Human interleukin-21 (IL-21) is a class I cytokine previously reported in clinical studies on immune responsive cancers. Here we report the effects of systemic IL-21 therapy on the immune system in two phase 1 trials with this novel cytokine.

Recombinant IL-21 was administered by intravenous bolus injection at dose levels from 1 to 100 μg/kg using two planned treatment regimens: thrice weekly for 6 weeks (3/week); or once daily for five consecutive days followed by nine dose-free days (5 + 9). The following biomarkers were studied in peripheral blood mononuclear cells (PBMC) during treatment: phosphorylation of STAT3, alterations in the composition of leukocyte subsets, ex vivo cytotoxicity, expression of effector molecules in enriched CD8⁺ T cells and CD56⁺ NK cells by quantitative RT-PCR, and gene array profiling of CD8⁺ T cells.

Effects of IL-21 were observed at all dose levels. In the 5 + 9 regimen IL-21 induced a dose dependent decrease in circulating NK cells and T cells followed by a return to baseline in resting periods. In both CD8⁺ T cells and CD56⁺ NK cells we found up-regulation of perforin and granzyme B mRNA. In addition, full transcriptome analysis of CD8⁺ T cells displayed changes in several transcripts associated with increased cell cycle progression, cellular motility, and immune activation. Finally, cytotoxicity assays showed that IL-21 enhanced the ability of NK cells to kill sensitive targets ex vivo.

IL-21 was biologically active at all dose levels administered with evidence of in vivo NK cell and CD8⁺ T cell activation.