Lactocepin as a protective microbial structure in the context of IBD

Abstract

Probiotics have been shown to exert beneficial effects in the context of different diseases including inflammatory bowel diseases (IBD). However, clinical use of probiotics is hampered by lack of understanding of the protective mechanisms and by safety concerns regarding the application of high numbers of live bacteria in patients. The identification of protective microbial structure-function relationships might enable to overcome these restraints and might lead to innovative therapies using the isolated active microbial structures. In our study, we aimed to characterize the protective mechanisms of VSL#3, a clinically relevant probiotic mixture in IBD. We found Lactobacillus casei/paracasei-produced lactocepin to selectively degrade pro-inflammatory chemokines, resulting in reduced immune cell infiltration and reduced inflammation in experimental IBD models. As immune cell recruitment is a major proinflammatory mechanism our findings suggest that lactocepin might be of broad therapeutic relevance in an array of inflammatory diseases like IBD, allergic skin inflammation and psoriasis.

Keywords: CEP; IBD; chemokines; inflammation; lactocepin; probiotic; protease; prtP.

Figure 1. Anti-inflammatory mechanism of prtP-encoded lactocepin. Chronic inflammatory diseases like IBD, allergy and autoimmune diseases are characterized by a vicious circle of chemokine secretion, immune cell recruitment and activity and tissue damage including barrier disruption. PrtP-encoded lactocepin produced by L. casei/L. paracasei interferes with this vicious circle by specifically degrading proinflammatory chemokines, resulting in reduced infiltration of immune cells and potentially healing. However, the location of luminal lactocepin production and the mechanisms underlying its penetration into the mucosal tissue remain to be elucidated.

Figure 2. Screening of lactobacilli for anti-IP-10 activity. Human fecal L.casei isolates (L.c BFLM14, L.c BFLM 100, L.c BFLM 218) and commercial strains (L.c shirota, L.c defensis) were screened for their ability to degrade murine IP-10. All tested strains secrete low amounts of IP-10-degrading prtP-encoded lactocepin compared with L.p. The anti-IP-10 activity was strongly increased after cultivation in higher cell densities (1x = 5x10⁷ cfu/ml, 10x = 5x10⁸ cfu/ml). L.c BFLM 218 prtP^dis served as negative control.

Figure 3. Domain structure of prtP-encoded lactocepin. PrtP-encoded lactocepins from lactobacilli and lactococci are highly similar but show different substrate specificities even within the same species. It has been found that the catalytic domain (PR), the insertion domain (I) and the A domain play a role in the casein specificity of certain prtP-encoded lactocepins. However, it is not known which residues of the active secreted protease (PR to H domain) determine the anti-inflammatory substrate profile of L.p-derived prtP-encoded lactocepin.