Importance of α- and β/α-linked mannooligosaccharides in antibody response against C. dubliniensis

A conjugate of C. dubliniensis cell-wall mannan and human serum albumin (HSA) induced significant level of anti-mannan IgGs in sera of immunized rabbits, whereas mannan alone was not immunogenic. Binding affinities of anti-mannan IgGs induced by the conjugate were evaluated by inhibition ELISA (iELISA) using mannooligosaccharides (dimer-octamer), derived from the side chains of C. dubliniensis mannan, as the inhibitors. Inhibition power of the mannooligosaccharides increased exponentially with their size, with dimer being the weakest (IC(50) = 4 mmol/L) and heptamer/octamer the strongest inhibitors (IC(50) = 0.01 mmol/L). In addition, the mannooligosaccharides proved effective as inhibitors against antiserum obtained from rabbits immunized with C. dubliniensis heat-killed cells, demonstrating a high correlation in the IC(50) values with anti-conjugate serum (Pearson's correlation coefficient r = 0.98; P < 0.01). These findings suggest that a) the mannooligosaccharides comprising the side chains of C. dubliniensis mannan may represent relevant points of interaction with host immune system during infection and b) anti-mannan antibodies induced by the two antigens (the mannan conjugate and the yeast) are of similar specificities.