ABSTRACTOur previous study demonstrated that treatment ofCoxiella burnetiiwith the phase I lipopolysaccharide (PI-LPS)-targeted monoclonal antibody (MAb) 1E4 significantly inhibitedC. burnetiiinfection in mice, suggesting that 1E4 is a protective MAb. To determine whether passive transfer of antibodies (Abs) can provide protection againstC. burnetiinatural infection, we examined if passive transfer of 1E4 would protect SCID mice againstC. burnetiiaerosol infection. The results indicated that 1E4 conferred significant protection against aerosolizedC. burnetii, suggesting that 1E4 may be useful for preventingC. burnetiinatural infection. To further understand the mechanisms of 1E4-mediated protection and to test the possibility of using humanized 1E4 to preventC. burnetiiinfection, we examined whether the Fab fragment of 1E4 (Fab1E4), a recombinant murine single-chain variable fragment (muscFv1E4), and a humanized single-chain variable fragment (huscFv1E4) retained the ability of 1E4 to inhibitC. burnetiiinfection. The results indicated that Fab1E4, muscFv1E4, and huscFv1E4 were able to inhibitC. burnetiiinfection in mice but that their ability to inhibitC. burnetiiinfection was lower than that of 1E4. In addition, treatment ofC. burnetiiwith Fab1E4, muscFv1E4, or huscFv1E4 can blockC. burnetiiinfection of macrophages. Interestingly, treatment ofC. burnetiiwith huscFv1E4 can significantly reduceC. burnetiiinfectivity in human macrophages. This report provides the first evidence to demonstrate that the humanized variable fragments of an LPS-specific MAb can neutralizeC. burnetiiinfection and appears to be a promising step toward the potential use of a humanized MAb as emergency prophylaxis againstC. burnetiiexposure.
Crystal Structure and Computational Modeling of the Fab Fragment from a Protective Anti-Ricin Monoclonal Antibody