2729. mRNA Vaccines Encoding Conserved Influenza Antigens Induce Robust and Durable Immunity in Rhesus Macaques

Abstract Background In response to immune pressure, influenza virus evolves, producing drifted variants capable of escaping immune recognition. One strategy for inducing a broad-spectrum immune response that can recognize multiple antigenically diverse strains is to target conserved proteins or protein domains. To that end, we assessed the immunogenicity of mRNA vaccines encoding the stem domain of hemagglutinin (HA) or nucleoprotein (NP) in nonhuman primates (NHPs). Methods Rhesus macaques were immunized three times intramuscularly, at 28 day intervals, with lipid nanoparticle-encapsulated mRNA encoding either HA stem (Yassine et al, 2015) or NP. Serum and PBMCs were collected up to 14 or 24 weeks, respectively, after the last vaccination. The magnitude and durability of humoral and cell-mediated immunity were evaluated. ELISA, competition ELISA, an in vitro antibody-dependent cell-mediated cytotoxicity (ADCC) reporter bioassay, and microneutralization assays were used to characterize serum immune responses. Intracellular cytokine staining (IFN-gamma and IL-2) was used to assess antigen-specific T-cell responses. Results HA stem-immunized NHPs developed a robust anti-stem binding titer after a single vaccine dose, and after two doses, serum antibodies recognized several antigenically distinct Group 1 HA proteins. This broad antibody response persisted for at least 14 weeks post-dose 3 (PD3). Serum antibodies showed ADCC activity and competed with a well-characterized broadly neutralizing antibody, CR9114, for binding to HA stem; however, the polyclonal serum had only minimal activity against a panel of H1N1 viruses in a microneutralization assay. HA-specific CD4+ T-cell responses were detectable PD3. A robust antibody binding response was also detected in NP-vaccinated NHPs, and titers remained high for at least 14 weeks PD3. Additionally, these animals developed robust NP-specific T-cell responses that persisted for at least 24 weeks PD3. On average, 0.5% of CD4+ and 4% of CD8+ T cells produced IFN-gamma in response to NP peptide stimulation at the peak of the response, 2 weeks after the last vaccine dose was administered. Conclusion Lipid nanoparticle-encapsulated mRNA vaccines encoding conserved influenza antigens induce a robust and durable immune response in NHPs. Disclosures All authors: No reported disclosures.