No abstract

The scavenger receptor cysteine-rich (SRCR) family comprises a group of membrane-attached or secreted proteins that contain one or more modules/domains structurally similar to the membrane distal domain of type I macrophage scavenger receptor. Although no all-inclusive biological function has been ascribed to the SRCR family, some of these receptors have been shown to recognize pathogen-associated molecular patterns (PAMP) of bacteria, fungi, or other microbes. SSc5D is a recently described soluble SRCR receptor produced by monocytes/macrophages and T lymphocytes, consisting of an N-terminal portion, which contains five SRCR modules, and a large C-terminal mucin-like domain. Toward establishing a global common role for SRCR domains, we interrogated whether the set of five SRCR domains of SSc5D displayed pattern recognition receptor (PRR) properties. For that purpose, we have expressed in a mammalian expression system the N-terminal SRCR-containing moiety of SSc5D (N-SSc5D), thus excluding the mucin-like domain likely by nature to bind microorganisms, and tested the capacity of the SRCR functional groups to physically interact with bacteria. Using conventional protein–bacteria binding assays, we showed that N-SSc5D had a superior capacity to bind to Escherichia coli strains RS218 and IHE3034 compared with that of the extracellular domains of the SRCR proteins CD5 and CD6 (sCD5 and sCD6, respectively), and similar E. coli-binding properties as Spα, a proven PRR of the SRCR family. We have further designed a more sensitive, real-time, and label-free surface plasmon resonance (SPR)-based assay and examined the capacity of N-SSc5D, Spα, sCD5, and sCD6 to bind to different bacteria. We demonstrated that N-SSc5D compares with Spα in the capacity to bind to E. coli and Listeria monocytogenes, and further that it can distinguish between pathogenic E. coli RS218 and IHE3034 strains and the non-pathogenic laboratory E. coli strain BL21(DE3). Our work thus advocates the utility of SPR-based assays as sensitive tools for the rapid screening of interactions between immune-related receptors and PAMP-bearing microbes. The analysis of our results suggests that SRCR domains of different members of the family have a differential capacity to interact with bacteria, and further that the same receptor can discriminate between different bacteria strains and species.

The effect of three dietary tannins (procyanidin B3, B6, and T2) on the bioavailability of the 32-mer gliadin-derived immunogenic peptide was evaluated. An enterocyte-like Caco-2 cell line was used to mimic the epithelial transport of the 32-mer peptide, which was modeled by kinetic parameters with a mass spectrometry approach. The hydrolysis pattern on the enterocytes was analyzed, and the released peptides were quantified during the assay. The transport flux was dose-dependent. Along with procyanidin T2 and B6, procyanidin B3 promoted a significant inhibition mainly at the 100 μM peptide concentration. The hydrolysis efficiency was affected by procyanidins, while the cleavage pattern was suggested to be promoted by brush-border membranes at the apical compartment. The ability of procyanidins to molecularly bind to immunogenic peptides able to induce an adaptive response arose as a mechanism able to modulate their bioavailability, bioaccesibility, and further T CD4+ cell activation and expansion in a celiac disease (CD) model.