Bortezomib-induced pro-inflammatory macrophages as a potential factor limiting anti-tumour efficacy.

Multiple myeloma (MM) is a chronic progressive malignancy of plasma cells. Although treatment with the novel proteasome inhibitor, bortezomib, significantly improves patient survival, some patients fail to respond due to the development of de novo resistance. We have previously shown that cytotoxic drugs can induce pro-tumorigenic host-mediated effects which contribute to tumour re-growth and metastasis, and thus limit anti-tumour efficacy. However, such effects and their impact on tumour cell aggressiveness have not been investigated using cytostatic agents such as bortezomib. Here we show that plasma from bortezomib-treated mice significantly increases migration, viability and proliferation of MM cells in vitro, compared to plasma from vehicle treated mice. In vivo, bortezomib induces the mobilization of pro-angiogenic bone marrow cells. Furthermore, mice treated with bortezomib and subsequently were used as recipients for an injection of MM cells succumb to MM earlier than mice treated with the vehicle. We show that bortezomib promotes pro-inflammatory macrophages which account for MM cell aggressiveness, an effect which is partially mediated by interleukin-16. Accordingly, co-inoculation of MM cells with pro-inflammatory macrophages from bortezomib-treated mice accelerates MM disease progression. Taken together, our results suggest that, in addition to the known effective anti-tumour activity of bortezomib, host-driven pro-tumorigenic effects generated in response to treatment can promote MM aggressiveness, and thus may contribute to the overall limited efficacy.