Adhesion assays of PRR2 transfectants. Adhesion assays of Da-1/PRR2 hi and Da1/PRR2δ hi on either PRR2-Fc (A, C, E, and G) or B7.1-Fc (B, D, F, and H) –coated petri dishes as described in Materials and Methods. Adhesion was observed by light microscopy. No or slight binding was observed with control Da-1 cells (A and B), whereas Da-1/PRR2 hi adhered specifically on PRR2-Fc–coated petri dish (C and D), similar to Da1/PRR2δ hi (E and F). The CD28-expressing cells adhere specifically on B7.1-Fc protein (G and H).