Sammy Bedoui

Cross-presentation of viral and self antigens by skin-derived CD103 + dendritic cells

Skin-derived dendritic cells (DCs) include Langerhans cells, classical dermal DCs and a langerin-positive CD103+ dermal subset. We examined their involvement in the presentation of skin-associated viral and self antigens. Only the CD103+ subset efficiently presented antigens of herpes simplex virus type 1 to naive CD8+ T cells, although all subsets presented these antigens to CD4+ T cells. This showed that CD103+ DCs were the migratory subset most efficient at processing viral antigens into the major histocompatibility complex class I pathway, potentially through cross-presentation. This was supported by data showing only CD103+ DCs efficiently cross-presented skin-derived self antigens. This indicates CD103+ DCs are the main migratory subtype able to cross-present viral and self antigens, which identifies another level of specialization for skin DCs.

Putative IKDCs are functionally and developmentally similar to natural killer cells, but not to dendritic cells.

Interferon-producing killer dendritic cells (IKDCs) have been described as possessing the lytic potential of NK cells and the antigen-presenting capacity of dendritic cells (DCs). In this study, we examine the lytic function and antigen-presenting capacity of mouse spleen IKDCs, including those found in DC preparations. IKDCs efficiently killed NK cell targets, without requiring additional activation stimuli. However, in our hands, when exposed to protein antigen or to MHC class II peptide, IKDCs induced little or no T cell proliferation relative to conventional DCs or plasmacytoid DCs, either before or after activation with CpG, or in several disease models. Certain developmental features indicated that IKDCs resembled NK cells more than DCs. IKDCs, like NK cells, did not express the transcription factor PU.1 and were absent from recombinase activating gene-2–null, common γ-chain–null (Rag2−/−Il2rg−/−) mice. When cultured with IL-15 and -18, IKDCs proliferated extensively, like NK cells. Under these conditions, a proportion of expanded IKDCs and NK cells expressed high levels of surface MHC class II. However, even such MHC class II+ IKDCs and NK cells induced poor T cell proliferative responses compared with DCs. Thus, IKDCs resemble NK cells functionally, and neither cell type could be induced to be effective antigen-presenting cells.

Relevance of Neuropeptide Y for the neuroimmune crosstalk

Both cellular and humoral functions of the immune system are modulated by the sympathetic nervous system (SNS). This interaction is mainly mediated by the release of catecholamines (CA) and their receptor-specific action on immune cells. However, neuropeptide Y (NPY), also present in sympathetic nerve terminals, is released upon SNS-stimulation. NPY modulates potent immunological effects in vitro and in vivo, such as differentiation of T helper cells, monocyte mediator release, NK cell activation, and immune cell redistribution. In addition to this direct action within the neuroimmune crosstalk, NPY is also able to modulate the immunomodulatory effects of other neurotransmitters, thereby acting as a neuroimmune co-transmitter. This review will discuss key findings from recent studies, provide implications for the clinical situation, and integrate the pleiotropic functions of NPY in the context of neuroimmune interactions.

Cutting Edge: CD69 Interference with Sphingosine-1-Phosphate Receptor Function Regulates Peripheral T Cell Retention

Tissue-resident memory T cells provide local immune protection in barrier tissues, such as skin and mucosa. However, the molecular mechanisms controlling effector T cell retention and subsequent memory formation in those locations are not fully understood. In this study, we analyzed the role of CD69, an early leukocyte activation marker, in regulating effector T cell egress from peripheral tissues. We provide evidence that CD69 surface expression by skin-infiltrating CD8 T cells can be regulated at multiple levels, including local Ag stimulation and signaling through type I IFNRs, and it coincides with the transcriptional downregulation of the sphingosine-1-phosphate receptor S1P1. Importantly, we demonstrate that expression of CD69, by interfering with sphingosine-1-phosphate receptor function, is a critical determinant of prolonged T cell retention and local memory formation. Our results define an important step in the generation of long-lived adaptive immune memory at body surfaces.

NLRC4 inflammasomes in dendritic cells regulate noncognate effector function by memory CD8 + T cells

Memory T cells exert antigen-independent effector functions, but how these responses are regulated is unclear. We discovered an in vivo link between flagellin-induced NLRC4 inflammasome activation in splenic dendritic cells (DCs) and host protective interferon-γ (IFN-γ) secretion by noncognate memory CD8+ T cells, which could be activated by Salmonella enterica serovar Typhimurium, Yersinia pseudotuberculosis and Pseudomonas aeruginosa. We show that CD8α+ DCs were particularly efficient at sensing bacterial flagellin through NLRC4 inflammasomes. Although this activation released interleukin 18 (IL-18) and IL-1β, only IL-18 was required for IFN-γ production by memory CD8+ T cells. Conversely, only the release of IL-1β, but not IL-18, depended on priming signals mediated by Toll-like receptors. These findings provide a comprehensive mechanistic framework for the regulation of noncognate memory T cell responses during bacterial immunity.

Selected Toll-like Receptor Ligands and Viruses Promote Helper-Independent Cytotoxic T Cell Priming by Upregulating CD40L on Dendritic Cells

CD40L (CD154) on CD4(+) T cells has been shown to license dendritic cells (DCs) via CD40 to prime cytotoxic T lymphocyte (CTL) responses. We found that the converse (CD40L on DCs) was also important. Anti-CD40L treatment decreased endogenous CTL responses to both ovalbumin and influenza infection even in the absence of CD4(+) T cells. DCs expressed CD40L upon stimulation with agonists to Toll-like receptor 3 (TLR3) and TLR9. Moreover, influenza infection, which stimulates CTLs without help, upregulated CD40L on DCs, but herpes simplex infection, which elicits CTLs through help, did not. CD40L-deficient (Cd40lg(-/-)) DCs are suboptimal both in vivo in bone marrow chimera experiments and in vitro in mixed lymphocyte reactions. In contrast, Cd40lg(-/-) CD8(+) T cells killed as effectively as wild-type cells. Thus, CD40L upregulation on DCs promoted optimal priming of CD8(+) T cells without CD4(+) T cells, providing a mechanism by which pathogens may elicit helper-independent CTL immunity.

Neuropeptide Y (NPY) Suppresses Experimental Autoimmune Encephalomyelitis: NPY1 Receptor-Specific Inhibition of Autoreactive Th1 Responses In Vivo

Prior studies have revealed that the sympathetic nervous system regulates the clinical and pathological manifestations of experimental autoimmune encephalomyelitis (EAE), an autoimmune disease model mediated by Th1 T cells. Although the regulatory role of catecholamines has been indicated in the previous works, it remained possible that other sympathetic neurotransmitters like neuropeptide Y (NPY) may also be involved in the regulation of EAE. Here we examined the effect of NPY and NPY receptor subtype-specific compounds on EAE, actively induced with myelin oligodendrocyte glycoprotein 35–55 in C57BL/6 mice. Our results revealed that exogenous NPY as well as NPY Y1 receptor agonists significantly inhibited the induction of EAE, whereas a Y5 receptor agonist or a combined treatment of NPY with a Y1 receptor antagonist did not inhibit signs of EAE. These results indicate that the suppression of EAE by NPY is mediated via Y1 receptors. Furthermore, treatment with the Y1 receptor antagonist induced a significantly earlier onset of EAE, indicating a protective role of endogenous NPY in the induction phase of EAE. We also revealed a significant inhibition of myelin oligodendrocyte glycoprotein 35–55-specific Th1 response as well as a Th2 bias of the autoimmune T cells in mice treated with the Y1 receptor agonist. Ex vivo analysis further demonstrated that autoimmune T cells are directly affected by NPY via Y1 receptors. Taken together, we conclude that NPY is a potent immunomodulator involved in the regulation of the Th1-mediated autoimmune disease EAE.

Characterization of an Immediate Splenic Precursor of CD8+ Dendritic Cells Capable of Inducing Antiviral T Cell Responses

Mouse spleens contain three major dendritic cell (DC) populations: plasmacytoid DC, conventional CD8+CD24+ DC (CD8+ DC), and conventional CD8−CD24− DC (CD8− DC). We have previously shown that CD8+ DC are the major cross-presenting subtype in vivo and are the main inducers of antiviral cytotoxic T lymphocyte responses. Here we show that after depletion of CD8+ DC, the only DC capable of viral Ag presentation was a small subset that expresses CD24 but not CD8. This CD8−CD24+ DC population is greatly expanded in mice treated with the DC growth factor FMS-like tyrosine kinase 3 ligand. The CD8−CD24+ DC represent an immediate precursor of CD8+ DC, as demonstrated by their expression pattern of characteristic markers of CD8+ DC, their capacity to cross-present in vitro, and their conversion into CD8+ DC upon adoptive transfer into recipient mice. Accordingly, the lifespan of transferred CD8−CD24+ DC in vivo was greatly enhanced as compared with terminally differentiated CD8+ DC. Moreover, in a vaccination protocol, CD8−CD24+ DC induced stronger T cell responses and accelerated viral clearance of HSV-1 compared with CD8+ DC. Our results demonstrate that the ability to cross-present first appears in an immediate precursor population of CD8+ DC that does not yet express CD8. The enhanced capacity of CD8−CD24+ DC to induce immune responses upon adoptive transfer makes them an attractive novel tool for DC-based immunotherapies.

NLRP3 inflammasome activation downstream of cytoplasmic LPS recognition by both caspase‐4 and caspase‐5

Humans encode two inflammatory caspases that detect cytoplasmic LPS, caspase‐4 and caspase‐5. When activated, these trigger pyroptotic cell death and caspase‐1‐dependent IL‐1β production; however the mechanism underlying this process is not yet confirmed. We now show that a specific NLRP3 inhibitor, MCC950, prevents caspase‐4/5‐dependent IL‐1β production elicited by transfected LPS. Given that both caspase‐4 and caspase‐5 can detect cytoplasmic LPS, it is possible that these proteins exhibit some degree of redundancy. Therefore, we generated human monocytic cell lines in which caspase‐4 and caspase‐5 were genetically deleted either individually or together. We found that the deletion of caspase‐4 suppressed cell death and IL‐1β production following transfection of LPS into the cytoplasm, or in response to infection with Salmonella typhimurium. Although deletion of caspase‐5 did not confer protection against transfected LPS, cell death and IL‐1β production were reduced after infection with Salmonella. Furthermore, double deletion of caspase‐4 and caspase‐5 had a synergistic effect in the context of Salmonella infection. Our results identify the NLRP3 inflammasome as the specific platform for IL‐1β maturation, downstream of cytoplasmic LPS detection by caspase‐4/5. We also show that both caspase‐4 and caspase‐5 are functionally important for appropriate responses to intracellular Gram‐negative bacteria.

GM-CSF increases cross-presentation and CD103 expression by mouse CD8⁺ spleen dendritic cells.

Resident CD8+ DCs perform several functions, including cross‐presenting antigen and rapidly engulfing the Gram‐positive intracellular pathogen Listeria monocytogenes. Little is known about how these functions of CD8+ DCs are modulated. Here, we show that granulocyte‐macrophage CSF (GM‐CSF), a cytokine that exists at low levels at steady state but is elevated during infection and inflammation, enhances cross‐presentation and rapid uptake of L. monocytogenes by resident CD8+ DCs. This previously unrecognized functional enhancement of CD8+ DCs by GM‐CSF was independent of promoting DC survival in vitro. Enhancement of these functions by GM‐CSF was also marked by CD103 expression on CD8+ DCs that was strongly regulated by GM‐CSF. Our findings not only identify GM‐CSF as a key molecule regulating CD8+ DC function, but also as a factor responsible for functional heterogeneity of CD8+ DCs that is at least substantially demarcated by CD103 expression.