Background Imprinting an effector or regulatory phenotype on naïve T cells requires education at induction sites by dendritic cells (DC).

Objectives To analyse the effect of inflammation on the frequency of mononuclear phagocytes (MP) and the effect of altering their frequency by administration of Flt3-L in chronic ileitis.

Methods Using a tumour necrosis factor (TNF) driven model of ileitis (ie, TNFΔARE) that recapitulates many features of Crohn's disease (CD), dynamic changes in the frequency and functional state of MP within the inflamed ileum were assessed by flow cytometry, immunofluorescence and real-time reverse-transcription PCR and by generating CX₃CR1 GFP-reporter TNFΔARE mice. The effect of Flt3-L supplementation on the severity of ileitis, and the frequency of CD103⁺ DC and of FoxP3⁺ regulatory T cells was also studied in TNFΔARE mice.

Results CD11c^Hi/MHCII⁺ MP accumulated in inflamed ilea, predominantly mediated by expansion of the CX₃CR1⁺ MP subpopulation. This coincided with a decreased pro-regulatory CD103⁺ DC. The phenotype of these MP was that of activated cells, as they expressed increased CD80 and CD86 on their surface. Flt3-ligand administration resulted in a preferential expansion of CD103⁺ DC that attenuated the severity of ileitis in 20-week-old TNFΔARE mice, mediated by increased CD4⁺/CD25⁺/FoxP3⁺ regulatory T cells.

Conclusions Results support a role for Flt3-L as a potential therapeutic agent in Crohn's-like ileitis.