The murine anti-human common gamma chain monoclonal antibody CP.B8 blocks the second step in the formation of the intermediate affinity IL-2 receptor.

A murine monoclonal antibody, CP.B8, specific for the extracellular portion of the human common gamma (gammac) chain, and its Fab fragment are shown to block the binding of IL-2 to COS-7 cells transfected with the cDNA for the full-length IL-2 receptor beta (IL-2Rbeta) and gammac chains, components which together comprise the intermediate affinity IL-2 receptor (IL-2R) expressed on the surface of resting T cells, NK cells, and on certain intestinal epithelial cells. To investigate the mechanism of this inhibition, the extracellular portions of the IL-2Rbeta and gammac chains were expressed and purified, and their interactions with each other and with IL-2 were studied by gel filtration and by surface plasmon resonance (SPR). By gel filtration, a stable ternary complex was formed by association of the three proteins, while no stable binary complexes were detected between any two of the three proteins. By SPR analysis, IL-2 was shown to associate rapidly with IL-2Rbeta, forming a binary complex with an equilibrium dissociation constant (Kd) of 800 nM, which permitted subsequent association of the gammac chain. Dissociation of the IL-2/IL-2Rbeta/gammac chain complex was significantly slower than dissociation of the IL-2/IL-2Rbeta complex. Using these model systems, we tested the ability of mAb CP.B8 to inhibit the association of the gammac chain with IL-2 and IL-2Rbeta. By gel filtration, mAb CP.B8 formed a stable complex with the gammac chain, preventing its association with IL-2 and IL-2Rbeta. MAb CP.B8 was also capable of dissociating the gammac chain already complexed with IL-2 and IL-2Rbeta. SPR analysis confirmed these findings and showed, in addition, that the Fab fragment of CP.B8 was also capable of inhibiting the association of the gammac chain with the IL-2/IL-2Rbeta complex. We conclude that mAb CP.B8 blocks the second step in the formation of the intermediate affinity IL-2R on the surface of transfected COS-7 cells by binding at or close to a region on the gammac chain that is involved in contact with IL-2 and/or IL-2Rbeta.