The biology of intestinal immunoglobulin A responses

Surface displaced alfa-enolase of Lactobacillus plantarum is a fibronectin binding protein

BACKGROUND

Lactic acid bacteria of the genus Lactobacillus and Bifidobacterium are one of the most important health promoting groups of the human intestinal microbiota. Their protective role within the gut consists in out competing invading pathogens for ecological niches and metabolic substrates. Among the features necessary to provide health benefits, commensal microorganisms must have the ability to adhere to human intestinal cells and consequently to colonize the gut. Studies on mechanisms mediating adhesion of lactobacilli to human intestinal cells showed that factors involved in the interaction vary mostly among different species and strains, mainly regarding interaction between bacterial adhesins and extracellular matrix or mucus proteins. We have investigated the adhesive properties of Lactobacillus plantarum, a member of the human microbiota of healthy individuals.

RESULTS

We show the identification of a Lactobacillus plantarum LM3 cell surface protein (48 kDa), which specifically binds to human fibronectin (Fn), an extracellular matrix protein. By means of mass spectrometric analysis this protein was identified as the product of the L. plantarum enoA1 gene, coding the EnoA1 alfa-enolase. Surface localization of EnoA1 was proved by immune electron microscopy. In the mutant strain LM3-CC1, carrying the enoA1 null mutation, the 48 kDa adhesin was not anymore detectable neither by anti-enolase Western blot nor by Fn-overlay immunoblotting assay. Moreover, by an adhesion assay we show that LM3-CC1 cells bind to fibronectin-coated surfaces less efficiently than wild type cells, thus demonstrating the significance of the surface displaced EnoA1 protein for the L. plantarum LM3 adhesion to fibronectin.

CONCLUSION

Adhesion to host tissues represents a crucial early step in the colonization process of either pathogens or commensal bacteria. We demonstrated the involvement of the L. plantarum Eno A1 alfa-enolase in Fn-binding, by studying LM3 and LM3-CC1 surface proteins. Isolation of LM3-CC1 strain was possible for the presence of expressed enoA2 gene in the L. plantarum genome, giving the possibility, for the first time to our knowledge, to quantitatively compare adhesion of wild type and mutant strain, and to assess doubtless the role of L. plantarum Eno A1 as a fibronectin binding protein.

Effect of Lactobacillus casei Shirota strain intraperitoneal administration in CD1 mice on the establishment of Trichinella spiralis adult worms and on IgA anti-T. spiralis production

The effect of the intraperitoneal (ip) administration of Lactobacillus casei Shirota strain (LcS) in CD1 mice on the establishment of Trichinella spiralis adult worms (TSAW), and on the generation of intestinal IgA anti-T. spiralis after challenge (AC) were evaluated. One hundred and twenty mice were allocated at random into two groups of 60 mice each: Treated group (T) and Non-treated group (NT). Each mouse in T group was inoculated with LcS at days -21, -14, and -7. On day 0 each mouse was challenged with 200 larvae of T. spiralis. At days 3, 5, 7, 10, 12, 14, 17, 19, 21, and 28 AC, six mice from each group were sacrificed to obtain TSAW. At days 7, 14, 21, and 28 IgA-s anti-T. spiralis levels in intestinal washings were evaluated by ELISA. From day five on AC, mice in LcS group showed significantly less TSAW (P<0.05) than animals from NT group. At days 7, 14, 21, and 28 AC IgA anti-T. spiralis levels were higher in mice from T group (P<0.05) than in the NT group. The results indicate that LcS inoculated into mice induces protection against T. spiralis and an increase in the production of IgA anti-T. spiralis.

Role of retinoic acid in the imprinting of gut-homing IgA-secreting cells

Antibody-secreting cells (ASCs) lodging in the mucosa of the small intestine are derived from activated B cells that are thought to arise in gut-associated lymphoid tissues (GALT). Upon leaving the GALT, B cells return to the blood where they must express the gut-homing receptors alpha4beta7 and CCR9 in order to emigrate into the small bowel. Recent evidence indicates that gut-associated dendritic cells (DCs) in GALT induce gut-homing receptors on B cells via a mechanism that depends on the vitamin A metabolite retinoic acid (RA). In addition, although ASC associated with other mucosal tissues secrete IgA in an RA-independent fashion, the presence of high levels of RA in intestine and GALT can promote B cell class switching to IgA and thus, boost the production of IgA in the intestinal mucosa. Here, we discuss the role of RA in the imprinting of gut-homing ASC and the evidence linking RA with the generation of intestinal IgA-ASCs.