Introduction There is increasing evidence that a defect in the handling of intracellular bacteria is involved in the pathogenesis of Crohn's disease (CD). In CD, we have previously demonstrated a population of intestinal macrophages harbouring viable E coli and it is therefore possible that this failure of bacterial killing contributes to the observed inflammatory response. Previous studies showed that E coli laden (E coli positive) macrophages produce less TNFα compared to those not infected with bacteria (E coli negative). T cells are seen clustering around these infected macrophages suggesting T cell activation is altered between the innate and adaptive immune systems.

Aims To use co-localisation immunofluorescent techniques to investigate the immunological profile of T cells associated with E coli positive and negative macrophages, and those from control mucosa.

Methods Snap frozen mucosal biopsies were taken at routine colonoscopy from patients with CD and controls with normal colorectal mucosa and cryostat section made. E coli positive/negative macrophages were identified using CD-68 staining and a polyclonal anti E coli antibody and the T cells were identified using an anti CD3 antibody. In serial sections the T cells were labelled with anti IL10, IL17, TNFα, TGFβ, FoxP3 or IL23 receptor and the results analysed using confocal microscopy.

Results The profile of T cells surrounding E coli positive macrophages compared to E coli negative macrophages demonstrated an elevated number of cells with a regulatory phenotype. More of these T cells expressed IL-10 (67% vs 11%; p=0.001) and FoxP3 (14% vs 2%; p= 0.001). In contrast, increased numbers of pro-inflammatory T cells were seen surrounding E coli negative macrophages as measured by cells staining positive for TNFα (22%), IL-17 (26%) and IL23 (30%), which were not seen in association with E coli positive macrophages.

Conclusion There is an increased number and specific distribution of regulatory or inflammatory T cells surrounding lamina propria macrophages in CD according to intracellular persistence of E coli. The results suggest that despite E coli persistence, an appropriate immune containment occurs while it is the macrophages and T cells free of E coli which make the main contribution to active inflammation. Further studies are needed to determine the immune interplay between these populations and whether other microbial factors are present in E coli negative macrophages.