Paired Siglec receptors generate opposite inflammatory responses to a human-specific pathogen

Hypersialylation in Cancer: Modulation of Inflammation and Therapeutic Opportunities

Cell surface glycosylation is dynamic and often changes in response to cellular differentiation under physiological or pathophysiological conditions. Altered glycosylation on cancers cells is gaining attention due its wide-spread occurrence across a variety of cancer types and recent studies that have documented functional roles for aberrant glycosylation in driving cancer progression at various stages. One change in glycosylation that can correlate with cancer stage and disease prognosis is hypersialylation. Increased levels of sialic acid are pervasive in cancer and a growing body of evidence demonstrates how hypersialylation is advantageous to cancer cells, particularly from the perspective of modulating immune cell responses. Sialic acid-binding receptors, such as Siglecs and Selectins, are well-positioned to be exploited by cancer hypersialylation. Evidence is also mounting that Siglecs modulate key immune cell types in the tumor microenvironment, particularly those responsible for maintaining the appropriate inflammatory environment. From these studies have come new and innovative ways to block the effects of hypersialylation by directly reducing sialic acid on cancer cells or blocking interactions between sialic acid and Siglecs or Selectins. Here we review recent works examining how cancer cells become hypersialylated, how hypersialylation benefits cancer cells and tumors, and proposed therapies to abrogate hypersialylation of cancer.

Coevolution of Siglec-11 and Siglec-16 via gene conversion in primates

Background

Siglecs-11 and -16 are members of the sialic acid recognizing Ig-like lectin family, and expressed in same cells. Siglec-11 functions as an inhibitory receptor, whereas Siglec-16 exhibits activating properties. In humans, SIGLEC11 and SIGLEC16 gene sequences are extremely similar in the region encoding the extracellular domain due to gene conversions. Human SIGLEC11 was converted by the nonfunctional SIGLEC16P allele, and the converted SIGLEC11 allele became fixed in humans, possibly because it provides novel neuroprotective functions in brain microglia. However, the detailed evolutionary history of SIGLEC11 and SIGLEC16 in other primates remains unclear.

Results

We analyzed SIGLEC11 and SIGLEC16 gene sequences of multiple primate species, and examined glycan binding profiles of these Siglecs. The phylogenetic tree demonstrated that gene conversions between SIGLEC11 and SIGLEC16 occurred in the region including the exon encoding the sialic acid binding domain in every primate examined. Functional assays showed that glycan binding preference is similar between Siglec-11 and Siglec-16 in all analyzed hominid species. Taken together with the fact that Siglec-11 and Siglec-16 are expressed in the same cells, Siglec-11 and Siglec-16 are regarded as paired receptors that have maintained similar ligand binding preferences via gene conversions. Relaxed functional constraints were detected on the SIGLEC11 and SIGLEC16 exons that underwent gene conversions, possibly contributing to the evolutionary acceptance of repeated gene conversions. The frequency of nonfunctional SIGLEC16P alleles is much higher than that of SIGLEC16 alleles in every human population.

Conclusions

Our findings indicate that Siglec-11 and Siglec-16 have been maintained as paired receptors by repeated gene conversions under relaxed functional constraints in the primate lineage. The high prevalence of the nonfunctional SIGLEC16P allele and the fixation of the converted SIGLEC11 imply that the loss of Siglec-16 and the gain of Siglec-11 in microglia might have been favored during the evolution of human lineage.

Electronic supplementary material

The online version of this article (10.1186/s12862-017-1075-z) contains supplementary material, which is available to authorized users.

Using the glycan toolbox for pathogenic interventions and glycan immunotherapy

Glycans play a crucial role to discern between self and foreign entities by providing key recognition elements for C-type lectin receptors (CLRs) and Siglec receptors expressed on immune cells. The glycan recognition of CLRs has illustrated a potent immune modulatory role affecting not only innate pathogen binding and immune signalling, but also Thelper differentiation, cytokine production and antigen presentation. This broad range of influence has implicated glycans in the pathogenesis of infectious diseases but also revealed their extraordinary properties in cancer. Glycan binding by CLRs and Siglecs can be exploited for immunotherapy and the design of glycan-based therapeutics and their multivalent requirements will aspire new biotechnological approaches to effectively interfere in immunological processes in cancer and infectious diseases.

A biosynthetic route for polysialylating proteins in Escherichia coli

Polysialic acid (polySia) is a posttranslational modification found on only a handful of proteins in the central nervous and immune systems. The addition of polySia to therapeutic proteins improves pharmacokinetics and reduces immunogenicity. To date, polysialylation of therapeutic proteins has only been achieved in vitro by chemical or chemoenzymatic strategies. In this work, we develop a biosynthetic pathway for site-specific polysialylation of recombinant proteins in the cytoplasm of Escherichia coli. The pathway takes advantage of a bacterial cytoplasmic polypeptide-glycosyltransferase to establish a site-specific primer on the target protein. The glucose primer is extended by glycosyltransferases derived from lipooligosaccharide, lipopolysaccharide and capsular polysaccharide biosynthesis from different bacterial species to synthesize long chain polySia. We demonstrate the new biosynthetic route by modifying green fluorescent proteins and a therapeutic DARPin (designed ankyrin repeat protein).

Inhibitory and Coactivating Receptors Recognising the Same Ligand: Immune Homeostasis Exploited by Pathogens and Tumours

Coactivating and inhibitory receptors that share at least one ligand interact with a wide variety of ligands, indicating their importance in a range of situations. Here, we discuss principles of mainly human paired receptor function and ligand recognition, and possible therapeutic implications of targeting these receptors in cancer, autoimmune diseases, and allergy. We summarise and emphasise the idea that these receptors, which have evolved in part in response to pathogen pressure, fine-tune the immune response, preserve homeostasis, and that pathogens and tumours use the dominance of the inhibitory receptors over the coactivating receptors to avoid immune elimination. Finally, we discuss the options of using paired receptors and their ligand for immune cell education and therapy.

Paired Siglec receptors generate opposite inflammatory responses to a human-specific pathogen

Paired immune receptors display near-identical extracellular ligand-binding regions but have intracellular sequences with opposing signaling functions. While inhibitory receptors dampen cellular activation by recognizing self-associated molecules, the functions of activating counterparts are less clear. Here, we studied the inhibitory receptor Siglec-11 that shows uniquely human expression in brain microglia and engages endogenous polysialic acid to suppress inflammation. We demonstrated that the human-specific pathogen Escherichia coli K1 uses its polysialic acid capsule as a molecular mimic to engage Siglec-11 and escape killing. In contrast, engagement of the activating counterpart Siglec-16 increases elimination of bacteria. Since mice do not have paired Siglec receptors, we generated a model by replacing the inhibitory domain of mouse Siglec-E with the activating module of Siglec-16. Siglec-E16 enhanced proinflammatory cytokine expression and bacterial killing in macrophages and boosted protection against intravenous bacterial challenge. These data elucidate uniquely human interactions of a pathogen with Siglecs and support the long-standing hypothesis that activating counterparts of paired immune receptors evolved as a response to pathogen molecular mimicry of host ligands for inhibitory receptors.