Interaction between Aspergillus fumigatus and basement membrane laminin: binding and substrate degradation

Aspergillus fumigatus, the causative agent of human aspergillosis, binds to and degrades basement membrane laminin. Using immunoelectron microscopy, laminin binding appeared to be associated with the cell wall expansions of resting conidia, and progressively extended to the outer electron dense layer of the conidial wall during the germination process. Labeling of thin sections revealed numerous binding sites in the cytoplasm, whereas the inner cell wall and the plasma membrane were not labeled. Attachment of A fumigatus conidia on microtiter plates coated with laminin and its fragments P1 and E8 was also investigated. Conidia cells showed good adhesion to wells coated with laminin. As indicated by inhibition experiments, the interaction was specific and fragment P1 represented the major binding site on the laminin molecule. In addition, since A fumigatus produced an extracellular serine protease, we determined the susceptibility of laminin to this enzyme. We demonstrated that protease extract was capable to degrade laminin in solution as well as in tissue sections. The laminin cleavage products were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. All the three chains were extensively degraded within 1 h. Treatment of the crude protease extract with the enzyme inhibitors, phenylmethylsulfonyl-fluoride and chymostatin, blocked the degradation of laminin, indicating a chymotrypsin-like serine protease activity. Immunofluorescence microscopy of cryostat sections of mouse and rat kidneys treated with the protease extract showed widespread loss of laminin epitopes from basement membranes. Enzyme treatment also removed immunoreactivity from lungs as observed after immunoperoxidase performed on paraffin sections. Binding and proteolytic degradation of laminin may together facilitate initial interaction of A fumigatus with the host tissues.