No abstract

Certain pathogenic bacteria are known to modulate the innate immune response by decorating themselves with sialic acids, which can engage the myelomonocytic lineage inhibitory receptor Siglec-9, thereby evading immunosurveillance. We hypothesized that the well-known up-regulation of sialoglycoconjugates by tumors might similarly modulate interactions with innate immune cells. Supporting this hypothesis, Siglec-9-expressing myelomonocytic cells found in human tumor samples were accompanied by a strong up-regulation of Siglec-9 ligands. Blockade of Siglec-9 enhanced neutrophil activity against tumor cells in vitro. To investigate the function of inhibitory myelomonocytic Siglecs in vivo we studied mouse Siglec-E, the murine functional equivalent of Siglec-9. Siglec-E-deficient mice showed increased in vivo killing of tumor cells, and this effect was reversed by transgenic Siglec-9 expression in myelomonocytic cells. Siglec-Edeficient mice also showed enhanced immunosurveillance of autologous tumors. However, once tumors were established, they grew faster in Siglec-E-deficient mice. In keeping with this, Siglec-E-deficient macrophages showed a propensity toward a tumor-promoting M2 polarization, indicating a secondary role of CD33-related Siglecs in limiting cancer-promoting inflammation and tumor growth. Thus, we define a previously unidentified impact of inhibitory myelomonocytic Siglecs in cancer biology, with distinct roles that reflect the dual function of myelomonocytic cells in cancer progression. In keeping with this, a human polymorphism that reduced Siglec-9 binding to carcinomas was associated with improved early survival in nonsmall-cell lung cancer patients, which suggests that Siglec-9 might be therapeutically targeted within the right time frame and stage of disease.hypersialylation | tumor-associated macrophages | tumor-associated neutrophils | immune evasion | tumor-associated inflammation

In contrast to the established role of blood vessel remodeling in inflammation, the biologic function of the lymphatic vasculature in acute inflammation has remained less explored. We studied 2 established models of acute cutaneous inflammation, namely, oxazolone-induced delayed-type hypersensitivity reactions and ultraviolet B irradiation, in keratin 14-vascular endothelial growth factor (VEGF)-C and keratin 14-VEGF-D transgenic mice. These mice have an expanded network of cutaneous lymphatic vessels. Transgenic delivery of the lymphangiogenic factors VEGF-C and the VEGFR-3 specific ligand mouse VEGF-D significantly limited acute skin inflammation in both experimental models, with a strong reduction of dermal edema. Expression of VEGFR-3 by lymphatic endothelium was strongly down-regulated at the mRNA and protein level in acutely inflamed skin, and no VEGFR-3 expression was detectable on inflamed blood vessels and dermal macrophages. There was no major change of the inflammatory cell infiltrate or the composition of the inflammatory cytokine milieu in the inflamed skin of VEGF-C or VEGF-D transgenic mice. However, the increased network of lymphatic vessels in these mice significantly enhanced lymphatic drainage from the ear skin. These results provide evidence that specific lymphatic vessel activation limits acute skin inflammation via promotion of lymph flow from the skin and reduction of edema formation. (Blood. 2011;117(17):4667-4678) IntroductionAcute and chronic inflammatory processes are associated with the growth and/or enlargement of blood and lymphatic vessels. 1 Indeed, vascular remodeling is a hallmark of a plethora of inflammatory diseases, such as chronic airway inflammation, rheumatoid arthritis, inflammatory bowel disease, and the chronic inflammatory skin disease psoriasis. [2][3][4][5] We previously identified an important role of the blood vasculature and in particular of vascular endothelial growth factor (VEGF)-A in the promotion of acute and chronic inflammatory reactions in different experimental skin inflammation models. 6-11 Recently, we found that activation of VEGFR-3 had a potent anti-inflammatory effect in a mouse model of psoriasis. 12 Conversely, inhibition of VEGFR-3 significantly prolonged edema and inflammation during chronic bacterial airway inflammation, in chronic inflammatory arthritis, and in chronic skin inflammation. 3,12,13 However, it has also been reported that the lymphatic vasculature plays an active role in promoting corneal and kidney transplant rejections, in part by facilitating dendritic cell transport to draining lymph nodes. 14,15 Furthermore, the inflamed lymphatic endothelium might actively modulate immune responses. 16,17 Together, these results indicate an important role of blood vessel angiogenesis in sustaining inflammation, whereas the functional role of the lymphatic vasculature in acute inflammation has remained less explored.The cutaneous lymphatic vasculature is involved in the afferent immune response and also maintains tissue fluid homeostasis. [1...

Rationale Lymphatic vasculature plays important roles in tissue fluid homeostasis maintenance and in the pathology of human diseases. Yet, the molecular mechanisms that control lymphatic vessel maturation remain largely unknown. Objective We analyzed the gene expression profiles of ex vivo isolated lymphatic endothelial cells to identify novel lymphatic vessel expressed genes and we investigated the role of Sema3A and neuropilin-1 (Nrp-1) in lymphatic vessel maturation and function. Methods and results Lymphatic and blood vascular endothelial cells from mouse intestine were isolated using fluorescence-activated cell sorting and transcriptional profiling was performed. We found that the axonal guidance molecules Sema3A and Sema3D were highly expressed by lymphatic vessels. Importantly, we found that the semaphorin receptor Nrp-1 is expressed on the perivascular cells of the collecting lymphatic vessels. Treatment of mice in utero (E12.5-E16.5) with an antibody that blocks Sema3A binding to Nrp-1, but not with an antibody that blocks VEGF-A binding to Nrp-1, resulted in a complex phenotype of impaired lymphatic vessel function, enhanced perivascular cell coverage and abnormal lymphatic vessel and valve morphology. Conclusions Together, these results reveal an unanticipated role of Sema3A/Nrp-1 signaling in the maturation of the lymphatic vascular network likely via regulating the perivascular cell coverage of the vessels thus affecting lymphatic vessel function and lymphatic valve development.

Background: Engagement of inhibitory CD33-related Siglecs on immune cells has been shown to influence interactions with cancer cells, including tumor immune evasion. Results:LGALS3BP binds with high affinity to CD33-related Siglecs and inhibits neutrophil activation. Conclusion: We identify LGALS3BP as novel, cancer-associated Siglec ligand that can influence neutrophil activation. Significance: The engagement of inhibitory CD33-related Siglecs by LGALS3BP could support immune evasion of tumor cells.

The immunomodulatory receptor Siglec-3/CD33 influences risk for late-onset Alzheimer's disease (LOAD), an apparently human-specific post-reproductive disease. generates two splice variants: a full-length CD33M transcript produced primarily by the "LOAD-risk" allele and a shorter CD33m isoform lacking the sialic acid-binding domain produced primarily from the "LOAD-protective" allele. An SNP that modulates CD33 splicing to favor CD33m is associated with enhanced microglial activity. Individuals expressing more protective isoform accumulate less brain β-amyloid and have a lower LOAD risk. How the CD33m isoform increases β-amyloid clearance remains unknown. We report that the protection by the CD33m isoform may not be conferred by what it does but, rather, from what it cannot do. Analysis of blood neutrophils and monocytes and a microglial cell line revealed that unlike CD33M, the CD33m isoform does not localize to cell surfaces; instead, it accumulates in peroxisomes. Cell stimulation and activation did not mobilize CD33m to the surface. Thus, the CD33m isoform may neither interact directly with amyloid plaques nor engage in cell-surface signaling. Rather, production and localization of CD33m in peroxisomes is a way of diminishing the amount of CD33M and enhancing β-amyloid clearance. We confirmed intracellular localization by generating a CD33m-specific monoclonal antibody. Of note, CD33 is the only Siglec with a peroxisome-targeting sequence, and this motif emerged by convergent evolution in toothed whales, the only other mammals with a prolonged post-reproductive lifespan. The allele that protects post-reproductive individuals from LOAD may have evolved by adaptive loss-of-function, an example of the less-is-more hypothesis.

Circulating platelets have important functions in thrombosis and in modulating immune and inflammatory responses. However, the role of platelets in innate immunity to bacterial infection is largely unexplored. While human platelets rapidly kill Staphylococcus aureus, we found the neonatal pathogen group B Streptococcus (GBS) to be remarkably resistant to platelet killing. GBS possesses a capsule polysaccharide (CPS) with terminal α2,3-linked sialic acid (Sia) residues that mimic a common epitope present on the human cell surface glycocalyx. A GBS mutant deficient in CPS Sia was more efficiently killed by human platelets, thrombin-activated platelet releasate, and synthetic platelet-associated antimicrobial peptides. GBS Sia is known to bind inhibitory Sia-recognizing Ig superfamily lectins (Siglecs) to block neutrophil and macrophage activation. We show that human platelets also express high levels of inhibitory Siglec-9 on their surface, and that GBS can engage this receptor in a Sia-dependent manner to suppress platelet activation. In a mouse i.v. infection model, antibody-mediated platelet depletion increased susceptibility to platelet-sensitive S. aureus but did not alter susceptibility to platelet-resistant GBS. Elimination of murine inhibitory Siglec-E partially reversed platelet suppression in response to GBS infection. We conclude that GBS Sia has dual roles in counteracting platelet antimicrobial immunity: conferring intrinsic resistance to platelet-derived antimicrobial components and inhibiting platelet activation through engagement of inhibitory Siglecs. We report a bacterial virulence factor for evasion of platelet-mediated innate immunity.

Siglecs (Sialic acid-binding immunoglobulin-type lectins) are a I-type lectin that typically binds sialic acid. Siglecs are predominantly expressed in immune cells and generate activating or inhibitory signals. They are also shown to be expressed on the surface of cells in the nervous system and have been shown to play central roles in neuroinflammation. There has been a plethora of reviews outlining the studies pertaining to Siglecs in immune cells. However, this review aims to compile the articles on the role of Siglecs in brain function and neurological disorders. In humans, the most abundant Siglecs are CD33 (Siglec-3), Siglec-4 (myelin-associated glycoprotein/MAG), and Siglec-11, Whereas in mice the most abundant are Siglec-1 (sialoadhesin), Siglec-2 (CD22), Siglec-E, Siglec-F, and Siglec-H. This review is divided into three parts. Firstly, we discuss the general biological aspects of Siglecs that are expressed in nervous tissue. Secondly, we discuss about the role of Siglecs in brain function and molecular mechanism for their function. Finally, we collate the available information on Siglecs and neurological disorders. It is intriguing to study this family of proteins in neurological disorders because they carry immunoinhibitory and immunoactivating motifs that can be vital in neuroinflammation.