T cell proliferation-augmenting activities of the gene 3 protein derived from a phage library clone with CD80-binding activity.

We have isolated a phage clone, F2, by panning a phage library with a CTLA4-conformation recognizing mAb (anti-CTLA4 mAb). The unique sequence of 15 amino acids with an internal disulfide bond was inserted in the gene 3 proteins of F2 phage clone (F2-g3p). We show here that 1) F2-g3p was recognized with anti-CTLA4 mAb but not with anti-CD28 mAb, and 2) F2-g3p bound to CD80 but not to CD86. The surface plasmon resonance analysis showed that F2-g3p strongly bound CD80. F2-g3p inhibited the binding of CTLA4 to CD80 but not to CD86. In contrast, F2-g3p weakly inhibited the binding of CD28 with CD80. When hen egg lysozyme (HEL)-primed lymph node cells were stimulated with HEL in the presence of F2-g3p in vitro, cell proliferation was highly potentiated. In the absence of antigenic stimulation, F2-g3p induced no T cell proliferation, indicating the costimulatory nature of F2-g3p. The T cell-augmenting activity of the F2 clone was eliminated when the F2 clone was preincubated with CD80-Ig before the addition to the cultures, indicating the involvement of CD80-binding in the F2-g3p-mediated immunopotentiation. Thus, the F2 motif conferred CD80-binding activity and an immunoregulatory function to the g3p.