Regulation of CD28 binding to SH2 domains of Grb2 and PI3K by trisubstituted carboranes for T-cell activation.

Binding of adaptor molecules, such as growth factor receptor-bound protein 2 (Grb2) and phosphoinositide 3-kinase (PI3K), to the cytoplasmic region of CD28 is critical for T-cell activation. The Src homology 2 (SH2) domains of Grb2 and PI3K interact with the cytoplasmic region, including phosphorylated Tyr, of CD28. We found that trisubstituted carboranes efficiently increased the proliferation of T cells obtained from C57BL/6 mice. The carboranes specifically increased the binding of Grb2 Src homology 2 (SH2) to CD28-derived phosphopeptide but decreased the binding of PI3K C-terminal SH2 (cSH2). Based on the crystal structures of CD28-derived phosphopeptides complexed with Grb2 SH2 and PI3K cSH2, the bound structures of compound 4 (CRL266481) were modeled to determine the molecular mechanism of the regulation.