Marques M B, Kasper D L, Pangburn M K, Wessels M R
Channing Laboratory, Brigham and Women's Hospital, Boston, Massachusetts.
Infect Immun. 1992 Oct;60(10):3986-93. doi: 10.1128/iai.60.10.3986-3993.1992.
Strains of type III group B streptococci isolated from patients with neonatal sepsis are generally resistant to complement-mediated phagocytic killing in the absence of specific antibody. It has been suggested that the resistance of type III group B streptococci to phagocytosis results from inhibition of alternative-complement-pathway activation by sialic acid residues of the type III polysaccharide. To better define the relationship between structural features of the type III capsule and resistance of type III group B streptococci to complement-mediated phagocytic killing, we measured deposition of human C3 on group B streptococcal strains with altered capsule phenotypes. C3 binding was quantified by incubating bacteria with purified human 125I-C3 in 10% serum. Wild-type group B Streptococcus sp. strain COH1 bound eightfold fewer C3 molecules than did either of two isogenic mutant strains, one expressing a sialic acid-deficient capsule and the other lacking capsule completely. Similar results were obtained when the incubation with 125I-C3 was performed in serum chelated with Mg-ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'- tetraacetic acid (MgEGTA), suggesting that the majority of C3 deposition occurred via the alternative pathway. In contrast to the wild-type strain, which was relatively resistant, both mutant strains were killed by human leukocytes in 10% serum with or without MgEGTA. We also measured C3 binding to 14 wild-type strains of type III group B streptococci expressing various amounts of capsule. Comparison of degree of encapsulation with C3 binding revealed a significant inverse correlation (r = -0.72; P less than 0.01). C3 fragments released by methylamine treatment of wild-type strain COH1 were predominantly in the form of C3bi, while those released from the acapsular mutant were predominantly C3b and those from the asialo mutant represented approximately equal amounts of C3b and C3bi. We conclude from these studies that the sialylated type III capsular polysaccharide inhibits alternative-pathway activation, prevents C3 deposition on group B streptococci, and protects the organisms from phagocytic killing.
从新生儿败血症患者中分离出的III型B族链球菌菌株，在缺乏特异性抗体的情况下，通常对补体介导的吞噬杀伤具有抗性。有人提出，III型B族链球菌对吞噬作用的抗性是由于III型多糖的唾液酸残基抑制了替代补体途径的激活。为了更好地界定III型荚膜的结构特征与III型B族链球菌对补体介导的吞噬杀伤的抗性之间的关系，我们测定了人C3在具有改变的荚膜表型的B族链球菌菌株上的沉积情况。通过将细菌与纯化的人125I-C3在10%血清中孵育来定量C3结合。野生型B族链球菌菌株COH1结合的C3分子比两个同基因突变菌株中的任何一个都少八倍，其中一个表达缺乏唾液酸的荚膜，另一个完全缺乏荚膜。当在与乙二醇双（β-氨基乙基醚）-N，N，N'，N'-四乙酸（MgEGTA）螯合的血清中进行与125I-C3的孵育时，获得了类似的结果，这表明大多数C3沉积是通过替代途径发生的。与相对抗性的野生型菌株相反，两种突变菌株在含有或不含有MgEGTA的10%血清中都被人白细胞杀死。我们还测定了C3与14株表达不同量荚膜的III型B族链球菌野生型菌株的结合情况。将包囊化程度与C3结合进行比较，发现存在显著的负相关（r = -0.72；P小于0.01）。用甲胺处理野生型菌株COH1释放的C3片段主要为C3bi形式，而从无荚膜突变体释放的主要是C3b，从无唾液酸突变体释放的C3b和C3bi量大致相等。我们从这些研究中得出结论，唾液酸化的III型荚膜多糖抑制替代途径的激活，阻止C3在B族链球菌上的沉积，并保护生物体免受吞噬杀伤。
