A safe and effective vaccine for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may be required to end the coronavirus disease 2019 (COVID-19) pandemic 1 – 8 . For global deployment and pandemic control, a vaccine that requires only a single immunization would be optimal. Here we show the immunogenicity and protective efficacy of a single dose of adenovirus serotype 26 (Ad26) vector-based vaccines expressing the SARS-CoV-2 spike (S) protein in nonhuman primates. 52 rhesus macaques were immunized with Ad26 vectors encoding S variants or sham control and were challenged with SARS-CoV-2 by the intranasal and intratracheal routes 9 , 10 . The optimal Ad26 vaccine induced robust neutralizing antibody responses and provided complete or near-complete protection in bronchoalveolar lavage and nasal swabs following SARS-CoV-2 challenge. Vaccine-elicited neutralizing antibody titers correlated with protective efficacy, suggesting an immune correlate of protection. These data demonstrate robust single-shot vaccine protection against SARS-CoV-2 in nonhuman primates. The optimal Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in clinical trials.

Natural killer (NK) cells contribute to control of HIV/SIV infection. We defined macaque NK-cell subsets based on expression of CD56 and CD16 and found their distribution to be highly disparate. CD16 ؉ NK cells predominated in peripheral blood, whereas most mucosal NK cells were CD56 ؉ , and lymph nodes contained both CD56 ؉ and CD16 ؊ CD56 ؊ (doublenegative [DN]) subsets. Functional profiles were also distinct among subsets-CD16 ؉ NK cells expressed high levels of cytolytic molecules, and CD56 ؉ NK cells were predominantly cytokine-secreting cells, whereas DN NK possessed both functions. In macaques chronically infected with SIV, circulating CD16 ؉ and DN NK cells were expanded in number and, although markers of cytoxicity increased, cytokine secretion decreased. Notably, CD56 ؉ NK cells in SIV-infected animals up-regulated perforin, granzyme B, and CD107a. In contrast, the lymph nodehoming molecules CD62 ligand (CD62L) and C-C chemokine receptor type 7 (CCR7), which are expressed primarily on CD56 ؉ and DN NK cells, were significantly down-regulated on NK cells from infected animals. These data demonstrate that SIV infection drives a shift in NK-cell function characterized by decreased cytokine production, expanded cytotoxicity, and trafficking away from secondary lymphoid organs, suggesting that the NK-cell repertoire is not only heterogeneous but also plastic. IntroductionSince their discovery in the 1970s, natural killer (NK) cells have been considered the major effector cells of the innate immune system because of their ability to kill virus-infected or neoplastic cells. Although NK cell-mediated killing does not require prior antigen sensitization, cell-to-cell contact between NK and target T cells occurs through a complex array of inhibitory and activating receptors. In humans, NK cells express both killer-cell immunoglobulin-like receptors (KIRs), which interact with major histocompatibility complex (MHC) class I molecules and can be either inhibitory or activating, and receptors belonging to the C-type lectin family such as natural killer group 2A (NKG2A), an inhibitory receptor that recognizes HLA-E and NKG2D, which recognizes the stress-induced ligands MHC class I chain-related gene A and B (MICA/MICB) and members of the ULBP family. 1 Human NK cells also express various natural cytotoxicity receptors including NKp46, NKp30, and NKp44, for which the ligands remain incompletely characterized. 2,3 However, increasing evidence suggests that the complexity of NK-cell function has been underappreciated and that in addition to cytolysis of aberrant T cells, NK cells also produce a wide array of cytokines, mediate tolerance to self-antigens, and regulate dendritic cell functions. 4 Most recently, murine studies have suggested that NK cells may even display characteristics of adaptive immune responses. 5 In humans, 2 primary phenotypically defined subsets of NK cells have been described, cytolytic CD56 dim CD16 ϩ and cytokinesecreting CD56 bright CD16 Ϫ subsets, of which the CD56 dim CD16 ϩ subset predomina...

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Natural killer (NK) cells play an essential role in antiviral immunity, but knowledge of their function in secondary lymphoid organs is incomplete. Lymph node follicles constitute a major viral reservoir during infections with HIV-1 and simian immunodeficiency virus of macaques (SIVmac). In contrast, during nonpathogenic infection with SIV from African green monkeys (SIVagm), follicles remain generally virus free. We show that NK cells in secondary lymphoid organs from chronically SIVagm-infected African green monkeys (AGMs) were frequently CXCR5 and entered and persisted in lymph node follicles throughout the follow-up (240 d post-infection). These follicles were strongly positive for IL-15, which was primarily presented in its membrane-bound form by follicular dendritic cells. NK cell depletion through treatment with anti-IL-15 monoclonal antibody during chronic SIVagm infection resulted in high viral replication rates in follicles and the T cell zone and increased viral DNA in lymph nodes. Our data suggest that, in nonpathogenic SIV infection, NK cells migrate into follicles and play a major role in viral reservoir control in lymph nodes.

Live-attenuated strains of simian immunodeficiency virus (SIV) routinely confer apparent sterilizing immunity against pathogenic SIV challenge in rhesus macaques. Understanding the mechanisms of protection by live-attenuated SIV may provide important insights into the immune responses needed for protection against HIV-1. Here we investigated the development of antibodies that are functional against neutralization-resistant SIV challenge strains, and tested the hypothesis that these antibodies are associated with protection. In the absence of detectable neutralizing antibodies, Env-specific antibody-dependent cell-mediated cytotoxicity (ADCC) emerged by three weeks after inoculation with SIVΔnef, increased progressively over time, and was proportional to SIVΔnef replication. Persistent infection with SIVΔnef elicited significantly higher ADCC titers than immunization with a non-persistent SIV strain that is limited to a single cycle of infection. ADCC titers were higher against viruses matched to the vaccine strain in Env, but were measurable against viruses expressing heterologous Env proteins. In two separate experiments, which took advantage of either the strain-specificity or the time-dependent maturation of immunity to overcome complete protection against SIVmac251 challenge, measures of ADCC activity were higher among the SIVΔnef-inoculated macaques that remained uninfected than among those that became infected. These observations show that features of the antibody response elicited by SIVΔnef are consistent with hallmarks of protection by live-attenuated SIV, and reveal an association between Env-specific antibodies that direct ADCC and apparent sterilizing protection by SIVΔnef.