Interactions between conidia of Aspergillus fumigatus and human complement component C3.

Activation and deposition of C3 on Aspergillus fumigatus conidia have been previously demonstrated. This study investigated in further detail the interactions between complement component C3 and the conidia of A. fumigatus. Immunoblotting and 125I-C3 binding studies showed that C3 deposition was rapid (less than 15 min) and parallel to the formation of iC3b. Immunoblotting experiments identified a 54- to 58-kDa conidial protein which binds human complement component C3 and/or a C3 fragment(s). 125I labeling of the outer layer of the conidia demonstrated that this protein doublet was present on the surface of the spore. The further degradation of C3 to low-molecular-mass fragments (40, 37, and 30 kDa), in the absence of plasma, by intact living conidia and a preparation of the outer conidial wall layer indicated the ability of fungal components to cleave C3. These data suggest that interactions between conidia and C3 are not limited only to deposition via activation of the alternative complement pathway; they also include degradation of bound C3.