Imatinib mesylate inhibits STAT5 phosphorylation in response to IL-7 and promotes T cell lymphopenia in chronic myelogenous leukemia patients.

Imatinib mesylate (IM) therapy has been shown to induce lower T cell counts in chronic myelogenous leukemia (CML) patients and an interference of IM with T cell receptor (TCR) signaling has been invoked to explain this observation. However, IL-7 and TCR signaling are both essential for lymphocyte survival. This study was undertaken to determine whether IM interferes with IL-7 or TCR signaling to explain lower T cell counts in patients. At diagnosis, CML patients have typically lower CD4 counts in their blood, yet CD8 counts are normal or even increased in some. Following the initiation of IM treatment, CD4 counts were further diminished and CD8 T lymphocytes were dramatically decreased. In vitro studies confirmed IM interference with TCR signaling through the inhibition of ERK phosphorylation and we showed a similar effect on IL-7 signaling and STAT5 phosphorylation (STAT5-p). Importantly however, using an in vivo mouse model, we demonstrated that IM impaired T cell survival through the inhibition of IL-7 and STAT5-p but not TCR signaling which remained unaffected during IM therapy. Thus, off-target inhibitory effects of IM on IL-7 and STAT5-p explain how T cell lymphopenia occurs in patients treated with IM.