By now, several hundreds of HSP90 client proteins have been identified, including a number of protooncogenes [2]. Based on the vital role of HSP90 to stabilize mutated oncogenic proteins and to promote accumulation of over-expressed oncogenes [3], and its high level expression in tumor cells [4], this chaperone has gained long-standing interest as a molecular target for cancer therapy [5]. In this regard, the prototypic HSP90 inhibitor geldanamycin (GA) exerted strong proapoptotic effects on tumor cells in vitro[6]. Derivatives of GA [7], and other HSP90 inhibitors [8], which are optimized in terms of metabolic stability and reduced hepato-toxicity,

are being tested in several clinical INCB024360 trials [9]. In light of the essential role of HSP90 in protein homeostasis in all cell types [10], it is of vital importance to elucidate consequences of drug-mediated inhibition BYL719 cost of HSP90 on the patients’ immune system as required to eradicate drug-resistant tumor cells [11]. In this respect, dendritic cells (DCs)

as the main inducers of primary immune responses play an essential role [12]. Stimulation of DCs by pathogen-derived molecular patterns and endogenous danger signals as well as by activated T cells results in the activation and upregulated expression of NF-κB transcription factors like RelB [13], which in turn orchestrate expression of genes required for functional DC maturation [14]. Inhibition of HSP90 by GA was shown to result in diminished NF-κB activity in tumor cells due to impaired functional activity of NF-κB signaling molecules [15–17]. This suggests a modulatory role of HSP90 for the DC activation state. Here we show that treatment of MO-DCs with GA at low concentration (0.1 μm) resulted in their partial activation. In contrast, GA interfered with stimulation of

MO-DCs. In addition, GA prevented the proliferation of stimulated T cells. These findings suggest that inhibition of HSP90 may differentially affect the DC activation state as well as T cell responses in individuals treated with HSP90-inhibiting chemotherapeutics. Methods Cell culture Peripheral blood Branched chain aminotransferase mononuclear cells (PBMCs) were derived from buffy coats of healthy donors by Ficoll density gradient centrifugation, and monocytes were isolated by plastic adherence for 1 h in 6-well tissue culture plates (Starlab, Hamburg, Germany) as described [18]. Monocytes were differentiated in culture medium (Gibco, Houston, TX), containing 2% (v/v) heat-inactivated (56°C, 30 min) autologous plasma, penicillin (100 U/ml)/streptomycin (100 μg/ml) (both PAA, Pasching, Austria), supplemented with recombinant human (rh) GM-CSF (200 U/ml, Berlex, Seattle, WA), IL-4 (1,000 U/ml; ImmunoTools, Friesoythe, Germany).