Sialic acids siglec interaction: a unique strategy to circumvent innate immune response by pathogens

Abstract

Sialic acids (Sias) are nine-carbon keto sugars primarily present on the terminal residue of cell surface glycans. Sialic acid binding immunoglobulins (Ig)-like lectins (siglecs) are generally expressed on various immune cells. They selectively recognize different linkage-specific sialic acids and undertake a variety of cellular functions. Many pathogens either synthesize or acquire sialic acids from the host. Sialylated pathogens generally use siglecs to manipulate the host immune response. The present review mainly deals with the newly developed information regarding mechanism of acquisition of sialic acids by pathogens and their biological relevance especially in the establishment of successful infection by impairing host innate immunity. The pathogens which are unable to synthesize sialic acids might adsorb these from the host as a way to engage the inhibitory siglecs. They promote association with the immune cells through sialic acids-siglec dependent manner. Such an association plays an important role to subvert host's immunity. Detailed investigation of these pathways has been discussed in this review. Particular attention has been focused on Pseudomonas aeruginosa (PA) and Leishmania donovani.

Fig. 1

Identification of sialic acids of Pseudomonas aeruginosa (PA) by analytical methods. (A) Fluorimetric-HPLC of 1,2-diamino-4,5-methyleneoxybenzene (DMB) derivatized sialic acids. A representative fluorimetric reverse phase HPLC profile of DMB derivatized sialic acids purified from PA. Three intense peaks were observed corresponding to Neu5Ac, Neu5Gc and Neu5,9Ac2. Reproduced with permission from John Wiley and Sons, Oxford, UK (FEBS Lett2010; 584: 555-61). B. MALDI-TOF-MS spectra of the total sialic acids from PA. Sialic acids were purified from PA^+Sias and derivatized with 1,2-diamino-4,5-methyleneoxybenzene (DMB). These were analyzed by MALDI-TOF MS on a target with an equal volume of matrix (2,5-dihydroxybenzoic acid). All mass spectra were recorded in the positive ion mode. Sodium cationized molecular ions of Neu5Ac, Neu5Gc and Neu5,9Ac₂ peaks having m/z at 448.7, 464.8 and 490.6, respectively are shown. Reproduced with permission from John Wiley and Sons, Oxford, UK (FEBS Lett 2010; 584: 555-61).

Fig. 2

Interaction of pathogens through linkage-specific sialic acids with different siglecs present on neutrophil and macrophages. The two pathogens namely L.donovani and P. aeruginosa contain α2,3 and α2,6 linked sialic acids on their surface. These linkage-specific sialic acids bind with siglec-1 and siglec-5 present on macrophages whereas siglec-5 and siglec-9 present on neutrophils.

Fig. 3

Adsorption of sialic acids by PA from normal human serum, used as a source of sialic acids. PA was cultured in tryptic soy broth (TSB) containing increasing concentrations of heat inactivated normal human serum (HI-NHS, 2-10%). Flow cytometric analysis showed adsorption of α2,6-linked sialic acids [Sambucus nigra agglutinin (SNA) positivity, filled square] and α2,3-linked sialic acids [Maackia amurensis agglutinin (MAA) positivity, filled circle] from HI-NHS. Data were presented as per cent of FITC-lectins positive PA cells. Reproduced with permission from Society for Leukocyte Biology, Bethesda, USA (J Leukoc Biol 2012; 91: 641-55).

Fig. 4

Visualization of neutrophil extracellular traps (NETs) by confocal microscopy. PA was incubated with neutrophils. Neutrophil extracellular traps (NETs) were visualized under confocal microscopy after stained with Sytox orange (DNA stain for dead cell) only as well as both with DAPI (nuclear stain) and Sytox orange. No NET formation was observed when PA^+Sias was incubated with neutrophils (A-C). In contrast, NETs formation was visualised after incubating PA^-Sias with neutrophils followed by staining with Sytox orange only (B) as well as DAPI and Sytox orange together (D). Reproduced with permission from Society for Leukocyte Biology, Bethesda, USA (J Leukoc Biol 2012; 91: 641-55).

Fig. 5

Schematic diagram of the proposed pathway of CD33 siglec-mediated suppression of cellular function in Leishmania infection. On macrophage, after ligand (present on various pathogen surface) binding with CD33 related siglecs, immunoreceptor tyrosine-based inhibition motif (ITIM) present on the cytosolic portion of siglecs get activated by phosphorylation with SH2 family kinases (Lyn, Syk). Then Src homology 2 domain-containing phosphatases 1 and 2 (SHP1 and SHP2) bind with the ITIM and get activated. These activated SHPs further dephosphorylate various signaling molecules and suppress cellular activation. The target of SHPs might be p38 mitogen-activated protein kinases (p38 MAPK) or Akt or nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kβ) and ultimately downregulate the effector functions of the cell. In macrophages this siglec-mediated pathway might regulate the polarization of macrophage function towards anti-inflammatory type by reducing Th1 cytokines (IFNγ) and increasing Th2 cytokines (IL-10) along with downregulating inducible nitric oxide synthase (iNOS) gene expression for nitric oxide (NO) secretion.

Fig. 6

Schematic diagram of proposed mechanism for survival of PA^+Sias in hostile environment. Schematic representation of interaction of PA^+Sias with immune cells through siglecs and their survival in host by dampening host innate immune response is shown. PA adsorbed sialic acids from host serum (PA^+Sias). PA^+Sias associated with immune cells (NK-cells, monocytes and neutrophils) through sialic acids-siglecs (siglec-7 and siglec-9, respectively) interaction. Neutrophils produced reduced amount of reactive oxygen species, serine proteases (elastase) and neutrophil extracellular traps (NETs) through siglec-9-sialic acids interaction with PA^+Sias. At the same time, due to such interaction neutrophils enhanced anti-inflammatory cytokines production leading to survival of PA^+Sias within host. Sialic acids on PA resist complement deposition directing survival of PA^+Sias. On the other hand, PA cultured in sialic acids free medium (PA^-Sias) exhibited no interaction with neutrophils. Therefore, neutrophils produced increased secretion of reactive oxygen species, serine proteases (elastase) followed by neutrophil extracellular traps (NETs) formation leading to effective clearance of PA. Reproduced with permission from Society for Leukocyte Biology, Bethesda, USA (J Leukoc Biol 2012; 91: 641-55).