Cyanovirin-N inhibits mannose-dependent Mycobacterium-C-type lectin interactions but does not protect against murine tuberculosis

Mevo lectin specificity towards high-mannose structures with terminal αMan(1,2)αMan residues and its implication to inhibition of the entry of Mycobacterium tuberculosis into macrophages

Mannose-binding lectins can specifically recognize and bind complex glycan structures on pathogens and have potential as anti-viral and anti-bacterial agents. We previously reported the structure of a lectin from an archaeal species, Mevo lectin, which has specificity towards terminal α1,2 linked manno-oligosaccharides. Mycobacterium tuberculosis (M. tuberculosis) expresses mannosylated structures including, lipoarabinomannan (ManLAM) on its surface and exploits C-type lectins to gain entry into the host cells. ManLAM structure has mannose capping with terminal αMan(1,2)αMan residues and is important for recognition by innate immune cells. Here, we aim to address the specificity of Mevo lectin towards high-mannose type glycans with terminal αMan(1,2)αMan residues and its effect on M. tuberculosis internalization by macrophages. ITC studies demonstrated that Mevo lectin shows preferential binding towards manno-oligosaccharides with terminal αMan(1,2)αMan structures, and showed a strong affinity for ManLAM, whereas it binds weakly to Mycobacterium smegmatis (M. smegmatis) lipoarabinomannan (MsmLAM), which displays relatively fewer and shorter mannosyl caps. Crystal structure of Mevo lectin complexed with a Man7D1 revealed the multivalent cross-linking interaction, which explains avidity-based high affinity for these ligands when compared to previously studied manno-oligosaccharides lacking the specific termini. Functional studies suggest that M. tuberculosis internalization by the macrophage was impaired by binding of Mevo lectin to ManLAM present on the surface of M. tuberculosis. Selectivity shown by Mevo lectin towards glycans with terminal αMan(1,2)αMan structures, and its ability to compromise the internalization of M. tuberculosis in vitro, underscore the potential utility of Mevo lectin as a research tool to study host-pathogen interactions.

Lectins as Promising Therapeutics for the Prevention and Treatment of HIV and Other Potential Coinfections

Human immunodeficiency virus-acquired immunodeficiency syndrome (HIV/AIDS) remains a global health problem. Current therapeutics specifically target the viral pathogen at various stages of its life cycle, although complex interactions between HIV and other pathogenic organisms are evident. Targeting HIV and concomitant infectious pathogens simultaneously, both by therapeutic regimens and in prevention strategies, would help contain the AIDS pandemic. Lectins, a ubiquitous group of proteins that specifically bind glycosylated molecules, are interesting compounds that could be used for this purpose, with demonstrated anti-HIV properties. In addition, potential coinfecting pathogens, including other enveloped viruses, bacteria, yeasts and fungi, and protozoa, display sugar-coated macromolecules on their surfaces, making them potential targets of lectins. This review summarizes the currently available findings suggesting that lectins should be further developed to simultaneously fight the AIDS pandemic and concomitant infections in HIV infected individuals.

Cyanobacterial lectins characteristics and their role as antiviral agents

Lectins are ubiquitous proteins/glycoproteins of non-immune origin that bind reversibly to carbohydrates in non-covalent and highly specific manner. These lectin-glycan interactions could be exploited for establishment of novel therapeutics, targeting the adherence stage of viruses and thus helpful in eliminating wide spread viral infections. Here the review focuses on the haemagglutination activity, carbohydrate specificity and characteristics of cyanobacterial lectins. Cyanobacterial lectins exhibiting high specificity towards mannose or complex glycans have potential role as anti-viral agents. Prospective role of cyanobacterial lectins in targeting various diseases of worldwide concern such as HIV, hepatitis, herpes, influenza and ebola viruses has been discussed extensively. The review also lays emphasis on recent studies involving structural analysis of glycan-lectin interactions which in turn influence their mechanism of action. Altogether, the promising approach of these cyanobacterial lectins provides insight into their use as antiviral agents.

Mannan core branching of lipo(arabino)mannan is required for mycobacterial virulence in the context of innate immunity

The causative agent of tuberculosis (TB), Mycobacterium tuberculosis, remains an important worldwide health threat. Although TB is one of the oldest infectious diseases of man, a detailed understanding of the mycobacterial mechanisms underlying pathogenesis remains elusive. Here, we studied the role of the α(1→2) mannosyltransferase MptC in mycobacterial virulence, using the Mycobacterium marinum zebrafish infection model. Like its M. tuberculosis orthologue, disruption of M. marinum mptC (mmar_3225) results in defective elongation of mannose caps of lipoarabinomannan (LAM) and absence of α(1→2)mannose branches on the lipomannan (LM) and LAM mannan core, as determined by biochemical analysis (NMR and GC-MS) and immunoblotting. We found that the M. marinum mptC mutant is strongly attenuated in embryonic zebrafish, which rely solely on innate immunity, whereas minor virulence defects were observed in adult zebrafish. Strikingly, complementation with the Mycobacterium smegmatis mptC orthologue, which restored mannan core branching but not cap elongation, was sufficient to fully complement the virulence defect of the mptC mutant in embryos. Altogether our data demonstrate that not LAM capping, but mannan core branching of LM/LAM plays an important role in mycobacterial pathogenesis in the context of innate immunity.

Receptor-mediated recognition of mycobacterial pathogens

Mycobacteria are a genus of bacteria that range from the non-pathogenic Mycobacterium smegmatis to Mycobacterium tuberculosis, the causative agent of tuberculosis in humans. Mycobacteria primarily infect host tissues through inhalation or ingestion. They are phagocytosed by host macrophages and dendritic cells. Here, conserved pathogen-associated molecular patterns (PAMPs) on the surface of mycobacteria are recognized by phagocytic pattern recognition receptors (PRRs). Several families of PRRs have been shown to non-opsonically recognize mycobacterial PAMPs, including membrane-bound C-type lectin receptors, membrane-bound and cytosolic Toll-like receptors and cytosolic NOD-like receptors. Recently, a possible role for intracellular cytosolic PRRs in the recognition of mycobacterial pathogens has been proposed. Here, we discuss currentideas on receptor-mediated recognition of mycobacterial pathogens by macrophages and dendritic cells.