Human intestinal pro-inflammatory CD11chighCCR2+CX3CR1+ macrophages, but not their tolerogenic CD11c-CCR2-CX3CR1- counterparts, are expanded in inflammatory bowel disease.

Although macrophages (Mϕ) maintain intestinal immune homoeostasis, there is not much available information about their subset composition, phenotype and function in the human setting. Human intestinal Mϕ (CD45+HLA-DR+CD14+CD64+) can be divided into subsets based on the expression of CD11c, CCR2 and CX3CR1. Monocyte-like cells can be identified as CD11chighCCR2+CX3CR1+ cells, a phenotype also shared by circulating CD14+ monocytes. On the contrary, their Mϕ-like tissue-resident counterparts display a CD11c-CCR2-CX3CR1- phenotype. CD11chigh monocyte-like cells produced IL-1ß, both in resting conditions and after LPS stimulation, while CD11c- Mϕ-like cells produced IL-10. CD11chigh pro-inflammatory monocyte-like cells, but not the others, were increased in the inflamed colon from patients with inflammatory bowel disease (IBD), including Crohn's disease and ulcerative colitis. Tolerogenic IL-10-producing CD11c- Mϕ-like cells were generated from monocytes following mucosal conditioning. Finally, the colonic mucosa recruited circulating CD14+ monocytes in a CCR2-dependent manner, being such capacity expanded in IBD. Mϕ subsets represent, therefore, transition stages from newly arrived pro-inflammatory monocyte-like cells (CD11chighCCR2+CX3CR1+) into tolerogenic tissue-resident (CD11c-CCR2-CX3CR1-) Mϕ-like cells as reflected by the mucosal capacity to recruit circulating monocytes and induce CD11c- Mϕ. The process is nevertheless dysregulated in IBD, where there is an increased migration and accumulation of pro-inflammatory CD11chigh monocyte-like cells.

The Innate Immune System in the Gastrointestinal Tract: Role of Intraepithelial Lymphocytes and Lamina Propria Innate Lymphoid Cells in Intestinal Inflammation.

Background: The gastrointestinal tract harbors the largest microbiota load in the human body, hence maintaining a delicate balance between immunity against invading pathogens and tolerance toward commensal. Such immune equilibrium, or intestinal homeostasis, is conducted by a tight regulation and cooperation of the different branches of the immune system, including the innate and the adaptive immune system. However, several factors affect this delicate equilibrium, ultimately leading to gastrointestinal disorders including inflammatory bowel disease. Therefore, here we decided to review the currently available information about innate immunity lymphocyte subsets playing a role in intestinal inflammation. Results: Intestinal innate lymphocytes are composed of intraepithelial lymphocytes (IELs) and lamina propria innate lymphoid cells (ILCs). While IELs can be divided into natural or induced, ILCs can be classified into type 1, 2, or 3, resembling, respectively, the properties of TH1, TH2, or TH17 adaptive lymphocytes. Noteworthy, the phenotype and function of both IELs and ILCs are disrupted under inflammatory conditions, where they help to exacerbate intestinal immune responses. Conclusions: The modulation of both IELs and ILCs to control intestinal inflammatory responses represents a major challenge, as they provide tight regulation among the epithelium, the microbiota, and the adaptive immune system. An improved understanding of the innate immunity mechanisms involved in gastrointestinal inflammation would therefore aid in the diagnosis and further treatment of gastrointestinal inflammatory disorders.