Siglec-targeted liposomes to identify sialoglycans present on fungal pathogens

The sialic acid Ig-like lectins Siglec-3 and Siglec-15 are pathogen receptors that bind sialic acid-modified glycoproteins, best characterized in metastatic cancers. Because fungi produce sialoglycans and sialo-glycoproteins, we wondered if Siglecs had the potential for targeted delivery of antifungal drugs. We purified the extracellular V-region Ig-like C2 ligand-binding domains and stalk regions of SIG3 and SIG15. We floated the two polypeptides on the surface of liposomes loaded with amphotericin B (AmB) and labeled with rhodamine B to prepare SIG3-Ls and SIG15-Ls. Using these two reagents, we explored the sialoglycans of two evolutionarily distant and deadly human fungal pathogens, the Mucormycete Rhizopus delemar and the Ascomycete Aspergillus fumigatus. We found that SIG3-Ls and SIG15-Ls localized in a continuous layer over the cell wall surface of germ tubes and hyphae of both fungal species and to the conidia of A. fumigatus. Binding was Neu5Ac-specific and appeared confined to N-linked sialoglycans on fungal proteins. SIG3 and SIG15 proteins bound to diverse sialo-glycoproteins extracted from the hyphae of both species. SIG3-Ls and SIG15-Ls delivering sub-micromolar concentrations of AmB were moderately more effective at inhibiting and/or killing both species relative to control liposomes. We discuss the roles that sialo-glycoproteins may play in fungal pathogens.