Dendritic cells (DCs) play a pivotal role in regulating innate and adaptive immunity and are key elements in the immunological response against tumors. Several specialized features make DCs uniquely efficient in priming anti-cancer T cell responses. DCs possess specialized pathways to acquire tumor antigens from apoptotic and necrotic cells and to present them to tumor specific T cells. Efficient T cell activation depends on the density of T cell receptors (TCR) ligands and co-stimulatory molecules on the cell surface, and on secretion of cytokines that determine T cell differentiation. It is well documented that DCs properties are suppressed by the tumor microenvironment and many evidences suggest that tolerogenic DCs may be a crucial element in the development of tumor-mediated immune anergy. The tumor microenvironment suppresses DCs functions by targeting different steps in the antigen presentation process, such as phagocytosis, antigen processing and presentation. Suppression also causes an important skewing in the pattern of secreted cytokines. In particular, how secretion of inflammatory cytokines becomes impaired in tumor-exposed DCs remains unclear.
We have recently undergone a study to identify the trafficking proteins SNAREs (soluble N-ethylmaleimide-sensitive factor-attachment protein receptor) implicated in secretion of cytokines in DCs and to examine whether they are modified by the tumor microenvironment. Using imaging and functional assays we identified the SNARE VAMP7 as a specific mediator of the secretion of interleukine-12 (IL-12), the major inflammatory cytokine implicated in T cell priming. Interestingly, depletion of VAMP7 does not affect secretion of the inhibitory cytokine IL-10, indicating that distinct secretory pathways direct the release of mediators with opposite cellular functions. We next analyzed lung DCs in healthy mice and in mice bearing Lewis lung carcinoma (LLC). We observed a consistent tumor-induced differentiation of the subpopulation CD11chigh/CD11b-/Gr-1- into a CD11bint phenotype, which was also confirmed in vitro by co-culturing conventional DCs on LLC monolayer. Both in vitro and in vivo, these cells show a normal maturation capacity and expression of surface co-stimulatory molecules, but a strong impairment in inflammatory cytokines secretion, in particular IL-12, upon restimulation with TLR agonists. We isolated CD11chigh/CD11b-/int/Gr-1- cells to high purity to analyze the expression of trafficking proteins. We found that tumor-exposed DCs downregulate, both at mRNA and protein level, the SNAREs VAMP7 and VAMP3, and Rab27. In addition, VAMP7 in tumor-educated DCs displays a different molecular weight suggesting post-transcriptional modification or expression of a different isoform.
These preliminary observations suggest that regulation/modification of trafficking proteins in DCs may represent a novel pathway targeted during suppression. Ongoing studies aim at extending the analysis to other trafficking proteins potentially involved in modulation of cytokine secretion.
Citation Format: Giulia Chiaruttini, Irene Soncin, Jennifer L. Stow, Andrew A. Peden, Federica Benvenuti. Tumor-educated CD11c+/CD11bint/Gr-1- regulatory dendritic cells show a mutated pattern of trafficking molecules implicated in cytokine secretion. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr B4.