Tumor-Derived Lactic Acid Modulates Dendritic Cell Activation and Differentiation.

The tumor milieu can influence DC differentiation in vivo and in vitro. We analyzed DC differentiation in a 3-dimensional tumor model and propose a new mechanism of DC modulation by the tumor environment. Monocytes were cultured in the presence of IL-4 and GM-CSF within multicellular tumor spheroids (MCTS) generated from urothelial carcinoma (J82, UMUC3) and melanoma cell lines (MelIm, Mel108). Monocytes invaded the tumor spheroids and differentiated into tumor-associated dendritic cells (TADC) that displayed an altered phenotype compared to DC generated without tumor contact. The expression of CD1a was reduced on TADC whereas CD80, CD86 and CD16 were upregulated. In addition, TADC generated within J82-MCS showed a reduced expression of HLA-DR and accordingly the antigen presentation in a mixed lympocyte reaction was decreased. We then analyzed the underlying mechanism. Supernatants of MCTS could partially transfer the suppressive effect. Conditioned media from urothelial carcinoma cell lines contained high levels of M-CSF and IL-6, both cytokines known to modulate DC differentiation. In contrast, melanoma MCTS cocultures produced little M-CSF and IL-6, but further analysis revealed that melanoma cells secreted high levels of lactic acid. Indeed, addition of lactic acid during DC differentiation in vitro induced a phenotype comparable to TADC generated within melanoma MCTS. Inhibition of melanoma cell lactic acid production by the addition of oxamic acid, an inhibitor of lactate dehydrogenase, reverted the TADC phenotype back to normal. We therefore conclude that tumor-derived lactic acid is an important factor modulating the DC phenotype in the tumor environment which may critically contribute to tumor escape mechanisms.