In:
Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, Vol. 109, No. 47 ( 2012-11-20)
Abstract:
Adding the discovery that PGT121 can bind complex-type N -glycans to the literature ( 1 – 5 ) suggests that the host immune system can respond to both high-mannose and complex-type glycans. Given that HIV-1 envelope spikes exhibit differing spectra of high-mannose and complex-type glycans on virions, promiscuity with respect to carbohydrate interactions would be a beneficial adaptation for HIV antibodies as a mechanism to ensure neutralization of all viruses within a given strain. This suggests that potential immunogens should contain complex-type, as well as high-mannose, N -glycans to elicit antibodies that can recognize these structures. Our results suggest that recognition of gp120 glycans by HIV antibodies, such as PGT121, can be promiscuous in accommodating both high-mannose and complex-type N -glycans in an epitope containing carbohydrate groups. Other HIV antibodies that recognize carbohydrate-containing epitopes, such as PGT128, may share this property. The PGT128/gp120 outer domain–V3 crystal structure ( 2 ) reveals a mechanism by which this might be accomplished, in that contacts between PGT128 and the Asn301 gp120 -linked high-mannose glycan involve only the core pentasaccharide ( 2 ), a portion that is common to both high-mannose and complex-type N -glycans. The immune system of the donor from which PGT121 was isolated developed a mechanism to overcome variations in the HIV-1 glycan shield, partly by producing clonal antibody variants within the PGT121-like and 10-1074–like families with different fine specificities for recognizing an epitope at or near the Asn332 gp120 PNGS ( Fig. P1 A ). The molecular basis for the differences between PGT121, 10-1074, and their germ-line progenitor (i.e., unmutated common ancestor) was revealed in part by their crystal structures ( Fig. P1 B ). The finding that the majority of somatic mutations in the light chain variable domain (V L ) were shared between PGT121 and 10-1074, whereas mutations in the heavy chain variable domain (V H ) differed, suggests that V L contacts shared portions of the gp120 epitope and V H recognizes distinct features ( Fig. P1 B ). All three antibodies exhibited an extended heavy-chain complementarity-determining region 3 (CDRH3) with a nonpolar tip that may allow access to cryptic epitopes. Differences in the antigen-binding site of the two mature antibodies were mainly localized to a cleft between CDRH2 and the extended CDRH3 ( Fig. P1 B ). We obtained structural information concerning glycan recognition by PGT121-like antibodies from a crystal structure in which a complex-type sialylated N -glycan attached to a V H domain residue interacted with the combining site of a neighboring PGT121 Fab. The glycan corresponds to the α2–6 sialylated glycan that PGT121 bound in microarrays and interacts with PGT121 by using the cleft between CDRH3 and CDRH2 that may be involved in epitope recognition, potentially explaining an unusual tilting of CDRH3 toward V L in the PGT121, 10-1074, and germ-line progenitor structures. Exchanging glycan contact residues between PGT121 and 10-1074 partially transferred their neutralization and glycan-recognition properties. PGT121 is a bNAb that was originally identified in the blood of an HIV-1–infected donor ( 1 ). We used soluble HIV-1 trimers as “bait” for single-cell sorting to isolate 29 new clonal variants of PGT121. The new bNAb molecules were classified into PGT121-like and 10-1074–like antibodies on the basis of differences in sequence, binding affinity, carbohydrate recognition, and neutralizing activity ( Fig. P1 A ). Antibodies belonging to the 10-1074–like group exhibit unusually potent virus-neutralizing activity, including broad reactivity against newly transmitted viruses. The epitopes, recognized by both groups, contain a potential N -linked glycosylation site (PNGS) at Asn332 gp120 and the base of the V3 loop of the gp120 subunit of the HIV spike ( Fig. P1 A ). However, the 10-1074–like antibodies required an intact PNGS at Asn332 gp120 for their neutralizing activity, whereas PGT121-like antibodies were able to neutralize some viral strains lacking the Asn332 gp120 PNGS. Moreover, the PGT121-like antibodies bound complex-type N -glycans in protein-free carbohydrate arrays, whereas the 10-1074–like antibodies showed no detectable binding to any of the protein-free N -glycans tested ( Fig. P1 A ). Although the recognition of complex-type N -glycans on gp120 by PGT121 was suggested by the glycan microarray data, both PGT121 and 10-1074 bound to isolated high-mannose-only–containing HIV-1 envelope spikes and to high-mannose-only virions. Antibodies are essential for the success of most vaccines. After several years of being infected with HIV-1, patients can develop broadly neutralizing antibodies (bNAbs) against the viral spike, a trimeric protein complex on the surface of virions that binds to host receptors. Some of these antibodies include N -linked carbohydrates in their binding site (epitope) on the envelope spike. Thus, they interact with a portion of the “glycan shield” on the spike that thwarts binding of many antibodies. From the peripheral B cells of an HIV-1–infected donor, we isolated a family of bNAbs related to PGT121, a previously described glycan-dependent bNAb ( 1 ). Our biochemical, structural, and virus neutralization data suggest that PGT121-like antibodies can accommodate either complex-type or high-mannose N -glycans in their epitope, an advantageous adaptation that would ensure neutralization of heterogeneously glycosylated viruses within a single HIV-1 strain.
Type of Medium:
Online Resource
ISSN:
0027-8424
,
1091-6490
DOI:
10.1073/pnas.1217207109
Language:
English
Publisher:
Proceedings of the National Academy of Sciences
Publication Date:
2012
detail.hit.zdb_id:
209104-5
detail.hit.zdb_id:
1461794-8
SSG:
11
SSG:
12
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