Beebe D P, Cooper N R
J Immunol. 1981 Apr;126(4):1562-8.
Neutralization of VSV by human serum ws previously shown to involve C1, C2, C3, and C4 of the classical complement (C) pathway. All normal human sera tested were equivalently active in this regard. However, purified C1, C2, C3, and C4 were unable to mediate VSV neutralization. In the present studies an additional factor required for C-mediated neutralization was isolated from normal human serum and identified as a natural IgM antibody specific for a viral encoded antigen. Purified IgM bound to the virus and formed a complex that activated component C1. Normal serum concentrations of purified IgM, C1, C2, C3, C4 neutralized VSV to the same extent as normal serum. Purified IgM did not neutralize VSV alone or in conjunction with C1, C2, and C4. Inclusion of C3 resulted in full neutralization and C3b binding to the virus was demonstrated. Thus, normal human serum contains a natural antibody of the IgM class that is directed toward a viral antigen. The antibody facilitates neutralization by forming an immune complex that activates C1 and thus efficiently initiates the classical pathway at the viral surface. Neutralization occurs with C3b deposition on the viral envelope and probably results from a blanket of C protein that interferes with viral attachment to susceptible cells.
先前已证明人血清对水疱性口炎病毒（VSV）的中和作用涉及经典补体（C）途径的C1、C2、C3和C4。所有测试的正常人血清在这方面的活性相当。然而，纯化的C1、C2、C3和C4无法介导VSV的中和作用。在本研究中，从正常人血清中分离出C介导的中和作用所需的另一种因子，并鉴定为一种针对病毒编码抗原的天然IgM抗体。纯化的IgM与病毒结合并形成激活C1成分的复合物。纯化的IgM、C1、C2、C3、C4在正常血清浓度下对VSV的中和程度与正常血清相同。纯化的IgM单独或与C1、C2和C4联合使用时均不能中和VSV。加入C3可导致完全中和，并证明C3b与病毒结合。因此，正常人血清中含有一种针对病毒抗原的IgM类天然抗体。该抗体通过形成激活C1的免疫复合物促进中和作用，从而在病毒表面有效地启动经典途径。中和作用伴随着C3b沉积在病毒包膜上，可能是由于一层C蛋白干扰了病毒与易感细胞的附着。
