The glycocalyx is a glycan layer found on the surfaces of host cells as well as microorganisms and enveloped virus. Its thickness may easily exceed 50 nm. The glycocalyx does not only serve as a physical protective barrier but also contains various structurally different glycans, which provide cell- or microorganism-specific 'glycoinformation'. This information is decoded by host glycan-binding proteins, lectins. The roles of lectins in innate immunity are well established, as exemplified by collectins, dectin-1, and dendritic cell (DC)-specific intracellular adhesion molecule-3-grabbing non-integrin (DC-SIGN). These mammalian lectins are synthesized in the secretory pathway and presented on the cell surface to bind to specific glycan 'epitopes'. As they recognize non-self glycans presented by microorganisms, they can be considered as receptors for pathogen-associated molecular patterns (PAMPs), i.e. pattern recognition receptors (PRRs). One notable exception is the galectin family. Galectins are synthesized and stored in the cytoplasm, but upon infection-initiated tissue damage and/or following prolonged infection, cytosolic galectins are either passively released by dying cells or actively secreted by inflammatory activated cells through a non-classical pathway, the 'leaderless' secretory pathway. Once exported, galectins act as PRR, as well as immunomodulators (or cytokine-like modulators) in the innate response to some infectious diseases. As galectins are dominantly found in the lesions where pathogen-initiated tissue damage signals appear, this lectin family is also considered as potential damage-associated molecular pattern (DAMP) candidates that orchestrate innate immune responses alongside the PAMP system.

Pneumonia can be caused by a variety of pathogens, among which Streptococcus pneumoniae causes one of the most common forms of community-acquired pneumonia. Depending on the invading pathogen, the elements of the immune response triggered will vary. For most pathogens, such as Escherichia coli, neutrophil recruitment involves a well-described family of adhesion molecules, β2-integrins. In the case of streptococcal pneumonia, however, neutrophil recruitment occurs mainly through a β2-integrin-independent pathway. Despite decades of research on this issue, the adhesion molecules involved in neutrophil recruitment during lung infection by S. pneumoniae have not been identified. We have previously shown that galectin-3, a soluble mammalian lectin, can be found in lungs infected by S. pneumoniae, but not by E. coli, and can mediate the adhesion of neutrophils on the endothelial cell layer, implying its role in the recruitment of neutrophils to lungs infected with S. pneumoniae. In this study, using galectin-3 null mice, we report further evidence of the involvement of this soluble lectin in the recruitment of neutrophils to S. pneumonia-infected lungs. Indeed, in the absence of galectin-3, lower numbers of leukocytes, mainly neutrophils, were recruited to the infected lungs during infection by S. pneumoniae. In the case of β2-integrin-dependent recruitment induced by lung infection with E. coli, the number of recruited neutrophils was not reduced. Thus, taken together, our data suggest that galectin-3 plays a role as a soluble adhesion molecule in the recruitment of neutrophils to lungs infected by S. pneumoniae, which induces β2-integrin-independent migration.

When infection occurs, neutrophils rapidly migrate to the affected site. Although the neutrophils neutralize microorganisms, they can also cause tissue damage or render invasion pathways to pathogens. Thus, the migration could be either beneficial or unfavorable in the initial control of infection. Studies on neutrophil recruitment revealed its complexity, especially in terms of the regulation of its initiation. Galectin-3 is a member of the galectin family that has an affinity for β-galactoside containing glycoconjugates. In this study, we investigated the role of galectin-3 in neutrophil migration and the biological significance of the rapid migration of neutrophils in an experimental parasitic cutaneous infection with Leishmania major. When the substrain of L. major, LV39, was infected, lack of galectin-3 impaired neutrophil recruitment in the footpads and the draining lymph nodes 1 d following infection. Reduced number of recruited neutrophils correlated with local high parasite burdens. In contrast, neutrophil migration, induced by the other L. major substrain, Friedlin, was unaffected, and the initial parasite burden remained similar in galectin-3 null mice as compared with wild-type mice. Infection with L. major LV39 but not Friedlin induced higher levels of extracellular release of galectin-3. Further, galectin-3 alone was able to initiate neutrophil migration even though galectin-3 is not a chemoattractant for neutrophils. Thus, our data suggest that once extracellularly released, galectin-3 can act as a damage-associated molecular pattern to facilitate early neutrophil migration, which is beneficial in the initial control of the Leishmania infection.

Galectin-3 belongs to a family of galectins, evolutionarily conserved glycan binding proteins (lectins) that have recently attracted much attention as modulators in adaptive immune responses. Previously, galectins have been considered lectins that bind only to endogenous "self" glycans. Further, galectins are synthesized and stored in the cytosol, where there are virtually no glycan-containing proteins, raising doubts over the biological significance of their glycan binding capacity. As discussed in this review, with particular emphasis on the role of galectin-3 in the innate immune response against the protozoan parasite Leishmania, several recent studies have suggested that galectin-3 could recognize L. major-specific pathogen-associated molecular pattern and, in parallel, facilitate the infiltration of neutrophils to the infected sites that helps reduce the initial parasite burden once galectin-3 is released as a damage-associated molecular pattern. Thus, while further investigation is necessary, based on the current results, it could be proposed that galectin-3 can hinge two areas of the innate immune recognition system, DAMP and PAMP pathways in the early host responses against various pathogens.