. Here we produced pseudovirions bearing the mutant envelope and a reporter gene to examine the mutant's infectious properties. These pseudovirions attach to cells expressing CD4 and coreceptor but infect only when triggered with reducing agent, implying that gp120-gp41 dissociation is necessary for infection. Further studies suggested that virus entry was arrested after CD4 and coreceptor engagement. By measuring the activities of various entry inhibitors against the arrested intermediate, we found that gp120-targeting inhibitors typically act prior to virus attachment, whereas gp41 inhibitors are able to act postattachment. Unexpectedly, a significant fraction of antibodies in HIV-1-positive sera neutralized virus postattachment, suggesting that downstream fusion events and structures figure prominently in the host immune response. Overall, this disulfide-shackled virus is a unique tool with potential utility in vaccine design, drug discovery, and elucidation of the HIV-1 entry process.Human immunodeficiency virus type 1 (HIV-1) enters susceptible target cells via a complex cascade of receptor-mediated events. A fine characterization of this process is complicated by the transient nature of the lipid and protein rearrangements involved. The envelope glycoprotein (Env) is responsible for viral attachment and fusion. Env consists of noncovalently associated trimers of heterodimers comprising gp120 surface and gp41 transmembrane glycoproteins (29, 39). During infection, gp120 attaches to the CD4 receptor and undergoes conformational changes that enable coreceptor binding (39). This leads to further changes in gp41 (22) to form a six-helix bundle consisting of three alpha-helical hairpins (7, 48) and culminates in lipid mixing and membrane fusion.The study of HIV-1 entry and the Env conformations involved has provided a rich source of targets for a new generation of antiretroviral therapies (6,16,18). The most clinically advanced HIV-1 entry inhibitor, the peptide T-20 (also known as DP178), blocks fusion at nanomolar concentrations (49) by binding to a structure known as the gp41 prehairpin intermediate that becomes available during the fusion process (22).Env represents the primary target for the neutralizing antibody response. Successful vaccines against many viral infections elicit neutralizing antibodies (4) but have been difficult to elicit against HIV-1. The virus evades host immunity by exposing hypervariable and heavily glycosylated regions on gp120, while the conserved domains that bind its cellular receptors are located in recessed cavities (29, 39). As a result, only a few monoclonal antibodies (MAbs) against Env isolated to date are both potently and broadly neutralizing (5,9,33,34,39,41,44,45,52). Since conserved domains and potential targets for neutralizing antibodies may become exposed after receptor binding, fusion intermediates may find utility in vaccine research (12,13,20,26,33,37,51).Until now, HIV-1 fusion intermediates have been generated by incubating virus or Env-expressing cells with tar...

To assess the potential of native Envelope glycoprotein (Env) trimers as neutralizing antibody vaccines, we immunized guinea pigs with three types of VLPs and soluble gp120. Particles included "SOS-VLPs" (bearing disulfide-shackled functional trimers), "UNC-VLPs" (bearing uncleaved nonfunctional Env) and "naked VLPs" (bearing no Env). The SOS-VLPs were found to have a density of about 27 native trimers per particle, approximately twice that of live inactivated HIV-1 preparations. As immunogens, UNC- and SOS-VLP rapidly elicited anti-gp120 antibodies focused on the V3 loop and the gp120 coreceptor binding site. Reactivity to the gp41 immunodominant domain was absent in SOS-VLP sera, presumably because gp120-gp41 association is stabilized, effectively covering this epitope. Gp120-immune sera reacted with the receptor binding sites of gp120 and were less focused on the V3 loop. Some Env-VLP sera neutralized primary isolates at modest titers. The measurement of neutralization was found to be affected by the cell lines used. Depending on the assay particulars, non-Env specific antibodies in VLP sera could enhance infection, or nonspecifically neutralize. However, a neutralization assay using TZM-BL cells was essentially clear of these effects. We also describe a native trimer binding assay to confirm neutralization activity in a manner that completely eliminates nonspecific effects. Overall, our data suggests that Env-VLP sera were primarily focused on nonfunctional forms of Env on VLP surfaces, possibly gp120/gp41 monomers and not the trimers. Therefore, to make progress toward a more effective VLP-based vaccine, we will need to find ways to refocus the attention of B cells on native trimers.

No abstract

Prostate-specific membrane antigen (PSMA) is a type 2 integral membrane glycoprotein that serves as an attractive target for cancer immunotherapy by virtue of its abundant and restricted expression on the surface of prostate carcinomas and the neovasculature of most other solid tumors. However, relatively little is known about the molecular structure of this target. Here, we report that PSMA is expressed on tumor cells as a noncovalent homodimer. A truncated PSMA protein, lacking transmembrane and cytoplasmic domains, also formed homodimers, indicating that the extracellular domain is sufficient for dimerization. PSMA dimers but not monomers displayed a native conformation and possessed high-level carboxypeptidase activity. A unique dimerspecific epitope was identified by using one of a panel of novel mAbs. When used to immunize animals, dimer but not monomer elicited antibodies that efficiently recognized PSMA-expressing tumor cells. These findings on PSMA structure and biology may have important implications for active and passive immunotherapy of prostate and other cancers.