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Saccharomyces cerevisiae is a commensal yeast colonizer of mucosal surfaces and an emerging opportunistic pathogen in the mucosa and bloodstream. The role of S. cerevisiae has been largely characterized in peripheral blood mononuclear cells and monocyte-derived dendritic cells, where yeast cells induce the production of inflammatory cytokines through the interaction with mannose receptors, chitin receptors, DC SIGN, and dectin1. However, the response of blood-circulating dendritic cells (DCs) to S. cerevisiae has never been investigated. Among blood DCs, conventional DCs (cDCs) are producers of inflammatory cytokines, while plasmacytoid DCs (pDCs) are a specialized population producing a large amount of interferon (IFN)-α, which is involved in the antiviral immune response. Here we report that both human DC subsets are able to sense S. cerevisiae. In particular, cDCs produce interleukin (IL)-6, express activation markers, and promotes T helper 17 cell polarization in response to yeasts, behaving similarly to monocyte-derived DCs as previously described. Interestingly, pDCs, not cDCs, sense fungal nucleic acids, leading to the generation of P1-pDCs (PD-L1+CD80–), a pDC subset characterized by the production of IFN-α and the induction of a Th profile producing IL-10. These results highlight a novel role of pDCs in response to S. cerevisiae that could be important for the regulation of the host microbiota–immune system balance and of anti-fungal immune response.

Background: Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease of the central nervous system. The cause of multiple sclerosis is unknown but there are several evidences that associate the genetic basis of the disease with environmental causes. An important association between viral infection and development of MS is clearly demonstrated. Viruses have a strong impact on innate immune cells. In particular, myeloid dendritic cells (mDCs) and plasmacytoid dendritic cells (pDCs), are able to respond to viruses and to activate the adaptive immune response. Methods: In this study we mimic viral infection using synthetic single-strand RNA, Resiquimod, and we compared the response of both DC subsets derived from healthy donors and MS patients by characterizing the expression of costimulatory molecules on the DC surface. Results: We found that pDCs from MS patients express higher levels of OX40-L, HLA-DR, and CD86 than healthy donors. Moreover, we found that blood cells from MS patients and healthy donors upon Resiquimod-stimulation are enriched in a subpopulation of pDCs, characterized by a high amount of costimulatory molecules. Conclusion: Overall, these results indicate that activation of pDCs is enhanced in MS, likely due to a latent viral infection, and that costimulatory molecules expressed on pDCs could mediate a protective response against the viral trigger of autoimmunity.

Multiple sclerosis (MS) is a chronic disease of the central nervous system (CNS) characterized by persistent inflammation orchestrated by cluster of differentiation (CD) 4 T helper (Th) cells. In particular, Th1 and Th17 cells amplify, whereas T regulatory (Treg) cells moderate inflammation. The role of other Th subsets in MS is not clear. In the present study, we investigated the generation of different Th responses by human dendritic cells (DCs) in MS. We compared the production of several Th cytokines by naive CD4+ T-cells polarized with myeloid and plasmacytoid DCs (mDCs and pDCs) in healthy donors (HD) and relapsing–remitting (RR)-MS patients. We found that resiquimod-stimulated mDCs were able to activate Th17 differentiation, whereas pDCs induced interleukin (IL)-10-producing Th cells. Surprisingly, resiquimod-stimulated pDCs from MS patients also significantly induced the differentiation of Th9 cells, which produce IL-9 and are known to be involved in allergic diseases. We investigated the potential role of IL-9 in MS. We found that IL-9 activated signal transducer and activator of transcription (STAT) 1 and STAT5 phosphorylation and interfered with IL-17 and interferon (IFN) regulatory transcription factor (IRF)-4 expression in Th17-polarized cells. Moreover, in the cerebrospinal fluid (CSF) of 107 RR-MS patients, IL-9 inversely correlated with indexes of inflammatory activity, neurodegeneration and disability progression of MS. High levels of IL-9 were associated with the absence of IL-17 in the CSF of RR-MS patients. Our results demonstrate a Th9-inducing potential of pDCs in MS, suggesting an immunoregulatory role leading to attenuation of the exaggerated Th17 inflammatory response.

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS). T helper (Th) 17 lymphocytes play a role in the pathogenesis of MS. Indeed, Th17 cells are abundant in the cerebrospinal fluid and peripheral blood of MS patients and promote pathogenesis in the mouse model of MS. To gain insight into the function of Th17 cells in MS, we tested whether Th17 cells polarized from na&iuml, ve CD4 T cells of healthy donors and MS patients display different features. To this end, we analysed several parameters that typify the Th17 profile during the differentiation process of na&iuml, ve CD4 T cells obtained from relapsing-remitting (RR)-MS patients (n = 31) and healthy donors (HD) (n = 28). Analysis of an array of cytokines produced by Th17 cells revealed that expression of interleukin (IL)-21, tumour necrosis factor (TNF)-&beta, IL-2 and IL-1R1 is significantly increased in Th17 cells derived from MS patients compared to healthy donor-derived cells. Interestingly, IL-1R1 expression is also increased in Th17 cells circulating in the blood of MS patients compared to healthy donors. Since IL-2, IL-21, TNF-&beta, and IL-1R1 play a crucial role in the activation of immune cells, our data indicate that high expression of these molecules in Th17 cells from MS patients could be related to their high inflammatory status.

Functionally distinct T-helper (Th) subsets orchestrate immune responses. Maintenance of homeostasis through the tight control of inflammatory Th cells is crucial to avoid autoimmune inflammation. Activation-Induced Cell Death (AICD) regulates homeostasis of T cells, and it has never been investigated in human Th cells. We generated stable clones of inflammatory Th subsets involved in autoimmune diseases, such as Th1, Th17 and Th1/17 cells, from healthy donors (HD) and multiple sclerosis (MS) patients and we measured AICD. We find that human Th1 cells are sensitive, whereas Th17 and Th1/17 are resistant, to AICD. In particular, Th1 cells express high level of FAS-ligand (FASL), which interacts with FAS and leads to caspases’ cleavage and ultimately to cell death. In contrast, low FASL expression in Th17 and Th1/17 cells blunts caspase 8 activation and thus reduces cell death. Interestingly, Th cells obtained from healthy individuals and MS patients behave similarly, suggesting that this mechanism could explain the persistence of inflammatory IL-17-producing cells in autoimmune diseases, such as MS, where their generation is particularly substantial.