells through CD5 and Ly49 co-receptors via Lyn kinase activity

CD5(+) B-1 cells are known to be unresponsive to B cell receptor (BCR)-mediated growth signals but instead undergo apoptosis. However, the B-1 cells from Lyn kinase-deficient (Lyn-/-) mice exhibited an enhanced proliferative response upon BCR cross-linking. It has been reported that BCR-mediated signaling in B-1 cells is negatively regulated by signals from CD22, CD5 and CD72 co-receptors, and that Lyn kinase plays a crucial role in tyrosine phosphorylation of immunoreceptor tyrosine-based inhibitory motifs on the CD22 and CD72, which recruits SHP-1 to the BCR complex. We found that Lyn kinase is also essential for the tyrosine phosphorylation of CD5 and subsequent recruitment of SHP-1 in B-1 cells upon cross-linking of BCR. Moreover, a distinct subpopulation of B-1 cells was found to express cell surface Ly49, which is known as a MHC class I-binding negative regulatory receptor on NK cells. Ly49 was rapidly tyrosine phosphorylated upon cross-linking of BCR and SHP-1 was found to recruit to the phosphorylated Ly49. Addition of F(ab')(2) fragments of anti-Ly49 antibodies partially blocked negative signals in B-1 cells. Thus two co-receptors, CD5 and Ly49, which are unique to B-1 cells, play a role in the regulation of B-1 cell activation. These results indicate that BCR-mediated signals in B-1 cells are strictly and negatively regulated through multiple pathways, that are dependent on Lyn kinase activity.

Activating and inhibitory signaling in mast cells: new opportunities for therapeutic intervention?

Immune responses are tightly controlled by the activities of both activating and inhibitory signals. At the cellular level, these signals are generated through engagement of membrane-associated receptors and coreceptors. The high-affinity IgE receptor FcepsilonRI is expressed on mast cells and basophils and, on cross-linking by multivalent antigen (allergen), stimulates the release of inflammatory mediators that induce acute allergic responses. Activation signals mediated by a variety of immune receptors (eg, B-cell receptor, T-cell receptor, and FcepsilonRI) are subject to negative regulation by a growing family of structurally and functionally related inhibitory receptors. Recent studies indicate that mast cells express multiple inhibitory receptors that may regulate FcepsilonRI-induced mast cell activation through similar mechanisms. The ability of inhibitory receptors to attenuate IgE-mediated allergic responses implicates them as potential targets for therapeutic intervention in the treatment of atopic disease. Indeed, coaggregation of activating and inhibitory receptors has been suggested as one possible mechanism to explain the beneficial effects of specific immunotherapy in the treatment of allergy. In this review we summarize the current knowledge of inhibitory receptors expressed in mast cells and the mechanisms through which they regulate mast cell function.

The protein-tyrosine phosphatase SHP-1 binds to and dephosphorylates p120 catenin

A prominent tyrosine-phosphorylated protein of approximately 100 kDa (designated pp100) in epidermal growth factor (EGF)-stimulated A431 cells was found to be a main interaction partner of the protein-tyrosine phosphatase SHP-1 in pull-down experiments with a glutathione S-transferase-SHP-1 fusion protein. Binding was largely mediated by the N-terminal SH2 domain of SHP-1 and apparently direct and independent from the previously described association of SHP-1 with the activated EGF receptor. pp100 was partially purified and identified by mass spectrometric analysis of tryptic fragments, partial amino acid sequencing, and use of authentic antibodies as the 3A isoform of the Armadillo repeat protein superfamily member p120 catenin (p120(ctn)). Different p120(ctn) isoforms expressed in human embryonal kidney 293 cells, exhibited differential binding to SHP-1 that correlated partly with the extent of EGF-dependent p120(ctn) tyrosine phosphorylation. Despite strong phosphorylation, p120(ctn) isoforms 3B and 3AB bound, however, less readily to SHP-1. SHP-1 associated transiently with p120(ctn) in EGF-stimulated A431 cells stably transfected with a tetracycline-responsive SHP-1 expression construct, and p120(ctn) exhibited elevated phosphorylation upon a tetracycline-mediated decrease in the SHP-1 level. Functions of p120(ctn), which are regulated by tyrosine phosphorylation, may be modulated by the described SHP-1-p120(ctn) interaction.

Roles of the SHP-1 tyrosine phosphatase in the negative regulation of cell signalling

The critical role for the SH2 domain-containing SHP-1 tyrosine phosphatase in regulating haemopoietic cell behaviour was initially revealed by data linking SHP-1 deficiency to the systemic autoimmunity and severe inflammation exhibited by motheaten mice. This discovery laid the groundwork for the identification of SHP-1 as an inhibitor of activation-promoting signalling cascades and for the coincident demonstration that protein tyrosine phosphatases (PTPs) such as SHP-1 show considerable specificity with respect to the mechanisms whereby they modulate the biochemical and biological sequelae of extracellular simulation. As outlined in this review, SHP-1 has now been implicated in the regulation of a myriad of signalling cascades and cell functions. As a result, the cumulative data generated from studies of this PTP have elucidated not only the functional relevance of SHP-1, but also a number of novel paradigms as to the molecular mechanisms whereby signalling cascades are regulated so as to either augment or abrogate specific cell behaviours.

Negative regulation of FcepsilonRI signaling by FcgammaRII costimulation in human blood basophils

BACKGROUND

Signaling through the antigen receptors of human B and T cells and the high-affinity IgE receptor FcepsilonRI of rodent mast cells is decreased by cross-linking these receptors to the low-affinity IgG receptor FcgammaRII. The inhibition is thought to involve the tyrosine phosphorylation of immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in the FcgammaRIIB cytoplasmic tail, creating binding sites for SH2-containing protein (Src homology domain containing protein tyrosine phosphatase 1 and 2 [SHP-1, SHP-2]) and/or lipid (SH2 domain-containing polyphosphatidyl-inositol 5-phosphatase) phosphatases that oppose activating signals from the costimulated antigen receptors.

OBJECTIVE

In human basophils and mast cells FcepsilonRI signaling generates mediators and cytokines responsible for allergic inflammation. We proposed to determine whether FcepsilonRI signaling is inhibited by FcgammaRII costimulation in human basophils and to explore the underlying mechanism as an approach to improving the treatment of allergic inflammation.

METHODS

FcgammaR expression on human basophils was examined using flow cytometry and RT-PCR analysis. FcgammaRII/FcepsilonRI costimulation was typically accomplished by priming cells with anti-dinitrophenol (DNP) IgE and anti-DNP IgG and stimulating with DNP-BSA. Phosphatases were identified by Western blotting, and their partitioning between membrane and cytosol was determined by cell fractionation. Biotinylated synthetic peptides and phosphopeptides corresponding to the FcgammaRIIB ITIM sequence were used for adsorption assays.

RESULTS

We report that peripheral blood basophils express FcgammaRII (in both the ITIM-containing FcgammaRIIB and the immunoreceptor tyrosine-based activation motif-containing FcgammaRIIA forms) and that costimulating FcgammaRII and FcepsilonRI inhibits basophil FcepsilonRI-mediated histamine release, IL-4 production, and Ca(2+) mobilization. The inhibition of basophil FcepsilonRI signaling by FcgammaRII/FcepsilonRI costimulation is linked to a significant decrease in Syk tyrosine phosphorylation. Human basophils express all 3 SH2-containing phosphatases.

CONCLUSIONS

Evidence that FcgammaRII/FcepsilonRI costimulation induces SHP-1 translocation from the cytosolic to membrane fractions of basophils and that biotinylated synthetic peptides corresponding to the phosphorylated FcgammaRIIB ITIM sequence specifically recruit SHP-1 from basophil lysates particularly implicates this protein phosphatase in the negative regulation of FcepsilonRI signaling by costimulated FcgammaRII.

Cytoplasmic protein tyrosine phosphatases SHP-1 and SHP-2: regulators of B cell signal transduction

One of the areas of greatest recent progress in immunology has been the elucidation of inhibitory receptors and their mode of signal transduction. A common feature of members of this growing family is expression of a conserved cytoplasmic sequence motif, the immunoreceptor tyrosine-based inhibitory motif, which functions to recruit and activate phosphatases that mediate the receptors' function. Family members include the protein tyrosine phosphatases SHP-1 (Src-homology-2-domain-containing protein tyrosine phosphatase 1) and SHP-2, which function to dephosphorylate key intermediaries in antigen receptor signaling pathways. Surprisingly, whereas most data to date support a role for SHP-1 in inhibitory signaling, SHP-2 exhibits distinct functions that appear to positively regulate receptor function.

Cytogenetic mapping of 31 functional genes on chicken chromosomes by direct R-banding FISH

Using direct R-banding fluorescence in situ hybridization, we determined the location of 31 functional genes on chicken chromosomes. Replication R-banded chromosomes were obtained by synchronizing splenocyte cultures with excessive thymidine, followed by BrdU treatment. Thirty-one functional genes were directly localized to banded chicken chromosomes using genomic DNA and cDNA fragments as probes. The possibility of conserved linkage homology between chicken and human chromosomes was demonstrated for seven chicken chromosome regions (1p, 1q, 2q, 4p, 4q, and 5q).

A novel function of B lymphocytes from normal mice to suppress autoimmunity in (NZB x NZW)F1 mice

In systemic autoimmune-prone (NZB x NZW)F1 (NZB/W F1) mice, B-cell abnormalities characterized by hypergammaglobulinaemia accompanying autoantibodies have been thought to be a main cause of the disease. To examine a possible regulatory role of B cells in the disease manifestations, we injected, intravenously (i.v.), normal or autoimmune B cells into non-irradiated NZB/W F1 mice. The injection of splenic B cells from major histocompatibility (MHC)-matched or allogeneic normal mice caused a marked decrease in serum immunoglobulin G (IgG) levels of autoantibodies, delayed the appearance of proteinuria and prolonged life span, whereas treatment with splenic B cells from NZB/W F1 or X-linked immunodeficient (Xid) mice failed to suppress the autoimmunity. Moreover, in vitro polyclonal antibody responses to lipopolysaccharide (LPS) of NZB/W F1-derived B cells from the treated mice were markedly reduced. Interestingly, the treatment of NZB/W F1 mice at 16, 18 and 20 or at 20, 22 and 24 weeks of age was more effective than that at 6, 8 and 10 weeks. The treatment also inhibited the development of surface IgG+ (sIgG+) B cells and splenomegaly, prominent in aged NZB/W F1 mice. In addition, when untreated NZB/W F1 responding B cells were precultured with normal B cells in vitro for 3 days, they also diminished the autoantibody production to subsequent LPS stimulation. Hence, the present results imply a novel function of normal B cells to ameliorate autoimmune disease in NZB/W F1 mice by correcting their B-cell abnormalities, and indicate that NZB/W F1 and Xid mice possess defects in this regulatory B-cell function.

Cytogenetic assignment of 29 functional genes to chicken microchromosomes by FISH

We assigned 29 functional genes to chicken microchromosomes by fluorescence in situ hybridization (FISH). Two linkage groups in the genetic linkage map of the East Lansing breed were identified in this study by localizing the genes AGRN and H2FA to microchromosomes. The frequency of the genes mapped on 30 pairs of microchromosomes, which account for roughly 30% of the whole chicken genome, was about 40% of the 73 genes randomly mapped in our laboratory. This result confirms the important role of microchromosomes for avian genome function and supports the likelihood of a high gene density on avian microchromosomes.

The CD2-subset of the Ig superfamily of cell surface molecules: receptor-ligand pairs expressed by NK cells and other immune cells

The CD2-subset of the immunoglobulin superfamily of cell surface receptors is an emerging family of proteins involved in cellular activation. Members of this family are CD2, CD48, CD58, CD84, signaling lymphocytic activation molecule (SLAM), 2B4 and Ly-9. These proteins are expressed on different leukocyte populations and the receptors of this family, specifically CD2, 2B4 and SLAM, contribute to the activation of T cells and natural killer cells. 2B4 and SLAM associate with a protein termed SLAM-associated protein that is the genetic defect in the immunodeficiency X-linked lymphoproliferative syndrome. Impaired signaling via these receptors may contribute to this often-fatal immunodeficiency.

Human inhibitory and activating Ig-like receptors which modulate the function of myeloid cells

There is increasing evidence that myeloid cells express several receptor families, which include both inhibitory and stimulatory isoforms. The expression of receptor isoforms with similar specificities but opposite functions on the same cell is intriguing. What might be the interplay between these receptors? Some clues to the answer to this question may come from recent studies on two myeloid receptor families: the ILT/LIR/MIR (immunoglobulin-like transcript/leukocyte Ig-like receptor/monocyte/macrophage Ig-like receptor) and the SIRP (signal-regulatory protein).

Dissecting the interaction of SHP-2 with PZR, an immunoglobulin family protein containing immunoreceptor tyrosine-based inhibitory motifs

Tyrosine phosphorylation of membrane proteins plays a crucial role in cell signaling by recruiting Src homology 2 (SH2) domain-containing signaling molecules. Recently, we have isolated a transmembrane protein designated PZR that specifically binds tyrosine phosphatase SHP-2, which has two SH2 domains (Zhao, Z. J., and Zhao, R. (1998) J. Biol. Chem. 273, 29367-29372). PZR belongs to the immunoglobulin superfamily. Its intracellular segment contains four putative sites of tyrosine phosphorylation. By site-specific mutagenesis, we found that the tyrosine 241 and 263 embedded in the consensus immunoreceptor tyrosine-based inhibitory motifs VIYAQL and VVYADI, respectively, accounted for the entire tyrosine phosphorylation of PZR. The interaction between PZR and SHP-2 requires involvement of both tyrosyl residues of the former and both SH2 domains of the latter, since its was disrupted by mutating a single tyrosyl residue or an SH2 domain. Overexpression of catalytically inactive but not active forms of SHP-2 bearing intact SH2 domains in cells caused hyperphosphorylation of PZR. In vitro, tyrosine-phosphorylated PZR was efficiently dephosphorylated by the full-length form of SHP-2 but not by its SH2 domain-truncated form. Together, the data indicate that PZR serves not only as a specific anchor protein of SHP-2 on the plasma membrane but also as a physiological substrate of the enzyme. The coexisting binding and dephosphorylation of PZR by SHP-2 may function to terminate signal transduction initiated by PZR and SHP-2 and to set a threshold for the signal transduction to be initiated.

CD72 negatively regulates signaling through the antigen receptor of B cells

The immunoreceptor tyrosine-based inhibition motif (ITIM) is found in various membrane molecules such as CD22 and the low-affinity Fc receptor for IgG in B cells and the killer cell-inhibitory receptor and Ly-49 in NK cells. Upon tyrosine phosphorylation at the ITIMs, these molecules recruit SH2 domain-containing phosphatases such as SH2-containing tyrosine phosphatase-1 and negatively regulate cell activity. The B cell surface molecule CD72 carries an ITIM and an ITIM-like sequence. We have previously shown that CD72 is phosphorylated and recruits SH2-containing tyrosine phosphatase-1 upon cross-linking of the Ag receptor of B cells (BCR). However, whether CD72 modulates BCR signaling has not yet been elucidated. In this paper we demonstrate that expression of CD72 down-modulates both extracellular signal-related kinase (ERK) activation and Ca2+ mobilization induced by BCR ligation in the mouse B lymphoma line K46micromlambda, whereas BCR-mediated ERK activation was not reduced by the ITIM-mutated form of CD72. Moreover, coligation with CD72 with BCR reduces BCR-mediated ERK activation in spleen B cells of normal mice. These results indicate that CD72 negatively regulates BCR signaling. CD72 may play a regulatory role in B cell activation, probably by setting a threshold for BCR signaling.

Early signaling via inhibitory and activating NK receptors

This review focuses on recent findings on the structural features of inhibitory NK cell receptors containing immunoreceptor tyrosine-based inhibition motif (ITIM) and of NK cell activating receptors, both in human and mouse. First, the study of the inhibitory killer cell immunoglobulin-like receptors (KIR) unveiled the presence of intracytoplasmic ITIM and their capacity to recruit protein tyrosine phosphatases such as SHP-1 in vivo. A brief summary of the known SHP-1 targets may help us to understand the inhibition mediated by the KIR. The characterization of ITIM thus allowed the definition of a large group of inhibitory cell surface receptors. The second part of the review describes the known NK cell activating receptors. Most of them require association with ITAM-containing polypeptides in order to mediate cell activation.

Constitutive tyrosine phosphorylation of the inhibitory paired Ig-like receptor PIR-B

PIR-A and PIR-B are activating and inhibitory Ig-like receptors on murine B lymphocytes, dendritic cells, and myeloid-lineage cells. The inhibitory function of PIR-B is mediated via its cytoplasmic immunoreceptor tyrosine-based inhibitory motifs, whereas PIR-A pairs with the Fc receptor common gamma chain to form an activating receptor complex. In these studies, we observed constitutive tyrosine phosphorylation of PIR-B molecules on macrophages and B lymphocytes, irrespective of the cell activation status. Splenocyte PIR-B molecules were constitutively associated with the SHP-1 protein tyrosine phosphatase and Lyn protein tyrosine kinase. In Lyn-deficient mice, PIR-B tyrosine phosphorylation was greatly reduced. Unexpectedly, tyrosine phosphorylation of PIR-B was not observed in most myeloid and B cell lines but could be induced by ligation of the PIR molecules. Finally, the phosphorylation status of PIR-B was significantly reduced in MHC class I-deficient mice, although not in mice deficient in TAP1 or MHC class II expression. These findings suggest a physiological inhibitory role for PIR-B that is regulated by endogenous MHC class I-like ligands.

Characterization of Paired Ig-Like Receptors in Rats

To explore the phylogenetic history of the murine paired Ig-like receptors of activating (PIR-A) and inhibitory (PIR-B) types, we isolated PIR homologues from a rat splenocyte cDNA library. The rat (ra) PIR-A and raPIR-B cDNA sequences predict transmembrane proteins with six highly conserved extracellular Ig-like domains and distinctive membrane proximal, transmembrane, and cytoplasmic regions. The raPIR-B cytoplasmic region contains prototypic inhibitory motifs, whereas raPIR-A features a charged transmembrane region and a short cytoplasmic tail. Southern blot analysis predicts the presence of multiple Pira genes and a single Pirb gene in the rat genome. Although raPIR-A and raPIR-B are coordinately expressed by myeloid cells, analysis of mRNA detected unpaired expression of raPIR-A by B cells and raPIR-B by NK cells. Collectively, these findings indicate that the structural hallmarks of the Pir gene family are conserved in rats and mice, yet suggest divergence of PIR regulatory elements during rodent speciation.

SHP-1 regulation of p62(DOK) tyrosine phosphorylation in macrophages

SHP-1 plays key roles in the modulation of hematopoietic cell signaling. To ascertain the impact of SHP-1 on colony-stimulating factor-1 (CSF-1)-mediated survival and proliferative signaling, we compared the CSF-1 responses of primary bone marrow macrophages (BMM) from wild-type and SHP-1-deficient motheaten (me/me) mice. CSF-1-induced protein tyrosine phosphorylation levels were similar in wild-type and me/me BMM, except for the constitutive hyperphosphorylation of a 62-kDa phosphoprotein (pp62) in me/me macrophages. pp62 was identified as the RASGAP-associated p62(DOK) and was shown to be the major CSF-1R-associated tyrosine-phosphorylated protein in CSF-1-treated BMM. p62(DOK) was found to be constitutively associated with SHP-1 in BMM and in 293T cells, co-expressing p62(dok) and either wild-type or catalytically inert SHP-1 (SHP-1 C453S). In both cell types, the interaction of SHP-1 with p62(DOK) occurred independently of p62(DOK) tyrosine phosphorylation, but only the tyrosine-phosphorylated p62(DOK) was bound by SHP-1 C453S in a far Western analysis. These findings suggest a constitutive association of SHP-1 and p62(DOK) that is either conformation-dependent or indirect as well as a direct, inducible association of the SHP-1 catalytic domain with tyrosine-phosphorylated p62(DOK). p62(DOK) hyperphosphorylation is not associated with altered CSF-1-induced RAS signaling or proliferation. However, whereas wild-type macrophages undergo cell death following CSF-1 removal, me/me macrophages exhibit prolonged survival in the absence of growth factor. Thus, p62(DOK) is a major SHP-1 substrate whose tyrosine phosphorylation correlates with growth factor-independent survival in macrophages.

Regulation of B-1 cell activation and its autoantibody production by Lyn kinase-regulated signallings

The src-family protein tyrosine kinase, Lyn, has been reported to play a crucial role in the regulation of B-cell antigen receptor (BCR)-mediated signalling. To elucidate the role of Lyn in the maintenance of immunological tolerance and the prevention of B-1 cell activation and its autoantibody production, Lyn-deficient mice were crossed with transgenic mice carrying the immunoglobulin heavy and light chain genes encoding an autoantibody against mouse red blood cells. In the transgenic mice, most peripheral B cells expressed the B-1 cell phenotype. When the transgenic mice were bred in specific pathogen-free (SPF) conditions, B-1 cells were anergic and did not produce any autoantibody. In contrast, Lyn-deficient transgenic mice kept in the same SPF conditions revealed markedly increased numbers of activated B-1 cells and developed severe autoimmune haemolytic anaemia. Moreover, the mice had a huge splenomegaly containing a remarkable accumulation of erythroblasts, resulted from extramedullary erythropoiesis, in addition to the increased numbers of lymphoblast-like cells of the B-1 cell lineages. The present study demonstrates a crucial role of Lyn kinase in the regulation of B-1 cell activation and maintenance of tolerance.

The sialoadhesin CD33 is a myeloid-specific inhibitory receptor

Activating and inhibitory receptors act in concert to regulate cellular activation. Inhibitory receptors are characterized by the presence of a characteristic sequence known as an immunoreceptor tyrosine-based inhibitory motif (ITIM) in their cytoplasmic tail. Phosphorylated ITIM serve as docking sites for the SH2-containing phosphatases which then inhibit signal transduction. CD33 is a member of the immunoglobulin superfamily and contains two immunoglobulin-like domains, a transmembrane region and a cytoplasmic tail that has two potential ITIM sequences. CD33 expression is restricted to cells of myelomonocytic lineage. The precise function of CD33 is unknown although it is a lectin that binds sialic acid residues in N- and O-glycans on cell surfaces. Co-immunoprecipitation studies demonstrate that CD33 associates with the SH2-containing tyrosine phosphatase SHP-1 in monocytes. The proximal ITIM is necessary and sufficient for SHP-1 binding which is mediated by the aminoterminal SH2 domain. Treatment of SHP-1 with a phosphopeptide representing the proximal CD33 ITIM results in increased SHP-1 enzymatic activity. CD33 exerts an inhibitory effect on tyrosine phosphorylation and Ca(2+) mobilization when co-engaged with the activating FcgammaRI receptor. This data indicates that CD33 is an inhibitory receptor that may regulate FcgammaRI signal transduction.

Stimulatory function of paired immunoglobulin-like receptor-A in mast cell line by associating with subunits common to Fc receptors

Paired Ig-like receptors (PIR) are polymorphic type I transmembrane proteins belonging to an Ig superfamily encoded by multiple isotypic genes. They are expressed on immune cells such as mast cells, macrophages, and B lymphocytes. Two subtypes of PIR have been classified according to the difference in the primary structure of the PIR transmembrane and cytoplasmic regions. These subtypes are designated as PIR-A and PIR-B. In this study, the transmembrane and cytoplasmic regions of the PIR-A subtype were shown to mediate activation signal events such as cytoplasmic calcium mobilization, protein tyrosine phosphorylations, and degranulation in rat mast cell line RBL-2H3. The association of the Fc receptor gamma and beta subunits with PIR-A was shown to be responsible for PIR-A function but not required for membrane expression of PIR-A on COS-7 cells. We further revealed the role of two charged amino acid residues in the transmembrane region, namely arginine and glutamic acid, in PIR-A function and its association with the above subunits. In contrast to the inhibitory nature of the PIR-B subtype, present findings reveal that PIR-A potentially acts as a stimulatory receptor in mast cells, suggesting a mechanism for regulation of mast cell functions by the PIR family.

Genetic Evidence for Lyn as a Negative Regulator of IL-4 Signaling

IL-4 has multiple effects on B lymphocytes, many of which are concentration dependent. This is particularly so for Ig isotype switching, where different thresholds of IL-4 stimulation are needed to induce switching from IgM to either IgG1 or IgE. In this report we describe a critical role for the tyrosine kinase Lyn in setting IL-4 signaling thresholds in mouse B lymphocytes. Upon CD40 ligand stimulation of lyn−/− B cells, 10-fold less IL-4 was required to induce switching from IgM to IgG1 and IgE and an increased proportion of B cells isotype switched at each IL-4 concentration. These in vitro results correlate with the in vivo findings that in lyn−/− mice, IgG1 Ab-forming cells develop prematurely in ontogeny and that adult lyn−/− mice have an abnormally high proportion of IgG1-expressing B cells in their spleens. Adult lyn−/− mice also have significantly higher levels of IgE in their serum. These results identify Lyn as a molecule involved in modulating the IL-4 signal in B cells and provide insights into its regulation and how a B cell signaling imbalance may contribute to atopy.

Regulation of acidification and apoptosis by SHP-1 and Bcl-2

Recruitment of the SH2 domain containing cytoplasmic protein-tyrosine phosphatase SHP-1 to the membrane by somatostatin (SST) is an early event in its antiproliferative signaling that induces intracellular acidification-dependent apoptosis in breast cancer cells. Fas ligation also induces acidification-dependent apoptosis in a manner requiring the presence of SHP-1 at the membrane. Moreover, we have recently reported that SHP-1 is required not only for acidification, but also for apoptotic events that follow acidification (Thangaraju, M., Sharma, K., Liu, D., Shen, S. H., and Srikant, C. B. (1999) Cancer Res. 59, 1649-1654). Here we show that ectopically expressed SHP-1 was predominantly membrane-associated and amplified the cytotoxic signaling initiated upon SST receptor activation and Fas ligation. The catalytically inactive mutant of SHP-1 (SHP-1C455S) abolished the ability of the SST agonists to signal apoptosis by preventing the recruitment of wild type SHP-1 to the membrane. Overexpression of the anti-apoptotic protein Bcl-2 in MCF-7 cells inhibited SST-induced apoptosis upstream of acidification by inhibiting p53-dependent induction of Bax as well as by raising the resting pH(i) and attenuating SST-induced decrease in pH(i). By contrast, Bcl-2 failed to prevent apoptosis triggered by direct acidification. These data demonstrate that (i) membrane-associated SHP-1 is required for receptor-mediated cytotoxic signaling that causes intracellular acidification and apoptosis, and (ii) Bcl-2 acts distal to SHP-1 and p53 to prevent SST-induced acidification but cannot inhibit the apoptotic events that ensue intracellular acidification.

PTPROt: An Alternatively Spliced and Developmentally Regulated B-Lymphoid Phosphatase That Promotes G0/G1 Arrest

Protein tyrosine phosphatases (PTP) regulate the proliferation, differentiation, and viability of lymphocytes by modulating their signaling pathways. By using the differential display assay, we have cloned a putative receptor-type PTP, which is predominantly expressed in B-lymphoid tissues (lymph nodes and spleen). This PTP, termed PTPROt (truncated), is a tissue-specific alternatively-spliced form of a human epithelial PTP, PTPRO (PTPU2/GLEPP1). Whereas the epithelial PTPRO includes an ≈800-amino acid extracellular domain, the major (3 kb) PTPROt cDNA predicts a unique 5′ untranslated region and truncated (8 amino acids) extracellular domain with a conserved transmembrane region and single catalytic domain. PTPROt cDNAs encode functional ∼47-kD and ∼43-kD PTPs, which are most abundant in normal naive quiescent B cells and decreased or absent in germinal center B cells and germinal center-derived diffuse large B-cell lymphomas. Because PTPROt was predominantly expressed in naive quiescent B cells, the enzyme’s effects on cell-cycle progression were examined. When multiple stable PTPROt sense, antisense, and vector only B-cell transfectants were grown in reduced serum and synchronized with nocodazole, PTPROt sense clones exhibited markedly increased G0/G1 arrest. Taken together, these data implicate PTPROt in the growth control of specific B-cell subpopulations.

Distribution of the src-homology-2-domain-containing inositol 5-phosphatase SHIP-2 in both non-haemopoietic and haemopoietic cells and possible involvement of SHIP-2 in negative signalling of B-cells

The termination of activation signals is a critical step in the control of the immune response; perturbation of inhibitory feedback pathways results in profound immune defects culminating in autoimmunity and overwhelming inflammation. FcgammaRIIB receptor is a well described inhibitory receptor. The ligation of B-cell receptor (BCR) and FcgammaRIIB leads to the inhibition of B-cell activation. Numerous studies have demonstrated that the SH2-domain-containing inositol 5-phosphatase SHIP (referred hereto as SHIP-1) is essential in this process. The cDNA encoding a second SH2-domain-containing inositol 5-phosphatase, SHIP-2, has been cloned [Pesesse, Deleu, De Smedt, Drayer and Erneux (1997) Biochem. Biophys. Res. Commun. 239, 697-700]. Here we report the distribution of SHIP-2 in mouse tissues: a Western blot analysis of mouse tissues reveals that SHIP-2 is expressed in both haemopoietic and non-haemopoietic cells. In addition to T-cell and B-cell lines, spleen, thymus and lung are shown to coexpress SHIP-1 and SHIP-2. Moreover, SHIP-2 is detected in fibroblasts, heart and different brain areas. SHIP-2 shows a maximal tyrosine phosphorylation and association to Shc after ligation of BCR to FcgammaRIIB but not after stimulation of BCR alone. Our results therefore suggest a possible role for SHIP-2 in the negative regulation of immunocompetent cells.

A New Role for Platelet-Endothelial Cell Adhesion Molecule-1 (CD31): Inhibition of TCR-Mediated Signal Transduction

Platelet-endothelial cell adhesion molecule-1 (PECAM-1) is a 130-kDa transmembrane glycoprotein expressed by endothelial cells, platelets, monocytes, neutrophils, and certain T cell subsets. The PECAM-1 extracellular domain has six Ig-homology domains that share sequence similarity with cellular adhesion molecules. The PECAM-1 cytoplasmic domain contains an immunoreceptor tyrosine-based inhibitory motif (ITIM) that, when appropriately engaged, becomes phosphorylated on tyrosine residues, creating docking sites for nontransmembrane, Src homology 2 domain-bearing protein tyrosine phosphatase (SHP)-1 and SHP-2. The purpose of the present study was to determine whether PECAM-1 inhibits protein tyrosine kinase (PTK)-dependent signal transduction mediated by the immunoreceptor tyrosine-based activation motif-containing TCR. Jurkat cells, which coexpress PECAM-1 and the TCR/CD3 complex, were INDO-1AM-labeled and then incubated with anti-CD3ε mAbs, anti-PECAM-1 mAbs, or both, and goat anti-mouse IgG was used to cross-link surface-bound mAbs. Calcium mobilization induced by CD3 cross-linking was found to be attenuated by coligation of PECAM-1 in a dose-dependent manner. PECAM-1-mediated inhibition of TCR signaling was attributable, at least in part, to inhibition of release of calcium from intracellular stores. These data provide evidence that PECAM-1 can dampen signals transduced by ITAM-containing receptors and support inclusion of PECAM-1 within the family of ITIM-containing inhibitors of PTK-dependent signal transduction.

Regulation of immune responses through inhibitory receptors

Major histocompatibility complex class I-specific inhibitory receptors on natural killer cells prevent the lysis of healthy autologous cells. The outcome of this negative signal is not anergy or apoptosis of natural killer cells but a transient abortion of activation signals. The natural killer inhibitory receptors fulfill this function by recruiting the tyrosine phosphatase SHP-1 through a cytoplasmic immunoreceptor tyrosine-based inhibition motif. This immunoreceptor tyrosine-based inhibition motif has become the hallmark of a growing family of receptors with inhibitory potential, which are expressed in various cell types such as monocytes, macrophages, dendritic cells, leukocytes, and mast cells. Most of the natural killer inhibitory receptors and two members of a monocyte inhibitory-receptor family bind major histocompatibility complex class I molecules. Ligands for many of the other receptors have yet to be identified. The inhibitory-receptor superfamily appears to regulate many types of immune responses by blocking cellular activation signals.

Paternal monoallelic expression of the paired immunoglobulin-like receptors PIR-A and PIR-B

A diverse pattern of polymorphism is defined for the paired Ig-like receptors (PIRs) that serve as activating (PIR-A) and inhibitory (PIR-B) receptors on B lymphocytes, dendritic cells, and myeloid-lineage cells in mice. The monoclonal anti-PIR antibody 10.4 is shown to recognize an allelic PIR-A/PIR-B determinant on cells from BALB/c but not C57BL/6 mice. Other strains of inbred mice also can be typed on the basis of their expression of this PIR allelic determinant. Analysis of (BALB/c x C57BL/6) F1 hybrid offspring indicates that PIR molecules bearing the paternal PIR allotype are expressed whereas PIR-A and PIR-B molecules bearing the maternal allotype are not. The monoallelic expression of the polymorphic PIR-A and PIR-B molecules, and possibly of their human Ig-like transcript/leukocyte Ig-like receptor/monocyte/macrophage Ig-like receptor and killer cell inhibitory receptor relatives, may influence innate and specific immune responses in outbred populations.

Genetic analysis of B cell antigen receptor signaling

In B lymphocytes, a signaling complex that contributes to cell fate decisions is the B cell antigen receptor (BCR). Data from knockout experiments in cell lines and mice have revealed distinct functions for the intracellular protein tyrosine kinases (Lyn, Syk, Btk) in BCR signaling and B cell development. Combinations of intracellular signaling pathways downstream of these PTKs determine the quality and quantity of BCR signaling. For example, concerted actions of the PLC-gamma 2 and PI3-K pathways are required for proper calcium responses. Similarly, the regulation of ERK and JNK responses involves both PLC-gamma 2 and GTPases pathways. Since the immune response in vivo is regulated by alteration of these signaling outcomes, achieving a precise understanding of intracellular molecular events leading to B lymphocyte proliferation, deletion, anergy, receptor editing, and survival still remains a challenge for the future.

Leukocyte-Associated Ig-Like Receptor-1 Functions as an Inhibitory Receptor on Cytotoxic T Cells

Leukocyte associated Ig-like receptor-1 (LAIR-1) is a surface molecule expressed on human mononuclear leukocytes that functions as an inhibitory receptor on human NK cells. In addition to NK cells, LAIR-1 is expressed on T cells, B cells, macrophages, and dendritic cells. Most cells express two biochemically distinct forms of LAIR-1, which we now show are likely alternative splice variants of the same gene. Cross-linking of LAIR-1 on human T cell clones results in inhibition of cytotoxicity only in T cell clones that lack CD28 and are able to spontaneously lyse certain targets in vitro. Moreover, the cytolytic activity of freshly isolated T cells, which is thought to be mainly due to “effector” T cells, can be inhibited by anti-LAIR-1 mAb. Thus, LAIR-1 functions as an inhibitory receptor not only on NK cells, but also on human T cells. This indicates that LAIR-1 provides a mechanism of regulation of effector T cells and may play a role in the inhibition of unwanted bystander responses mediated by Ag-specific T cells.

Immunoreceptor tyrosine-based inhibition motif-bearing receptors regulate the immunoreceptor tyrosine-based activation motif-induced activation of immune competent cells

ITIM-bearing receptors, a family which only recently has been recognized, play a key role in the regulation of the ITAM-induced activation of immune competent cells. The mechanism of ITM-mediated regulation in various cells was recently clarified. The present review focuses on ITIM bearing membrane proteins that negatively regulate the activation of cells when co-crosslinked with ITAM containing receptors, illustrates the inhibitory processes by the negative regulation of B-, NK-, T-cells and mast cells and summarizes current views on the mechanism of ITIM-mediated inhibition.

SHP2-interacting transmembrane adaptor protein (SIT), a novel disulfide-linked dimer regulating human T cell activation

T lymphocytes express several low molecular weight transmembrane adaptor proteins that recruit src homology (SH)2 domain-containing intracellular molecules to the cell membrane via tyrosine-based signaling motifs. We describe here a novel molecule of this group termed SIT (SHP2 interacting transmembrane adaptor protein). SIT is a disulfide-linked homodimeric glycoprotein that is expressed in lymphocytes. After tyrosine phosphorylation by src and possibly syk protein tyrosine kinases SIT recruits the SH2 domain-containing tyrosine phosphatase SHP2 via an immunoreceptor tyrosine-based inhibition motif. Overexpression of SIT in Jurkat cells downmodulates T cell receptor- and phytohemagglutinin-mediated activation of the nuclear factor of activated T cells (NF-AT) by interfering with signaling processes that are probably located upstream of activation of phospholipase C. However, binding of SHP2 to SIT is not required for inhibition of NF-AT induction, suggesting that SIT not only regulates NF-AT activity but also controls NF-AT unrelated pathways of T cell activation involving SHP2.

Paired immunoglobulin-like receptor B (PIR-B) inhibits BCR-induced activation of Syk and Btk by SHP-1

Coligation of paired immunoglobulin-like receptor B (PIR-B) with B cell antigen receptor (BCR) blocks antigen-induced B cell activation. This inhibition is mediated in part by recruitment of SHP-1 and SHP-2 to the phosphorylated ITIMs in the cytoplasmic domain of PIR-B; however the molecular target(s) of these phosphatases remain elusive. Here we show that PIR-B ligation inhibits the BCR-induced tyrosine phosphorylation of Igalpha/Igbeta, Syk, Btk and phospholipase C (PLC)-gamma2. Overexpression of a catalytically inactive form of SHP-1 prevents the PIR-B-mediated inhibition of tyrosine phosphorylation of Syk, Btk, and PLC-gamma2. Dephosphorylation of Syk and Btk mediated by SHP-1 leads to a decrease of their kinase activity, which in turn inhibits tyrosine phosphorylation of PLC-gamma2. Furthermore, we define a requirement for Lyn in mediating tyrosine phosphorylation of PIR-B. Based on these results, we propose a model of PIR-B-mediated inhibitory signaling in which coligation of PIR-B and BCR results in phosphorylation of ITIMs by Lyn, subsequent recruitment of SHP-1, and a resulting inhibition of the BCR-induced inositol 1,4,5-trisphosphate generation by dephosphorylation of Syk and Btk.