Background:: Ileal involvement in Crohn's disease (CD) is associated with NOD2 mutations and granulocyte‐macrophage colony stimulating factor autoantibodies (GM‐CSF Ab), and GM‐CSF blockade promotes ileitis in Nod2/Card15‐deficient (C15KO) mice. RALDH2‐expressing dendritic cells (DC) and IL‐4 promote CCR9 imprinting and small bowel homing of T lymphocytes, in conjunction with CCL25 expression by ileal epithelial cells (IEC). We hypothesized that GM‐CSF neutralization promotes ileal disease by modulating expression of CCL25 by IEC and CCR9 by T lymphocytes via Nod2‐dependent and independent pathways.
Methods:: CCL25 and CCR9 expression were determined in pediatric CD patients stratified by GM‐CSF Ab. Ileitis was induced in C15KO mice via GM‐CSF Ab administration followed by nonsteroidal antiinflammatory drug (NSAID) exposure, and expression of CCL25, CCR9, FOXP3, intracellular cytokines, and RALDH2 was determined in IEC and immune cell populations.
Results:: The frequency of CCL25+ IEC and CCR9+ T lymphocytes was increased in CD patients with elevated GM‐CSF Ab. In the murine model, GM‐CSF blockade alone induced IEC CCL25 expression, and reduced the frequency of mesenteric lymph node (MLN) CD4+FOXP3+ cells, while Card15 deficiency alone enhanced MLN DC RALDH2 expression. Both GM‐CSF neutralization and Card15 deficiency were required for downregulation of MLN DC IL‐10 expression; under these conditions NSAID exposure led to an expansion of IL‐4+ and IL‐17+ CCR9+ lymphocytes in the ileum.
Conclusions:: GM‐CSF prevents ileal expansion of CCR9+ lymphocytes via Nod2‐dependent and independent pathways. CCR9 blockade may be beneficial in CD patients with elevated GM‐CSF Ab.