Structure of the Fab Fragment of F105, a Broadly Reactive Anti-Human Immunodeficiency Virus (HIV) Antibody That Recognizes the CD4 Binding Site of HIV Type 1 gp120

A key step in the development of a successful vaccine against human immunodeficiency virus (HIV) will be the design of immunogens capable of generating an effective humoral immune response (3). Antibodies with two features characterize such a response. They must show, at the same time, both potency and broad specificity. A number of human antibodies that recognize elements of the conserved CD4 binding site of HIV type 1 (HIV-1) gp120 have now been isolated from HIV-infected patients (2, 11, 16, 24, 38, 40, 47, 49, 59, 65). This includes immunoglobulin G1 (IgG1) b12 (38, 47, 53), one of the most potent and broadly reactive anti-HIV antibodies known. In contrast, other CD4 binding-site antibodies are less potent towards many clinical isolates, even though they may be broadly cross-reactive. F105, the subject of this paper, is representative of the latter group. F105 is an IgG1 κ human monoclonal antibody isolated from an HIV-infected individual (49). It binds to the CD4 binding sites of both trimeric and monomeric gp120 and is capable of neutralizing various strains of HIV (e.g., IIIB [HXBc2], MN, RF, and SF2) (7, 49, 58) but is less successful against many primary clinical isolates (12, 32). F105 did not show evidence of anti-HIV-1 activity or a viral load decrease in a phase I dose-escalation study (5, 70). However, in triple and quadruple combination therapies with anti-HIV monoclonal antibodies (2F5, 2G12, and 694/98D) with other specificities, a complete and synergistic neutralization of the SHIV-Vpu+ chimeric simian-human immunodeficiency virus was seen in macaque peripheral blood mononuclear cells in vitro and in an in vivo macaque model that mimics mucosal exposure during intrapartum virus transmission (1, 31). Crystallographic studies of the ternary complex of the HIV gp120 core, CD4, and antibody 17b provided the first look at the structure of gp120 and its interactions with CD4 (26-28, 65). CD4 was found to bind at the nexus of the inner domain, the outer domain, and the bridging sheet of gp120 (26, 27). A large body of biochemical and biophysical data indicates a considerable conformational change in gp120 upon binding to CD4 (4, 6, 15, 24, 26, 27, 39, 41, 50, 55, 60, 62-69, 72, 73). The conformational change results in the formation and/or exposure of the chemokine receptor sites (62, 64), thus promoting further viral attachment and membrane fusion. The molecular reorganization that results upon binding of CD4 is revealed by the structure of an unliganded simian immunodeficiency virus (SIV) gp120 core (6). With a few important exceptions, the structure of the outer domain is quite similar to that seen in the CD4-bound state. In contrast, the structure of the inner domain is markedly different. A comparison of CD4-bound and unliganded gp120 shows that the conformational change is not a simple movement of the inner domain as a rigid body. Rather, the inner domain is comprised of a set of distinct substructures that move relatively independently of one another (6). The binding of CD4 results in a rearrangement of these secondary structural elements within the inner domain (6). As predicted (24), the bridging sheet is not present in the structure of unliganded gp120 (6). The structure of antibody b12 was determined by Saphire et al. (52-54). The structure revealed an extended CDR H3 loop with an apical tryptophan residue that is thought to recognize the Phe43 binding pocket of gp120 (53, 75). This putative interaction places significant constraints on possible gp120/b12 interactions, allowing an interaction between b12 and the CD4-bound conformation of gp120 to be modeled (53, 75). This model suggests that the broad neutralizing activity of b12 lies in its ability to interact with conserved features of the CD4 binding site through recognition of the Phe43 pocket and interactions with main chain atoms of gp120 and in its ability to recognize trimeric gp120 on the native viral surface. A further understanding of the molecular properties that confer broad reactivity and potency upon a CD4 binding-site antibody are of substantial interest. The underlying principles are likely to impact the design of new immunogens with the potential to elicit a successful immune response. In this regard, the structure of F105 provides an opportunity to compare and contrast the structure of a broadly reactive but nonpotent CD4 binding-site antibody (F105) with that of a broadly neutralizing antibody with an overlapping epitope (b12). The comparison provides significant insight into the molecular properties that impart broad reactivity and potency upon a CD4 binding-site antibody.