Effect of plasma proteins on the growth of Blastomyces dermatitidis in vitro: A possible role of albumin in innate immunity against B. dermatitidis yeast

Blastomyces dermatitidis is a thermally dimorphic fungal pathogen that primarily causes disease in humans and dogs. In this project, I identified a plasma protein that inhibited the growth of B. dermatitidis yeast in vitro and characterized its mechanism of action. Serum inhibitory activity against fungi has predominantly been associated with transferrin mediated iron-binding. However, I found that apo-transferrin had virtually no inhibitory activity against B. dermatitidis yeast, and that serum from hypotransferrinemic mice had inhibitory activity that was equivalent to that of normal mouse serum. Thus, serum inhibitory activity against B. dermatitidis yeast appears to be largely transferrin independent. These results are explained in part by the observation that B. dermatitidis yeast can grow under iron-limiting, but not calcium limiting, conditions. To identity the serum inhibitory factor, I used fractionation techniques. I found a correlation between fractions with serum inhibitory activity against B. dermatitidis yeast and the presence of albumin. Furthermore, serum albumins from several animal species exhibited inhibitory activities against B. dermatitidis yeast that were comparable to the inhibitory activities of their respective whole serums. These observations were confirmed using serum from Nagase analbuminemic rats, which did not inhibit the growth of B. dermatitidis yeast. In contrast, analbuminemic rat serum supplemented with rat albumin exhibited inhibitory activity against B. dermatitidis yeast that was equivalent to that of rat albumin alone, or whole rat serum. Albumin inhibitory activity was not due to sequestration of nutrients from the tissue culture medium. Nor was it dependent on other serum proteins or fatty acids in Fraction V serum albumin preparations. The mechanism of inhibition is dependent on the ability of the albumin site 1 binding domain to bind a B. dermatitidis factor that presumably regulates the growth of B. dermatitidis yeast by an autocrine or paracrine signaling system. These results demonstrate a novel mechanism by which albumin may contribute to innate immunity against B. dermatitidis.