Lamarre A, Talbot P J
Virology Research Center, Armand-Frappier Institute, University of Quebec, Laval, Canada.
J Immunol. 1995 Apr 15;154(8):3975-84.
Molecular mechanisms of in vitro and in vivo virus neutralization by specific Ab remain largely undefined. Murine coronaviruses provide an excellent animal model for such studies. To determine the role of Ab bivalency and the contribution of its Fc portion in the neutralization of viral infectivity and passive protection of mice by an in vitro neutralizing and in vivo protective mAb (7-10A), F(ab')2 and Fab fragments were generated and their biologic properties were examined. The two fragments reacted in ELISA like the whole Ab against viral Ag or specific anti-idiotypic Abs. The affinity constants of the different Ab preparations were determined by surface plasmon resonance using immobilized anti-idiotypic Abs. The apparent affinity constant of the whole Ab molecule was 7.0 x 10(9) M-1 and was reduced 2-fold for F(ab')2 fragments and 14-fold for Fab molecules. Like whole Ab, both F(ab')2 and Fab fragments could neutralize virus in vitro and passively protect mice in vivo. However, the efficiency of in vivo neutralization by Fab fragments was reduced compared with the bivalent molecules, despite almost identical half-lives of both types of Ab fragments. These results demonstrate that in vitro and in vivo virus neutralization mechanisms by this Ab are independent of Fc-mediated functions and bivalency, but are probably influenced by Ab avidity. Also, this is the first report of in vivo protection against a viral infection by Fab fragments of antiviral Ab.
特异性抗体在体外和体内中和病毒的分子机制在很大程度上仍不明确。鼠冠状病毒为此类研究提供了一个极佳的动物模型。为了确定抗体二价性的作用及其Fc部分在体外中和病毒感染性以及通过一种体外中和且体内具有保护作用的单克隆抗体（7-10A）对小鼠进行被动保护中的贡献，制备了F(ab')2和Fab片段并检测了它们的生物学特性。这两种片段在ELISA中与针对病毒抗原或特异性抗独特型抗体的完整抗体反应方式相同。使用固定化抗独特型抗体通过表面等离子体共振测定了不同抗体制剂的亲和常数。完整抗体分子的表观亲和常数为7.0×10⁹ M⁻¹，F(ab')2片段降低了2倍，Fab分子降低了14倍。与完整抗体一样，F(ab')2和Fab片段均可在体外中和病毒并在体内对小鼠起到被动保护作用。然而，尽管两种类型的抗体片段半衰期几乎相同，但Fab片段在体内的中和效率与二价分子相比有所降低。这些结果表明，该抗体在体外和体内的病毒中和机制与Fc介导的功能和二价性无关，但可能受抗体亲和力的影响。此外，这是关于抗病毒抗体的Fab片段在体内对病毒感染具有保护作用的首次报道。
