The indirect modulation of sensitivity by other regions of the envelope likely provides a means of viral escape from antibodies targeting conserved regions of the envelope (40), as well as affecting the sensitivity of heterologous viruses to such specificities. Unlike the V1 loop, which is likely to be structurally disordered and is very variable in length, the V2 loop contains a number of subdomains that are very conserved across all subtypes and shows a greater constraint around the minimum length tolerated (48). R166, D167, K169, and K171 (forming the FN/LRD-K-K motif) in the V2 region as crucial to the CAP256 epitope. However, the fine specificity of the BCN response varied over time and, while consistently dependent on R166 and K169, Rabbit Polyclonal to APC1 became gradually less dependent on D167 and K171, possibly contributing to the incremental increase in breadth over 4 years. The presence of an intact FN/LRD-K-K motif in heterologous viruses was associated with sensitivity, although the length of the adjacent V1 loop modulated the degree of sensitivity, with a shorter V1 region significantly associated with higher titers. Repair of the FN/LRD-K-K motif in resistant heterologous viruses conferred sensitivity, with titers sometimes exceeding 1:10,000. Comparison of the CAP256 epitope with that of the PG9/PG16 monoclonal antibodies suggested that these epitopes overlapped, adding to the mounting evidence that this may represent a common neutralization target that should be further investigated as a potential vaccine candidate. The development of an HIV-1 vaccine is usually widely assumed to require both a cellular and a neutralizing antibody (nAb) component. Despite substantial efforts to produce a vaccine immunogen capable of inducing broadly nAb responses, little success has been achieved (16,18,29). The inability to design an appropriate antigen is usually hindered by the enormous antigenic variability, as well as a myriad of masking mechanisms, exhibited by the envelope glycoprotein (5,19). The identification of new nAb targets around the HIV-1 envelope, as well as unraveling how antibodies to these targets develop in natural contamination, is usually therefore a high priority for HIV vaccinology. The majority of individuals mount a potent nAb response within the first few months of HIV-1 contamination that largely targets variable regions of the envelope glycoprotein and is very type specific (4,14,20,33,45). However, only a subset of infected people (20 to 30%) develop broadly cross-neutralizing (BCN) antibodies effective against diverse heterologous viruses (23,35,38,39). The concerted efforts by numerous groups to understand the polyclonal specificities of these BCN plasmas have defined the contributions of antibodies against known targets such as the CD4 binding site, CD4-induced epitopes, and the membrane-proximal region (MPER), as well as unknown targets on gp120 and gp41 (6,11,23,35,38,39). Identification of the targets of such BCN antibodies is crucial, as these epitopes likely represent novel vaccine targets that may be more amenable to Flunixin meglumine vaccine design than current targets, many of which have been defined on the basis of the specificities of a few well-characterized monoclonal antibodies (MAbs) (e.g., 4E10, 2F5, IgG1b12, and 2G12) (5). The presence of quaternary neutralizing epitopes (QNEs), those that are present only within the context of a native trimeric envelope spike, was first demonstrated by the isolation of MAb 2909, which binds an epitope encompassing parts of the V2 and V3 loops but does not bind monomeric gp120 (10,17). Similar neutralizing MAbs from rhesus macaques infected with simian/human immunodeficiency computer virus SF162P4 (2.2G, 2.3E, and 2.5B) target the same overall QNE, though each possesses distinct amino acid and glycan requirements (34). Although these antibodies are very potent, they are also extremely type specific, neutralizing only SF162. However, the isolation of MAbs Flunixin meglumine PG9 and PG16, somatic variants isolated from an HIV-1 subtype A-infected individual, which recognize QNEs and show substantial breadth and potency across subtypes, confirmed the notion that BCN antibodies against these types of epitopes exist and possibly define a valuable new vaccine target (43). The epitope recognized by PG9/PG16 is dependent around the N160 glycosylation motif in V2, as well as other residues in the stem of V1/V2, V2, and V3 (with PG16 showing much greater dependence on residues in V3 than PG9) (43). PG9/PG16-like antibodies are increasingly being defined as those that are not generally adsorbed by Flunixin meglumine gp120 and are sensitive to mutations at N160 and to treatment with kifunensine (a mannose analogue that inhibits -mannosidases in the type 1 endoplasmic reticulum and the Golgi apparatus) (7,44). The binding of such antibodies to a region largely described as highly variable confirms the fact that within V2 there are regions and residues that are highly conserved, even between different subtypes (48). Crystallographic and structure-function analyses of PG16 identified extensive affinity maturation and an unusually long complementarity-determining region (CDR) H3 (28 residues).
