Exclusive increase of CX3CR1⁺CD28^-CD4⁺ T cells in inflammatory bowel disease and their recruitment as intraepithelial lymphocytes

Background: CX3CL1/Fractalkine (FKN) has been reported to play important roles in various inflammatory diseases. We examined the role of FKN and its receptor CX3CR1 in T-cell migration in the inflammatory bowel diseases (IBDs), ulcerative colitis (UC) and Crohn's disease (CD). Methods: CX3CR1 expression on peripheral CD4⁺ cells from normal controls (NL n = 24) and IBD patients (UC n = 28, CD n = 26) was examined using flow cytometry. CX3CR1 ⁺CD4⁺ T cells were further characterized for surface antigens, cytokine production, and cytotoxic granule release by flow cytometry and ELISA. FKN expression in 53 colonic biopsy specimens (UC n = 20, CD n = 23, NL n = 10) was analyzed by quantitative PCR and immunohistochemistry. Isolated lamina propria and intraepithelial lymphocytes were also analyzed by flow cytometry (UC n = 10, CD n = 10, NL n = 6). Results: CX3CR1⁺CD4 ⁺ cells were increased in IBD while they were virtually absent in controls. Upregulation of CX3CR1 on CD4⁺ T cells was positively correlated with disease activity. These unique T cells expressed markers for both effector memory and cytotoxic cells. Interestingly, CX3CR1 was expressed on CD4⁺ T cells lacking CD28. CX3CR1⁺CD28 ^-CD4⁺ cells produced more IFN-γ and TNF-α than CX3CR1^- counterparts and released cytotoxic granules. FKN mRNA was upregulated in inflamed colonic tissues and robust expression of FKN was immunohistochemically observed on epithelial cells. Although CX3CR1⁺ CD4⁺ cells could not be detected in the gut, CD28^-CD4 ⁺ cells were found in IBD mainly as intraepithelial lymphocytes. Conclusions: FKN/CX3CR1 may contribute to the pathogenesis of IBD through the emergence of unique CX3CR1⁺CD28^-CD4⁺ T cells that can act both as proinflammatory and cytotoxic cells.