Protection from lethal coronavirus infection by immunoglobulin fragments

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.