Antibody secreting cells are critically dependent on integrin α4ß7/MAdCAM-1 for intestinal recruitment and control of the microbiota during chronic colitis.

T and B cells employ integrin α4ß7 to migrate to intestine under homeostatic conditions. Whether those cells differentially rely on α4ß7 for homing during inflammatory conditions has not been fully examined. This may have implications for our understanding of the mode of action of anti-integrin therapies in inflammatory bowel disease (IBD). Here, we examined the role of α4ß7 integrin during chronic colitis using IL-10-/- mice, ß7-deficient IL-10-/-, IgA-deficient IL-10-/- mice, and antibody blockade of MAdCAM-1. We found that α4ß7 was predominantly expressed by B cells. ß7 deficiency and MAdCAM-1 blockade specifically depleted antibody secreting cells (ASC) (not T cells) from the colonic LP, leading to a fecal pan-immunoglobulin deficit, severe colitis, and alterations of microbiota composition. Colitis was not due to defective regulation, as dendritic cells (DC), regulatory T cells, retinaldehyde dehydrogenase (RALDH) expression, activity, and regulatory T/B-cell cytokines were all comparable between the strains/treatment. Finally, an IgA deficit closely recapitulated the clinical phenotype and altered microbiota composition of ß7-deficient IL-10-/- mice. Thus, a luminal IgA deficit contributes to accelerated colitis in the ß7-deficient state. Given the critical/nonredundant dependence of IgA ASC on α4ß7:MAdCAM-1 for intestinal homing, B cells may represent unappreciated targets of anti-integrin therapies.

An integrin αEß7-dependent mechanism of IgA transcytosis requires direct plasma cell contact with intestinal epithelium.

Efficient IgA transcytosis is critical for the maintenance of a homeostatic microbiota. In the canonical model, locally-secreted dimeric (d)IgA reaches the polymeric immunoglobulin receptor (pIgR) on intestinal epithelium via simple diffusion. A role for integrin αE(CD103)ß7 during transcytosis has not been described, nor its expression by intestinal B cell lineage cells. We found that αE-deficient (αE-/-) mice have a luminal IgA deficit, despite normal antibody-secreting cells (ASC) recruitment, local IgA production and increased pIgR expression. This deficit was not due to dendritic cell (DC)-derived retinoic acid (RA) nor class-switching defects, as stool from RAG-/- mice reconstituted with αE-/- B cells was also IgA deficient. Flow cytometric, ultrastructural and transcriptional profiling showed that αEß7-expressing ASC represent an undescribed subset of terminally-differentiated intestinal plasma cells (PC) that establishes direct cell to cell contact with intestinal epithelium. We propose that IgA not only reaches pIgR through diffusion, but that αEß7+ PC dock with E-cadherin-expressing intestinal epithelium to directly relay IgA for transcytosis into the intestinal lumen.

Inherent Immune Cell Variation Within Colonic Segments Presents Challenges for Clinical Trial Design.

BACKGROUND AND AIMS: Intestinal biopsy sampling during IBD trials represents a valuable adjunct strategy for understanding drug responses at the tissue level. Given the length and distinctive embryonic origins of the proximal and distal colon, we investigated whether inherent regional differences of immune cell composition could introduce confounders when sampling different disease stages, or pre/post drug administration. Here, we capitalise on novel mass cytometry technology to perform deep immunophenotyping of distinct healthy colonic segments, using the limited numbers of biopsies that can be harvested from patients. METHODS: Biopsies [2.8 mm] were collected from the caecum, transverse colon, descending colon, and rectum of normal volunteers. Intestinal leukocytes were isolated, stained with a panel of 37 antibodies, and mass cytometry data acquired. RESULTS: Site-specific patterns of leukocyte localisation were observed. The proximal colon featured increased CD8+ T cells [particularly resident memory], monocytes, and CD19+ B cells. Conversely, the distal colon and rectum tissues exhibited enrichment for CD4+ T cells and antibody-secreting cells. The transverse colon displayed increased abundance of both γδ T cells and NK cells. Subsets of leukocyte lineages also displayed gradients of expression along the colon length. CONCLUSIONS: Our results show an inherent regional immune cell variation within colonic segments, indicating that regional mucosal signatures must be considered when assessing disease stages or the prospective effects of trial drugs on leukocyte subsets. Precise protocols for intestinal sampling must be implemented to allow for the proper interpretation of potential differences observed within leukocyte lineages present in the colonic lamina propria.

Sphingosine-1-Phosphate Lyase Inhibition Alters the S1P Gradient and Ameliorates Crohn's-Like Ileitis by Suppressing Thymocyte Maturation.

BACKGROUND: Lymphocytes recirculate from tissues to blood following the sphingosine-1-phosphate (S1P) gradient (low in tissues, high in blood), maintained by synthetic and degradative enzymes, among which the S1P lyase (SPL) irreversibly degrades S1P. The role of SPL in the intestine, both during homeostasis and IBD, is poorly understood. We hypothesized that modulation of tissue S1P levels might be advantageous over S1P receptor (S1PR) agonists (eg, fingolimod, ozanimod, etrasimod), as without S1PR engagement there might be less likelihood of potential off-target effects. METHODS: First we examined SPL mRNA transcripts and SPL localization in tissues by quantitative reverse transcription polymerase chain reaction and immunohistochemistry. The in vivo effects of the SPL inhibitors 4-deoxypyridoxine hydrochloride (30 mg/L) and 2-acetyl-4 (tetrahydroxybutyl)imidazole (50 mg/L) were assessed through their oral administration to adult TNF∆ARE mice, which spontaneously develop Crohn's-like chronic ileitis. The effect of SPL inhibition on circulating and tissue lymphocytes, transcriptional regulation of proinflammatory cytokines, and on the histological severity of ileitis was additionally examined. Tissue S1P levels were determined by liquid chromatography-mass spectrometry. Mechanistically, the potential effects of high S1P tissue levels on intestinal leukocyte apoptosis were assessed via terminal deoxynucleotidyl transferase dUTP nick end-labeling assay and annexin 5 staining. Finally, we examined the ability of T cells to home to the intestine, along with the effects of SPL inhibition on cellular subsets within immune compartments via flow and mass cytometry. RESULTS: S1P lyase was ubiquitously expressed. In the gut, immunohistochemistry predominantly localized it to small intestinal epithelia, although the lamina propria leukocyte fraction had higher mRNA transcripts. Inhibition of SPL markedly increased local intestinal S1P levels, induced peripheral lymphopenia, downregulated proinflammatory cytokines, and attenuated chronic ileitis in mice. SPL inhibition reduced T and myeloid cells in secondary lymphoid tissues and the intestine and decreased naïve T-cell recruitment. The anti-inflammatory activity of SPL inhibition was not mediated by leukocyte apoptosis, nor by interference with the homing of lymphocytes to the intestine, and was independent of its peripheral lymphopenic effect. However, SPL inhibition promoted thymic atrophy and depleted late immature T cells (CD4+CD8+ double positive), with accumulation of mature CD4+CD8- and CD4-CD8+ single-positive cells. CONCLUSIONS: Inhibition of the S1P lyase alters the S1P gradient and attenuates chronic ileitis via central immunosuppression. SPL inhibition could represent a potential way to tame an overactive immune response during IBD and other T-cell-mediated chronic inflammatory diseases.