Difficulties in eliciting broadly neutralizing anti-HIV antibodies are not explained by cardiolipin autoreactivity

Antigenic mimicry of the HIV envelope by AIDS-associated pathogens

Only one broadly neutralizing anti-HIV antibody, 2G12, recognises the envelope sugars of HIV. In the present study, we show that 2G12 also recognises Candida albicans and Candida tropicalis with high affinity (11 nmol/l) through a carbohydrate-dependent interaction (50% inhibitory concentration for D-fructose, 12 mmol/l). This is the first report of a neutralizing HIV antibody displaying cross-reactivity with another pathogen, revealing that the carbohydrate neutralization determinant of HIV, defined by 2G12, is more widespread amongst immunogenic, microbial surfaces than previously recognized.

The broadly neutralizing anti-human immunodeficiency virus type 1 4E10 monoclonal antibody is better adapted to membrane-bound epitope recognition and blocking than 2F5

The broadly neutralizing 2F5 and 4E10 monoclonal antibodies (MAbs) recognize epitopes within the membrane-proximal external region (MPER) that connects the human immunodeficiency virus type 1 (HIV-1) envelope gp41 ectodomain with the transmembrane anchor. By adopting different conformations that stably insert into the virion external membrane interface, such as helical structures, a conserved aromatic-rich sequence within the MPER is thought to participate in HIV-1-cell fusion. Recent experimental evidence suggests that the neutralizing activity of 2F5 and 4E10 might correlate with the MAbs' capacity to recognize epitopes inserted into the viral membrane, thereby impairing MPER fusogenic activity. To gain new insights into the molecular mechanism underlying viral neutralization by these antibodies, we have compared the capacities of 2F5 and 4E10 to block the membrane-disorganizing activity of MPER peptides inserted into the surface bilayer of solution-diffusing unilamellar vesicles. Both MAbs inhibited leakage of vesicular aqueous contents (membrane permeabilization) and intervesicular lipid mixing (membrane fusion) promoted by MPER-derived peptides. Thus, our data support the idea that antibody binding to a membrane-inserted epitope may interfere with the function of the MPER during gp41-induced fusion. Antibody insertion into a cholesterol-containing, uncharged virion-like membrane is mediated by specific epitope recognition, and moreover, partitioning-coupled folding into a helix reduces the efficiency of 2F5 MAb binding to its epitope in the membrane. We conclude that the capacity to interfere with the membrane activity of conserved MPER sequences is best correlated with the broad neutralization of the 4E10 MAb.

Antibody elicited against the gp41 N-heptad repeat (NHR) coiled-coil can neutralize HIV-1 with modest potency but non-neutralizing antibodies also bind to NHR mimetics

Following CD4 receptor binding to the HIV-1 envelope spike (Env), the conserved N-heptad repeat (NHR) region of gp41 forms a coiled-coil that is a precursor to the fusion reaction. Although it has been a target of drug and vaccine design, there are few monoclonal antibody (mAb) tools with which to probe the antigenicity and immunogenicity specifically of the NHR coiled-coil. Here, we have rescued HIV-1-neutralizing anti-NHR mAbs from immune phage display libraries that were prepared (i) from b9 rabbits immunized with a previously described mimetic of the NHR coiled-coil, N35(CCG)-N13, and (ii) from an HIV-1 infected individual. We describe a rabbit single-chain Fv fragment (scFv), 8K8, and a human Fab, DN9, which specifically recognize NHR coiled-coils that are unoccupied by peptide corresponding to the C-heptad repeat or CHR region of gp41 (e.g. C34). The epitopes of 8K8 and DN9 were found to partially overlap with that of a previously described anti-NHR mAb, IgG D5; however, 8K8 and DN9 were much more specific than D5 for unoccupied NHR trimers. The mAbs, including a whole IgG 8K8 molecule, neutralized primary HIV-1 of clades B and C in a pseudotyped virus assay with comparable, albeit relatively modest potency. Finally, a human Fab T3 and a rabbit serum (both non-neutralizing) were able to block binding of D5 and 8K8 to a gp41 NHR mimetic, respectively, but not the neutralizing activity of these mAbs. We conclude from these results that NHR coiled-coil analogs of HIV-1 gp41 elicit many Abs during natural infection and through immunization, but that due to limited accessibility to the corresponding region on fusogenic gp41 few can neutralize. Caution is therefore required in targeting the NHR for vaccine design. Nevertheless, the mAb panel may be useful as tools for elucidating access restrictions to the NHR of gp41 and in designing potential improvements to mimetics of receptor-activated Env.

Cross-reactive human immunodeficiency virus type 1-neutralizing human monoclonal antibody that recognizes a novel conformational epitope on gp41 and lacks reactivity against self-antigens

Broadly cross-reactive human immunodeficiency virus (HIV)-neutralizing antibodies are infrequently elicited in infected humans. The two best-characterized gp41-specific cross-reactive neutralizing human monoclonal antibodies, 4E10 and 2F5, target linear epitopes in the membrane-proximal external region (MPER) and bind to cardiolipin and several other autoantigens. It has been hypothesized that, because of such reactivity to self-antigens, elicitation of 2F5 and 4E10 and similar antibodies by vaccine immunogens based on the MPER could be affected by tolerance mechanisms. Here, we report the identification and characterization of a novel anti-gp41 monoclonal antibody, designated m44, which neutralized most of the 22 HIV type 1 (HIV-1) primary isolates from different clades tested in assays based on infection of peripheral blood mononuclear cells by replication-competent virus but did not bind to cardiolipin and phosphatidylserine in an enzyme-linked immunosorbent assay and a Biacore assay nor to any protein or DNA autoantigens tested in Luminex assays. m44 bound to membrane-associated HIV-1 envelope glycoproteins (Envs), to recombinant Envs lacking the transmembrane domain and cytoplasmic tail (gp140s), and to gp41 structures containing five-helix bundles and six-helix bundles, but not to N-heptad repeat trimers, suggesting that the C-heptad repeat is involved in m44 binding. In contrast to 2F5, 4E10, and Z13, m44 did not bind to any significant degree to denatured gp140 and linear peptides derived from gp41, suggesting a conformational nature of the epitope. This is the first report of a gp41-specific cross-reactive HIV-1-neutralizing human antibody that does not have detectable reactivity to autoantigens. Its novel conserved conformational epitope on gp41 could be helpful in the design of vaccine immunogens and as a target for therapeutics.

Assessment of antibody responses against gp41 in HIV-1-infected patients using soluble gp41 fusion proteins and peptides derived from M group consensus envelope

Human immunodeficiency virus type 1 (HIV-1) transmembrane glycoprotein gp41 is targeted by broadly-reactive neutralizing antibodies 2F5 and 4E10, making it an attractive target for vaccine development. To better assess immunogenic properties of gp41, we generated five soluble glutathione S-transferase fusion proteins encompassing C-terminal 30, 64, 100, 142, or 172 (full-length) amino acids of gp41 ectodomain from M group consensus envelope sequence. Antibody responses in HIV-1-infected patients were evaluated using these proteins and overlapping peptides. We found (i) antibody responses against different regions of gp41 varied tremendously among individual patients, (ii) patients with stronger antibody responses against membrane-proximal external region exhibit broader and more potent neutralizing activity, and (iii) several patients mounted antibodies against epitopes that are near, or overlap with, those targeted by 2F5 or 4E10. These soluble gp41 fusion proteins could be an important source of antigens for future vaccine development efforts.

Lipid A and liposomes containing lipid A as antigens and adjuvants

Lipid A derived from Gram-negative bacterial lipopolysaccharide is a potent adjuvant and antigen. Incorporation of lipid A into liposomes renders the liposomes themselves immunogenic, resulting in generation of specific antibodies that recognize either the individual liposomal lipids, or the unique pattern presented by the combination of lipids. Using liposomes containing lipid A, numerous polyclonal antisera and monoclonal antibodies have been produced against phospholipids, cholesterol, glycosphingolipids, and lipid A. Many of these antibodies have binding characteristics that are apparently similar to natural antibodies that are normally present in all human sera, and also antibodies that arise in response to various infections. Such antibodies probably represent a bridge between innate and adaptive immunity. The possible utility of liposomes containing lipid A as a constituent of certain types of novel vaccines was suggested by the observation that murine monoclonal antibodies to liposomal phosphatidylinositol-4-phosphate neutralized primary isolates of two different clades of HIV-1 in a human peripheral blood mononuclear cell neutralization assay.