Impaired activation of plasmacytoid dendritic cells via toll-like receptor 7/9 and STING is mediated by melanoma-derived immunosuppressive cytokines and metabolic drift

Abstract

IntroductionPlasmacytoid dendritic cells (pDCs) infiltrate a large set of human cancers. Interferon alpha (IFN-α) produced by pDCs induces growth arrest and apoptosis in tumor cells and modulates innate and adaptive immune cells involved in anti-cancer immunity. Moreover, effector molecules exert tumor cell killing. However, the activation state and clinical relevance of pDCs infiltration in cancer is still largely controversial. In Primary Cutaneous Melanoma (PCM), pDCs density decreases over disease progression and collapses in metastatic melanoma (MM). Moreover, the residual circulating pDC compartment is defective in IFN-α production.MethodsThe activation of tumor-associated pDCs was evaluated by in silico and microscopic analysis. The expression of human myxovirus resistant protein 1 (MxA), as surrogate of IFN-α production, and proximity ligation assay (PLA) to test dsDNA-cGAS activation were performed on human melanoma biopsies. Moreover, IFN-α and CXCL10 production by in vitro stimulated (i.e. with R848, CpG-A, ADU-S100) pDCs exposed to melanoma cell lines supernatants (SN-mel) was tested by intracellular flow cytometry and ELISA. We also performed a bulk RNA-sequencing on SN-mel-exposed pDCs, resting or stimulated with R848. Glycolytic rate assay was performed on SN-mel-exposed pDCs using the Seahorse XFe24 Extracellular Flux Analyzer.ResultsBased on a set of microscopic, functional and in silico analyses, we demonstrated that the melanoma milieu directly impairs IFN-α and CXCL10 production by pDCs via TLR-7/9 and cGAS-STING signaling pathways. Melanoma-derived immunosuppressive cytokines and a metabolic drift represent relevant mechanisms enforcing pDC-mediated melanoma escape.DiscussionThese findings propose a new window of intervention for novel immunotherapy approaches to amplify the antitumor innate immune response in cutaneous melanoma (CM).

Plasmacytoid dendritic cells at the forefront of anti-cancer immunity: rewiring strategies for tumor microenvironment remodeling

“…Specifically, the synergy between these cytokines was accountable for the TLR7/9 signaling inhibition, including IRF7 downregulation, impairing the I-IFN production by pDCs [ 124 , 125 ]. Accordingly, our group proposed a TGF-β-mediated functional impairment of TLR/MyD88-dependent signaling in melanoma-associated pDCs through IRF7 downregulation [ 126 ]. Wnt5a is an immunosuppressive molecule identified in the melanoma TME affecting pDC function [ 127 ].…”

“…As thoroughly described in the following section of this review, pDC functionality has been linked to glycolysis in order to provide the high energy demand for rapid I-IFN production [ 153 ]. Our group has demonstrated that IFN-α production by melanoma-exposed pDCs was hijacked and could derive from their metabolic drift [ 126 ] (Fig. 4 ).…”

“…Interestingly, an increase in pDC recruitment was observed in BRAFV600E melanomas compared to BRAF wild-type melanomas [ 99 , 213 ]. In silico analysis of a pDC signature in pan-cancer TCGA datasets showed that a high pDC content was limited to TDLNs and stage I primary tumors, whereas pDC signature was poor in CM as compared to primary carcinomas [ 99 , 126 ]. Instead, the pDC-specific signature has been significantly associated to a favorable survival outcome in the EBV-associated nasopharyngeal carcinoma, suggesting the potential role of pDCs in inducing antitumor immune responses [ 214 ].…”

“…In pan-cancer TCGA datasets, the signatures of pDC and I-IFNs were not correlated, suggesting that pDCs might be partly dysfunctional in I-IFNs production in the TME [ 126 ]. The above-mentioned studies are limited to immunohistochemistry or flow cytometry approach and lack functional data on the role of pDCs in the TME (Table 1 ).…”

“…We found that the lactate concentration was inversely associated to the concentration of glucose in melanoma-derived supernatants [ 212 ]. Mechanistically, in vitro exposure to melanoma soluble factors or lactic acidosis induced an IFN-α-defective tolerogenic state on fully differentiated pDCs [ 99 , 126 , 212 ]. Our recent data suggested that glucose deprivation and lactic acidosis by melanoma cells hindered glycolytic metabolism in pDCs, resulting ultimately in a detrimental effect on their IFN-α production [ 126 , 212 , 235 ] (Fig.…”