Functional and Structural Characterization of Human V3-Specific Monoclonal Antibody 2424 with Neutralizing Activity against HIV-1 JRFL

Abstract

The V3 region of HIV-1 gp120 is important for virus-coreceptor interaction and highly immunogenic. Although most anti-V3 antibodies neutralize only the sensitive tier 1 viruses, anti-V3 antibodies effective against the more resistant viruses exist, and a better understanding of these antibodies and their epitopes would be beneficial for the development of novel vaccine immunogens against HIV. The HIV-1 isolate JRFL with its cryptic V3 is resistant to most V3-specific monoclonal antibodies (MAbs). However, the V3 MAb 2424 achieves 100% neutralization against JRFL. 2424 is encoded by IGHV3-53 and IGLV2-28 genes, a pairing rarely used by the other V3 MAbs. 2424 also has distinct binding and neutralization profiles. Studies of 2424-mediated neutralization of JRFL produced with a mannosidase inhibitor further revealed that its neutralizing activity is unaffected by the glycan composition of the virus envelope. To understand the distinct activity of 2424, we determined the crystal structure of 2424 Fab in complex with a JRFL V3 peptide and showed that the 2424 epitope is located at the tip of the V3 crown ( IMPORTANCEHIV/AIDS vaccines are crucial for controlling the HIV epidemics that continue to afflict millions of people worldwide. However, HIV vaccine development has been hampered by significant scientific challenges, one of which is the inability of HIV vaccine candidates evaluated thus far to elicit production of potent and broadly neutralizing antibodies. The V3 loop is one of the few immunogenic targets on the virus envelope glycoprotein that can induce neutralizing antibodies, but in many viruses, parts of V3 are inaccessible for antibody recognition. This study examined a V3-specific monoclonal antibody that can completely neutralize HIV-1 JRFL, a virus isolate resistant to most V3 antibodies. Our data reveal that this antibody recognizes the most distal tip of V3, which is not as occluded as other parts of V3. Hence, the epitope of 2424 is in one of the vulnerable sites on the virus that may be exploited in designing HIV vaccine immunogens. The HIV-1 envelope glycoprotein (Env) is the only virus-encoded protein expressed on the surface of the virus and is the sole target for virus-neutralizing antibodies (Abs). On the virion surface, the HIV Env spike is a compact heterodimeric trimer made up of gp120 and gp41 subunits (1-3). The surface gp120 subunit is responsible for interacting with the host cell through binding to CD4 and the coreceptor, the chemokine receptor CCR5 or CXCR4 (4-7). On the basis of primary amino acid sequences, gp120 is divided into five conserved regions (C1 to C5), which are interspersed with five variable regions (V1 to V5) (8). The CD4-binding site and the chemokine receptor-binding site are both highly conformational and discontinuous. The chemokine receptor binding site in particular is composed of the invariant ␤2 and ␤3 strands of the V1V2 stem region, ␤20 and ␤21 strands in the conserved C4 region, and the third variable (V3) region of gp120 (3, 9). Vulnerable sites on t...

Structure/Function Studies Involving the V3 Region of the HIV-1 Envelope Delineate Multiple Factors That Affect Neutralization Sensitivity

“…Notably, however, JR-FL has an aspartate at position 197 (D197), abrogating the glycan motif. Given the relative resistance of JR-FL to V3 MAbs ( 62 ), it did not appear that the glycan at this position played a role in the occlusion of the V3 loop. To test this, a JR-FL D197N mutant was made to restore the putative glycosylation site; the infectivities of the D197N mutant and the WT were comparable (27,940 TCID 50 /ml).…”

Differential induction of anti-V3 crown antibodies with cradle- and ladle-binding modes in response to HIV-1 envelope vaccination

“…The highly immunogenic site on V3 is the 13 amino acid-long crown; this region is targeted by the anti-V3 Abs generated during infection [13–15] and after immunization [16–20]. The V3 crown is also recognized by a large panel of V3 monoclonal Abs (mAbs), most of which neutralize many Tier 1 viruses and few Tier 2 viruses [5, 6, 21–24]. Although the V3 crown is often occluded in the membrane-bound native Env trimers of Tier 2 viruses, this region is accessible to Abs in the soluble Env gp120 monomers and gp140 oligomers, including the stabilized gp140 trimers of BG505 SOSIP.664 which are strongly bound in ELISA by anti-V3 mAbs such as 39F, 19b and 14e [25].…”

Plasticity and Epitope Exposure of the HIV-1 Envelope Trimer

“…As V3 is highly flexible relative to other Env domains and V3 is among the most immunogenic regions of the Env glycoproteins (8)(9)(10)(15)(16)(17), we hypothesized that release of the V3 loop in various mutants from its occluded position in the Env spike occurs independently of substantial conformational alterations in the V1V2 loop. We also sought to determine if the same mutations that released V3 would affect the exposure of various epitopes presented by the V2 region of gp120, using the JR-FL pseudovirus, which exhibits a "closed" trimer conformation and is therefore relatively resistant to neutralization by V2i and V3 MAbs (18)(19)(20)(21)(22)(23). The effects of these mutations were subsequently tested, as well, to determine how they affected the potency of MAbs specific for the CD4 binding site (CD4bs) and the CD4-induced (CD4i) epitopes.…”