Characterisation of the fungal ligands, structure and function of the C-type lectin receptor, MelLec

Abstract

C-type lectin-receptors (CLRs) are a family of receptors that play a critical role in antifungal immunity. CLRs contain one or more C-type lectin like domains (CTLD) that recognise carbohydrates in fungal cell walls, leading to stimulation of several innate and adaptive immune responses. The novel CLR, MelLec, does not recognise carbohydrates but it recognises DHN-melanin in the cell wall of the major human fungal pathogen, A. fumigatus. Importantly, mice and humans are protected against disseminated A. fumigatus infection through recognition of melanin by this receptor. MelLec is not expressed on myeloid cells in mice but it is found to be expressed by CD31+ endothelial cells and a sub-population of these cells that co-express EpCAM. These novel double positive (CD31+EpCAM+) cells are only found in the mouse lung and liver, and still need to be characterised. Although the interaction of MelLec with DHN-melanin has been established, the downstream immune responses, including signalling pathways and cellular functions, remain unknown. In this thesis, I characterised further the mechanism of function, structure and ligands of MelLec. I demonstrated that the co expression of Dectin-1 in MelLec expressing reporter cells is required for MelLec to sense the Dectin-1-captured fungal ligands and induce MelLec signalling. Additionally, mMelLec forms homotypic interactions that are mediated via the stalk region and seem to be also essential for the induction of MelLec signalling. Using a chimaeric Fc-MelLec fusion protein, I demonstrated that the chemically synthesised heptaketide naphthopyrone, the first product in the DHN-melanin biosynthetic pathway, and several other intermediates in this pathway are recognised by MelLec. Finally, I also identified MelLec as a receptor for toxic fungal secondary metabolites, including aflatoxin B1 and its precursors. Together, these findings add to our current knowledge of the structure, functions and ligands of MelLec, which plays a critical role in antifungal immunity.