P093 Dysregulation of cellular vs humoral immunity to the intestinal microbiota in inflammatory bowel disease

Abstract

Background: The intestinal microbiota is altered in inflammatory bowel disease (IBD), but how interactions between microbes and the host immune system differ in health versus IBD is not understood. Large numbers of lymphocytes reside in gut mucosal tissues and play a key role in barrier function and immune surveillance. We have analysed immune cells and microbes at the mucosal surface and compared acquired memory responses to specific commensal bacteria. Methods: Colonic biopsies were obtained from healthy donors, or Crohn's disease and ulcerative colitis patients not receiving corticos-teroid or biologic therapies. Intraepithelial lymphocytes (IEL) and lamina propria lymphocytes (LPL) were isolated by digestion, phe-notyped and counted by flow cytometry. Intraepithelial microbes were labelled with SYBR Green and anti-IgA and analysed by flow cytometry. Commensal bacteria were isolated from the GI tract of healthy donors and identified by 16S rRNA gene sequencing. PBMC were CellTrace VioletTM-labelled and cultured with heat-killed bacteria to determine microbe-specific CD4+/CD8+ T cell and B cell re-sponses after 7 days culture. Results: Numbers of intraepithelial lymphocytes (IEL), of both resident memory CD8+ and γδ T cell subsets, were dramatically reduced (75-95%, p<0.006) in IBD, and there were 70% fewer resident memory type CD4 T cells in the lamina propria in ulcerative colitis (p<0.017). IgA responses to bacteria on the gut epithelium, and circulating memory B cells responsive to certain intestinal bacteria were increased in IBD, while healthy PBMC demonstrated CD4 memory T cell and occasional CD8 T cell responses but few B cell responses. There were fewer CD8 memory T cell responses to commensals in Crohn's disease, correlating with the low numbers of CD8 T cells in IEL. Circulating B cells from IBD donors showed signs of activation including increased proportions of plasmablasts in Crohn's donors and IgA-switched memory B cells. Our data suggest an imbalance between humoral and cellular immunity towards the microbiota in IBD, with enhanced mucosal antibody secretion concomitant with a loss of mucosal T cell-mediated barrier immunity. Conclusions: Strong CD4 T cell memory to commensal bacteria de-velops in health, contradicting the concept of tolerance to intestinal bacteria. Lack of CD8 T cell immunity and deficiency in IEL in IBD may result in compromised barrier function at the mucosal surface, leading to excessive B cell activation and antibody secretion against the microbiota. Therefore, rather than blanket immunosuppression, stimulation of mucosal T cell immunity may improve barrier function and counteract abnormal T cell inflammation and B cell IgA secretion against intestinal microbes in IBD.