The relationship between sport commitment and physical self-concept: Evidence for the self-enhancement hypothesis among adolescent females.

Perceptions of physical self-concept are critical to physical activity participation. In line with the reciprocal effects model of causal ordering (REM), higher perceptions of physical self-concept can function as a facilitator to physical activity, and can arise as a result of engaging in physical activity. While this relationship has been predominantly tested in physical activity contexts, directional tests between physical self-concept and sport specific outcomes are limited. The current study aimed to evaluate the generalizability of the REM to sport commitment and physical self-concept in youth athletes. Over 24 months, adolescent females (N = 215) completed self-report questionnaires at Time 1 (T1) and two years later (Time 2; T2). Using structural equation modeling, the reciprocal effects model demonstrated that the path leading from T1 physical self-concept to T2 sport commitment was significant (p = .02), whereas the path leading from T1 sport commitment to T2 physical self-concept was not significant (p = .23). The results suggest a unidirectional relationship and may underscore the importance of focusing on the physical self-concept in the development of strategies geared towards improving adolescent female's sport participation.

Applying self-compassion in sport: an intervention with women athletes.

This study investigated the effects of a self-compassion intervention on negative cognitive states and self-compassion in varsity women athletes. Athletes who self-identified as being self-critical were randomly assigned to a self-compassion intervention (n = 29) or attention control group (n = 22). The self-compassion intervention consisted of a psychoeducation session and writing components completed over a 7-day period. Measures of self-compassion, state self-criticism, state rumination, and concern over mistakes were collected pretreatment, at 1 week posttreatment, and at a 4-week follow-up. A mixed factorial MANOVA with follow-up post hoc tests demonstrated moderate-to-strong effects for the intervention at posttest and follow-up (Wilks's Λ = .566, F (8, 42) = 4.03, p < .01, η2 = .43). The findings demonstrate the effectiveness of the self-compassion intervention in managing self-criticism, rumination, and concern over mistakes. Fostering self-compassionate mind frames is a potential coping resource for women athletes dealing with negative events in sport.

Liposome induction of CD8+ T cell responses depends on CD169+ macrophages and Batf3-dependent dendritic cells and is enhanced by GM3 inclusion.

Cancer vaccines aim to efficiently prime cytotoxic CD8+ T cell responses which can be achieved by vaccine targeting to dendritic cells. CD169+ macrophages have been shown to transfer antigen to dendritic cells and could act as an alternative target for cancer vaccines. Here, we evaluated liposomes containing the CD169/Siglec-1 binding ligand, ganglioside GM3, and the non-binding ligand, ganglioside GM1, for their capacity to target antigens to CD169+ macrophages and to induce immune responses. CD169+ macrophages demonstrated specific uptake of GM3 liposomes in vitro and in vivo that was dependent on a functional CD169 receptor. Robust antigen-specific CD8+ and CD4+ T and B cell responses were observed upon intravenous administration of GM3 liposomes containing the model antigen ovalbumin in the presence of adjuvant. Immunization of B16-OVA tumor bearing mice with all liposomes resulted in delayed tumor growth and improved survival. The absence of CD169+ macrophages, functional CD169 molecules, and cross-presenting Batf3-dependent dendritic cells (cDC1s) significantly impaired CD8+ T cell responses, while B cell responses were less affected. In conclusion, we demonstrate that inclusion of GM3 in liposomes enhance immune responses and that splenic CD169+ macrophages and cDC1s are required for induction of CD8+ T cell immunity after liposomal vaccination.

Mouse macrophage hemagglutinin (sheep erythrocyte receptor) with specificity for sialylated glycoconjugates characterized by a monoclonal antibody.

An inhibitory rat mAb, SER-4, has been raised to the mouse macrophage (M phi)-restricted hemagglutinin, sheep erythrocyte receptor (SER), which binds unopsonized sheep erythrocytes through recognition of sialylated glycoconjugates. This receptor was originally defined on mouse resident bone marrow M phi where it was implicated in adhesive interactions of these cells with proliferating hematopoietic cells. In the present study using mouse serum-induced thioglycollate-elicited peritoneal M phi (TPM) as a model system for SER expression, mAb SER-4 IgG2a completely blocked rosette formation at 1 microgram/ml. The inhibition was likely to be via steric hindrance rather than through a direct interaction with the putative sialic acid binding site of SER because F(ab')2 and Fab fragments of mAb SER-4 gave a maximum inhibition of 50-60% and 0% respectively, despite binding effectively to the SER-4 antigen (Ag). Immunoprecipitation and Western blotting experiments with cultured M phi or tissue extracts demonstrated that the Ag recognized by SER-4 mAb is a single chain molecule with an apparent Mr by SDS-PAGE of 185 x 10(3) (reduced) or 170 x 10(3) (non-reduced) and is distinct from members of the leukocyte common Ag family. Expression of SER and SER-4 Ag in culture were closely correlated and depended on the presence of mouse serum for optimal induction. Further evidence that the SER-4 Ag is functionally equivalent to SER was provided by immunocytochemistry in which the overall pattern of staining in tissues was consistent with previous rosetting experiments. In the bone marrow, expression of the SER-4 Ag was restricted to the resident bone marrow M phi population with no expression on monocytes. High expression was also observed on stromal M phi within the subcapsular sinus and medullary cords in lymph nodes and on marginal metallophils in the spleen. These results therefore confirm that SER is a novel M phi-restricted receptor whose distribution and properties indicate a role in cellular interactions in hematopoietic and lymphoid tissues.

Isolation and characterization of resident stromal macrophages and hematopoietic cell clusters from mouse bone marrow.

In situ studies with the mouse macrophage (M phi)-specific antibody, F4/80, have shown that resident M phi in femoral bone marrow (RBMM) form hematopoietic islands with immature myelomonocytic and erythroid cells (Hume, D. A., et al. 1983. J. Exp. Med. 158: 1522). We have isolated these islands (clusters) by collagenase digestion, purified them from single cells by velocity sedimentation, and analyzed their cellular content. The clusters, ranging from 5- to 100 cells, constituted approximately 7% of the total nucleated cells, and greater than 70% contained at least one strongly staining, F4/80+ central M phi. In comparison, less than 26% showed reactivity for alkaline phosphatase, a marker of fibroblastoid reticulum cells. Compared with the nonclustering population, clusters were enriched with RBMM, fibroblastoid cells, and immature hematopoietic cells, but depleted of mature granulocytes and erythrocytes. The RBMM population was purified from other cells in clusters by selective adherence to glass and was compared with resident peritoneal M phi (RPM) for morphology and the presence of antigens, receptors, and enzymes. RBMM spread more extensively than RPM and frequently extended delicate plasma membrane processes. These and subsequent differences were not attributable to the collagenase treatment. Both M phi populations stained positively with antibodies F4/80 and 2.4G2 (Fc receptor IgG1/2b), bore mannosyl/fucosyl receptors, and showed reactivity for acid phosphatase and nonspecific esterase I. In contrast to RPM, RBMM had no detectable Mac-1 antigen (CR3) or complement receptors, but bore higher levels of Fc receptors (IgG2a and IgG2b) and Ia antigens. In addition, RBMM possessed a novel hemagglutinin activity for unopsonized sheep erythrocytes, which was not present on RPM. RBMM showed no respiratory burst activity in response to zymosan particles, but ingested them avidly. The growth properties of clustering and nonclustered populations were compared by measurement of [3H]thymidine incorporation and progenitor assays. Cells in clusters incorporated three- to fourfold more thymidine than nonclustered cells even in the absence of exogenous growth factors, and autoradiography demonstrated that RBMM made contact with proliferating cells. In contrast, the clusters contained over threefold fewer granulocyte/M phi progenitors compared with nonclustering cells. When clusters were cultivated for up to 3 d, there was rapid outgrowth of monocytes and fibroblastoid cells. These studies demonstrate that RBMM bear a distinct morphology and phenotype.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties and distribution of a lectin-like hemagglutinin differentially expressed by murine stromal tissue macrophages.

We describe a novel hemagglutinin which is differentially expressed on murine stromal tissue macrophages. Resident bone marrow macrophages (RBMM), which are physically associated with immature, proliferating hematopoietic cells in vivo, formed striking rosettes with unopsonized sheep erythrocytes (E) in vitro, unlike resident peritoneal macrophages (RPM). Binding of E was macrophage (M phi) specific, not accompanied by ingestion and independent of temperature (0-37 degrees C), divalent cations, and the metabolic inhibitors azide and iodoacetate. Pretreatment of RBMM with trypsin prevented rosette formation, but neuraminidase enhanced it. Conversely, binding was virtually abrogated if E were pretreated with neuraminidase, whereas trypsin pretreatment of the ligand resulted in a slight enhancement. The lectin-like nature of the E receptor (SER), with specificity for sialylated glycoconjugates, was consistent with the inhibition of binding we saw with neuraminyllactose or the ganglioside GD1a (50% inhibition at 5-10 mM and 11 microM, respectively). Expression of SER on freshly isolated RBMM was heterogeneous and exhibited a striking inverse correlation with expression of Ia antigens. During cultivation in 10% FCS, levels of SER on RBMM declined with a half-life of approximately 24 h. Other cell surface changes induced by cultivation included a transient increase in expression of Ia antigen and acquisition of Mac-1. To determine whether SER was expressed on other stromal M phi populations, adherent cells were isolated from various tissues by collagenase digestion or lavage. Binding of E was highest on RBMM and lymph node stromal M phi, at intermediate levels on Kupffer cells and splenic stromal M phi, but was low or undetectable on blood monocytes and thymic, peritoneal, pleural, and bronchoalveolar M phi. SER therefore appeared to be expressed on certain M phi populations embedded in solid tissues but was largely absent from M phi recoverable by lavage. Its absence from monocytes implies that SER is acquired by M phi after entering tissues where it may perform adhesive functions. In bone marrow, SER on RBMM could interact with an appropriate sialylated ligand on murine hematopoietic cells, and could influence their rate of growth and differentiation.

Macrophage plasma membrane and secretory properties in murine malaria. Effects of Plasmodium yoelii blood-stage infection on macrophages in liver, spleen, and blood.

We have studied the effect of infection with the blood-stage of Plasmodium yoelii 17X, a nonlethal parasite, on plasma membrane antigens, receptors, and secretory properties of macrophages (M phi) in murine liver, spleen, and blood. mAb F4/80 (M phi specific), F7/4 (a marker for immature and immunologically activated M phi, as well as neutrophils), and Mac-1, which binds to the type 3 complement receptor, were used to measure the distribution and total content of antigens in situ and to assay surface expression of antigens on M phi isolated by collagenase perfusion-digestion and adherence. We also examined respiratory burst activity after stimulation with PMA, FcR activity, Ia antigen expression, and binding of 125I-mannose-BSA and unopsonized sheep erythrocytes by isolated M phi. In the normal animal, spleen M phi expressed Mac-1 and F7/4 antigens and relatively high levels of respiratory burst activity, in contrast to Kupffer cells in liver, where all three features were virtually absent. The introduction of parasitized erythrocytes into the circulation resulted in a large influx of F4/80+ M phi into the blood, liver, and spleen, where local M phi proliferation could also contribute. Liver M phi during malaria infection showed increased Mac-1 and 7/4 antigen and an increased respiratory burst potential compared with uninfected controls. Increases in total, but not specific activity of FcR, Ia antigen, and binding of unopsonized sheep erythrocytes were found in spleen and liver M phi populations after infection. In both populations, there was an early but persistent marked reduction in specific binding and uptake of 125I-mannose-BSA. These results confirm and extend observations that normal Kupffer cells are relatively homogeneous in morphology, surface markers, and anatomical location, in contrast to M phi in normal spleen, and that both of these populations differ from resident M phi elsewhere, including the peritoneal cavity. In the course of infection by P. yoelii, M phi with high levels of opsonic receptors (CR3, FcR) and respiratory burst potential are mobilized in large numbers at specific sites such as liver and spleen, in accordance with an important role for M phi in the clearance of parasitized erythrocytes from blood.

Identification of CD22 ligands on bone marrow sinusoidal endothelium implicated in CD22-dependent homing of recirculating B cells.

CD22 is a B cell-specific transmembrane protein known to function as a negative regulator of B cell signaling. It has also been implicated in cell adhesion through recognition of alpha2,6-linked sialic acids on glycans of target cells. Previous studies showed that CD22-deficient mice had a strongly reduced population of mature recirculating B cells in the bone marrow despite normal B cell development. Using a soluble recombinant form of the receptor (CD22-Fc), we demonstrate here that sialylated ligands for CD22 are expressed on sinusoidal endothelial cells of murine bone marrow but not on endothelial cells in other tissues examined. Injection of CD22-Fc revealed that the CD22 ligands in the bone marrow were accessible to the circulation. Treatment of mice with either CD22-Fc or affinity-purified anti-CD22 antibody led to an approximately 50% reduction in mature recirculating B cells in the bone marrow without affecting numbers in the spleen. Finally, consistent with the notion that CD22 is a homing receptor, we show that compared with wild-type mice, CD22-deficient animals have a lower number of immunoglobulin M-secreting plasma cells in the bone marrow.

Species heterogeneity in macrophage expression of the CD4 antigen.

The CD4 antigen is expressed on T cells of all mammalian species examined and appears to play an important role in the response of T cells to antigen. In humans, the molecule acts as a receptor for the AIDS virus. Previous studies have demonstrated that M phi in the rat and human also express the CD4 antigen, which is indistinguishable from that on T cells. In this paper we demonstrate by FACS analysis, Northern blot hybridization, and immunoperoxidase labeling that, in striking contrast to the rat and human, mouse M phi do not express the CD4 (L3T4) antigen. This species heterogeneity indicates that T cells and M phi regulate CD4 antigen expression independently and that CD4 may not be essential for M phi function.

Isolation and immunocytochemical characterization of human bone marrow stromal macrophages in hemopoietic clusters.

Stromal macrophages (M phi) have been localized in situ and isolated within erythroid clusters from human marrow. Stromal M phi arborize in an extensive network uniformly distributed throughout marrow interstitium, and express the phenotype CD4+, CD11a+, CD11c+, CD13+, CD14+, CD16+, CD18+, CD31+, CD32+, FcRI+, HLA-DR+, and CD35-, transferrin receptor-negative, and CD11b (weak). They express endocytic receptor antigens, but show significant differences in myeloid antigen expression compared with freshly harvested or cultured monocytes. Human stromal M phi are therefore specialized mature marrow M phi that are accessible for further investigations in infectious, storage, or hemopoietic disorders.

Sialoadhesin on macrophages: its identification as a lymphocyte adhesion molecule.

In this study we present evidence that the mouse and rat sialoadhesin (originally named sheep erythrocyte receptor) on macrophages can function as a lymphocyte adhesion molecule. Lymphocytes were shown to bind to the splenic marginal zone, and lymph node subcapsular sinus and medulla in a frozen section assay. Selective depletion experiments showed that binding was mediated by macrophages. Adhesion was blocked by preincubation of the sections with monoclonal antibodies against mouse or rat sialoadhesin. Binding was temperature dependent, divalent cation independent, and involved sialic acid residues on the lymphocyte, as it could be inhibited by prior neuraminidase treatment or addition of the ganglioside GD1a. Binding to sialoadhesin was confirmed using the purified receptor and was observed among T cells, T blasts, B cells, and B blasts. Isolated macrophages or dendritic cells showed little binding. Sialoadhesin provides the first example of a macrophage-restricted lymphocyte adhesion molecule.

Changes in body-related self-conscious emotions over time among youth female athletes.

The current study explored change in body-related self-conscious emotions (e.g., shame, guilt, authentic pride, hubristic pride) over three years, and tested body surveillance, age, weight status, years in sport, and competitive status as baseline predictors of change. Adolescent females engaged in organized sport (N = 518 at baseline, Mage = 14.02, SD = 1.38 years) completed a self-report survey once a year for three years (n = 293 and n = 215 in Years 2 and 3, respectively). Based on the unconditional latent growth model, body-related shame and guilt increased over time, and authentic and hubristic pride decreased over time. There was substantial between-person variability in the intercepts for all emotions and slopes for shame, guilt, and hubristic pride. In the conditional parallel process latent growth model, body surveillance predicted shallower change in shame and guilt over time. Female athletes high in body surveillance also reported higher body-related shame and guilt and lower authentic and hubristic pride at baseline. These findings highlight the importance of studying changes in self-conscious emotions over time in sport, and demonstrate that body surveillance may be an important factor to explore in interventions early in development.

Unravelling the health status of brachycephalic dogs in the UK using multivariable analysis.

Brachycephalic dog breeds are regularly asserted as being less healthy than non-brachycephalic breeds. Using primary-care veterinary clinical data, this study aimed to identify predispositions and protections in brachycephalic dogs and explore differing inferences between univariable and multivariable results. All disorders during 2016 were extracted from a random sample of 22,333 dogs within the VetCompass Programme from a sampling frame of 955,554 dogs under UK veterinary care in 2016. Univariable and multivariable binary logistic regression modelling explored brachycephaly as a risk factor for each of a series of common disorders. Brachycephalic dogs were younger, lighter and less likely to be neutered than mesocephalic, dolichocephalic and crossbred dogs. Brachycephalic differed to non-brachycephalic types in their odds for 10/30 (33.33%) common disorders. Of these, brachycephalic types were predisposed for eight disorders and were protected for two disorders. Univariable and multivariable analyses generated differing inference for 11/30 (30.67%) disorders. This study provides strong evidence that brachycephalic breeds are generally less healthy than their non-brachycephalic counterparts. Results from studies that report only univariable methods should be treated with extreme caution due to potential confounding effects that have not been accounted for during univariable study design or analysis.

Murine fetal liver macrophages bind developing erythroblasts by a divalent cation-dependent hemagglutinin.

During mammalian development the fetal liver plays an important role in hematopoiesis. Studies with the macrophage (M phi)-specific mAb F4/80 have revealed an extensive network of M phi plasma membranes interspersed between developing erythroid cells in fetal liver. To investigate the interactions between erythroid cells and stromal M phi, we isolated hematopoietic cell clusters from embryonic day-14 murine fetal liver by collagenase digestion and adherence. Clusters of erythroid cells adhered to glass mainly via M phi, 94% of which bound 19 +/- 11 erythroblasts (Eb) per cell. Bound Eb proliferated vigorously on the surface of fetal liver M phi, with little evidence of ingestion. The M phi could be stripped of their associated Eb and the clusters then reconstituted by incubation with Eb in the presence of divalent cations. The interaction required less Ca++ than Mg++, 100 vs. 250 microM for half-maximal binding, and was mediated by a trypsin-sensitive hemagglutinin on the M phi surface. After trypsin treatment fetal liver M phi recovered the ability to bind Eb and this process could be selectively inhibited by cycloheximide. Inhibition tests showed that the Eb receptor differs from known M phi plasma membrane receptors and fetal liver M phi did not bind sheep erythrocytes, a ligand for a distinct M phi hemagglutinin. We propose that fetal liver M phi interact with developing erythroid cells by a novel nonphagocytic surface hemagglutinin which is specific for a ligand found on Eb and not on mature red cells.

Myelin-associated glycoprotein interacts with neurons via a sialic acid binding site at ARG118 and a distinct neurite inhibition site.

Inhibitory components in myelin are largely responsible for the lack of regeneration in the mammalian CNS. Myelin-associated glycoprotein (MAG), a sialic acid binding protein and a component of myelin, is a potent inhibitor of neurite outgrowth from a variety of neurons both in vitro and in vivo. Here, we show that MAG's sialic acid binding site is distinct from its neurite inhibitory activity. Alone, sialic acid-dependent binding of MAG to neurons is insufficient to effect inhibition of axonal growth. Thus, while soluble MAG-Fc (MAG extracellular domain fused to Fc), a truncated form of MAG-Fc missing Ig-domains 4 and 5, MAG(d1-3)-Fc, and another sialic acid binding protein, sialoadhesin, each bind to neurons in a sialic acid- dependent manner, only full-length MAG-Fc inhibits neurite outgrowth. These results suggest that a second site must exist on MAG which elicits this response. Consistent with this model, mutation of arginine 118 (R118) in MAG to either alanine or aspartate abolishes its sialic acid-dependent binding. However, when expressed at the surface of either CHO or Schwann cells, R118-mutated MAG retains the ability to inhibit axonal outgrowth. Hence, MAG has two recognition sites for neurons, the sialic acid binding site at R118 and a distinct inhibition site which is absent from the first three Ig domains.

A novel role for myelin-associated glycoprotein as an inhibitor of axonal regeneration.

Following nerve injury, axons in the CNS do not normally regenerate. It has been shown that CNS myelin inhibits neurite outgrowth, though the nature of the molecules responsible for this effect are not known. Here, we demonstrate that the myelin-associated glycoprotein (MAG), a transmembrane protein of both CNS and PNS myelin, strongly inhibits neurite outgrowth from both developing cerebellar and adult dorsal root ganglion (DRG) neurons in vitro. This inhibition is reversed by an anti-MAG antibody. In contrast, MAG promotes neurite outgrowth from newborn DRG neurons. These results suggest that MAG may be responsible, in part, for the lack of CNS nerve regeneration in vivo and may influence, both temporally and spatially, regeneration in the PNS.

Siglec-F antibody administration to mice selectively reduces blood and tissue eosinophils.

BACKGROUND: Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of receptors that bind sialic acid and mostly contain immunoreceptor tyrosine-based inhibitory motifs, suggesting that these molecules possess inhibitory functions. We have recently identified Siglec-8 as an eosinophil-prominent Siglec, and cross-linking of Siglec-8 on human eosinophils induces apoptosis. In this article, we address the in vivo consequences of Siglec engagement. We and others have identified mouse Siglec-F as the closest functional paralog of human Siglec-8, based on shared ligand-binding and expression pattern. We therefore hypothesized that Siglec-F engagement would affect levels and viability of eosinophils in vivo. METHODS: Wild type and hypereosinophilic mice were administered Siglec-F antibody and levels of eosinophils in peripheral blood and tissue were measured. Eosinophil apoptosis (in vivo and in vitro) was determined by binding of Annexin-V. RESULTS: Studies in IL-5 transgenic mice, displaying hypereosinophilia, show that administration of a single dose of Siglec-F antibody results in rapid reductions in quantum of eosinophils in the blood. This decrease was accompanied by reductions in tissue eosinophils. Quantum of eosinophils in blood was decreased using two separate antibodies, as well as in other mouse models (wild type mice and in a mouse model of chronic eosinophilic leukemia). Mechanistic studies demonstrated that Siglec-F antibody administration induced apoptosis of eosinophils in vivo and in vitro. CONCLUSION: These data demonstrate that activation of innate immune receptors, like Siglec-F, can significantly reduce mouse eosinophil viability. As such, targeting Siglec-8/F may be a therapeutic approach for eosinophilic disorders.

Sialoadhesin binds preferentially to cells of the granulocytic lineage.

Sialoadhesin is a macrophage-restricted, sialic acid-dependent receptor of 185 kD that binds to the oligosaccharide sequence NeuAc alpha 2,3Gal on cell surface glycoconjugates. Recent cDNA cloning has shown that sialoadhesin is a new member of the immunoglobulin superfamily with sequence similarity to CD22, a sialic acid-dependent receptor of B lymphocytes. Sialoadhesin has been implicated in cellular interactions of stromal macrophages with developing myeloid cells. In this study, direct evidence for this interaction was obtained in cell-cell binding assays using both native and recombinant forms of the protein. In all assays, sialoadhesin exhibited specific, differential binding to various murine cell populations of hemopoietic origin. In rank order, sialoadhesin bound neutrophils > bone marrow cells = blood leukocytes > lymphocytes > thymocytes. Single-cell analyses confirmed that sialoadhesin selectively bound myeloid cells in complex cell mixtures obtained from the bone marrow and blood. In comparison, a recombinant Fc-chimeric form of murine CD22 showed high binding to B and T lymphocytes, but very low binding to immature and mature myeloid cells. These results are consistent with the notion that sialoadhesin in involved in interactions with granulocytes at different stages of their life histories.

Carbohydrate recognition systems: functional triads in cell-cell interactions.

Considerable progress is being made in our understanding of the molecular basis for mammalian carbohydrate recognition systems. Selectins, related proteins and sialoadhesins are carbohydrate-binding proteins which serve as receptors in the orchestration of innate and acquired immune responses, inflammation and other forms of cell-cell communication. Protein structural studies and gain-of-function and loss-of-function mutations are providing clues to ways in which the receptors interact with monosaccharide elements of the oligosaccharide ligands. Binding experiments using oligosaccharides on lipid or protein carriers indicate that modes of presentation such as the clustered state and the manner of display on proteins are crucial factors determining whether a functional triad of receptor and ligand + carrier (counter-receptor) is formed.

Sialic acid binding receptors (siglecs) expressed by macrophages.

Sialic acids are structurally and topographically well-suited to function as ligands in cellular recognition events. Sialoadhesin (Sn) is a sialic acid binding receptor uniquely expressed by macrophage subsets. It is a member of the immunoglobulin (Ig) superfamily with 17 extracellular domains. Sn is a prototypical member of the siglec family of sialic acid binding proteins, which includes CD22, myelin-associated glycoprotein, CD33, and siglec-5. These membrane proteins are involved in discrete functions in the hemopoietic, immune, and nervous systems. The sialic acid binding region of siglecs is localized within the membrane-distal, amino-terminal domain and in the case of Sn, it has been characterized in atomic detail by X-ray crystallography, nuclear magnetic resonance, and site-directed mutagenesis. Our studies on Sn indicate that this receptor is likely to function as a macrophage accessory molecule in a variety of cell-cell and cell-extracellular matrix interactions. CD33 and siglec-5 are also expressed on macrophage subsets as well as other myeloid cells. However, unlike Sn, the properties of these molecules indicate a predominant role in signaling functions rather than in cell-cell interactions.