STAT3 blockade decreases baseline NKG2D expression of primary human NK cells. (A) To assess the potential direct toxicity of STAT3 inhibitors, freshly-purified human NK cells were cultured in the presence of the STAT3 inhibitors, JSI-124 and S3I-201. NK-cell viability was evaluated at 24, 48, and 72 hours by flow-cytometric analysis of 7-AAD staining. (B) To assess the effect of STAT3 inhibition on NKG2D expression, NK cells were treated with the nontoxic concentrations of STAT3 inhibitors (JSI-124 and S3I-201) for 24 hours, after which NKG2D expression was evaluated by flow cytometry. Replicates are from 3 different donors. P values indicated are for 2-tailed Student t tests of comparisons to untreated NK cells with Bonferroni correction. *P < .05; ***P < .001. NS, no statistical significance.