PROJECT SUMMARY Although the advent of combination antiretroviral therapy (cART) has dramatically improved the prognosis of people living with human immunodeficiency virus (HIV), cART alone cannot eradicate the infection and, therefore, daily treatment must be maintained for life to prevent relapse of uncontrolled viral replication and resumption of disease progression. However, lifelong treatment entails both health risks to treated individuals and a significant economic burden to society. As such, there is a pressing need to develop novel therapeutic interventions to cure HIV. Since most examples of stringent, long-term, spontaneous and vaccine-associated immune control of HIV and simian immunodeficiency virus (SIV) infection are either known, or strongly suspected to be CD8+ T cell-mediated, therapeutic strategies designed to exploit CD8+ T cell immunity hold great promise for achieving durable control of virus replication in the absence of cART, often referred to as a “functional cure”. However, in most people, naturally occurring CD8+ T cell responses induced by HIV infection are often ineffective at controlling the virus. As such, any HIV cure strategy based on enhancing CD8+ T cell immunity would need to elicit immune responses that are qualitatively and/or quantitatively different from those that emerge during primary infection and are subsequently maintained during cART. In addition, in the setting of cure where the rebound-competent viral reservoir is systemically distributed, having high frequencies of effector- differentiated and functionally potent anti-viral CD8+ T cell responses pre-positioned in sites of potential viral rebound (even in immune privileged sites such as B cell follicles) at the time of cART cessation is likely to be critical for achieving durable post-cART viral control. In this project, we will determine whether a therapeutic vaccination strategy that utilizes the messenger ribonucleic acid (mRNA)-based vaccine platform RNActive, can enhance cellular immunity in SIV-infected rhesus macaques (RM) on cART and establish high-level, long- term control of SIV replication after cART cessation. The choice of this vector is based on preliminary data demonstrating RNActivevaccines with SIV gene inserts (mRNA/SIV) are highly immunogenic in RM, with the capacity to elicit potent, systemically distributed, SIV-specific CD8+ T cells with broad epitope recognition. Here we will assess whether mRNA/SIV vaccination alone or in combination with anti-CD20 B cell depletion (to disrupt B cell follicles) can facilitate immediate interception of rebounding viral reservoirs to facilitate durable control of SIV replication after cART cessation. Any finding of vaccine efficacy in this project will provide strong impetus for clinical assessment of the RNActiveplatform in cART-suppressed HIV+ patients and potentially lead to a clinically translatable therapeutic approach to achieve HIV infection remission off cART.