Abstract

The potential role of probiotic bacteria as adjuvants in vaccine trials led to their use as
nonparenteral live mucosal vaccine vectors. Yet, interactions between these vectors, the
host and the microbiome are poorly understood. In this presentation we evaluate impact of
three probiotic, Lactobacillus acidophilus, vector strains, and their interactions with the host’s
immune response, on the gut microbiome. One strain expressed the membrane proximal
external region from HIV-1 (MPER). The other two expressed MPER and either secreted
interleukin-1 (IL-1β) or expressed the surface flagellin subunit C (FliC) as adjuvants. We also
used MPER with rice bran as prebiotic supplement. We observed a strain dependent,
differential effect suggesting that MPER and IL-1β induced a shift of the microbiome while
FliC had minimal impact. Joint probiotic and prebiotic use resulted in a compound effect. We
concluded that careful consideration of constitutive adjuvants and use of prebiotics is
needed depending on whether or not to target microbiome modulation to improve vaccine
efficacy. No clear associations were observed between total or MPER-specific IgA and the

microbiome suggesting a role for other immune mechanisms or a need to focus on IgA-
bound, resident microbiota, most affected by an immune response