Human immunodeficiency virus type 1 (HIV-1) spike is a trimeric complex of gp120-gp41 heterodi-mers. Gp120, a surface attachment protein, and gp41, the membrane-spinning protein, are non-covalently linked. They are initially produced as a single glycoprotein precursor gp160, which is cleaved by a cellular protease. Gp120 serves as a ligand for virus attachment and for interaction with receptor and co-receptor. By so doing, it determines HIV-1 tropism. Gp41 mediates fusion process between virus mem-brane and target cell membrane. By so doing, it facilitates the formation of fusion pores in target cell membrane, which in turn serves as a gateway for viral core to be delivered into cytosol of target cells. It has been shown that both gp120 and gp41 go through many conformational changes from virus attachment to target cells to the formation of fusion pore (42).
Gp120 and Gp41 not only play an essential role in virus entry, they are also major targets for antibody responses during natural infection and vaccination. Neutralizing antibodies block viral entry by reco-gnizing epitopes on the envelope spike critical for their interaction with receptor and co-receptors, or for the fusion process. Antibodies that neutralize a broad range of primary isolates of HIV-1 have been extremely difficult to generate. Despite almost two decades of effort, only a limited number of such antibodies have been identified. They include two antibodies 2F5 and 4E10 that direct against the membrane proximate region of gp41 (26, 27, 35, 45) and three antibodies 2G12, b12, and 447-52D that direct against gp120 (6-12). The antibody b12 interacts with the epitope that overlaps with CD4 binding site of gp120 (4). The antibody 2G12 recognizes cluster α1 to 2 linked mannose residues on the distal ends of oligomannose sugar located on the carbohydrate-covered silent face of gp120 (38). The antibody 447-52D recognizes gly-pro-gly-arg (GPGR) motif at the center of V3 loop (12). The GPGR motif is conserved among clade B viruses, but not non-clade B viruses (13). In non-clade B virus the arg residue is replaced by the gln residue (11). Data derived from the crystal structure of envelope-antibody complexes suggest that the neutralizing activity of these antibodies (except for 2G12 and 447-52D) is mediated by an unusually long CDR3 H3 loop that penetrates deeply into the antigen cleft, which is obscured in the heterotrimic envelope (30). Unfortunately, antibodies with such broadly neutrali-zing activity occur very rarely in patients and attempts to induce such antibody responses by immunization were unsuccessful (30). Thus, so far we still do not know how to induce such antibodies through rational immunogen design.
In contrast, nonneutralizing antibodies are almost always generated during natural infection or vacci-nation. Some have very high affinity, but fail to block viral entry, suggesting that nonneutralization epitopes are wither buried within the intact envelope spike or exposed but not critical for viral entry. By molecular modeling, Poignard et al. described that gp120 monomer exists three faces-neutralizing, nonneutrali-zing, and silent (31). The neutralizing face corres-ponds to the surface of gp120 trimer that interacts with its receptor and co-receptors. This face is exposed at the surface of the intact trimeric envelope spike and conserved. The silent face is heavily glycosylated and does not elicit antibody responses, though it is well exposed at the surface of the intact trimeric envelope spike. The non-neutralizing face elicits strong antibody response. However, since this face is buried within the intact trimeric envelope spike, antibodies that bind to this face do not bind to the intact spike on virions and have no neutralizing activity. This model also implies that the nonneu-tralizing antibodies to gp120 are elicited by either monomeric gp120 shed from the virions or infected cells or by gp160 precursor proteins found in the debris of dying HIV-1-infected cells. Since no similar modeling has been done for gp41, we do not know the structural basis of neutralization versus nonneu-tralization epitopes in gp41. A few studies have shown that most of the epitopes of gp41 are buried underneath the gp120 trimer in the intact envelope spikes (6, 18, 33, 40, 42), with notable exception of epitopes in cluster Ⅰ determinant (28) and 2F5 (26, 27) and 4E10 epitopes (35, 45). Interaction of gp120 with CD4 causes conformational changes of the envelope spikes, resulting in exposure of some of these hidden epitopes and making them accessible to antibody binding (34). From an evolution point of view, because non-neutralizing antibodies do not exert selective pressure on virus survival, non-neutralization epitopes are usually more conserved than neutralization epitopes.
Historically, whether an antibody has neutralizing activity is determined using "conventional" neutrali-zation assay in vitro. The assay uses phytohemag-glutinin (PHA)-stimulated human PBMCs as target cells. Thus, in essence it evaluates inhibitory effect of antibody on HIV-1 replication in CD4+ T cells.However, recently, several laboratories inclu-ding us demonstrated that some antibodies and polyclonal IgG purified from patient sera, although they do not have neutralizing activity when tested by the "conventi-onal" neutralization assay, do exhibit potent neutra-lizing activity in "unconventional" ways (7, 8, 15-17, 19, 20, 23, 41). The neutralizing activity of these antibodies and purified polyclonal IgG is acquired through post-translational modifications (7, 8, 23), through opsonization of virus particles into macrophages and immature dendritic cells (iDCs) (15-17, 41), or through expression of antibodies on the surface of HIV-1-susceptible cells (19, 20). These findings not only shed new light on our understanding neutralizing activity of antibodies against HIV, but also high-lights potential applications in the develop-ment of preventive strategies against HIV. Thus, our following review will focus on some of recent findings of the "unconventional" neutralizing activity of these antibodies and purified polyclonal IgG.
"Unconventional" Neutralizing Activity of Antibodies Against HIV
- Received Date: 03 July 2007
- Accepted Date: 25 September 2007
Abstract: Neutralizing antibodies are recognized to be one of the essential elements of the adaptive immune response that must be induced by an effective vaccine against HIV. However, only a limited number of antibodies have been identified to neutralize a broad range of primary isolates of HIV-1 and attempts to induce such antibodies by immunization were unsuccessful. The difficulties to generate such antibodies are mainly due to intrinsic properties of HIV-1 envelope spikes, such as high sequence diversity, heavy glycosylation, and inducible and transient nature of certain epitopes. In vitro neutralizing antibodies are identified using “conventional” neutralization assay which uses phytohe- magglutinin (PHA)-stimulated human PBMCs as target cells. Thus, in essence the assay evaluates HIV-1 replication in CD4+ T cells. Recently, several laboratories including us demonstrated that some monoclonal antibodies and HIV-1-specific polyclonal IgG purified from patient sera, although they do not have neutralizing activity when tested by the “conventional” neutralization assay, do exhibit potent and broad neutralizing activity in “unconventional” ways. The neutralizing activity of these antibodies and IgG fractions is acquired through post-translational modifications, through opsonization of virus particles into macrophages and immature dendritic cells (iDCs), or through expression of antibodies on the surface of HIV-1-susceptible cells. This review will focus on recent findings of this area and point out their potential applications in the development of preventive strategies against HIV.