CROI 2025 Abstract eBook

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Poster Abstracts

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immunization, the animals were challenged with weekly low-dose intra-rectal inoculations of SHIV-AD8. Infection was monitored by plasma viremia using TaqMan PCR. Results: We further optimized our mRNA vaccine platform by addition of the viral protease (using full-length gag-pol transcript) to produce mature VLPs, and by priming with the germline (gl)-engaging Env 426c-DG3, which in the VLP/mRNA formulation showed exceptional potency in VRC01-gl knock-in mice. In two pre-clinical studies in macaques, sequential immunization with VLP/ mRNA formulations expressing Envs from 3 clades (A,B,C) reproducibly elicited heterologous tier-2 neutralizing antibodies, along with strong Env- and Gag specific CD4+ T-cell responses, including T-follicular helper cells, while CD8+ T-cell responses were less pronounced. Inclusion of the protease induced better protection from SHIV-AD8 compared to Env+Gag alone. Protection correlates included heterologous tier-2 neutralizing antibodies, as well as Env-specific antibodies present in mucosal secretions. Conclusions: A VLP-forming mRNA vaccine supplemented with the viral protease and utilizing an optimized priming immunogen was effective in inducing broad-spectrum neutralization and protection in a relevant nonhuman primate model. A Phase-I clinical protocol (HVTN 310) is under development to evaluate the safety and immunogenicity of the VLP-forming mRNA vaccine in healthy human volunteers. Cytoplasmic Tail Engineering of Stabilized mRNA Env Immunogens Enhances Neutralizing Response Edward Kreider 1 , Ashwin N. Skelly 1 , Weimin Liu 1 , Yuhong Xiao 1 , Wenge Ding 1 , Amie Albertus 1 , Natalia Perez Devia 1 , Barton Haynes 2 , Pamela J. Bjorkman 3 , George M. Shaw 1 , James Hoxie 1 , Beatrice H. Hahn 1 , Drew Weissman 1 1 University of Pennsylvania, Philadelphia, PA, USA, 2 Duke Human Vaccine Institute, Durham, NC, USA, 3 California Institute of Technology, Pasadena, CA, USA Background: Certain Env features, such as low cell surface expression and conformational lability, have hampered mRNA vaccine development for HIV. We previously found that ablation of Env endocytosis motifs and incorporation of a truncated HIV/SIV chimeric cytoplasmic tail (SIV.CT) increased surface expression on virions and cells transduced with vaccinia-vectored immunogens. Here, we incorporated this SIV.CT modification into mRNA immunogens and measured its impact on cell surface expression and immunogenicity. Methods: mRNAs encoding prefusion-stabilized versions of Subtype A, AE, C, and D HIV-1 Envs, including germline targeting Envs RC1 and 11MUTB, were produced. Cell surface protein expression from mRNA was assessed following transfection of 293F cells and staining with conformationally-dependent antibodies ( e.g. , PGDM1400, 17b) using flow cytometry. To test immunogenicity, BALB/c mice were immunized intramuscularly at weeks 0 and 4 with 1 ug of mRNA-lipid nanoparticles encoding SIV.CT or gp160 variants of RC1 or 11MUTB. Serum drawn at weeks 2, 3, and 6 was evaluated using autologous antigen specific ELISAs and neutralization assays. Results: Properly folded, prefusion RC1 and 11MUTB trimers were expressed on the surface of mRNA-transfected cells as demonstrated by high PGDM1400 (V2-apex) signal and low 1393A (V2i) and 17b (CD4i) signals. Notably, DH270.6 (V3) and PGDM1400 signals were ~1 log higher for Envs containing the SIV. CT modification as compared to gp160 Envs. Application of our stabilization + SIV.CT approach to 8 additional HIV-1 strains yielded similarly high cell surface expression of properly folded trimers. Sera from RC1- and 11MUTB-vaccinated mice drawn 2 weeks post-prime demonstrated autologous binding antibodies in all animals, with SIV.CT-immunized animals showing consistently higher titers than gp160-immunized ones (RC1 SIV.CT- vs. gp160-primed geomean AUC: 59.2 vs 33.4; 11MUTB-primed: 67.7 vs 23.9). Further, SIV.CT variants elicited 10-28-fold greater neutralizing titers 3 weeks post-prime (geomean reciprocal ID50 RC1-primed animals: 2,155 vs. 219; 11MUTB-primed: 7,246 vs. 251) that remained 4.2-11.7-fold higher following homologous boost at week 6. Conclusions: Engineering of the mRNA-encoded Env CT increases cell surface expression and, in combination with trimer stabilization, confers enhanced immunogenicity in mice. This strategy may be applied to generate mRNA vaccines from genetically diverse Env strains for preventative and therapeutic vaccine applications.

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Enhanced HIV Antibody Precursor Development With Early-Life Germline Targeting Immunization Yasmine Issah 1 , Ashley N. Nelson 1 , Xintao Hu 1 , Xiaoying Shen 2 , Gabriel Ozorowski 3 , Leigh M. Sewall 3 , Shiyu Zhang 3 , Andrew B. Ward 3 , David Montefiori 2 , Rogier W. Sanders 4 , John P. Moore 1 , Koen K. Van Rompay 5 , Kristina De Paris 6 , Sallie Permar 1 1 Weill Cornell Medicine, New York, NY, USA, 2 Duke University, Durham, NC, USA, 3 The Scripps Research Institute, La Jolla, CA, USA, 4 Amsterdam University Medical Centers, Amsterdam, Netherlands, 5 University of California Davis, Davis, CA, USA, 6 University of North Carolina at Chapel Hill, Chapel Hill, NC, USA Background: HIV is a highly mutable virus, therefore a vaccine that induces protective, broadly-neutralizing antibodies (bnAbs) before sexual debut is critical to eliminate the ~410,000 new infections annually among adolescents worldwide. Recent work has established that children living with HIV develop bnAbs earlier and at a higher frequency than adults. In this study we compared the ability of a CD4 binding site (CD4bs) germline-targeting SOSIP trimer immunization strategy to induce precursor bnAbs in infant and juvenile rhesus macaques (RMs). Methods: Infant (n=5) and juvenile (n=4) RMs received 3 immunizations of the germline-targeting BG505 GT1.1 SOSIP trimer (50mg) with the 3M-052 SE adjuvant 6 weeks apart. All RMs were then boosted 12 weeks later with the BG505.664 WT SOSIP trimer 3 times in 6-month intervals. After an over one-year follow-up, all animals received two boosts with a mosaic Clade B SOSIP nanoparticle. Vaccine-elicited antibody responses were monitored through 2.5 years after the 1st vaccination. Results: BG505 GT1.1 SOSIP trimer immunization consistently induced higher magnitude vaccine-specific IgG binding in infants compared to juvenile RMs yet, plasma autologous tier 1 and 2 neutralization responses were similar between the groups. Additionally, analysis of circulating B cells at week 14 showed that the infants had higher frequencies of BG505 GT1.1-specific B cells than the juveniles following the last BG505 GT1.1 prime. Plasma nsEMPEM revealed 4 infants developed CD4bs targeting antibodies compared to 3 juveniles. Similarly 3 of 5 GT1.1 SOSIP-immunized infants exhibited a plasma neutralization signature indicating CD4bs bnAb precursor development, compared to only 1 of 4 juvenile RMs. Interestingly in the RMs, higher tier 1 neutralization titers showed a trend in predicting the likelihood of an animal to develop bnAb precursors. Following the nanoparticle boost, an additional infant developed the CD4bs precursor bnAb signature, while this response was not maintained in the one juvenile. Nanoparticle boosting also improved the breadth of heterologous neutralization in 4 out of the 5 infants and 2 out of the 4 juveniles. Conclusions: Our data indicates that sequential immunization with germline targeting BG505 SOSIP trimers may elicit CD4bs bnAb precursors more frequently in infants compared to juveniles. Our results highlight the potential for an HIV immunization strategy in early life to induce protective bnAb responses and can inform future human pediatric clinical trials. The figure, table, or graphic for this abstract has been removed. A VLP-Forming mRNA Vaccine Protects Macaques From Heterologous SHIV Infection Peng Zhang 1 , Vinay Gopan 1 , Mamta Singh 1 , Flavio Matassoli 1 , Parul Agrawal 2 , Shayne Andrew 1 , Kathryn Foulds 1 , Sunny Himansu 3 , Leonidas Stamatatos 2 , Andrea Carfi 3 , Paolo Lusso 1 1 National Institutes of Health, Bethesda, MD, USA, 2 Fred Hutchinson Cancer Center, Seattle, WA, USA, 3 Moderna, Inc, Cambridge, MA, USA Background: The development of a protective vaccine remains one of the top priorities for the control of the HIV/AIDS pandemic on a global scale. Due to its versatility and ability to drive endogenous protein production, mRNA is emerging as a promising platform for an HIV vaccine. We previously reported that a virus-like particle (VLP)-forming HIV-1 env-gag mRNA vaccine elicited heterologous tier-2 neutralizing antibodies and protection from heterologous tier-2 SHIV (AD8) in rhesus macaques ( Nat. Med. 27:2234, 2021). Here, we report further optimization of this vaccine platform. Methods: In two separate studies, juvenile rhesus macaques were immunized repeatedly over the course of 12 months with mRNA expressing different HIV-1 Envs in combination with Gag and GagPol to produce mature VLPs. Immunized animals were longitudinally studied for trimer-binding and neutralizing antibodies, virus-specific T-cell responses and other immune parameters. After

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Poster Abstracts

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CROI 2025 104