Escherichia coli ghosts promote innate immune responses in human keratinocytes

Bacterial ghosts (BGs) as non-living bacterial envelopes devoid of cytoplasmic content with preserved and intact inner and outer membrane structures of their living counterparts have been used to study the ability of their surface components for the induction of antimicrobial peptides and pro-inflammatory cytokines in human primary keratinocytes (KCs). Quantitative real-time PCR analysis revealed that incubation of KCs with BGs generated from wild-type Escherichia coli induced the mRNA expression of antimicrobial psoriasin (S100A7c) in a BGs particle concentration-dependent manner. Using immunoblot analysis we showed that BGs generated from the flagellin-deficient (ΔFliC) E. coli strain NK9375 were as effective as its isogenic wild-type (wt) E. coli strain NK9373 to induce psoriasin expression when normalized to BG particles being taken up by KCs. However, results obtained from endocytic activity of KCs reflect that internalization of BGs is greatly dependent on the presence of flagellin on the surface of BGs. Moreover, BGs derived from wt E. coli NK9373 strongly induced the release of the pro-inflammatory cytokines IL-6 and IL-8, compared to ΔFliC E. coli NK9375 BGs. Taken together, obtained data demonstrate that non-living BGs possessing all bacterial bio-adhesive surface properties in their original state while not posing any infectious threat have the capacity to induce the expression of innate immune modulators and that these responses are partially dependent on the presence of flagellin.

Research highlights
► Activation of KCs by BGs up-regulates the expression of the antimicrobial psoriasin.
► KCs respond to BGs by the production of psoriasin in a particle-dependent manner.
► Flagellin on the surface of BGs enhances the expression of psoriasin and cytokines.
► Flagellin plays an important role during internalization of BGs by KCs.
► BGs enhance the innate immune defence system of the skin.