Figure 1.

The interaction between the microbiome and the immune system. Illustration of jejunal enterocyte histology with particular attention to the lamina propria and its resident immune cells (dendritic cells, macrophages, innate lymphoid cells (ILC) and T cells. The enteric immune system provides CD4 T cells with both potential for survival and polarization into different effector subtypes. In terms of survival, naïve CD4 T cells have the capacity to home to the gut and interact with cross-reactive Ag derived from the biomass in the lumen via Ag presentation from a MHC II expressing cell (either traditional APC, specialized intestinal epithelia or due to induced expression of MHC II on enterocytes). In addition, survival factors such as TSLP may be expressed by the enterocytes. Memory CD4 T cells, which can be directly reactive or cross-reactive to the Ag in question, can receive survival cues from similar immune components in the gut environment. Both cell subsets can receive polarizing cues (e.g. IL-6 and IL-21) predisposing the CD4 T cells in question to adopt a Th17 phenotype, which is central in both gut immunity and inflammatory bowel diseases.