Influence of Gut Microbiota on Immune Responses and Protection in Volunteers Receiving the Live Attenuated Oral ETEC Vaccine ACE257 followed by Virulent ETEC H10407 Challenge

Enterotoxigenic Escherichia coli (ETEC) remains a major cause of diarrheal morbidity with no licensed vaccines. Role of gut microbiota in vaccine immunogenicity and protection was investigated using 16S rRNA sequencing from the stool samples of 27 volunteers receiving two doses of the live attenuated oral ETEC vaccine ACE527 followed by virulent ETEC H10407 challenge. Systemic and mucosal IgG and IgA responses to heat-labile toxin-B (LTB) and colonization-factor-antigen-I (CFA/I) were quantified by ELISA in serum and antibody-in-lymphocyte-supernatant (ALS). Microbiome α-diversity, β-diversity, and taxa–immune associations were evaluated using regression models, MiRKAT, and relaxed LASSO. Vaccination increased (~ 25–30%) Eubacterium_brachy_group, Family_XIII_AD3011 and Actinomyces. Higher α-diversity (inverse-Simpson) was associated with reduced ALS anti-LTB and CFA/I IgA responses, whereas β-diversity correlated with increased serum anti-CFA/I IgA. Members of Anaerovoraceae, Peptostreptococcaceae, Oscillospiraceae, and Veillonellaceae enhanced immune responses and protection against severe diarrhea and ETEC colonization, while Ruminococcaceae, Sutterellaceae, Coriobacteria, Clostridia, and Actinobacteria showed antagonistic associations.