2034. Natural Antibodies Affects the Formation of Titan Cells in Cryptococcus neoformans In Vitro

BACKGROUND: An important feature of Cryptococcus neoformans (CN) is an ability to undergo morphological changes that enhance virulence and development of cryptococcal disease (CD). CN can change its size by capsule enlargement alone or capsule and cell body enlargement, resulting in “titan cells.” Titan cells enable CN to evade host defense mechanisms. Human and mouse β-glucan antibodies bind and inhibit CN growth in vitro. Naturally occurring antibodies in human serum bind β-glucans. In this study, we determined the effect of human IgM and IgG on CN size and titan cell formation in vitro. METHODS: Experiments were performed with CN var. grubii H99 (serotype A) grown in liquid Sabouraud media at 30ºC. First, we established that human IgM (Sigma Aldrich) binds H99 and Laminarin (a polymer consisting primarily of β (1–3) glucan with occasional β (1–6) branching (Sigma Aldrich) by ELISA using Goat Anti-Human IgM-AP Then, we cultured CN in titan cell medium (TCM, 5% sabouraud and 5% fetal bovine serum diluted in MOPS 50 mM at pH 7.3 plus 15 µM sodium azide) at 37ºC with CO(2) for 18 hours with and without human IgM or IgG (Sigma Aldrich), after which cell size was evaluated using India Ink in a Zeiss microscope. RESULTS: We found that IgM-treated cells exhibited a significant reduction in CN capsule size and titan cell formation (total cell size) compared with controls without IgM or with IgG. Median total cell size (µm) were: IgM (15.04), IgG (20) and PBS (22.24), P < 0.05 using the Kolmogorov–Smirnow test to estimate normality and one-way ANOVA to compare between groups. There were no statistical differences in cell size after incubation with human IgG or PBS. To gain insight into how IgM may mediate its effect, we demonstrated that it bound mainly to the CN cell wall with some diffuse punctuate to the capsule by immunofluorescence. CONCLUSION: Our results reveal that natural IgM has the ability to inhibit CN titan cell formation in cultured cells. Given the importance of titan cell formation in virulence, our results suggest that direct effects of natural antibody on CN biology may contribute to human resistance to CD. This hypothesis is under investigation in our laboratory. DISCLOSURES: All authors: No reported disclosures.