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

Counteracting immunotyrosine-based signaling motifs augment zebrafish leukocyte immune-type receptor-mediated phagocytic activity

Leukocyte immune-type receptors (LITRs) represent a polymorphic and polygenic family of immunoregulatory proteins originally discovered in channel catfish (Ictalurus punctatus; IpLITRs). Belonging to the immunoglobulin superfamily (IgSF), IpLITRs are generally classified as stimulatory or inhibitory types based on their utilization of various intracellular tyrosine-based signaling motifs. While research has shown that IpLITRs can activate as well as abrogate different immune cell effector responses including phagocytosis, recent identification of LITRs within the zebrafish genome (Danio rerio; DrLITRs) revealed the existence of fish LITR-types uniquely containing counteracting stimulatory and inhibitory cytoplasmic tail (CYT) region motifs (i.e., an immunoreceptor tyrosine-based activation motif; ITAM, and immunoreceptor tyrosine-based inhibitory motif; ITIM) within the same receptor. This arrangement is unusual as these motifs typically exist on separate stimulatory (i.e., ITAM-containing) or inhibitory (i.e., ITIM-containing) immunoregulatory receptors that then co-engage to fine-tune cellular signaling and effector responses. Using a flow cytometric-based phagocytosis assay, we show here that engagement of DrLITR 1.2-expressing cells with antibody coated 4.5 μm beads causes a robust ITAM-dependent phagocytic response and reveal that its tandem ITIM motif surprisingly enhances the DrLITR 1.2-induced phagocytic activity while simultaneously decreasing the receptors ability to bind the beads. Confocal microscopy studies also revealed that the ITIM-associated inhibitory signaling molecule SHP-2 is localized to the phagocytic synapse during the phagocytic response. Overall, these results provide the first functional characterization of teleost immune receptors containing a tandem ITAM and ITIM and allow for the proposal of an intracytoplasmic tail signaling model for ITIM-mediated enhancement of ITAM-dependent cellular activation.

Inhibitory pattern recognition receptors

Pathogen- and damage-associated molecular patterns are sensed by the immune system's pattern recognition receptors (PRRs) upon contact with a microbe or damaged tissue. In situations such as contact with commensals or during physiological cell death, the immune system should not respond to these patterns. Hence, immune responses need to be context dependent, but it is not clear how context for molecular pattern recognition is provided. We discuss inhibitory receptors as potential counterparts to activating pattern recognition receptors. We propose a group of inhibitory pattern recognition receptors (iPRRs) that recognize endogenous and microbial patterns associated with danger, homeostasis, or both. We propose that recognition of molecular patterns by iPRRs provides context, helps mediate tolerance to microbes, and helps balance responses to danger signals.

Functional categories of immune inhibitory receptors

The human genome encodes more than 300 potential immune inhibitory receptors. The reason for this large number of receptors remains unclear. We suggest that inhibitory receptors operate as two distinct functional categories: receptors that control the signalling threshold for immune cell activation and receptors involved in the negative feedback of immune cell activation. These two categories have characteristic receptor expression patterns: 'threshold' receptors are expressed at steady state and their expression remains high or is downregulated upon activation, whereas 'negative feedback' receptors are induced upon immune cell activation. We use mathematical models to illustrate their possible modes of operation in different scenarios for different purposes. We discuss how this categorization may impact the choice of therapeutic targets for immunotherapy of malignant, infectious and autoimmune diseases.

Porcine FcγRIIb mediated PRRSV ADE infection through inhibiting IFN-β by cytoplasmic inhibitory signal transduction

Antibody-dependent enhancement (ADE) in porcine reproductive and respiratory syndrome virus (PRRSV) infection is a significant obstacle to the development of effective vaccines for controlling PRRS. Our previous results have demonstrated that porcine FcγRIIb (poFcγRIIb) play an important role in mediating ADE of PRRSV infection in vitro. However, the underlying mechanisms involved in poFcγRIIb mediated-ADE are still not clear. In this study, MARC-145 cel1 lines stably expressing mutated poFcγRIIb (MARC-poFcγRIIb-T and MARC-poFcγRIIb-CT) in cytoplasm were established and the capacity of poFcγRIIb mutants in mediating ADE of PRRSV was investigated. Our results showed that removal of cytoplasmic domain or disruption the tyrosine residue within ITIM (immunoreceptor tyrosine-based inhibition motif) of the poFcγRIIb abolished the ability of poFcγRIIb to mediate ADE of PRRSV. Furthermore, we found that SHIP1 and TBK1 were involved in poFcγRIIb-mediated ADE of PRRSV infection. Taken together, our findings indicated that poFcγRIIb mediated the ADE pathway of PRRSV infection through recruiting SHIP-1, which further inhibited of TBK-1-IRF3-IFN-β signaling pathway to enhance PRRSV infection. These findings will contribute to the molecular mechanism of ADE infection and provide some implications for vaccine development.

Interaction of an Iridium(III) Complex with G-Quadruplex DNA and Its Application in Luminescent Switch-On Detection of Siglec-5

Sialic acid (Sia) binding immunoglobulin (Ig)-like lectin-5 (Siglec-5) is a type-I transmembrane protein, and it has been demonstrated as a biomarker of granulocytic maturation and acute myeloid leukemia phenotype. Herein we aimed to construct a method that could sensitively detect Siglec-5 by taking advantage of the high affinity and selectivity of the K19 aptamer for its cognate target, and the selective interaction of luminescent iridium(III) transition metal complexes with G-quadruplex DNA. The iridium(III) complex 1 [Ir(tpyd)₂(2,9-dmphen)]PF₆ (where tpyd =2-(m-tolyl)pyridine; 2,9-dmphen =2,9-dimethyl-1,10-phenanthroline) was synthesized, and it displayed high luminescence for G-quadruplex DNA compared to dsDNA and ssDNA. Additionally, complex 1 exhibited a blue shift luminescence response to c-kit2 G-quadruplex, and the interaction between 1 and G-quadruplexes was discussed based on the results of G-tetrad assay, loop effect assay, and other assays. Then complex 1 was utilized to develop a G-quadruplex-based sensing platform for Siglec-5 in aqueous solution. Upon the addition of Siglec-5, the specific binding of the K19 aptamer sequence results in a conformational change that generates a split G-quadruplex structure, which is then recognized by the G-quadruplex-specific iridium(III) complex with an enhanced luminescent response. Futhermore, the use of the assay for detecting Siglec-5 in cellular debris was demonstrated.

C-Type Lectin Receptors

C-type lectins, originally defined as proteins binding carbohydrates in a Ca²⁺-dependent manner, form a large family containing soluble and membrane-bound proteins. Among them, those expressed on phagocytes and working as pathogen pattern-recognition receptors were designated as C-type lectin receptors (CLRs), in accordance with Toll-like receptors (TLRs), NOD-like receptors (NLRs), and RIG-I–like receptors (RLRs). Most of the genes for CLRs are clustered in human chromosome 12 close to the natural killer gene complex. Similar to the killer lectin-like receptors whose genes are clustered in this complex, most of the CLRs induce activating or regulatory signal cascades in response to distinct pathogen- or self-derived components, through the immunoreceptor tyrosine-based activating or inhibitory motif, respectively. In this chapter, some representative CLRs are picked up and their structural features leading to the functional consequences are discussed, especially on the signaling cascades and pathogen interactions, including some impacts on cutaneous pathophysiology. These CLRs should provide targets to develop effective vaccination and therapeutics for distinct infectious and autoimmune/inflammatory diseases.

The role of soluble fibrinogen-like protein 2 in transplantation: protection or damage

Soluble fibrinogen-like protein 2 (sFGL2) is the soluble form of fibrinogen like protein 2. As a novel immunoregulatory molecule, sFGL2 is secreted mainly by T cells, especially regulatory T cells, and exerts an immunoregulatory property rather than a prothrombinase function in the immune system. sFGL2 changes not only the proliferation and differentiation of T cells but also the maturation of antigen presenting cells. Besides its innate and adaptive immunoregulatory functions, sFGL2 also induces apoptosis in cells including renal tubular epithelial cells through Fcγ receptors (FcγRs). It may affect transplantation via regulation of immunity and induction of apoptosis of different cells in a spatiotemporal manner. Here, we review the research progresses on sFGL2 including its structure, functions, and molecular mechanisms via which sFGL2 might affect organ transplantation, as well as discuss its characteristics and potential of becoming a therapeutic target in patients with rejection.

Dissecting the antibody constant region protective immune parameters in HIV infection

ABSTRACT: 

RV144 vaccine immune-correlates analysis has generated a renewed interest in understanding the potentially protective role of non-neutralizing antibodies in HIV infection and vaccine design. Antibodies consist of a variable region involved in antigen binding and a constant region. While both ends of the antibody collaborate to induce protective immunity, it is through the constant portion that an antibody provides instructions to the innate immune system on how the recognized antigen should be processed, contributing directly to antiviral immunity. Antibody constant regions, despite their name, are not uniform structures, but can vary both in protein sequence and glycosylation, together modulating antibody functionality via conformational changes that alter antibody affinity for Fc receptors, complement and so on. This review will focus on how the immune system naturally modulates the Fc domain of antibodies to achieve optimum protective Fc effector responses for vaccine and monoclonal therapeutic design efforts aimed at preventing or curing HIV infection.

Group B Streptococcus engages an inhibitory Siglec through sialic acid mimicry to blunt innate immune and inflammatory responses in vivo

Group B Streptococcus (GBS) is a common agent of bacterial sepsis and meningitis in newborns. The GBS surface capsule contains sialic acids (Sia) that engage Sia-binding immunoglobulin-like lectins (Siglecs) on leukocytes. Here we use mice lacking Siglec-E, an inhibitory Siglec of myelomonocytic cells, to study the significance of GBS Siglec engagement during in vivo infection. We found GBS bound to Siglec-E in a Sia-specific fashion to blunt NF-κB and MAPK activation. As a consequence, Siglec-E-deficient macrophages had enhanced pro-inflammatory cytokine secretion, phagocytosis and bactericidal activity against the pathogen. Following pulmonary or low-dose intravenous GBS challenge, Siglec-E KO mice produced more pro-inflammatory cytokines and exhibited reduced GBS invasion of the central nervous system. In contrast, upon high dose lethal challenges, cytokine storm in Siglec-E KO mice was associated with accelerated mortality. We conclude that GBS Sia mimicry influences host innate immune and inflammatory responses in vivo through engagement of an inhibitory Siglec, with the ultimate outcome of the host response varying depending upon the site, stage and magnitude of infection.

The bacterium Group B Streptococcus (GBS) causes serious infections such as meningitis in human newborn babies. The surface of GBS is coated with a capsule made of sugar molecules. Prominent among these is sialic acid (Sia), a human-like sugar that interacts with protein receptors called Siglecs on the surface of our white blood cells. In a test tube, GBS Sia binding to human Siglecs can suppress white blood cell activation, reducing their bacterial killing abilities; however, the significance of this during actual infection was unknown. To answer this question, we studied mice for which a key white blood cell Siglec has been genetically deleted. When infected with GBS, white blood cells from the mutant mice are not shut off by the pathogen's Sia-containing sugar capsule. The white blood cells from the Siglec-deficient mice are better at killing GBS and are able to clear infection more quickly than a normal mouse. However, if the mice are given an overwhelming dose of GBS bacteria, exaggerated white blood activation can trigger shock and more rapid death. These studies show how “molecular mimicry” of sugar molecules in the host can influence a bacterial pathogen's interaction with the immune system and the outcome of infection.

Channel catfish leukocyte immune-type receptor mediated inhibition of cellular cytotoxicity is facilitated by SHP-1-dependent and -independent mechanisms

Channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are immunoregulatory proteins belonging to the immunoglobulin superfamily that likely play an important role in the regulation of teleost immune cell effector responses. IpLITRs are expressed by myeloid and lymphoid subsets and based on their structural features can be classified as either putative stimulatory or inhibitory forms. We have recently demonstrated at the biochemical and functional levels that stimulatory IpLITR-types induced intracellular signaling cascades resulting in immune cell activation. Alternatively, we have shown that putative inhibitory IpLITRs may abrogate immune cell responses by recruiting teleost Src homology 2 (SH2) domain-containing cytoplasmic phosphatases (SHP) to their tyrosine-containing cytoplasmic tails. In the present study, we used vaccinia virus to express recombinant chimeric proteins encoding the extracellular and transmembrane regions of human KIR2DL3 fused with the cytoplasmic tails of two putative inhibitory IpLITRs (i.e. IpLITR1.2a and IpLITR1.1b) in mouse spleen-derived cytotoxic lymphocytes. This approach allowed us to study the specific effects of IpLITR-induced signaling on lymphocyte killing of B cell targets (e.g. 721.221 cells) using a standard chromium release assay. Our results suggest that both IpLITR1.2a and IpLITR1.1b are potent inhibitors of lymphocyte-mediated cellular cytotoxicity. Furthermore, using a catalytically inactive SHP-1 mutant in combination with site-directed mutagenesis and co-immunoprecipitations, we also demonstrate that the IpLITR1.2a-mediated functional inhibitory response is SHP-1-dependent. Alternatively, IpLITR1.1b-mediated inhibition of cellular cytotoxicity is facilitated by both SHP-1-dependent and independent mechanisms, possibly involving the C-terminal Src kinase (Csk). The involvement of this inhibitory kinase requires binding to a tyrosine residue encoded in the unique membrane proximal cytoplasmic tail region of IpLITR1.1b. Overall, this represents the first functional information for inhibitory IpLITR-types and reveals that catfish LITRs engage SHP-dependent and -independent inhibitory signaling pathways to abrogate lymphocyte-mediated killing.

Differences in endocytosis mediated by FcγRIIA and FcγRIIB2

An important function of Fcγ receptors is the removal of IgG-containing immune complexes from the circulation. The activating receptor FcγRIIA and inhibitory receptor FcγRIIB2 are both expressed on human myeloid cells, and are both capable of mediating endocytosis of immune complexes. We studied endocytosis of these two receptors expressed by transfection in ts20 Chinese hamster fibroblasts. We find that while FcγRIIA-mediated endocytosis requires the participation of the ubiquitin-conjugating system, the endocytosis of FcγRIIB2 does not. Little if any ubiquitylation of FcγRIIB2 was observed in response to immune complex binding. FcγRIIB2 mediates internalization of immune complexes at a faster rate than FcγRIIA, and facilitates the endocytosis of FcγRIIA upon co-engagement of both receptors. This may represent a novel mechanism by which the inhibitory receptor can reduce signalling from the activating Fcγ receptor.

Differential recruitment of activating and inhibitory Fc gamma RII during phagocytosis

Human myeloid cells express both activating and inhibitory receptors of the FcgammaRII family. FcgammaRIIA mediates processes associated with cell activation, including phagocytosis of IgG-opsonized particles, whereas coengagement of the inhibitory FcgammaRIIB downregulates such signaling. We analyzed the relative recruitment of these two receptors during phagocytosis of IgG-coated particles by ts20 Chinese hamster fibroblast cells cotransfected with both receptors carrying distinguishable fluorescent protein tags. We found that FcgammaRIIA is substantially enriched at sites of particle binding relative to its inhibitory counterpart, with a greater than 2-fold increase in the local ratio of activating to inhibitory receptor compared with that for the plasma membrane as a whole. Experiments with chimeric receptors revealed that the preferential enrichment of FcgammaRIIA results from differences between the extracellular domains of the receptors, and indicated that the lesser recruitment of FcgammaRIIB limits its ability to effectively inhibit FcgammaRIIA-mediated phagocytosis. Mutagenesis studies indicated that FcgammaRIIA residues leucine 132 and phenylalanine 160, which lie in IgG-binding regions of FcgammaRIIA and which differ in FcgammaRIIB, both contribute to the local relative enrichment of FcgammaRIIA by increasing its affinity for IgG1 relative to that of FcgammaRIIB. In human monocytes, engagement of approximately equal amounts of FcgammaRIIB was required to substantially inhibit FcgammaRIIA-mediated phagocytosis. These results demonstrate that differences in affinity for IgG between activating and inhibitory FcgammaR can result in substantial local changes in their relative concentrations during phagocytosis, with important functional consequences.

Group B Streptococcus suppression of phagocyte functions by protein-mediated engagement of human Siglec-5

Group B Streptococcus (GBS) is a leading cause of invasive bacterial infections in human newborns. A key GBS virulence factor is its capsular polysaccharide (CPS), displaying terminal sialic acid (Sia) residues which block deposition and activation of complement on the bacterial surface. We recently demonstrated that GBS Sia can bind human CD33-related Sia-recognizing immunoglobulin (Ig) superfamily lectins (hCD33rSiglecs), a family of inhibitory receptors expressed on the surface of leukocytes. We report the unexpected discovery that certain GBS strains may bind one such receptor, hSiglec-5, in a Sia-independent manner, via the cell wall-anchored beta protein, resulting in recruitment of SHP protein tyrosine phosphatases. Using a panel of WT and mutant GBS strains together with Siglec-expressing cells and soluble Siglec-Fc chimeras, we show that GBS beta protein binding to Siglec-5 functions to impair human leukocyte phagocytosis, oxidative burst, and extracellular trap production, promoting bacterial survival. We conclude that protein-mediated functional engagement of an inhibitory host lectin receptor promotes bacterial innate immune evasion.

Cell surface expression of channel catfish leukocyte immune-type receptors (IpLITRs) and recruitment of both Src homology 2 domain-containing protein tyrosine phosphatase (SHP)-1 and SHP-2

Channel catfish leukocyte immune-type receptors (IpLITRs) are immunoglobulin superfamily (IgSF) members believed to play a role in the control and coordination of cellular immune responses in teleost. Putative stimulatory and inhibitory IpLITRs are co-expressed by different types of catfish immune cells (e.g. NK cells, T cells, B cells, and macrophages) but their signaling potential has not been determined. Following cationic polymer-mediated transfections into human cell lines we examined the surface expression, tyrosine phosphorylation, and phosphatase recruitment potential of two types of putative inhibitory IpLITRs using 'chimeric' expression constructs and an epitope-tagged 'native' IpLITR. We also cloned and expressed the teleost Src homology 2 domain-containing protein tyrosine phosphatases (SHP)-1 and SHP-2 and examined their expression in adult tissues and developing zebrafish embryos. Co-immunoprecipitation experiments support the inhibitory signaling potential of distinct IpLITR-types that bound both SHP-1 and SHP-2 following the phosphorylation of tyrosine residues within their cytoplasmic tail (CYT) regions. Phosphatase recruitment by IpLITRs represents an important first step in understanding their influence on immune cell effector functions and suggests that certain inhibitory signaling pathways are conserved among vertebrates.

EBV LMP-2A employs a novel mechanism to transactivate the HERV-K18 superantigen through its ITAM

EBV encodes latent membrane protein (LMP)-2A that functions as a homologue of the activated BCR. We have previously shown that LMP-2A transactivates a human endogenous retrovirus, HERV-K18, in infected B-lymphocytes. The Env protein of HERV-K18 encodes a superantigen that strongly stimulates a large number of T cells. To delineate the mechanism through which LMP-2A transactivates HERV-K18 env, we tested a panel of tyrosine mutants of LMP-2A in a murine B lymphoma that stably harbors HERV-K18. Our analysis revealed that the immunoreceptor tyrosine-based activation motif (ITAM) of LMP-2A is important for HERV-K18 env transactivation. ITAM contains 2 tyrosines that initiate signaling cascades when both residues are phosphorylated. However, in our study, single-tyrosine mutants of ITAM still retained full induction of HERV-K18 env. After truncating 25 kb of genomic sequence downstream of HERV-K18, LMP-2A failed to transactivate HERV-K18 env. Thus, an LMP-2A-inducible element is located downstream of HERV-K18.

The diverse functions of Src family kinases in macrophages

Macrophages are key components of the innate immune response. These cells possess a diverse repertoire of receptors that allow them to respond to a host of external stimuli including cytokines, chemokines, and pathogen-associated molecules. Signals resulting from these stimuli activate a number of macrophage functional responses such as adhesion, migration, phagocytosis, proliferation, survival, cytokine release and production of reactive oxygen and nitrogen species. The cytoplasmic tyrosine kinase Src and its family members (SFKs) have been implicated in many intracellular signaling pathways in macrophages, initiated by a diverse set of receptors ranging from integrins to Toll-like receptors. However, it has been difficult to implicate any given member of the family in any specific pathway. SFKs appear to have overlapping and complementary functions in many pathways. Perhaps the function of these enzymes is to modulate the overall intracellular signaling network in macrophages, rather than operating as exclusive signaling switches for defined pathways. In general, SFKs may function more like rheostats, influencing the amplitude of many pathways.

Novel roles for murine complement receptors type 1 and 2 I. Regulation of B cell survival and proliferation by CR1/2

Innate components of the immune system, such as complement are known to have a modulatory effect on adaptive immune responses. Complement receptors are expressed by both B and T lymphocytes and play part in antigen presentation and cellular activation and adhesion events. On murine B cells type 1 and 2 complement receptors (CR1/2) are expressed and form a co-receptor complex together with CD19 and CD81. We used CR1/2 specific antibodies to assess the role these receptors might play in regulating cell cycling events of B cells. We show that a CR1/2 specific antibody fragment, 7G6 scFv can induce the proliferation of mature B cells. This effect is countermodulated by FcR crosslinkage and enhanced by BCR engagement. The proliferative effect is severely impaired in Cr2-/- animals, strengthening the involvement of CR1/2. Transitional B cells are prone to apoptotic death by selection events, yet they are rescued from apoptosis by CR1/2 crosslinkage. CR1/2 ligation by 7G6 scFv alone can induce nuclear translocation of NF-kappaB, supporting the above observations. We conclude that engagement of complement receptor 2 of B cells promotes the survival of both mature and transitional B cells. This activity supplements the previously described adjuvant effects of complement.

The acute‐phase protein α1‐acid glycoprotein (AGP) induces rises in cytosolic Ca2+in neutrophil granulocytesviasialic acid binding immunoglobulin‐like lectins (Siglecs)

We studied whether the acute-phase protein alpha1-acid glycoprotein (AGP) induces rises in [Ca2+]i in neutrophils and sought to identify the corresponding AGP receptor (or receptors). We found that AGP elicited a minimal rise in [Ca2+]i in Fura-2-loaded neutrophils, and this response was markedly enhanced by pretreatment with anti-L-selectin antibodies. (The EC50 value of the AGP-induced Ca2+ response was 9 microg/ml.) Activation of phospholipase-C, Src tyrosine kinases, and PI3 kinases proved to be essential for the AGP-mediated increase in [Ca2+]i, whereas the p38 MAPK and SYK signaling pathways were not involved. Furthermore, antibodies against sialic acid binding, immunoglobulin-like lectin 5 (Siglec-5) and oligosaccharide 3'-sialyl-lactose both antagonized the AGP-induced response and caused an immediate increase in [Ca2+]i in anti-L-selectin-treated neutrophils, which indicates a signaling capacity of Siglec-5. We used modified forms of AGP (treated with mild periodate or neuraminidase) to establish the importance of sialic acid residues. The modified forms of AGP caused a much smaller rise in [Ca2+]i than did unaltered AGP. Affinity chromatography confirmed that unchanged AGP, but not neuraminidase-treated AGP, bound to Siglec-5. Our report provides the first evidence for a signaling capacity by AGP through a defined receptor. Pre-engagement of L-selectin significantly enhanced this signaling capacity.

Regulation of the mast cell response to the type 1 Fc epsilon receptor

The type I Fc epsilon receptor (Fc epsilon RI) is one of the better understood members of its class and is central to the immunological activation of mast cells and basophils, the key players in immunoglobulin E (IgE)-dependent immediate hypersensitivity. This review provides background information on several distinct regulatory mechanisms controlling this receptor's stimulus-response coupling network. First, we review the current understanding of this network's operation, and then we focus on the inhibitory regulatory mechanisms. In particular, we discuss the different known cytosolic molecules (e.g. kinases, phosphatases, and adapters) as well as cell membrane proteins involved in negatively regulating the Fc epsilon RI-induced secretory responses. Knowledge of this field is developing at a fast rate, as new proteins endowed with regulatory functions are still being discovered. Our understanding of the complex networks by which these proteins exert regulation is limited. Although the scope of this review does not include addressing several important biochemical and biophysical aspects of the regulatory mechanisms, it does provide general insights into a central field in immunology.

Fc receptor-like 5 inhibits B cell activation via SHP-1 tyrosine phosphatase recruitment

The Fc receptor-like protein 5 (FCRL5) on B cells has both an immunoreceptor tyrosine-based activation motif (ITAM)-like sequence and two consensus immunoreceptor tyrosine-based inhibitory motifs (ITIM) in its cytoplasmic region. To evaluate its signaling potential, we expressed constructs for chimeric molecules composed of the cytoplasmic region of FCRL5 and the extracellular and transmembrane regions of the IgG Fc receptor FcgammaRIIB in a B cell line lacking an endogenous Fc receptor. Coligation of this fusion protein with the B cell receptor (BCR) inhibited BCR-mediated calcium mobilization, intracellular tyrosine phosphorylation, and Erk kinase activation. Our mutational analysis indicated that, whereas tyrosines in both the inhibitory and activation motifs are phosphorylated after ligation, only those in ITIMs influence BCR-mediated signaling. This FCRL5 inhibitory effect was mediated through dual ITIM recruitment of the SH2-containing protein tyrosine phosphatase, SHP-1, which in turn dephosphorylates the ITAM-based tyrosines in BCR Igalpha/Igbeta heterodimers. An FCRL5 inhibitory effect on BCR signaling was likewise demonstrable for primary B cells. Although its ligand is presently unknown, we conclude that FCRL5 has the functional potential to serve as an inhibitory coreceptor on mature B cells in humans.

Induction of the nuclear IκB protein IκB-ζ upon stimulation of B cell antigen receptor

The nuclear IkappaB protein IkappaB-zeta is barely detectable in resting cells and is induced in macrophages and fibroblasts following stimulation of innate immunity via Toll-like receptors. The induced IkappaB-zeta associates with nuclear factor (NF)-kappaB in the nucleus and plays crucial roles in its transcriptional regulation. Here, we examined the induction of IkappaB-zeta in B lymphocytes, one of the major players in adaptive immunity. Upon crosslinking of the surface immunoglobulin complex, IkappaB-zeta mRNA was robustly induced in murine B-lymphoma cell line A20 cells. While the crosslinking activated NF-kappaB and induced its target gene, IkappaB-alpha, co-crosslinking of Fcgamma receptor IIB to the surface immunoglobulin complex inhibited NF-kappaB activation and the induction of IkappaB-zeta and IkappaB-alpha, suggesting critical roles for NF-kappaB in the induction. These results indicate that IkappaB-zeta is also induced by stimulation of B cell antigen receptor, suggesting that IkappaB-zeta is involved in the regulation of adaptive immune responses.

An immunoreceptor tyrosine activation motif in the mouse mammary tumor virus envelope protein plays a role in virus-induced mammary tumors

Mouse mammary tumor virus (MMTV) induces breast cancer with almost 100% efficiency in susceptible strains through insertional activation of protooncogenes, such as members of the wnt and fibroblast growth factor (fgf) families. We previously showed that expression of the MMTV envelope protein (Env) in normal immortalized mammary epithelial cells grown in three-dimensional cultures caused their morphological transformation, and that this phenotype depended on an immunoreceptor tyrosine-based activation motif (ITAM) present in Env and signaling through the Syk tyrosine kinase (E. Katz, M. H. Lareef, J. C. Rassa, S. M. Grande, L. B. King, J. Russo, S. R. Ross, and J. G. Monroe, J. Exp. Med. 201:431-439, 2005). Here, we examined the role of the Env protein in virus-induced mammary tumorigenesis in vivo. Similar to the effect seen in vitro, Env expression in the mammary glands of transgenic mice bearing either full-length wild-type provirus or only Env transgenes showed increased lobuloalveolar budding. Introduction of the ITAM mutation into the env of an infectious, replication-competent MMTV or into MMTV/murine leukemia virus pseudotypes had no effect on incorporation of Env into virus particles or on in vitro infectivity. Moreover, replication-competent MMTV bearing the ITAM mutation in Env infected lymphoid and mammary tissue at the same level as wild-type MMTV and was transmitted through milk. However, mammary tumor induction was greatly attenuated, and the pattern of oncogene activation was altered. Taken together, these studies indicate that the MMTV Env protein participates in mammary epithelial cell transformation in vivo and that this requires a functional ITAM in the envelope protein.

Dendritic cell immunoreceptors: C-type lectin receptors for pattern-recognition and signaling on antigen-presenting cells

C-type lectin receptors are equipped on phagocytes for antigen capturing. Some of them seem to have a major role in cellular activation, rather than antigen internalization. The dendritic cell (DC) immunoreceptor (DCIR) and DC-associated C-type lectin (dectin)-1 have been identified as prototypic DC-associated C-type lectin receptors, characterized by their signaling mechanisms through distinct intracellular motifs; the former contains the immunoreceptor tyrosine-based inhibitory motif (ITIM), to act as an inhibitory receptor, whereas the latter works as an activating receptor via its immunoreceptor tyrosine-based activation motif (ITAM). Genes of both receptors are localized very close to the natural killer (NK) gene complex (NKC), in which genes of lectin-type activating and inhibitory NK cell receptors are clustered. Recently, the gene of the DC immunoactivating receptor (DCAR) has been identified next to the DCIR gene, and this acts as a putative activating pair of DCIR through association with an ITAM-bearing Fc receptor (FcR) gamma chain. On the other hand, the gene of an ITIM-bearing myeloid inhibitory C-type lectin-like receptor (MICL) has been found close to the dectin-1 gene. The genes of other homologous DC-associated C-type lectin receptors, dectin-2 and blood DC antigen (BDCA)-2, form a cluster with those of DCIR and DCAR, while the dectin-1 gene cluster contains lectin-like oxidized low-density lipoprotein receptor (LOX)-1, C-type lectin-like receptor (CLEC)-1 and 2, as well as MICL. Although no ligand of DCIR has yet been identified, dectin-1 recognizes fungal beta-glucan and its critical role in the biological effects of beta-glucan has been vigorously investigated. In this review, the characteristic features of these DCIR and dectin-1 family lectins, including the signaling mechanisms, ligand recognition and regulation of cellular functions, are summarized and the term "DC immunoreceptors" is applied to a distinct set of signaling pattern-recognition receptors described here.

ITAM-mediated tonic signalling through pre-BCR and BCR complexes

Studies carried out over the past few years provide strong support for the idea that Ig alpha-Ig beta-containing complexes such as the pre-B-cell receptor and the B-cell receptor can signal independently of ligand engagement, and this has been termed tonic signalling. In this Review, I discuss recent literature that is relevant to the potential mechanisms by which tonic signals are initiated and regulated, and discuss views on how tonic and ligand-dependent (aggregation-mediated) signalling differ. These mechanisms are relevant to the possibility that tonic signals generated through immunoreceptor tyrosine-based activation motif (ITAM)-containing proteins that are expressed by oncogenic viruses induce transformation in non-haematopoietic cells.

Genomic organization, promoter activity, and expression of the human choline transporter-like protein 1

Choline transporter-like (CTL) proteins of the CTL1 family are novel transmembrane proteins implicated in choline transport for phospholipid synthesis. In this study, we characterized the 5'-flanking region of the human (h)CTL1 gene and examined some of the possible mechanisms of its regulation, including promoter activity, splicing, and expression. The transcription start site of the hCTL1 gene was mapped by 5'-rapid amplification of cDNA ends (RACE), and the presence of two splice variants, hCTL1a and hCTL1b, was investigated using isoform-specific PCR and 3'-RACE. The hCTL1 promoter region of approximately 900 bp was isolated from MCF-7 human breast cancer cells. The promoter was TATA-less and driven by a long stretch of GC-rich sequence in accordance with widespread expression of hCTL1 at both mRNA and protein levels. Deletion analyses demonstrated that a very strong promoter is contained within 500 bp of the transcription start site, and more upstream regions did not increase its activity. The core promoter that conferred the minimal transcription is within the -188/+27-bp region, and its activity varied in human breast cancer and mouse skeletal muscle cells. Multiple motifs within the promoter regulatory region bound nuclear factors from both cultured cells and normal human skeletal muscle. The motifs within the three regions [S1 (-92/-61 bp), S2 (-174/-145 bp), and S3 (-289/-260 bp)] contained overlapping binding sites for hematopoietic transcription factors and ubiquitous transcription factors, in line with the expected gene function. Genomic analyses demonstrated a high conservation of hCTL1 and mouse CTL1 proximal promoters. Accordingly, mRNA profiles demonstrated that human splice variants were expressed ubiquitously, as demonstrated for the mouse transcripts; however, they differed from the profiles of rat CTL1 transcripts, which were more restricted to neurons and intestinal tissues. The shorter hCTL1b variant contained the cytosolic COOH-terminal motif L651KKR654 for endoplasmic reticulum retrieval/retention. This retention signal was conserved in hCTL1b and rat and mouse CTL1b and is typical for transmembrane proteins of type 1 topology.

Mast cells as "tunable" effector and immunoregulatory cells: recent advances

This review focuses on recent progress in our understanding of how mast cells can contribute to the initiation, development, expression, and regulation of acquired immune responses, both those associated with IgE and those that are apparently expressed independently of this class of Ig. We emphasize findings derived from in vivo studies in mice, particularly those employing genetic approaches to influence mast cell numbers and/or to alter or delete components of pathways that can regulate mast cell development, signaling, or function. We advance the hypothesis that mast cells not only can function as proinflammatory effector cells and drivers of tissue remodeling in established acquired immune responses, but also may contribute to the initiation and regulation of such responses. That is, we propose that mast cells can also function as immunoregulatory cells. Finally, we show that the notion that mast cells have primarily two functional configurations, off (or resting) or on (or activated for extensive mediator release), markedly oversimplifies reality. Instead, we propose that mast cells are "tunable," by both genetic and environmental factors, such that, depending on the circumstances, the cell can be positioned phenotypically to express a wide spectrum of variation in the types, kinetics, and/or magnitude of its secretory functions.

The LILR family: modulators of innate and adaptive immune pathways in health and disease

Leukocyte immunoglobulin (Ig)-like receptors [LILRs, also known as Ig-like transcripts (ILTs)] are a family of inhibitory and stimulatory receptors encoded within the leukocyte receptor complex and are expressed by immune cell types of both myeloid and lymphoid lineage. Several members of the LILR family recognize major histocompatibility complex class I. The immunomodulatory role of LILR receptors indicates that they may exert an influence on signaling pathways of both innate and adaptive immune systems. LILR activity can also influence the antigen-presenting properties of macrophages and dendritic cells and may thus play a role in T-cell tolerance. The wide-ranging effects of LILR signaling on immune cell activity imply that these receptors are likely to play an important role in a range of clinical situations including pregnancy, transplantation, and arthritis (including the human leukocyte antigen B27-associated spondyloarthropathies). In this review, we summarize current knowledge on the nature and function of LILRs, focusing on their regulation of immune cell activity and their potential role in disease.

Chicken Ig-like receptor B2, a member of a multigene family, is mainly expressed on B lymphocytes, recruits both Src homology 2 domain containing protein tyrosine phosphatase (SHP)-1 and SHP-2, and inhibits proliferation

Ig-like inhibitory receptors have been the focus of intensive research particularly in mouse and human. We report the cloning and characterization of three novel inhibitory chicken Ig-like receptors (CHIR) that display a two Ig-domain extracellular structure, a transmembrane region lacking charged residues and a cytoplasmic domain containing two ITIM. The localization of all receptors to a small genomic region and the hybridization pattern indicated that they belong to a multigene family. The genomic structure of the extracellular domain with two exons encoding the signal peptide and single exons for each Ig domain resembled that of all human leukocyte Ig-like receptors and killer cell Ig-like receptors, whereas the exons encoding the C terminus displayed a structure closely resembling killer cell Ig-like receptor genes. A mAb generated against one receptor designated CHIR-B2 reacted with all B cells and a small T cell subset, but not with monocytes, thrombocytes, or various leukocyte-derived cell lines. The mAb immunoprecipitated a 46-kDa protein from bursal cells and transfected cells. The Src homology 2 domain containing protein tyrosine phosphatase (SHP)-2 bound to CHIR-B2 even in unstimulated cells, whereas pervanadate treatment induced the tyrosine phosphorylation and recruitment of several CHIR-B2-associated proteins including SHP-1 and increased levels of SHP-2. Moreover, mAb cross-linking of CHIR-B2 reduced the proliferation of a stable transfected cell line. Together, we have identified a multigene family containing multiple CHIR including one receptor designated CHIR-B2 that is mainly expressed on B lymphocytes and inhibits cellular proliferation by recruitment of SHP-1 and SHP-2.

Signaling and immune regulatory role of the dendritic cell immunoreceptor (DCIR) family lectins: DCIR, DCAR, dectin-2 and BDCA-2

This review focuses on a distinct family of dendritic cells (DC) expressing C-type lectins that include DC immunoreceptor (DCIR), DC immunoactivating receptor (DCAR), DC-associated C-type lectin (dectin)-2 and blood DC antigen (BDCA)-2. DCIR is a type II C-type lectin expressed on antigen presenting cells and granulocytes and acts as an inhibitory receptor via an intracellular immunoreceptor tyrosine-based inhibitory motif (ITIM). In contrast, DCAR has been identified as a molecule that forms a putative pair with DCIR. While both molecules share the highly homologous extracellular lectin domain, DCAR lacks the ITIM in its short cytoplasmic tail and acts as an activating receptor through association with the Fc receptor gamma chain. Two other lectins, dectin-2 and BDCA-2, are highly related to DCAR by similarities of their amino acid sequence, molecular structure and chromosomal localization. Although they also lack the ITIM, they are capable of transducing signal to regulate cellular functions positively or negatively. Here we propose to designate these four highly related molecules as the "DCIR family lectins" and discuss their signaling mechanisms, carbohydrate recognition, and other features that contribute to the function of DC to control immunity.

Dendritic cell immunoactivating receptor, a novel C-type lectin immunoreceptor, acts as an activating receptor through association with Fc receptor gamma chain

An increasing number of C-type lectin receptors are being discovered on dendritic cells, but their signaling abilities and underlying mechanisms require further definition. Among these, dendritic cell immunoreceptor (DCIR) induces negative signals through an inhibitory immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytoplasmic tail. Here we identify a novel C-type lectin receptor, dendritic cell immunoactivating receptor (DCAR), whose extracellular lectin domain is highly homologous to that of DCIR. DCAR is expressed similarly in tissues to DCIR, but its short cytoplasmic portion lacks signaling motifs like ITIM. However, a positively charged arginine residue is present in the transmembrane region of the DCAR, which may explain its association with Fc receptor gamma chain and its stable expression on the cell surface. Furthermore, cross-linking of DCAR in the presence of gamma chain activates calcium mobilization and tyrosine phosphorylation of cellular proteins. These signals are mediated by the immunoreceptor tyrosine-based activating motif (ITAM) of the gamma chain. Thus, DCAR is closely related to DCIR, but it introduces activating signals into antigen-presenting cells through its physical and functional association with ITAM-bearing gamma chain. The identification of this activating immunoreceptor provides an example of signaling via a dendritic cell-expressed C-type lectin receptor.

CEACAM1 is a potent regulator of B cell receptor complex-induced activation

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1, CD66a) is a member of the immunoglobulin (Ig) superfamily, previously characterized as an adhesion and signaling molecule in epithelial, endothelial, and hematopoietic cells. Here, we show that the CEACAM1 isoform expression pattern is different in nonactivated and activated primary mouse B lymphocytes and that CEACAM1 influences B cell receptor complex-mediated activation. A CEACAM1-specific monoclonal antibody strongly triggered proliferation of mouse B cells when combined with surface IgM cross-linking. However, anti-CEACAM1 was not mitogenic when added alone. The proliferation was more pronounced and lasted longer as compared with other activators of B cells, such as anti-IgM in the presence of interleukin-4 or lipopolysaccharide. A similar, costimulatory effect was exerted by CEACAM1-expressing fibroblasts, indicating that homophilic CEACAM1-CEACAM1 cell-mediated binding is the physiological stimulus for CEACAM1-triggered B cell signaling. The anti-CEACAM1/anti-IgM-activated cells aggregated in a lymphocyte function-associated antigen-1-dependent manner. Furthermore, cells that were activated by anti-CEACAM1/anti-IgM secreted Ig but did not go through Ig class-switching. Anti-CEACAM1 induced phosphorylation of c-Jun N-terminal kinase (stress-activated protein kinase) but did not activate the extracellular signal-regulated kinase or p38 mitogen-activated protein kinases.

YXXL motifs in SH2-Bbeta are phosphorylated by JAK2, JAK1, and platelet-derived growth factor receptor and are required for membrane ruffling

SH2-Bbeta binds to the activated form of JAK2 and various receptor tyrosine kinases. It is a potent stimulator of JAK2, is required for growth hormone (GH)-induced membrane ruffling, and increases mitogenesis stimulated by platelet-derived growth factor (PDGF) and insulin-like growth factor I. Its domain structure suggests that SH2-Bbeta may act as an adapter protein to recruit downstream signaling proteins to kinase.SH2-Bbeta complexes. SH2-Bbeta is tyrosyl-phosphorylated in response to GH and interferon-gamma, stimulators of JAK2, as well as in response to PDGF and nerve growth factor. To begin to elucidate the role of tyrosyl phosphorylation in the function of SH2-Bbeta, we used phosphopeptide mapping, mutagenesis, and a phosphotyrosine-specific antibody to identify Tyr-439 and Tyr-494 in SH2-Bbeta as targets of JAK2 both in vitro and in intact cells. SH2-Bbeta lacking Tyr-439 and Tyr-494 inhibits GH-induced membrane ruffling but still activates JAK2. We provide evidence that JAK1, like JAK2, phosphorylates Tyr-439 and Tyr-494 in SH2-Bbeta and that PDGF receptor phosphorylates SH2-Bbeta on Tyr-439. Therefore, phosphorylated Tyr-439 and/or Tyr-494 in SH2-Bbeta may provide a binding site for one or more proteins linking cytokine receptor.JAK2 complexes and/or receptor tyrosine kinases to the actin cytoskeleton.

Fc receptor homologs: newest members of a remarkably diverse Fc receptor gene family

Newfound relatives of the classical Fc receptors (FcR) have been provisionally named the Fc receptor homologs (FcRH). The recent identification of eight human and six mouse FcRH genes substantially increases the size and functional potential of the FcR family. The extended family of FcR and FcRH genes spans approximately 15 Mb of the human chromosome 1q21-23 region, whereas in mice this family is split between chromosomes 1 and 3. The FcRH genes encode molecules with variable combinations of five subtypes of immunoglobulin (Ig) domains. The presence of a conserved sequence motif in one Ig domain subtype implies Ig Fc binding capability for many FcRH family members that are preferentially expressed by B lineage cells. In addition, most FcRH family members have consensus tyrosine-based activating and inhibitory motifs in their cytoplasmic domains, while the others lack features typical of transmembrane receptors. The FcRH family members, like the classical FcRs, come in multiple isoforms and allelic variations. The unique individual and polymorphic properties of the FcR/FcRH members indicate a remarkably diverse Fc receptor gene family with immunoregulatory function.

Bovine leukemia virus gp30 transmembrane (TM) protein is not tyrosine phosphorylated: examining potential interactions with host tyrosine-mediated signaling

Bovine leukemia virus (BLV) causes persistent lymphocytosis, a preneoplastic, polyclonal expansion of B lymphocytes. The expansion increases viral transmission to new hosts, but the mechanisms of this expansion have not been determined. We hypothesized that BLV infection contributes to B-cell expansion by signaling initiated via viral transmembrane protein motifs undergoing tyrosine phosphorylation. Viral mimicry of host cell proteins is a well-demonstrated mechanism by which viruses may increase propagation or decrease recognition by the host immune system. The cytoplasmic tail of BLV transmembrane protein gp30 (TM) has multiple areas of homology to motifs of host cell signaling proteins, including two immunoreceptor tyrosine-based activation motifs (ITAMs) and two immunoreceptor tyrosine-based inhibition motifs (ITIMs), which are homologous to B-cell receptor and inhibitory co-receptor motifs. Signaling by these motifs in B cells typically relies on tyrosine phosphorylation, followed by interactions with Src-homology-2 (SH2) domains of nonreceptor protein tyrosine kinases or phosphatases. Phosphorylation of tyrosine residues in the cytoplasmic tail of TM was tested in four systems including ex vivo cultured peripheral blood mononuclear cells from BLV infected cows, BLV-expressing fetal lamb kidney cell and bat lung cell lines, and DT40 B cells transfected with a fusion of mouse extracellular CD8alpha and cytoplasmic TM. No phosphorylation of TM was detected in our experiments in any of the cell types utilized, or with various stimulation methods. Detection was attempted by immunoblotting for phosphotyrosines, or by metabolic labeling of cells. Thus BLV TM is not likely to modify host signal pathways through interactions between phosphorylated tyrosines of the ITAM or ITIM motifs and host-cell tyrosine kinases or phosphatases.

On the mechanism of tolerance to the Rh D antigen mediated by passive anti-D (Rh D prophylaxis)

Anti-D prophylaxis is the most successful clinical application of antibody-mediated immune suppression. Passive IgG anti-D is given to Rh D-negative women to prevent immunisation to foetal Rh D-positive red blood cells (RBC) and subsequent haemolytic disease of the newborn. Despite its widespread use and efficacy, the mechanism of action of this therapy is unproven. The known facts about the antigen, antibody response, dose of anti-D, RBC clearance and effects of the passive anti-D on subsequent primary and secondary immune responses are discussed in relation to recent information on ways by which immune responses may be suppressed. Most Rh D antigen sites on RBC are not bound by passive anti-D, and thus epitope masking (which may occur in experimental murine models using xenogeneic RBC) is not the reason why anti-D responses are prevented by administration of prophylactic anti-D. It is hypothesised that although clearance and destruction of the antigenic RBC may be a contributing factor in preventing immunisation, down-regulation of antigen-specific B cells through co-ligation of B cell receptors and inhibitory IgG Fc receptors must also occur.

Structure and function of major histocompatibility complex (MHC) class I specific receptors expressed on human natural killer (NK) cells

Natural killer (NK) cells express receptors that are specific for MHC class I molecules. These receptors play a crucial role in regulating the lytic and cytokine expression capabilities of NK cells. In humans, three distinct families of genes have been defined that encode for receptors of HLA class I molecules. The first family identified consists of type I transmembrane molecules belonging to the immunoglobulin (Ig) superfamily and are called killer cell Ig-like receptors (KIR). A second group of receptors belonging to the Ig superfamily, named ILT (for immunoglobulin like transcripts), has more recently been described. ILTs are expressed mainly on B, T and myeloid cells, but some members of this group are also expressed on NK cells. They are also referred to as LIRs (for leukocyte Ig-like receptor) and MIRs (for macrophage Ig-like receptor). The ligands for the KIR and some of the ILT receptors include classical (class Ia) HLA class I molecules, as well as the nonclassical (class Ib) HLA-G molecule. The third family of HLA class I receptors are C-type lectin family members and are composed of heterodimers of CD94 covalently associated with a member of the NKG2 family of molecules. The ligand for most members is the nonclassical class I molecule HLA-E. NKG2D, a member of the NKG2 family, is expressed as a homodimer, along with the adaptor molecule DAP10. The ligands of NKG2D include the human class I like molecules MICA and MICB, and the recently described ULBPs. Each of these three families of receptors has individual members that can recognize identical or similar ligands yet signal for activation or inhibition of cellular functions. This dichotomy correlates with particular structural features present in the transmembrane and intracytoplasmic portions of these molecules. In this review we will discuss the molecular structure, specificity, cellular expression patterns, and function of these HLA class I receptors, as well as the chromosomal location and genetic organization.

Characterization of CDw92 as a member of the choline transporter-like protein family regulated specifically on dendritic cells

CDw92 is a 70-kDa surface protein broadly expressed on leukocytes and endothelial cells. In this manuscript, we present the molecular cloning of the CDw92 molecule by using a highly efficient retroviral expression cloning system. Sequence analysis of the CDw92 cDNA revealed a length of 2679 bp. The 1959-bp open reading frame encodes a protein of 652 amino acids. Computational analysis of the CDw92 protein sequence indicates 10 transmembrane domains, three potential N-linked glycosylation sites, and an amino acid stretch in the C-terminal region that is related to the immunoreceptor tyrosine-based inhibitory motif. Comparison of the sequence of the CDw92 clone presented in this study with various database entries show that it is a C-terminal variant of human choline transporter-like protein 1, a member of a recently identified family of multitransmembrane surface proteins. Furthermore, we found that CDw92 is stably expressed on monocytes, PBLs, and endothelial cells, as we did not yet find modulation of expression by various stimuli on these cells. In contrast to this factor-independent expression of CDw92, we detected a specific regulation of CDw92 on monocyte-derived dendritic cells (Mo-DCs). Maturation of Mo-DCs by ionomycin or calcium ionophore resulted in down-regulation of CDw92 and incubation of these cells with IL-10 in a specific re-expression. Moreover, targeting of CDw92 on LPS-treated Mo-DCs by CDw92 mAb VIM15b augmented the LPS-induced IL-10 production 2.8-fold. Together, these data suggest a crucial role of the CDw92 protein in the biology and regulation of the function of leukocytes in particular DCs.

DIgR1, a novel membrane receptor of the immunoglobulin gene superfamily, is preferentially expressed by antigen-presenting cells

A novel membrane receptor of immunoglobulin gene superfamily (IgSF) has been identified from mouse dendritic cells (DC) and designated as DC-derived Ig-like receptor 1 (DIgR1). It encodes a 228-amino-acid (aa) residue polypeptide with a 21-aa signal peptide, a 20-aa transmembrane region, a 189-aa extracellular region, and a 19 aa intracellular region. Its extracellular region contains a single V domain of Ig. So it is a novel type I transmembrane glycoprotein of IgSF. DIgR1 shows significant homologies to human CMRF-35 antigens and polymeric immunoglobulin receptors (pIgR). The mRNA expression of DIgR1 was highly abundant in mouse spleen. The preferential expression of DIgR1 mRNA is observed in the known antigen-presenting cells (APC) including DC, monocytes/macrophages, and B lymphocytes. A 40 kDa of protein in NIH/3T3 cells transfected with the DIgR1 cDNA was detected by Western blot analysis using anti-DIgR1 polyclonal antibodies. The expression of DIgR1 protein on DC is not regulated by LPS stimulation. Further study should be conducted to investigate what were biological functions of DIgR1 in the immunobiology of APC.

Identification of a family of Fc receptor homologs with preferential B cell expression

Investigation of human genome sequences with a consensus sequence derived from receptors for the Fc region of Igs (FcR) led to the identification of a subfamily of five Ig superfamily members that we term the Fc receptor homologs (FcRHs). The closely linked FcRH genes are located in a chromosome 1q21 region in the midst of previously recognized FcR genes. This report focuses on the FcRH1, FcRH2, and FcRH3 members of this gene family. Their cDNAs encode type I transmembrane glycoproteins with 3-6 Ig-like extracellular domains and cytoplasmic domains containing consensus immunoreceptor tyrosine-based activating and/or inhibitory signaling motifs. The five FcRH genes are structurally related, and their protein products share 28-60% extracellular identity with each other. They also share 15-31% identity with their closest FcR relatives. The FcRH genes are expressed primarily, although not exclusively, by mature B lineage cells. Their conserved structural features, patterns of cellular expression, and the inhibitory and activating signaling potential of their transmembrane protein products suggest that the members of this FcRH multigene family may serve important regulatory roles in normal and neoplastic B cell development.

Expression of Ly49A on T cells alters the threshold for T cell responses

In this study we investigated the balance between activating and inhibitory signals during T cell activation. We have used transgenic mice in which CD8+ T cells expressed an inhibitory receptor, Ly49A, and a specific activating alphabeta TCR. This TCR recognizes an lymphocytic choriomeningitis virus peptide in combination with H-2Db. We observed a quantitative influence on cellular responses that depended upon the activating signals received through the TCR and the inhibitory signals received through Ly49A. By varying the peptide concentration given to stimulating cells or target cells, we could adjust the amount of ligand available to trigger the TCR. At low doses of peptide, Ly49A-expressing T cells were unresponsive on target cells that expressed H-2Dd, but responded against target cells without H-2Dd. However, this inhibition could be overcome by increasing the peptide concentration or by addition of anti-Ly49A F(ab')2 fragments. Thus, rather than behaving as simple "off" switches, our data indicate that Ly49 receptors modulate T cell signaling so that higher amounts of activating signals are required for effector-cell responses.

Myeloid specific human CD33 is an inhibitory receptor with differential ITIM function in recruiting the phosphatases SHP-1 and SHP-2

CD33 is a myeloid specific member of the sialic acid-binding receptor family and is expressed highly on myeloid progenitor cells but at much lower levels in differentiated cells. Human CD33 has two tyrosine residues in its cytoplasmic domain (Y340 and Y358). When phosphorylated, these tyrosines could function as docking sites for the phosphatases, SHP-1 and/or SHP-2, enabling CD33 to function as an inhibitory receptor. Here we demonstrate that CD33 is tyrosine phosphorylated in the presence of the phosphatase inhibitor, pervanadate, and recruits SHP-1 and SHP-2. Co-expression studies suggest that the Src-family kinase Lck is effective at phosphorylating Y340, but not Y358, suggesting that these residues may function in the selective recruitment of adapter molecules and have distinct functions. Further support for overlapping, but nonredundant, roles for Y340 and Y358 comes from peptide-binding studies that revealed the recruitment of both SHP-1 and SHP-2 to Y340 but only SHP-2 to Y358. Analysis using mutants of SHP-1 demonstrated that binding Y340 of CD33 was primarily to the amino Src homology-2 domain of SHP-1. The potential of CD33 to function as an inhibitory receptor was demonstrated by its ability to down-regulate CD64-induced calcium mobilization in U937. The dependence of this inhibition on SHP-1 was demonstrated by blocking CD33-mediated effects with dominant negative SHP-1. This result implies that CD33 is an inhibitory receptor and also that SHP-1 phosphatase has a significant role in mediating CD33 function. Further studies are essential to identify the receptor(s) that CD33 inhibits in vivo and its function in myeloid lineage development.

Myeloid specific human CD33 is an inhibitory receptor with differential ITIM function in recruiting the phosphatases SHP-1 and SHP-2

CD33 is a myeloid specific member of the sialic acid-binding receptor family and is expressed highly on myeloid progenitor cells but at much lower levels in differentiated cells. Human CD33 has two tyrosine residues in its cytoplasmic domain (Y340 and Y358). When phosphorylated, these tyrosines could function as docking sites for the phosphatases, SHP-1 and/or SHP-2, enabling CD33 to function as an inhibitory receptor. Here we demonstrate that CD33 is tyrosine phosphorylated in the presence of the phosphatase inhibitor, pervanadate, and recruits SHP-1 and SHP-2. Co-expression studies suggest that the Src-family kinase Lck is effective at phosphorylating Y340, but not Y358, suggesting that these residues may function in the selective recruitment of adapter molecules and have distinct functions. Further support for overlapping, but nonredundant, roles for Y340 and Y358 comes from peptide-binding studies that revealed the recruitment of both SHP-1 and SHP-2 to Y340 but only SHP-2 to Y358. Analysis using mutants of SHP-1 demonstrated that binding Y340 of CD33 was primarily to the amino Src homology-2 domain of SHP-1. The potential of CD33 to function as an inhibitory receptor was demonstrated by its ability to down-regulate CD64-induced calcium mobilization in U937. The dependence of this inhibition on SHP-1 was demonstrated by blocking CD33-mediated effects with dominant negative SHP-1. This result implies that CD33 is an inhibitory receptor and also that SHP-1 phosphatase has a significant role in mediating CD33 function. Further studies are essential to identify the receptor(s) that CD33 inhibits in vivo and its function in myeloid lineage development.