Kozel Thomas R, Levitz Stuart M, Dromer Françoise, Gates Marcellene A, Thorkildson Peter, Janbon Guilhem
Department of Microbiology and Immunology, University of Nevada School of Medicine, Reno, Nevada 89557, USA.
Infect Immun. 2003 May;71(5):2868-75. doi: 10.1128/IAI.71.5.2868-2875.2003.
Cryptococcus neoformans is surrounded by an antiphagocytic polysaccharide capsule whose primary constituent is glucuronoxylomannan (GXM). Three prominent structural features of GXM are single xylosyl and glucuronosyl side chains and O acetylation of the mannose backbone. Isogenic pairs of O-acetyl-positive and O-acetyl-negative strains (cas1 Delta) as well as xylose-positive and xylose-negative strains (uxs1 Delta) of serotype D have been reported. The cas1 Delta strains were hypervirulent, and the uxs1 Delta strains were avirulent. The goal of this study was to examine the effects of the cas1 Delta and uxs1 Delta mutations on the following: (i) binding of anti-GXM monoclonal antibodies (MAbs) in capsular quellung reactions, (ii) activation of the complement system and binding of C3, (iii) phagocytosis by neutrophils, and (iv) clearance of GXM in vivo. The results showed that loss of O acetylation produced dramatic changes in the reactivities of five of seven anti-GXM MAbs. In contrast, loss of xylosylation produced a substantive alteration in the binding behavior of only one MAb. O-acetyl-negative strains showed no alteration in activation and binding of C3 from normal serum. Xylose-negative strains exhibited accelerated kinetics for C3 deposition. Loss of O acetylation or xylosylation had no effect on phagocytosis of serum-opsonized yeast cells by human neutrophils. Finally, loss of O acetylation or xylosylation altered the kinetics for clearance of GXM from serum and accumulation of GXM in the liver and spleen. These results show that O acetylation and/or xylosylation are important for binding of anti-GXM MAbs, for complement activation, and for tissue accumulation of GXM but do not impact phagocytosis by neutrophils.
新型隐球菌被一层抗吞噬多糖荚膜所包围，其主要成分是葡糖醛酸木糖甘露聚糖（GXM）。GXM的三个显著结构特征是单个木糖基和葡糖醛酸基侧链以及甘露糖主链的O-乙酰化。已报道了血清型D的O-乙酰化阳性和O-乙酰化阴性菌株（cas1Δ）以及木糖阳性和木糖阴性菌株（uxs1Δ）的同基因对。cas1Δ菌株具有高毒力，而uxs1Δ菌株无毒力。本研究的目的是研究cas1Δ和uxs1Δ突变对以下方面的影响：（i）荚膜肿胀反应中抗GXM单克隆抗体（MAb）的结合；（ii）补体系统的激活和C3的结合；（iii）中性粒细胞的吞噬作用；（iv）体内GXM的清除。结果表明，O-乙酰化的缺失使七种抗GXM MAb中的五种的反应性发生了显著变化。相比之下，木糖基化的缺失仅使一种MAb的结合行为发生了实质性改变。O-乙酰化阴性菌株在正常血清中C3的激活和结合方面没有改变。木糖阴性菌株表现出C3沉积的加速动力学。O-乙酰化或木糖基化的缺失对人中性粒细胞吞噬血清调理的酵母细胞没有影响。最后，O-乙酰化或木糖基化的缺失改变了GXM从血清中清除的动力学以及GXM在肝脏和脾脏中的积累。这些结果表明，O-乙酰化和/或木糖基化对于抗GXM MAb的结合、补体激活以及GXM在组织中的积累很重要，但不影响中性粒细胞的吞噬作用。
