cIdentifying immune correlates of protection is important to develop vaccines against infectious diseases. We designed a novel, universally applicable strategy to profile the antibody (Ab) repertoire of protected vaccine recipients, using recombinant phages encoding random peptide libraries. The new approach, termed "protection-linked (PL) biopanning," probes the Ab paratopes of protected vaccinees versus those with vaccine failure. As proof of concept, we screened plasma samples from vaccinated rhesus macaques (RMs) that had completely resisted multiple mucosal challenges with R5-tropic simian-human immunodeficiency viruses (SHIVs). The animals had been immunized with a multicomponent vaccine (multimeric HIV-1 gp160, HIV-1 Tat, and SIV Gag-Pol particles). After PL biopanning, we analyzed the phagotopes selected for amino acid homologies; in addition to the expected Env mimotopes, one recurring motif reflected the neutralizing Ab epitope at the N terminus (NT) of HIV-1 Tat. Subsequent binding and functional assays indicated that anti-Tat NT Abs were present only in completely or partially protected RMs; peak viremia of the latter was inversely correlated with anti-Tat NT Ab titers. In contrast, highly viremic, unvaccinated controls did not develop detectable Abs against the same epitope. Based upon the protective effect observed in vivo, we suggest that Tat should be included in multicomponent HIV-1 vaccines. Our data highlight the power of the new PL-biopanning strategy to identify Ab responses with significant association to vaccine protection, regardless of the mechanism(s) or targets of the protective Abs. PL biopanning is also unbiased with regard to pathogens or disease model, making it a universal tool.T he design of a safe, effective vaccine is desirable to control the AIDS pandemic. In order to improve future vaccine candidates, it is important to understand the protective mechanism(s) leading to reduced virus acquisition (1). Thus, correlates of infection risk for the recent RV144 vaccine efficacy trial (2) are the subject of intense study (3,4). However, additional information can be gained by dissecting immune responses in biologically relevant animal models where immunization induced complete protection from immunodeficiency virus challenge(s) (5, 6).Recent results of active (7,8) and passive (9) immunization trials in humans and in nonhuman primates suggested that antihuman immunodeficiency virus type 1 (HIV-1) Env antibodies (Abs) can have protective roles even in the absence of virus neutralization, implying that other Ab-mediated mechanisms may be involved in preventing mucosal virus acquisition. Thus, the ability to profile Ab epitopes associated with protection might help to identify potential targets for future HIV-1 vaccines.Random peptide phage display has been used to dissect Ab responses in the context of primate immunodeficiency virus infections, and several studies have identified HIV-1 or simian-human immunodeficiency virus (SHIV)-specific peptides (10-14). We developed a novel epitope...