FabALACTICA Facilitates the Structural Insight into SARS-CoV-2 Neutralizing Antibodies

The first steps of infection with SARS-CoV-2 is binding of a viral Spike protein to a host receptor angiotensin-converting enzyme 2 (ACE2), followed by fusion of viral and host membranes. Antibodies that block this interaction are emerging as early COVID-19 therapies, however, the neutralization potencies of the antibodies are less studied.

The authors screened for antibodies that target the receptor-binding domain of the Spike protein, consequently blocking ACE2-binding. Six monoclonal antibodies were identified with a high blockade potency. To interrogate determinants of binding mode and neutralization, the potency of each antibody was compared to its respective monomeric Fab.
Immobilized FabLACTICA was used to convert the antibodies to Fab fragments. FabALACTICA is a cysteine protease that digests human IgG1 at a specific site above the hinge, generating intact Fab and Fc fragments. The Immobilized FabALACTICA spin columns contain the FabALACTICA enzyme covalently coupled to agarose beads, for digestion of human IgG1 without contaminating the final preparation with enzyme.
The six identified antibodies were found to exhibit different avidity, binding mode and neutralization of the live SARS-CoV-2. Strikingly, the neutralizing antibodies were also found to either inhibit or enhance Spike-mediated membrane fusion and formation of syncytia (multinucleated giant cells), a sign of tissue damage in individuals with COVID-19. These findings suggest that the potency of a neutralizing antibody is influenced by several possibly counteracting factors, proposing a complex basis for viral neutralization effectiveness.

Asarnow et al., 2021. Structural insight into SARS-CoV-2 neutralizing antibodies and modulation of syncytia. Cell.



FabALACTICA – Above hinge digestion of human IgG1.