ICOS blockade inhibits expansion of GFP⁺ effectors in secondary lymphoid organs and GVHD target tissues. Effect of ICOS blockade in both a Tg and a polyclonal GVHD model is shown. Images of sublethally irradiated (6.5 Gy by ¹³⁷Cs) CB6 mice infused with purified 2C GFP⁺ CD8⁺ and TEa GFP⁺ CD4⁺ T cells (2 × 10⁶ each) are shown in left-hand panels of each organ. Images of lethally irradiated (8.0 Gy) BR mice infused with B6 non-GFP BM (20 × 10⁶) and purified B6 GFP⁺ T cells (3 × 10⁶) are shown in right-hand panels of each organ. Rat IgG or anti-ICOS mAb was administered. Representative images from 1 of 3 mice per group imaged at 1 and 2 weeks are shown. Stereomicroscope was set to × 1.0 zoom factor for intestinal loops in abdomen (A); × 3.2 for inguinal LN (B), skin (C), colon (D), and ileum (E); × 4.5 for Peyer patch (F); × 7.0 for liver (G) and spleen (H); and × 10.0 for lung (I). Exposure times were optimized for rIgG-treated mice for each organ and identical times were used for anti-ICOS-treated mice. Control mice not receiving GFP⁺ effectors to verify lack of autofluorescence resulted in dark images (not shown). NA indicates not available.