Antibodies to the superantigenic site of HIV-1 gp120: Hydrolytic and binding activities of the light chain subunit

Antibodies (Abs) to the superantigenic determinant of HIV gp120 (gp120SAg) are potential protective agents against HIV infection. We report that the light chain subunits of Abs cloned from lupus patients using phage library methods bind and hydrolyze gp120SAg independent of the heavy chain. Unlike frequent gp120SAg recognition by intact Abs attributable to VH domain structural elements, the isolated light chains expressed this activity rarely. Four light chains capable of gp120SAg recognition were identified by fractionating phage displayed light chains using peptide probes containing gp120 residues 421-433, a gp120SAg component. Three light chains expressed non-covalent gp120SAg binding and one expressed gp120SAg hydrolyzing activity. The hydrolytic light chain was isolated by covalent phage fractionation using an electrophilic analog of residues 421-433. This light chain hydrolyzed a reporter gp120SAg substrate and full-length gp120. Other peptide substrates and proteins were hydrolyzed at lower rates or not at all. Consistent with the expected nucleophilic mechanism of hydrolysis, the light chain reacted selectively and covalently with the electrophilic gp120SAg peptide analog. The hydrolytic reaction entailed a fast initial step followed by a slower rate limiting step, suggesting rapid substrate acylation and slow deacylation. All four gp120SAg-recognizing light chains contained sequence diversifications relative to their germline gene counterparts. These observations indicate that in rare instances, the light chain subunit can bind and hydrolyze gp120SAg without the participation of the heavy chain. The pairing of such light chains with heavy chains capable of gp120SAg recognition represents a potential mechanism for generating protective Abs with enhanced HIV binding strength and anti-viral proteolytic activity.