Simian Immunodeficiency Virus Challenge Research Articles

Pre-challenge gut microbial signature predicts RhCMV/SIV vaccine efficacy in rhesus macaques.

Rhesus cytomegalovirus expressing simian immunodeficiency virus (RhCMV/SIV) vaccines protect ~59% of vaccinated rhesus macaques against repeated limiting-dose intra-rectal exposure with highly pathogenic SIVmac239M, but the exact mechanism responsible for the vaccine efficacy is unknown. It is becoming evident that complex interactions exist between gut microbiota and the host immune system. Here, we aimed to investigate if the rhesus gut microbiome impacts RhCMV/SIV vaccine-induced protection. Three groups of 15 rhesus macaques naturally pre-exposed to RhCMV were vaccinated with RhCMV/SIV vaccines. Rectal swabs were collected longitudinally both before SIV challenge (after vaccination) and post-challenge and were profiled using 16S rRNA based microbiome analysis. We identified ~2,400 16S rRNA amplicon sequence variants (ASVs), representing potential bacterial species/strains. Global gut microbial profiles were strongly associated with each of the three vaccination groups, and all animals tended to maintain consistent profiles throughout the pre-challenge phase. Despite vaccination group differences, by using newly developed compositional data analysis techniques, we identified a common gut microbial signature predictive of vaccine protection outcome across the three vaccination groups. Part of this microbial signature persisted even after SIV challenge. We also observed a strong correlation between this microbial signature and an early signature derived from whole blood transcriptomes in the same animals. Our findings indicate that changes in gut microbiomes are associated with RhCMV/SIV vaccine-induced protection and early host response to vaccination in rhesus macaques.IMPORTANCEThe human immunodeficiency virus (HIV) has infected millions of people worldwide. Unfortunately, still there is no vaccine that can prevent or treat HIV infection. A promising pre-clinical HIV vaccine based on rhesus cytomegalovirus (RhCMV) expressing simian immunodeficiency virus (SIV) antigens (RhCMV/SIV) provides sustained, durable protection against SIV challenge in ~59% of vaccinated rhesus macaques. There is an urgent need to understand the cause of this protection vs non-protection outcome. In this study, we profiled the gut microbiomes of 45 RhCMV/SIV vaccinated rhesus macaques and identified gut microbial signatures that were predictive of RhCMV/SIV vaccination groups and vaccine protection outcomes. These vaccine protection-associated microbial features were significantly correlated with early vaccine-induced host immune signatures in whole blood from the same animals. These findings show that the gut microbiome may be involved in RhCMV/SIV vaccine-induced protection, warranting further research into the impact of the gut microbiome in human vaccine trials.

Statistical design and analysis of controlled human malaria infection trials

BackgroundMalaria is a potentially life-threatening disease caused by Plasmodium protozoa transmitted by infected Anopheles mosquitoes. Controlled human malaria infection (CHMI) trials are used to assess the efficacy of interventions for malaria elimination. The operating characteristics of statistical methods for assessing the ability of interventions to protect individuals from malaria is uncertain in small CHMI studies. This paper presents simulation studies comparing the performance of a variety of statistical methods for assessing efficacy of intervention in CHMI trials.MethodsTwo types of CHMI designs were investigated: the commonly used single high-dose design (SHD) and the repeated low-dose design (RLD), motivated by simian immunodeficiency virus (SIV) challenge studies. In the context of SHD, the primary efficacy endpoint is typically time to infection. Using a continuous time survival model, five statistical tests for assessing the extent to which an intervention confers partial or full protection under single dose CHMI designs were evaluated. For RLD, the primary efficacy endpoint is typically the binary infection status after a specific number of challenges. A discrete time survival model was used to study the characteristics of RLD versus SHD challenge studies.ResultsIn a SHD study with the continuous time survival model, log-rank test and t-test are the most powerful and provide more interpretable results than Wilcoxon rank-sum tests and Lachenbruch tests, while the likelihood ratio test is uniformly most powerful but requires knowledge of the underlying probability model. In the discrete time survival model setting, SHDs are more powerful for assessing the efficacy of an intervention to prevent infection than RLDs. However, additional information can be inferred from RLD challenge designs, particularly using a likelihood ratio test.ConclusionsDifferent statistical methods can be used to analyze controlled human malaria infection (CHMI) experiments, and the choice of method depends on the specific characteristics of the experiment, such as the sample size allocation between the control and intervention groups, and the nature of the intervention. The simulation results provide guidance for the trade off in statistical power when choosing between different statistical methods and study designs.

Iterative hypothesis testing in HIV vaccine research: moving towards success.

Iterative hypothesis testing in HIV vaccine research: moving towards success.

Effect of Passive Administration of Monoclonal Antibodies Recognizing Simian Immunodeficiency Virus (SIV) V2 in CH59-Like Coil/Helical or β-Sheet Conformations on Time of SIVmac251 Acquisition.

The monoclonal antibodies (MAbs) NCI05 and NCI09, isolated from a vaccinated macaque that was protected from multiple simian immunodeficiency virus (SIV) challenges, both target an overlapping, conformationally dynamic epitope in SIV envelope variable region 2 (V2). Here, we show that NCI05 recognizes a CH59-like coil/helical epitope, whereas NCI09 recognizes a β-hairpin linear epitope. In vitro, NCI05 and, to a lesser extent, NCI09 mediate the killing of SIV-infected cells in a CD4-dependent manner. Compared to NCI05, NCI09 mediates higher titers of antibody-dependent cellular cytotoxicity (ADCC) to gp120-coated cells, as well as higher levels of trogocytosis, a monocyte function that contributes to immune evasion. We also found that passive administration of NCI05 or NCI09 to macaques did not affect the risk of SIVmac251 acquisition compared to controls, demonstrating that these anti-V2 antibodies alone are not protective. However, NCI05 but not NCI09 mucosal levels strongly correlated with delayed SIVmac251 acquisition, and functional and structural data suggest that NCI05 targets a transient state of the viral spike apex that is partially opened, compared to its prefusion-closed conformation. IMPORTANCE Studies suggest that the protection against SIV/simian-human immunodeficiency virus (SHIV) acquisition afforded by the SIV/HIV V1 deletion-containing envelope immunogens, delivered by the DNA/ALVAC vaccine platform, requires multiple innate and adaptive host responses. Anti-inflammatory macrophages and tolerogenic dendritic cells (DC-10), together with CD14+ efferocytes, are consistently found to correlate with a vaccine-induced decrease in the risk of SIV/SHIV acquisition. Similarly, V2-specific antibody responses mediating ADCC, Th1 and Th2 cells expressing no or low levels of CCR5, and envelope-specific NKp44+ cells producing interleukin 17 (IL-17) also are reproducible correlates of decreased risk of virus acquisition. We focused on the function and the antiviral potential of two monoclonal antibodies (NCI05 and NCI09) isolated from vaccinated animals that differ in antiviral function in vitro and recognize V2 in a linear (NCI09) or coil/helical (NCI05) conformation. We demonstrate that NCI05, but not NCI09, delays SIVmac251 acquisition, highlighting the complexity of antibody responses to V2.

Myeloid cell tropism enables MHC-E-restricted CD8+ T cell priming and vaccine efficacy by the RhCMV/SIV vaccine.

The strain 68-1 rhesus cytomegalovirus (RhCMV)-based vaccine for simian immunodeficiency virus (SIV) can stringently protect rhesus macaques (RMs) from SIV challenge by arresting viral replication early in primary infection. This vaccine elicits unconventional SIV-specific CD8+ T cells that recognize epitopes presented by major histocompatibility complex (MHC)-II and MHC-E instead of MHC-Ia. Although RhCMV/SIV vaccines based on strains that only elicit MHC-II- and/or MHC-Ia-restricted CD8+ T cells do not protect against SIV, it remains unclear whether MHC-E-restricted T cells are directly responsible for protection and whether these responses can be separated from the MHC-II-restricted component. Using host microRNA (miR)-mediated vector tropism restriction, we show that the priming of MHC-II and MHC-E epitope-targeted responses depended on vector infection of different nonoverlapping cell types in RMs. Selective inhibition of RhCMV infection in myeloid cells with miR-142-mediated tropism restriction eliminated MHC-E epitope-targeted CD8+ T cell priming, yielding an exclusively MHC-II epitope-targeted response. Inhibition with the endothelial cell-selective miR-126 eliminated MHC-II epitope-targeted CD8+ T cell priming, yielding an exclusively MHC-E epitope-targeted response. Dual miR-142 + miR-126-mediated tropism restriction reverted CD8+ T cell responses back to conventional MHC-Ia epitope targeting. Although the magnitude and differentiation state of these CD8+ T cell responses were generally similar, only the vectors programmed to elicit MHC-E-restricted CD8+ T cell responses provided protection against SIV challenge, directly demonstrating the essential role of these responses in RhCMV/SIV vaccine efficacy.

RhCMV/SIV tropism modulation programs unconventional CD8+ T cell priming and vaccine efficacy

Abstract Strain 68-1 rhesus cytomegalovirus (RhCMV) vectors expressing simian immunodeficiency virus (SIV) antigens demonstrate a vaccine efficacy where 50–60% of vaccinated rhesus macaques are protected from SIV challenge. Intriguingly, RhCMV/SIV vectors elicit CD8+ T cells recognizing epitopes presented by MHC-II and MHC-E instead of MHC-Ia. We are studying how these unconventional T cell responses are elicited and contribute to the efficacy against SIV challenge. Here we utilize host microRNA (miRNA)-mediated vector tropism restriction to show that MHC-II- and MHC-E-restricted responses are primed by directly infected, non-overlapping cell types in rhesus macaques. Targeting essential RhCMV genes with myeloid cell-selective miR-142-3p eliminated MHC-E-restricted CD8+ T cell priming, yielding an exclusively MHC-II-restricted response, whereas endothelial cell-selective miR-126-3p targeting eliminated MHC-II-restricted CD8+ T cell priming, yielding an exclusively MHC-E-restricted response. Incorporation of both restriction elements reverts CD8+ T cell responses back to conventional MHC-Ia restriction. Using these otherwise isogenic vectors we show that although they demonstrate similar overall immunogenicity, only the vectors programmed to elicit MHC-E-restricted CD8+ T cell responses provided protection against SIV challenge. The MHC-E-only RhCMV/SIV vaccine efficacy did not exceed that of the parental 68-1 RhCMV/SIV vectors (that elicits both MHC-II and MHC-E responses) indicating that while the MHC-II-restricted CD8+ T cell responses are neutral to overall vaccine efficacy, an additional component of 68-1 RhCMV/SIV-induced immunity contributes to overall vaccine efficacy. This work was supported by the National Institute of Allergy and Infectious Diseases (NIAID) grants UM1 AI124377 and U19 AI128741 to LJP; the Oregon National Primate Research Center Core grant from the National Institutes of Health, Office of the Director (P51 OD011092); contracts from the National Cancer Institute (# HHSN261200800001E) to JDL; and the Bill and Melinda Gates Foundation grant OPP1107409.

Env-independent protection of intrarectal SIV challenge by vaccine induction of Gag/Vif-specific CD8+ T cells but not CD4+ T cells

Effective Tcell induction is an important strategy in HIV-vaccine development. However, it has been indicated that vaccine-induced HIV-specific CD4+ Tcells, the preferential targets of HIV infection, might increase viral acquisition after HIV exposure. We have recently developed an immunogen (CaV11), tandemly connected overlapping 11-mer peptides spanning the simian immunodeficiency virus (SIV) Gag capsid and Vif proteins, to selectively induce Gag- and Vif-specific CD8+ Tcells but not CD4+ Tcells. Here, we show protective efficacy of a CaV11-expressing vaccine against repeated intrarectal low-dose SIVmac239 challenge in rhesus macaques. Eight ofthe twelve vaccinated macaques were protected after eightchallenges. Kaplan-Meier analysis indicated significant protection in the vaccinees compared to the unvaccinated macaques. Vaccine-induced Gag-specific CD8+ Tcell responses were significantly higher in the protected than the unprotected vaccinees. These results suggest that classical CD8+ Tcell induction by viral Env-independent vaccination can confer protection from intrarectal SIV acquisition, highlighting the rationale for this immunogen design to induce virus-specific CD8+ Tcells but not CD4+ Tcells in HIV-vaccine development.

The transcription factor CREB1 is a mechanistic driver of immunogenicity and reduced HIV-1 acquisition following ALVAC vaccination.

Development of effective HIV-1 vaccines requires synergy between innate and adaptive immune cells. We show that induction of the transcription factor CREB1 and its target genes by the recombinant canarypox vector ALVAC+Alum augments immunogenicity in Non-human primates (NHPs) and predicts reduced HIV-1 acquisition in the RV144 trial. These target genes contain cytokines/chemokines associated with heightened protection from SIV challenge in NHPs. CREB1 gene expression likely results from direct cGAMP (STING agonist) modulated p-CREB1 activity which drives the recruitment of CD4+ T cells and B cells to the site of antigen presentation. Importantly, unlike NHPs immunized with ALVAC+Alum, immunization with ALVAC+MF59, the regimen in the HVTN702 trial which showed no protection from HIV infection, exhibited significantly reduced CREB1 target gene expression. Our integrated systems biology approach has validated CREB1 as a critical driver of vaccine efficacy and highlights that adjuvants which trigger CREB1 signaling may be critical for efficacious HIV-1 vaccines.

Monocyte-derived transcriptome signature indicates antibody-dependent cellular phagocytosis as a potential mechanism of vaccine-induced protection against HIV-1.

A gene signature was previously found to be correlated with mosaic adenovirus 26 vaccine protection in simian immunodeficiency virus and simian-human immunodeficiency virus challenge models in non-human primates. In this report, we investigated the presence of this signature as a correlate of reduced risk in human clinical trials and potential mechanisms of protection. The absence of this gene signature in the DNA/rAd5 human vaccine trial, which did not show efficacy, strengthens our hypothesis that this signature is only enriched in studies that demonstrated protection. This gene signature was enriched in the partially effective RV144 human trial that administered the ALVAC/protein vaccine, and we find that the signature associates with both decreased risk of HIV-1 acquisition and increased vaccine efficacy (VE). Total RNA-seq in a clinical trial that used the same vaccine regimen as the RV144 HIV vaccine implicated antibody-dependent cellular phagocytosis (ADCP) as a potential mechanism of vaccine protection. CITE-seq profiling of 53 surface markers and transcriptomes of 53,777 single cells from the same trial showed that genes in this signature were primarily expressed in cells belonging to the myeloid lineage, including monocytes, which are major effector cells for ADCP. The consistent association of this transcriptome signature with VE represents a tool both to identify potential mechanisms, as with ADCP here, and to screen novel approaches to accelerate the development of new vaccine candidates.

Interleukin-15 response signature predicts RhCMV/SIV vaccine efficacy.

Simian immunodeficiency virus (SIV) challenge of rhesus macaques (RMs) vaccinated with strain 68-1 Rhesus Cytomegalovirus (RhCMV) vectors expressing SIV proteins (RhCMV/SIV) results in a binary outcome: stringent control and subsequent clearance of highly pathogenic SIV in ~55% of vaccinated RMs with no protection in the remaining 45%. Although previous work indicates that unconventionally restricted, SIV-specific, effector-memory (EM)-biased CD8+ T cell responses are necessary for efficacy, the magnitude of these responses does not predict efficacy, and the basis of protection vs. non-protection in 68-1 RhCMV/SIV vector-vaccinated RMs has not been elucidated. Here, we report that 68-1 RhCMV/SIV vector administration strikingly alters the whole blood transcriptome of vaccinated RMs, with the sustained induction of specific immune-related pathways, including immune cell, toll-like receptor (TLR), inflammasome/cell death, and interleukin-15 (IL-15) signaling, significantly correlating with subsequent vaccine efficacy. Treatment of a separate RM cohort with IL-15 confirmed the central involvement of this cytokine in the protection signature, linking the major innate and adaptive immune gene expression networks that correlate with RhCMV/SIV vaccine efficacy. This change-from-baseline IL-15 response signature was also demonstrated to significantly correlate with vaccine efficacy in an independent validation cohort of vaccinated and challenged RMs. The differential IL-15 gene set response to vaccination strongly correlated with the pre-vaccination activity of this pathway, with reduced baseline expression of IL-15 response genes significantly correlating with higher vaccine-induced induction of IL-15 signaling and subsequent vaccine protection, suggesting that a robust de novo vaccine-induced IL-15 signaling response is needed to program vaccine efficacy. Thus, the RhCMV/SIV vaccine imparts a coordinated and persistent induction of innate and adaptive immune pathways featuring IL-15, a known regulator of CD8+ T cell function, that support the ability of vaccine-elicited unconventionally restricted CD8+ T cells to mediate protection against SIV challenge.

Cytomegaloviral determinants of CD8+ T cell programming and RhCMV/SIV vaccine efficacy.

Simian immunodeficiency virus (SIV) insert-expressing, 68-1 rhesus cytomegalovirus (RhCMV/SIV) vectors elicit major histocompatibility complex E (MHC-E)- and MHC-II-restricted, SIV-specific CD8+ T cell responses, but the basis of these unconventional responses and their contribution to demonstrated vaccine efficacy against SIV challenge in the rhesus monkeys (RMs) have not been characterized. We show that these unconventional responses resulted from a chance genetic rearrangement in 68-1 RhCMV that abrogated the function of eight distinct immunomodulatory gene products encoded in two RhCMV genomic regions (Rh157.5/Rh157.4 and Rh158-161), revealing three patterns of unconventional response inhibition. Differential repair of these genes with either RhCMV-derived or orthologous human CMV (HCMV)-derived sequences (UL128/UL130; UL146/UL147) leads to either of two distinct CD8+ T cell response types-MHC-Ia-restricted only or a mix of MHC-II- and MHC-Ia-restricted CD8+ T cells. Response magnitude and functional differentiation are similar to RhCMV 68-1, but neither alternative response type mediated protection against SIV challenge. These findings implicate MHC-E-restricted CD8+ T cell responses as mediators of anti-SIV efficacy and indicate that translation of RhCMV/SIV vector efficacy to humans will likely require deletion of all genes that inhibit these responses from the HCMV/HIV vector.

Protection against SIV in Rhesus Macaques Using Albumin and CD4-Based Vector-Mediated Gene Transfer

Antibody-like molecules were evaluated with potent simian immunodeficiency virus (SIV) neutralizing properties (immunoadhesins) that were delivered by a recombinant adeno-associated virus (rAAV) vector in the SIV-infected rhesus macaque model. When injected intramuscularly into the host, the vector directs in vivo production of the transgenes with antibody-like binding properties that lead to serum neutralizing activity against SIV. To extend the half-life of the immunoadhesins, rhesus cluster of differentiation 4 (CD4) and a single-chain antibody (4L6) were fused with albumin molecules, and these constructs were tested in our model of SIV infection. Antibody-based immunoadhesins provided high serum neutralizing titers against the original SIV strain. CD4-based immunoadhesins provided a wider spectrum of neutralization against different SIV strains in comparison to antibody-based therapeutics and had the potential to protect against high viral challenging doses. Although the albumin-antibody fusion immunoadhesin provided strong and prolonged protection of the immunized animals against SIV challenge, the albumin-CD4 fusion altered the specificity and decreased the overall protection effectiveness of the immunoadhesin in comparison to the antibody-based molecules. Albumin-based immunoadhesins increase in vivo longevity of the immune protection; however, they present challenges likely linked to the induction of anti-immunoadhesin antibodies.

Vaccination of Macaques with DNA Followed by Adenoviral Vectors Encoding Simian Immunodeficiency Virus (SIV) Gag Alone Delays Infection by Repeated Mucosal Challenge with SIV.

Vaccines aimed at inducing T cell responses to protect against human immunodeficiency virus (HIV) infection have been under development for more than 15 years. Replication-defective adenovirus (rAd) vaccine vectors are at the forefront of this work and have been tested extensively in the simian immunodeficiency virus (SIV) challenge macaque model. Vaccination with rAd vectors coding for SIV Gag or other nonenvelope proteins induces T cell responses that control virus load but disappointingly is unsuccessful so far in preventing infection, and attention has turned to inducing antibodies to the envelope. However, here we report that Mauritian cynomolgus macaques (MCM), Macaca fascicularis, vaccinated with unmodified SIV gag alone in a DNA prime followed by an rAd boost exhibit increased protection from infection by repeated intrarectal challenge with low-dose SIVmac251. There was no evidence of infection followed by eradication. A significant correlation was observed between cytokine expression by CD4 T cells and delayed infection. Vaccination with gag fused to the ubiquitin gene or fragmented, designed to increase CD8 magnitude and breadth, did not confer resistance to challenge or enhance immunity. On infection, a significant reduction in peak virus load was observed in all vaccinated animals, including those vaccinated with modified gag These findings suggest that a nonpersistent viral vector vaccine coding for internal virus proteins may be able to protect against HIV type 1 (HIV-1) infection. The mechanisms are probably distinct from those of antibody-mediated virus neutralization or cytotoxic CD8 cell killing of virus-infected cells and may be mediated in part by CD4 T cells.IMPORTANCE The simian immunodeficiency virus (SIV) macaque model represents the best animal model for testing new human immunodeficiency virus type 1 (HIV-1) vaccines. Previous studies employing replication-defective adenovirus (rAd) vectors that transiently express SIV internal proteins induced T cell responses that controlled virus load but did not protect against virus challenge. However, we show for the first time that SIV gag delivered in a DNA prime followed by a boost with an rAd vector confers resistance to SIV intrarectal challenge. Other partially successful SIV/HIV-1 protective vaccines induce antibody to the envelope and neutralize the virus or mediate antibody-dependent cytotoxicity. Induction of CD8 T cells which do not prevent initial infection but eradicate infected cells before infection becomes established has also shown some success. In contrast, the vaccine described here mediates resistance by a different mechanism from that described above, which may reflect CD4 T cell activity. This could indicate an alternative approach for HIV-1 vaccine development.

Transient Immune Activation in BCG-Vaccinated Infant Rhesus Macaques Is Not Sufficient to Influence Oral Simian Immunodeficiency Virus Infection.

BCG vaccination has been demonstrated to increase levels of activated CD4+ T cells, thus potentially influencing mother-to-child transmission of human immunodeficiency virus (HIV). To assess the risk of BCG vaccination in HIV infection, we randomly assigned newborn rhesus macaques to receive BCG vaccine or remain unvaccinated and then undergo oral simian immunodeficiency virus (SIV) challenges 3 weeks later. We observed elevated levels of activated peripheral CD4+ T cells (ie, HLA-DR+CD38+CCR5+ CD4+ T cells) by week 3 after vaccination. BCG was also associated with an altered immune gene expression profile, as well as with monocyte activation in both peripheral blood and the draining axillary lymph node, indicating significant BCG vaccine-induced immune activation. Despite these effects, BCG vaccination did not increase the rate of SIV oral transmission or disease progression. Our findings therefore identify patterns of T-cell and monocyte activation that occur after BCG vaccination but do not support the hypothesis that BCG vaccination is a risk factor for postnatal HIV transmission or increased pathogenesis in infants.

Building a safer CMV vector

Vaccines Vaccine vectors based on cytomegalovirus (CMV) show strong T cell induction and protection against a multitude of pathogens. However, such vaccines may not be safe for people who are immunodeficient or immunosuppressed. Marshall et al. genetically modified rhesus CMV to allow engagement of host intrinsic immunity. The modified ΔRh110 vector did not spread once administered to nonhuman primates but still induced robust T cell immunity. Hansen et al. showed in a simian immunodeficiency virus challenge model that the ΔRh110 vector provided durable protection that was comparable to the parental vector. Mutations in the human CMV vector could thus yield a potent but restrained CMV for human vaccination. Sci. Transl. Med. 11 , eaaw2603, eaaw2607 (2019).

A live-attenuated RhCMV/SIV vaccine shows long-term efficacy against heterologous SIV challenge.

Previous studies have established that strain 68-1-derived rhesus cytomegalovirus (RhCMV) vectors expressing simian immunodeficiency virus (SIV) proteins (RhCMV/SIV) are able to elicit and maintain cellular immune responses that provide protection against mucosal challenge of highly pathogenic SIV in rhesus monkeys (RMs). However, these efficacious RhCMV/SIV vectors were replication and spread competent and therefore have the potential to cause disease in immunocompromised subjects. To develop a safer CMV-based vaccine for clinical use, we attenuated 68-1 RhCMV/SIV vectors by deletion of the Rh110 gene encoding the pp71 tegument protein (ΔRh110), allowing for suppression of lytic gene expression. ΔRh110 RhCMV/SIV vectors are highly spread deficient in vivo (~1000-fold compared to the parent vector) yet are still able to superinfect RhCMV+ RMs and generate high-frequency effector-memory-biased T cell responses. Here, we demonstrate that ΔRh110 68-1 RhCMV/SIV-expressing homologous or heterologous SIV antigens are highly efficacious against intravaginal (IVag) SIVmac239 challenge, providing control and progressive clearance of SIV infection in 59% of vaccinated RMs. Moreover, among 12 ΔRh110 RhCMV/SIV-vaccinated RMs that controlled and progressively cleared an initial SIV challenge, 9 were able to stringently control a second SIV challenge ~3 years after last vaccination, demonstrating the durability of this vaccine. Thus, ΔRh110 RhCMV/SIV vectors have a safety and efficacy profile that warrants adaptation and clinical evaluation of corresponding HCMV vectors as a prophylactic HIV/AIDS vaccine.

Antibody Fab‐Fc properties outperform titer in predictive models of SIV vaccine‐induced protection

Characterizing the antigen‐binding and innate immune‐recruiting properties of the humoral response offers the chance to obtain deeper insights into mechanisms of protection than revealed by measuring only overall antibody titer. Here, a high‐throughput, multiplexed Fab‐Fc Array was employed to profile rhesus macaques vaccinated with a gp120‐CD4 fusion protein in combination with different genetically encoded adjuvants, and subsequently subjected to multiple heterologous simian immunodeficiency virus (SIV) challenges. Systems analyses modeling protection and adjuvant differences using Fab‐Fc Array measurements revealed a set of correlates yielding strong and robust predictive performance, while models based on measurements of response magnitude alone exhibited significantly inferior performance. At the same time, rendering Fab‐Fc measurements mathematically independent of titer had relatively little impact on predictive performance. Similar analyses for a distinct SIV vaccine study also showed that Fab‐Fc measurements performed significantly better than titer. These results suggest that predictive modeling with measurements of antibody properties can provide detailed correlates with robust predictive power, suggest directions for vaccine improvement, and potentially enable discovery of mechanistic associations.

What Is the Predictive Value of Animal Models for Vaccine Efficacy in Humans?

Simian immunodeficiency virus (SIV) challenge of rhesus macaques provides an invaluable tool to evaluate the clinical prospects of HIV-1 vaccine concepts. However, as with any animal model of human disease, it is crucial to understand the advantages and limitations of this system to maximize the translational value of SIV vaccine studies. Here, we discuss the importance of assessing the efficacy of vaccine prototypes using stringent SIV challenge regimens that mimic HIV-1 transmission and pathogenesis. We also review some of the cautionary tales of HIV-1 vaccine research because they provide general lessons for the preclinical assessment of vaccine candidates.

Cabotegravir long acting injection protects macaques against intravenous challenge with SIVmac251.

We evaluated the effectiveness of cabotegravir (CAB; GSK1265744 or GSK744) long acting as preexposure prophylaxis (PrEP) against intravenous simian immunodeficiency virus (SIV) challenge in a model that mimics blood transfusions based on the per-act probability of infection. CAB long acting is an integrase strand transfer inhibitor formulated as a 200 mg/ml injectable nanoparticle suspension that is an effective PrEP agent against rectal and vaginal simian/human immunodeficiency virus transmission in macaques. Three groups of rhesus macaques (n = 8 per group) were injected intramuscularly with CAB long acting and challenged intravenously with 17 animal infectious dose 50% SIVmac251 on week 2. Group 1 was injected with 50 mg/kg on week 0 and 4 to evaluate the protective efficacy of the CAB long-acting dose used in macaque studies mimicking sexual transmission. Group 2 was injected with 50 mg/kg on week 0 to evaluate the necessity of the second injection of CAB long acting for protection against intravenous challenge. Group 3 was injected with 25 mg/kg on week 0 and 50 mg/kg on week 4 to correlate CAB plasma concentrations at the time of challenge with protection. Five additional macaques remained untreated as controls. CAB long acting was highly protective with 21 of the 24 CAB long-acting-treated macaques remaining aviremic, resulting in 88% protection. The plasma CAB concentration at the time of virus challenge appeared to be more important for protection than sustaining therapeutic plasma concentrations with the second CAB long acting injection. These results support the clinical investigation of CAB long acting as PrEP in people who inject drugs.

Balancing Trained Immunity with Persistent Immune Activation and the Risk of Simian Immunodeficiency Virus Infection in Infant Macaques Vaccinated with Attenuated Mycobacterium tuberculosis or Mycobacterium bovis BCG Vaccine.

Our goal is to develop a pediatric combination vaccine to protect the vulnerable infant population against human immunodeficiency virus type 1 (HIV-1) and tuberculosis (TB) infections. The vaccine consists of an auxotroph Mycobacterium tuberculosis strain that coexpresses HIV antigens. Utilizing an infant rhesus macaque model, we have previously shown that this attenuated M. tuberculosis (AMtb)-simian immunodeficiency virus (SIV) vaccine is immunogenic, and although the vaccine did not prevent oral SIV infection, a subset of vaccinated animals was able to partially control virus replication. However, unexpectedly, vaccinated infants required fewer SIV exposures to become infected compared to naive controls. Considering that the current TB vaccine, Mycobacterium bovis bacillus Calmette-Guérin (BCG), can induce potent innate immune responses and confer pathogen-unspecific trained immunity, we hypothesized that an imbalance between enhanced myeloid cell function and immune activation might have influenced the outcome of oral SIV challenge in AMtb-SIV-vaccinated infants. To address this question, we used archived samples from unchallenged animals from our previous AMtb-SIV vaccine studies and vaccinated additional infant macaques with BCG or AMtb only. Our results show that vaccinated infants, regardless of vaccine strain or regimen, had enhanced myeloid cell responses. However, CD4+ T cells were concurrently activated, and the persistence of these activated target cells in oral and/or gastrointestinal tissues may have facilitated oral SIV infection. Immune activation was more pronounced in BCG-vaccinated infant macaques than in AMtb-vaccinated infant macaques, indicating a role for vaccine attenuation. These findings underline the importance of understanding the interplay of vaccine-induced immunity and immune activation and its effect on HIV acquisition risk and outcome in infants.