Mutant B7-1–transfected tumor cells that bind to CTLA4, but not to CD28, were selectively eliminated by P1CTL.

(A) Characterization of receptor binding of WT and mutant B7-1 by flow cytometry. COS cells were transiently transfected with plasmid expressing GFP (i), GFP-tagged wild-type B7 (B7-GFP) (ii), or mutant B7(W88→A)(iii) were stained with 100 μg/mL of either CD28 immunoglobulin or CTLA4 immunoglobulin mixed with PE-conjugated goat–antihuman IgG. Two-color flow cytometry was used to determine B7 expression versus receptor binding. Green indicates binding of CTLA4 immunoglobulin; blue, CD28 immunoglobulin; red, control. (B) Receptor binding and (C) tumorigenicity of J558-Neo and J558-B7W. (B) Binding to anti–B7-1 mAb (i), CD28 immunoglobulin (ii), and CTLA4 immunoglobulin (iii). (Ci, iii) (n = 4): tumor incidence (i) and growth kinetics (iii) of J558-Neo and J558-B7W tumors in RAG-2^−/− mice that received no T cells. (Cii, iv) (n = 5): incidence (ii) and growth kinetics (iv) in the presence of 15 × 10⁶CD28^+/− P1CTL.