The live attenuated simian immunodeficiency virus (LASIV) vaccine SIVnef is one of the most effective vaccines in inducing protection against wild-type lentiviral challenge, yet little is known about the mechanisms underlying its remarkable protective efficacy. depth in successful vaccine design. Author Summary Annually, more than two million people are infected NSC 74859 with HIV, the virus that causes AIDS. Due to the ability of the virus to escape host immune responses, designing a successful HIV vaccine has been elusive. Similar to HIV in humans, rhesus NSC 74859 macaques can be infected with SIV, a close relative and ancestor of Tlr4 HIV, resulting in simian AIDS. SIVnef, a live attenuated form of SIV, protects rhesus macaques from subsequent challenge with pathogenic SIV and is widely viewed as the most effective SIV vaccine. Here, we demonstrate that after vaccination of macaques with SIVnef, the immune response initially targets more variable regions of the virus, which the virus rapidly escapes. However, as the virus escapes, the immune response evolves to target more conserved regions of the virus as well as escape variants. This refocused targeting of conserved regions by the immune response provides a new mechanistic model that contributes to our understanding of how SIVnef vaccination protects animals from pathogenic challenge with SIV. Our findings also reinforce the importance of developing HIV vaccines that target conserved regions of the virus as well as their potential variants. Introduction Two decades ago, a report described a cohort of rhesus macaques infected with the live attenuated simian immunodeficiency virus (LASIV), SIVnef, and subsequently protected from pathogenic wild-type SIV challenge [1]. Since then, numerous studies have demonstrated the efficacy of SIVnef-induced protection, ranging from complete protection with sterilizing immunity to partial protection with two or more logs reduction in peak and set-point viremia [2C6]. Remarkably, presumably in part due to the replication of SIVnef in mucosal sites [7], robust protection has been also been documented following mucosal challenges [3C5]. SIVnef has also induced significant protection against heterologous challenge, albeit less effectively than against homologous challenge [4,5]. However, studies describing disease progression in SIVnef-vaccinated infant macaques and a subset of adult non-human primates precluded live attenuated HIV from being developed as a vaccine in human subjects [8C10]. Due to concerns over safety, research on SIVnef and related LASIV vaccines has shifted from safety-and-efficacy determination to mechanism-of-action delineation. As the most effective lentiviral vaccine, SIVnef has been extensively studied in order to shed light on the correlates of vaccine-mediated protection. However, no immunological correlate or mode of action has consistently been identified as being responsible for protection against pathogenic challenge. SIVnef generates a diverse SIV-specific antibody response [11,12] and macaques vaccinated with the related attenuated virus SIV3 and lacking the MHC I allele show effective control of pathogenic viral challenge despite CD8 T cell NSC 74859 depletion [13], implying that humoral immunity may play a significant role, at least in some genotypic backgrounds. Innate immunity has also been implicated as a correlate of SIVnef-induced protection [3]. SIVnef infection induces potent CD8 T cell responses similar in magnitude to wild-type SIV infection [14], and multiple studies have implicated SIV-specific CD8 T cells in the protection induced by SIVnef [4C6,15]. Most recently, Fukazawa et al. [6] correlated the magnitude of lymph node SIV-specific T cell responses with protection elicited by a range of different LASIV strains, including SIVnef. Finally, studies in a related vaccine challenge model involving vaccination.