The RV144 vaccine trial implicated epitopes in the C1 region of gp120 (A32-like epitopes) as targets of potentially protective antibody-dependent cellular cytotoxicity (ADCC) responses. determinants of ADCC potency, with the latter process having the greater impact. These studies provide atomic-level definition of BIX02188 A32-like epitopes implicated as targets of protective antibodies in RV144. Moreover, these studies establish that epitope structure and mode of antibody binding can dramatically affect the potency of Fc-mediated effector function against HIV-1. These results provide key insights for understanding, refining, and improving the outcome of HIV vaccine trials, in which relevant immune responses are facilitated by A32-like elicited responses. IMPORTANCE HIV-1 Env is a primary target for antibodies elicited during infection. Although a small number of infected individuals elicit broadly neutralizing antibodies, the bulk of the humoral response consists of antibodies that do not neutralize or do so with limited breadth but may effect protection through Fc receptor-dependent processes, such as antibody-dependent cellular cytotoxicity (ADCC). Understanding these nonneutralizing responses is an important aspect of elucidating the complete KLF10 spectrum of immune response against HIV-1 infection. With this report, we provide the first atomic-level definition of nonneutralizing CD4-induced epitopes in the N-terminal region of the BIX02188 HIV-1 gp120 (A32-like epitopes). Further, our studies point to the dominant role of precise epitope targeting and mode of antibody attachment in ADCC responses even when largely overlapping epitopes are involved. Such information provides key insights into the mechanisms of Fc-mediated function of antibodies to HIV-1 and will help us understand the outcome of vaccine trials based on humoral immunity. INTRODUCTION Antibodies contribute significantly to protection against HIV-1, but how they do so is only partially understood. Existing evidence suggests that protective antibody responses can involve neutralizing activity as well as Fc receptor-dependent processes, such as antibody-dependent cellular cytotoxicity (ADCC) (1,C10). A role of Fc-mediated effector function by nonneutralizing antibodies (nnAbs) in blocking HIV-1 BIX02188 acquisition is supported by vaccine trials in nonhuman primates (4, 11, 12) and humans (3, 13, 14), as well as by a breast milk transmission study of BIX02188 mother-infant pairs (2). In contrast, unlike an early study of passive immunization against simian immunodeficiency virus (SIV) using polyclonal sera (15, 16), more recent passive immunization studies using well-defined monoclonal antibodies (MAbs) showed no protection against acquisition (17, 18). Postinfection control of viremia was observed in both studies, suggesting that nnAbs can impact the transmitted virus (17, 18) without blocking acquisition. Postinfection control is often seen in nonhuman primate (NHP) models when protective levels of anti-retroviral drugs (19) or MAbs (20, 21) are too low to block acquisition. If postinfection control and blocking acquisition are a continuum of protection, there is reason to believe that nnAbs could protect against infection in NHPs with the right MAb(s) or vaccine. Thus, an understanding of Fc-mediated effector function, including the epitopes and mechanisms by which BIX02188 potent antibodies mediate ADCC, is critical for clarifying the role of nnAbs in protection. ADCC escape variants emerging in HIV-1-infected people (22) and ADCC responses correlating with reduced risk of infection in the RV144 vaccine trial (3, 13, 14) point to nonneutralizing epitopes in the C1 region of gp120 (A32-like epitopes) (23, 24) as relevant targets for potentially protective antibodies. The gp120 regions recognized by MAb A32 were also shown to be immunogenic during HIV-1 infection, as infected individuals frequently produce antibodies specific for these determinants (25,C27). Antibody titers, as measured by enzyme-linked immunosorbent assay (ELISA) against these epitopes, however, do not consistently correlate with protection (3). This discordance between ADCC, antibody-binding responses, and protection suggests that ADCC reactions to the A32-like epitopes (and ADCC epitopes in general) are governed by a mechanism(s) more complex than simply antibody binding. Here we.