Toll-like receptors and innate antifungal responses.

The mammalian Toll-like receptors (TLRs) are homologues of Drosophila Toll and constitute a novel protein family involved in the mediation of innate immunity and the activation of adaptive immunity. Analysis of infection with human pathogenic fungi Candida albicans and Aspergillus fumigatus implicated TLR2 and TLR4 in elicitation of immune responses. Cryptococcus neoformans is recognized by a process that uses TLR4. C. albicans induces immunostimulation through causative agents, such as mannan or its structural derivatives (e.g. phospholipomannan), which are recognized by the immune system as pathogen-associated molecular patterns and are located in the cell wall of fungi. Secreted aspartic proteinases represent a key virulence factor that contributes to the ability of C. albicans to cause mucosal and disseminated infections, and might be a further potential stimulator of TLRs. Simultaneous activation of other pattern recognition receptors collaborating with TLRs illustrates the cooperation of various chains within ligand-specific receptor complexes for the recognition of fungal pathogens and their cell wall components.