Interactions of Blastomyces dermatitidis with human monocyte-derived macrophages: The WI-1 surface antigen of yeasts mediates binding to human macrophage CD11b/CD18 (CR3) and CD14

Three genetically related strains of Blastomyces dermatitidis yeasts that differ in their expression of WI-1, an immunodominant cell wall antigen, were tested for their capacity to bind to human macrophages in the absence of serum. These strains included ATCC 26199 that is virulent for mice; an attenuated mutant strain ATCC 60915, that expresses 2.5 fold more WI-1 than strain 26199; and an avirulent mutant strain ATCC 60916 that has 6 fold more WI-1 than strain 26199. Attachment of both mutant strains to macrophages was rapid, and was maximum after 10 min at 37 degrees C. Attachment of strain 26199 to macrophages was about 40% of that obtained with the mutants. Binding of B. dermatitidis to macrophages was temperature and Mg++ dependent, and heat-killed yeasts bound to macrophages as well as viable yeasts. Experiments with receptor-specific monoclonal antibodies demonstrated that 26199 yeasts bound predominantly to the lipopolysaccharide binding site on receptor CD11b/CD18 (CR3). However, the mutants bound to macrophage CD14 as well as CR3 receptors. Fab anti-WI-l inhibited the binding of all strains to macrophages by 69-78%. Latex microspheres coated with purified WI-1, or a 25-amino acid tandem repeat located within WI-1 also bound to macrophage CR3 and CD14 receptors. These data demonstrate that: 1) WI-l is a major ligand on B. dermatitidis that mediates attachment of yeasts to human macrophages; 2) the binding activity of WI-1 is located within the 25-amino acid tandem repeat; and 3) binding of B. dermatitidis yeasts to macrophages is mediated through the lipopolysaccharide binding site on CR3, and CD14.