Department of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA.
Department of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA
mSphere. 2018 Oct 24;3(5):e00437-18. doi: 10.1128/mSphere.00437-18.
Phagosomal acidification is a critical cellular mechanism for the inhibition and killing of ingested microbes by phagocytic cells. The acidic environment activates microbicidal proteins and creates an unfavorable environment for the growth of many microbes. Consequently, numerous pathogenic microbes have developed strategies for countering phagosomal acidification through various mechanisms that include interference with phagosome maturation. The human-pathogenic fungus resides in acidic phagosomes after macrophage ingestion that actually provides a favorable environment for replication, since the fungus replicates faster at acidic pH. We hypothesized that the glucuronic acid residues in the capsular polysaccharide had the capacity to affect phagosomal acidity through their acid-base properties. A ratiometric fluorescence comparison of imaged phagosomes containing to phagosomes containing beads showed that the latter were significantly more acidic. Similarly, phagosomes containing nonencapsulated cells were more acidic than those containing encapsulated cells. Acid-base titrations of isolated polysaccharide revealed that it behaves as a weak acid with maximal buffering capacity around pH 4 to 5. We interpret these results as indicating that the glucuronic acid residues in the capsular polysaccharide can buffer phagosomal acidification. Interference with phagosomal acidification represents a new function for the cryptococcal capsule in virulence and suggests the importance of considering the acid-base properties of microbial capsules in the host-microbe interaction for other microbes with charged residues in their capsules. is the causative agent of cryptococcosis, a devastating fungal disease that affects thousands of individuals worldwide. This fungus has the capacity to survive inside phagocytic cells, which contributes to persistence of infection and dissemination. One of the major antimicrobial mechanisms of host phagocytes is to acidify the phagosomal compartment after ingestion of microbes. This study shows that the capsule of can interfere with full phagosomal acidification by serving as a buffer.
吞噬体酸化是吞噬细胞抑制和杀死吞噬微生物的关键细胞机制。酸性环境激活杀菌蛋白，并为许多微生物的生长创造不利环境。因此，许多致病性微生物已经通过各种机制发展出了对抗吞噬体酸化的策略，包括干扰吞噬体成熟。人致病性真菌 在被巨噬细胞吞噬后存在于酸性吞噬体中，实际上为其复制提供了有利环境，因为真菌在酸性 pH 值下繁殖更快。我们假设荚膜多糖中的葡萄糖醛酸残基通过其酸碱性质具有影响吞噬体酸度的能力。对含有 和含有珠子的吞噬体进行成像的比率荧光比较表明，后者的酸度明显更高。同样，含有未包裹的 细胞的吞噬体比含有包裹细胞的吞噬体更酸性。分离的 多糖的酸碱滴定表明，它表现为一种弱酸，最大缓冲能力约为 pH4 到 5。我们将这些结果解释为表明荚膜多糖中的葡萄糖醛酸残基可以缓冲吞噬体酸化。干扰吞噬体酸化代表了 cryptococcal 荚膜在毒力中的一个新功能，并表明在宿主-微生物相互作用中，考虑带电荷残基的微生物荚膜的酸碱性质对于其他具有荚膜的微生物的重要性。 是隐球菌病的病原体，隐球菌病是一种毁灭性的真菌病，影响着全球数千人。这种真菌有在吞噬细胞内生存的能力，这有助于感染的持续存在和传播。宿主吞噬细胞的主要抗菌机制之一是在吞噬微生物后酸化吞噬体隔室。这项研究表明， 的荚膜可以通过充当缓冲液来干扰完全的吞噬体酸化。
