Figure 5.

Binding kinetics correlate with efficacy in the IL-21-mediated pSTAT3 assay. Serial dilutions of IL-21 antagonists were pre-incubated with 96 pM IL-21 for 0 (filled squares), 5 (open triangles), 15 (filled inverted triangles) or 30 (open diamonds) min prior to initiation of the pSTAT3 assay. Recombinant CHO-expressed mAbs were used in these experiments. (A) With a relatively slow association rate, clone 366.552 requires a longer pre-incubation period than clone 362.78 or IL-21R to completely neutralize IL-21-mediated STAT3 phosphorylation. (B) IL-21 mAb clone 362.78 (very fast association rate) demonstrated the most potent inhibition of IL-21 mediated pSTAT3 with complete neutralization following 5, 15 and 30 min pre-incubation periods. (C) Soluble IL-21R/γ_c neutralized IL-21-mediated pSTAT3 in a time-dependent manner, but not as effectively as clone 362.78.