Dendritic cell based antitumor vaccination: impact of functional indoleamine 2,3-dioxygenase expression.

Background: Recent reports have demonstrated that the enzyme indoleamine 2,3-dioxygenase (IDO) is upregulated in human dendritic cells (DCs) upon in vitro maturation. IDO is supposed to convey immunosuppressive effects by degrading the essential amino acid tryptophan, thereby downregulating T-cell functions. Hence, we evaluated IDO expression in DC preparations used for therapeutic DC vaccination and its in vivo effects.
Patients, Methods and Results: IDO expression was detected by real-time-PCR in a series of human clinical grade DCs (n = 28) prior to vaccination of advanced melanoma patients (n = 11). These analyses revealed an intra- and interpersonal variation in IDO mRNA levels. IDO was strongly upregulated in human DCs on RNA and on protein level upon in vitro maturation by Interleukin-1beta (IL-1beta), tumour necrosis factor-alpha (TNF-alpha), Interleukin-6 (IL-6) and Prostaglandin E2 (PGE2) over a time course of 24 h. The enzymatic activity of induced IDO was demonstrated by measuring tryptophan degradation. Moreover, in biopsies obtained 24 h after application of the DC vaccine a prominent infiltrate of IDO-positive cells was observed by immunohistochemistry. The inflammatory infiltrate of these sites stained positive for the transcription factor Forkhead box P3 (FoxP3), suggesting an IDO-mediated induction of regulatory T-cells. All analysed melanoma patients (n = 11) receiving DC based immunotherapy exhibited rapid disease progression with a short overall survival due to advanced tumour stage.
Conclusion: The presented observations suggest a potential clinical relevance of IDO expression in DC-based therapeutic vaccines via the attraction or induction of FoxP3(+) T-cells.