Objectives:
Aspergillus fumigatus is a common airborne fungal pathogen of humans and a significant source of mortality in immunocompromised individuals. Conidia, the asexually produced spores, are the first form of A. fumigatus to come in contact with lung epithelial and immune cells. Thus, the composition of the conidial surface has the potential to directly influence the outcome of an A. fumigatus infection. The cell wall polysaccharides that form the cell wall of A. fumigatus conidia and the pigmented melanin layer have been extensively studied. Little is known about conidial surface proteins with immune-modulatory functions; apart from the RodA hydrophobin that forms a hydrophobic surface layer on dormant conidia. Here, we aimed to identify additional conidial surface proteins which may play a role during host pathogen interaction.

Methods:
By either hydrogen flouride-pyridine extraction of condial proteins or "trypsin-shaving" of surface-exposed proteins and subsequent LC-MS/MS analysis we characterised the A. fumigatus spore proteome.

Results:
We identified 116 proteins in the conidial cell wall. One protein, designated conidial cell wall protein A (CcpA) was nearly as abundant as the hydrophobin layer-forming protein RodA. CcpA peaks in expression during sporulation on resting conidia. Despite a high surface abundance, the cell surface of ΔccpA resting conidia appeared normal. However, trypsin shaving of ΔccpA conidia revealed novel surface-exposed proteins normally masked in the wild-type strain, suggesting a role for CcpA as an important structural conidial protein. Interestingly, swollen ΔccpA conidia led to higher activation of neutrophils and dendritic cells than wild-type conidia, suggesting a role for CcpA in the pathogenicity of A. fumigatus. In addition, virulence was highly attenuated when cortisone-treated mice were infected with ΔccpA conidia. Although CcpA played an important function in overcoming the innate immune response, recombinant CcpA protein was unremarkable when presented to the adaptive immune response, where it was neither immunosuppressive nor immunologically inert.

Conclusion:
These data suggest that CcpA is a major structural conidial protein that may hide immunogenic structures of the conidial surface from the host innate immune system.

Conidial surface proteome analysis reveals the highly abundant CcpA protein important for virulence of Aspergillus fumigatus