Enhanced dendritic cell development through long-term proteasome inhibition

Bortezomib (BTZ) is a reversible proteasome inhibitor. It affects NF-κB activity and has shown clinical efficacy in multiple myeloma. However, the same mechanisms through which it curtails malignant growth may compromise antitumor immunity. As dendritic cells (DC) play a vital role in the elicitation and maintenance of antitumor immunity, we studied the long-term effects of BTZ on human DC development and function. The CD34 + MUTZ-3 cell line, stably transduced with human telomerase reverse transcriptase (hTERT), was employed as a sustainable model to study human steady-state DC development and was grown in the presence of consecutively rising BTZ concentrations over a period of 4 months. The thus generated BTZ adapted cells were prospectively assessed for their ability to develop into mature Langerhans cells (LC). Long-term exposure to BTZ (>10 months) at a clinically relevant and apoptosis-inducing concentration (10 nM) provoked spontaneous differentiation in surviving precursors, evidenced by increased expression levels of CD14, TNF receptors and immuno-proteasome subunits. Cytokine-dependent differentiation was also enhanced, resulting in increased numbers of mature DC with higher levels of co-stimu-latory molecules and an increased capacity for T cell induction. Assessment of nuclear NF-κB subunit levels provided evidence for a role of canonical RelB/ p50 activation in the enhanced DC differentiation and maturation established through prolonged BTZ exposure. We conclude that long-term BTZ treatment is compatible with DC-dependent induction of antitumor immunity.