Rakesh Vineet, Schweitzer Andrew D, Zaragoza Oscar, Bryan Ruth, Wong Kevin, Datta Ashim, Casadevall Arturo, Dadachova Ekaterina
Department of Biological and Environmental Engineering, Cornell University, Ithaca, New York 14853, USA.
J Phys Chem B. 2008 Jul 24;112(29):8514-22. doi: 10.1021/jp8018205. Epub 2008 Jun 28.
Many microorganisms such as bacteria and fungi possess so-called capsules made of polysaccharides which protect these microorganisms from environmental insults and host immune defenses. The polysaccharide capsule of Cryptococcus neoformans, a human pathogenic yeast, is capable of self-assembly, composed mostly of glucuronoxylomannan (GXM), a polysaccharide with a molecular weight of approximately 2,000,000, and has several layers with different densities. The objective of this study was to model pore-hindered diffusion and binding of the GXM-specific antibody within the C. neoformans capsule. Using the finite-element method (FEM), we created a model which represents the in vivo binding of a GXM-specific antibody to a C. neoformans cell taking into account the intravenous infusion time of antibody, antibody diffusion through capsular pores, and Michaelis-Menten kinetics of antibody binding to capsular GXM. The model predicted rapid diffusion of antibody to all regions of the capsule where the pore size was greater than the Stokes diameter of the antibody. Binding occurred primarily at intermediate regions of the capsule. The GXM concentration in each capsular region was the principal determinant of the steady-state antibody-GXM complex concentration, while the forward binding rate constant influenced the rate of complex formation in each region. The concentration profiles predicted by the model closely matched experimental immunofluorescence data. Inclusion of different antibody isotypes (IgG, IgA, and IgM) into the modeling algorithm resulted in similar complex formation in the outer capsular regions, but different depths of binding at the inner regions. These results have implications for the development of new antibody-based therapies.
许多微生物，如细菌和真菌，都拥有由多糖构成的所谓荚膜，这些荚膜可保护这些微生物免受环境侵害和宿主免疫防御的影响。新型隐球菌是一种人类致病酵母，其多糖荚膜能够自我组装，主要由葡糖醛酸木甘露聚糖（GXM）组成，这是一种分子量约为2,000,000的多糖，并且有几层不同密度的结构。本研究的目的是模拟新型隐球菌荚膜内GXM特异性抗体的孔阻碍扩散和结合。我们使用有限元方法（FEM）创建了一个模型，该模型考虑了抗体的静脉输注时间、抗体通过荚膜孔的扩散以及抗体与荚膜GXM结合的米氏动力学，来表示GXM特异性抗体与新型隐球菌细胞的体内结合。该模型预测抗体迅速扩散到荚膜中孔径大于抗体斯托克斯直径的所有区域。结合主要发生在荚膜的中间区域。每个荚膜区域的GXM浓度是稳态抗体 - GXM复合物浓度的主要决定因素，而正向结合速率常数影响每个区域复合物形成的速率。该模型预测的浓度分布与实验免疫荧光数据密切匹配。将不同的抗体同种型（IgG、IgA和IgM）纳入建模算法会导致在荚膜外层区域形成相似的复合物，但在内部区域结合深度不同。这些结果对基于抗体的新疗法的开发具有启示意义。
