Background & Aims: Inducible costimulator (ICOS)/B7RP-1 represents a newly described receptor/ligand pair involved in costimulation of T cells by antigen-presenting cells. We investigated the involvement of the ICOS/B7RP-1 interaction in the pathogenesis of colitis and the therapeutic potential of anti-ICOS monoclonal antibody (mAb) in experimental colitis. Methods: We administered anti-ICOS or anti-B7RP-1 mAb to mice with experimental colitis induced by transfer of CD4+CD45RBhigh T cells from normal mice into SCID mice. The ability of CD4+CD45RBhigh cells derived from ICOS-/- mice to induce colitis was assessed. Th2 cytokine production and apoptosis in infiltrating T cells was examined after administration of anti-ICOS mAb. Results: ICOS was strongly induced on CD4+ T cells, and B7RP-1 was expressed by macrophages in the inflamed mucosa of colitic mice. Anti-ICOS mAb, but not anti-B7RP-1, ameliorated chronic colitis when administered in prevention or therapeutic protocols. Transfer of CD4+CD45RBhigh T cells from ICOS-/- mice induced colitis. Treatment with anti-ICOS mAb did not enhance the production of Th2 cytokines, but a single dose of anti-ICOS mAb induced massive apoptosis of infiltrating ICOS-expressing T cells. Conclusions: ICOS/B7RP-1 interactions are not required for the development of colitis. However, treatment with anti-ICOS mAb can prevent and reverse intestinal inflammation by inducing apoptosis of ICOS-expressing T lymphocytes.
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