The tumour microenvironment shapes dendritic cell plasticity in a human organotypic melanoma culture.

The tumour microenvironment (TME) forms a major obstacle in effective cancer treatment and for clinical success of immunotherapy. Conventional co-cultures have shed light onto multiple aspects of cancer immunobiology, but they are limited by the lack of physiological complexity. We develop a human organotypic skin melanoma culture (OMC) that allows real-time study of host-malignant cell interactions within a multicellular tissue architecture. By co-culturing decellularized dermis with keratinocytes, fibroblasts and immune cells in the presence of melanoma cells, we generate a reconstructed TME that closely resembles tumour growth as observed in human lesions and supports cell survival and function. We demonstrate that the OMC is suitable and outperforms conventional 2D co-cultures for the study of TME-imprinting mechanisms. Within the OMC, we observe the tumour-driven conversion of cDC2s into CD14+ DCs, characterized by an immunosuppressive phenotype. The OMC provides a valuable approach to study how a TME affects the immune system.

Retinoic acid primes human dendritic cells to induce gut-homing, IL-10-producing regulatory T cells.

The vitamin A metabolite all-trans retinoic acid (RA) is an important determinant of intestinal immunity. RA primes dendritic cells (DCs) to express CD103 and produce RA themselves, which induces the gut-homing receptors α4ß7 and CCR9 on T cells and amplifies transforming growth factor (TGF)-ß-mediated development of Foxp3(+) regulatory T (Treg) cells. Here we investigated the effect of RA on human DCs and subsequent development of T cells. We report a novel role of RA in immune regulation by showing that RA-conditioned human DCs did not substantially enhance Foxp3 but induced α4ß7(+) CCR9(+) T cells expressing high levels of interleukin (IL)-10, which were functional suppressive Treg cells. IL-10 production was dependent on DC-derived RA and was maintained when DCs were stimulated with toll-like receptor ligands. Furthermore, the presence of TGF-ß during RA-DC-driven T-cell priming favored the induction of Foxp3(+) Treg cells over IL-10(+) Treg cells. Experiments with naive CD4(+) T cells stimulated by anti-CD3 and anti-CD28 antibodies in the absence of DCs emphasized that RA induces IL-10 in face of inflammatory mediators. The data thus show for the first time that RA induces IL-10-producing Treg cells and postulates a novel mechanism for IL-10 in maintaining tolerance to the intestinal microbiome.