Identifying settings to study the natural response to HIV can inform how to elicit a protective response through vaccination. HIV-1 superinfection (SI), defined as a second heterologous HIV-1 reinfection, is one such setting that enables studying two unique aspects of the immune response to HIV. Interrogating the pre-SI immune response enables examining how different components of the preexisting anti-HIV immunity impact susceptibility to reinfection, and analyzing the post-SI immune response allows us to examine how infection with multiple strains affects the immune response. Initial infection has recently been shown to be partially protective of SI, and identification of the immune correlates of SI is imperative to identify what specific immunological mechanisms mediate this protection and thus should be elicited with a vaccine. I propose to examine if neutralizing antibodies (NAbs) elicited by the initial infection are involved with this protection by investigating the relationship between pre-SI NAb activity and risk of SI using a case-control study. The post-SI immune response is a valuable surrogate for the response we hope to elicit with a vaccine composed of multiple diverse immunogens. NAb responses post-SI have been shown to broaden, yet they do not target known broadly NAb epitopes, suggesting SI elicits a broad polyclonal NAb response. To further characterize this poorly understood response, I propose to use deep mutational scanning of Env combined with high-throughput, quantitative neutralization-escape assays to comprehensively map the targets of post-SI NAbs in a donor that exhibits elite NAb activity.