Abstract
Primarily arising from their well understood beneficial health effects, many lactobacilli strains are considered good candidates for use as probiotics in humans and animals. Lactobacillar probiosis can itself be best typified by the Lactobacillus rhamnosus GG strain, which, with its well-documented clinical benefits, has emerged as one of the most widely used probiotics in the food and health-supplement industries. Even so, many facets of its molecular mechanisms and limitations as a beneficial commensal bacterium still remain to be thoroughly explored and dissected. Because L. rhamnosus GG is one of only a few such strains exhibiting surface piliation (called SpaCBA), we sought to examine whether this particular type of cell-surface appendage has a discernible immunomodulating capacity and is able to trigger targeted responses in human immune-related cells. Thus, presented herein for this study, we recombinantly engineered Lactococcus lactis to produce native (and pilin-deleted) SpaCBA pili that were assembled in a structurally authentic form and anchored to the cell surface, and which had retained mucus-binding functionality. By using these recombinant lactococcal constructs, we were able to demonstrate that the SpaCBA pilus can be a contributory factor in the activation of Toll-like receptor 2-dependent signaling in HEK cells as well as in the modulation of pro- and anti-inflammatory cytokine (TNF-α, IL-6, IL-10, and IL-12) production in human monocyte-derived dendritic cells. From these data, we suggest that the recombinant-expressed and surface-anchored SpaCBA pilus, given its projected functioning in the gut environment, might be viewed as a new microbe-associated molecular pattern (MAMP)-like modulator of innate immunity. Accordingly, our study has brought some new insight to the molecular immunogenicity of the SpaCBA pilus, thus opening the way to a better understanding of its possible role in the multifaceted nature of L. rhamnosus GG probiosis within the human gut.
Highlights
Gram-positive lactobacilli are recurrent members of the gut commensal community and among the earliest inhabitants of the gastrointestinal (GI) tract [1]
Genes for the SpaCBA pilus are arranged in the L. rhamnosus GG genome as an operon (Figure 1A) and encode for three pilin proteins and the pilin-specific sortase, an enzyme that is needed for pilus polymerization [23]
The coding region of the spaCBA pilus operon was inserted into a nisin-inducible expression vector to generate recombinant plasmid pKTH5391, which was propagated as the GRS1185 lactococcal construct (Figure 1A)
Summary
Gram-positive lactobacilli are recurrent members of the gut commensal community and among the earliest inhabitants of the gastrointestinal (GI) tract [1]. Stemming from the century-old Metchnikoff concept of lactobacilli, a certain proportion of intestinal strains, as well as isolates from other habitats, are used routinely as probiotics in humans and animals. Such bacteria are understood to possess a strong propensity to help promote good health as well as alleviate a variety of health problems [2,3,4]. Despite some genetic shortcomings associated with their overall adhesion capacity, when such lactobacilli are replenished constantly in high numbers in the gut, they can perform key functions in sustaining a normal microbial balance and in helping to preclude certain pathogen-borne infections [8]
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