Moore F D, Austen K F, Fearon D T
J Immunol. 1982 Mar;128(3):1302-6.
Activation of the human alternative pathway of complement (C) by surfaces requires the initial deposition of C3b by fluid-phase C3 convertase and sustained C3 cleavage by C3 convertases fixed to the surface. Nonactivating particles have previously been characterized by an inability to sustain the function of C3 convertases on their surfaces because these sites were susceptible to the regulatory action of the control proteins. Pronase converts the mouse erythrocyte (E) from an activator to a nonactivator by markedly decreasing the ability of the cell to affix C3b generated by a fluid-phase C3 convertase; this conversion is unrelated to the action of the control proteins on bound C3b. The capacity of antibody and its F(ab')2 or Fab' fragments to restore the alternative pathway-activating function of pronase-treated mouse E indicates that the contribution of antibody is mediated by its combining site without a requirement for bridging or for the Fc portion. The fab'-dependent activation of the alternative pathway of C relates to the deposition of C3b on the particle surface, which is a first and continuing step in alternative pathway activation. The antibody effect is not directed to the action of the regulatory proteins. Thus, particle-dependent activation of the alternative C pathway has been shown for the first time to be abolished and restored by cell surface-directed mechanisms that function independently of the regulatory proteins.
补体（C）的人替代途径被表面激活需要通过液相C3转化酶初始沉积C3b，并由固定在表面的C3转化酶持续裂解C3。非激活颗粒以前的特征是无法维持其表面上C3转化酶的功能，因为这些位点易受控制蛋白的调节作用影响。链霉蛋白酶通过显著降低细胞附着液相C3转化酶产生的C3b的能力，将小鼠红细胞（E）从激活剂转变为非激活剂；这种转变与控制蛋白对结合的C3b的作用无关。抗体及其F（ab'）2或Fab'片段恢复链霉蛋白酶处理的小鼠E的替代途径激活功能的能力表明，抗体的作用是由其结合位点介导的，不需要桥接或Fc部分。Fab'依赖的补体替代途径激活与C3b在颗粒表面的沉积有关，这是替代途径激活的第一步且是持续步骤。抗体效应并非针对调节蛋白的作用。因此，首次证明了颗粒依赖的补体替代途径激活可通过独立于调节蛋白起作用的细胞表面定向机制被消除和恢复。
