Oral immunization of a rAd vector expressing norovirus VP1 elicits a potent mucosal immune response without an increase in anti-vector immunity

Abstract

Background: Noroviruses are the leading cause of epidemic acute gastroenteritis and foodborne diarrheal disease in humans. Potential correlates of protection identified through human challenge studies include mucosal IgA, memory B cells, and serum blocking (BT50) antibody titers. We hypothesized that fecal and memory IgA responses would be significantly elevated following oral immunization. Methods & Materials: We conducted a single-site, randomized, double-blind, placebo-controlled trial to determine the safety and immunogenicity of a norovirus vaccine consisting of a non-replicating adenovirus vector expressing VP1 delivered orally by tablet. Subjects that met inclusion/exclusion criteria were enrolled and randomized 2:1 to receive either a single dose of vaccine or placebo, respectively. Two dose levels were evaluated. The primary objective was to assess safety. The secondary objective was to determine immunogenicity, primarily assessed by measuring serum BT50. Exploratory immunological assessments included serum, fecal and saliva antibody titers measured by and the frequency of memory and antibody secreting B cells (ASCs). Anti-vector immunity was assessed by measuring Ad5 neutralizing antibody responses pre and post immunization (ClinicalTrials.gov number NCT02868073). Results: Sixty-six subjects were enrolled between July and Sept 2016. The vaccine was well-tolerated. The primary immunological endpoint (BT50 titers) was met in low and high dose groups (P = 0.0014, P < 0.0001). In the high-dose group, 18/23 (78%) subjects had a two-fold or greater increase in BT50 titer after a single vaccine dose. An average of 561 IgA ASCs x106 PBMC were induced post-immunization in the high dose group. Post immunization, a 15 fold increase in the geometric mean was observed, along with a greater than 10 fold average increase in the fecal IgA VP1 specific antibody response. Anti-Ad5 neutralizing antibody responses did not significantly increase compared to the placebo group. Conclusion: The norovirus vaccine was well-tolerated (with no dose limiting toxicities), and generated immune responses to norovirus VP1, without a corresponding increase in anti-Ad5 neutralizing antibody responses. It particular, the vaccine generated memory and local effector IgA + B cells. Because mucosal IgA is the front-line defense against enteric pathogens, vaccine-derived norovirus-specific intestinal IgA may translate to superior efficacy. This is a very promising advance in the development of an oral norovirus vaccine.