conducted on CLR assigned to the so-called Dectin-1 or Dectin-2 clusters, localized within 51 the NK cell gene cluster on human chromosome 12 or mouse chromosome 6 (1-3). Several 52 excellent reviews on the function of these CLR in anti-microbial defense and homeostasis are 53 available (4, 5). In this review we summarize the current knowledge about signaling 54 downstream of the activating CLR Dectin-1 (Clec7a), Dectin-2 (human Clec6a, mouse 55 Clec4n), Mincle (Clec4e) and Mcl (Clec4d) that is largely dependent on the kinase spleen 56 tyrosine kinase (Syk). Table I provides an overview of defined ligands and microorganisms 57 bound by this group of PRR. In addition to microbial carbohydrate and glycolipid structures 58 acting as PAMP, several CLR bind endogenous ligands such as SAP130 released by dying 59 cells or cholesterol crystals. Thus, these CLR are involved in homeostatic responses and 60 inflammatory conditions (6-9), in addition to host response to pathogens and commensals (10-61 (Fig. 1). 103 Syk-Card9 coupled CLR is observed across cell-types and species, promoting the 111 differentiation of IL-17-producing CD4+ T cells (23, 24, 26, 27). Remarkably, to date there is 112 only very limited information about the effects of combined stimulation of CLR and TLR 113 pathways on global gene expression. 114

Trehalose-6,6-dimycolate (TDM), the mycobacterial cord factor, is an abundant cell wall glycolipid and major virulence factor of Mycobacterium tuberculosis. Its synthetic analog trehalose-6,6-dibehenate (TDB) is a new adjuvant currently in phase I clinical trials. In rodents, the C-type lectin receptors Mincle and Mcl bind TDB/TDM and activate macrophages and dendritic cells (DC) through the Syk-Card9 pathway. However, it is unknown whether these glycolipids activate human innate immune cells through the same mechanism. We performed in vitro analysis of TDB/TDM-stimulated primary human monocytes, macrophages, and DC; determined C-type lectin receptor expression; and tested the contribution of SYK, MINCLE, and MCL by small interfering RNA knockdown and genetic complementation. We observed a robust chemokine and cytokine release in response to TDB or TDM. MCSF-driven macrophages secreted higher levels of IL-8, IL-6, CCL3, CCL4, and CCL2 after stimulation with TDM, whereas DC responded more strongly to TDB and GM-CSF-driven macrophages were equally responsive to TDB and TDM. SYK kinase and the adaptor protein CARD9 were essential for glycolipid-induced IL-8 production. mRNA expression of MINCLE and MCL was high in monocytes and macrophages, with MINCLE and MCL proteins localized intracellularly under resting conditions. Small interfering RNA-mediated MINCLE or MCL knockdown caused on average reduced TDB- or TDM-induced IL-8 production. Conversely, retroviral expression in murine Mincle-deficient DC revealed that human MINCLE, but not MCL, was sufficient to confer responsiveness to TDB/TDM. Our study demonstrates that SYK-CARD9 signaling plays a key role in TDB/TDM-induced activation of innate immune cells in man as in mouse, likely by engagement of MINCLE.

Membrane microparticles (MMP) released from apoptotic cells deliver signals that secure the anti-inflammatory response beyond the nearest proximity of the apoptotic cell. Plasmacytoid dendritic cells (pDC) are sentinels prepared to detect cellular processes that endanger the organism. They play a key role in the regulation of both pro- and anti-inflammatory immune responses. Based on the assumption that pDC could participate in the initiation of the anti-inflammatory response to apoptotic cells, we investigated the effects of apoptotic cell-derived MMP on human pDC. The results obtained in our experiments confirmed that MMP released from apoptotic cells trigger IFN-α secretion from human pDC. They further suggest that pDC activation results from sensing of DNA contained in MMP. MMP-DNA displays a particularly strong stimulatory activity compared with MMP-RNA and other sources of DNA. Inhibition of MMP-induced IFN-α secretion by cytochalasin D, chloroquine, and an inhibitory G-rich oligodeoxynucleotide identify TLR9 as the receptor for MMP-DNA. In marked contrast to the pDC response in autoimmune patients, in healthy subjects MMP-mediated stimulation of pDC-derived IFN-α was found to be independent of FcγRIIA (CD32A). Based on our findings, we conclude that induction of pDC-derived IFN-α by MMP is a physiological event; future investigations are necessary to elucidate whether pDC activation promotes inflammation or propagates tolerance in the context of apoptotic cell clearance.

Intestinal epithelial homeostasis is maintained by adult intestinal stem cells, which, alongside Paneth cells, appear after birth in the neonatal period. We aimed to identify regulators of neonatal intestinal epithelial development by testing a small library of epigenetic modifier inhibitors in Paneth cell–skewed organoid cultures. We found that lysine-specific demethylase 1A (Kdm1a/Lsd1) is absolutely required for Paneth cell differentiation. Lsd1-deficient crypts, devoid of Paneth cells, are still able to form organoids without a requirement of exogenous or endogenous Wnt. Mechanistically, we find that LSD1 enzymatically represses genes that are normally expressed only in fetal and neonatal epithelium. This gene profile is similar to what is seen in repairing epithelium, and we find that Lsd1-deficient epithelium has superior regenerative capacities after irradiation injury. In summary, we found an important regulator of neonatal intestinal development and identified a druggable target to reprogram intestinal epithelium toward a reparative state.

The C-type lectin receptors (CLRs) Mincle, Mcl, and Dectin-2 bind mycobacterial and fungal cell wall glycolipids and carbohydrates. Recently, we described that expression of these CLR is downregulated during differentiation of human monocytes to dendritic cells (DC) in the presence of GM-CSF and IL-4. Here, we demonstrate that the Th2 cytokine IL-4 specifically inhibits expression of Mincle, Mcl, and Dectin-2 in human antigen-presenting cells (APC). This inhibitory effect of IL-4 was observed across species, as murine macrophages and DC treated with IL-4 also downregulated these receptors. IL-4 blocked upregulation of Mincle and Mcl mRNA expression and cell surface protein by murine macrophages in response to the Mincle ligand Trehalose-6,6-dibehenate (TDB), whereas the TLR4 ligand LPS overcame inhibition by IL-4. Functionally, downregulation of Mincle expression by IL-4 was accompanied by reduced cytokine production upon stimulation with TDB. These inhibitory effects of IL-4 were dependent on the transcription factor Stat6. Together, our results show that the key Th2 cytokine IL-4 exerts a negative effect on the expression of Mincle and other Dectin-2 cluster CLR in mouse and human macrophages and DC, which may render these sentinel cells less vigilant for sensing mycobacterial and fungal ligands.

Treatment of inflammatory bowel disease (IBD) is challenging, with a series of available drugs each helping only a fraction of patients. Patients may face time-consuming drug trials while the disease is active, thus there is an unmet need for biomarkers and assays to predict drug effect. It is well known that the intestinal epithelium is an important factor in disease pathogenesis, exhibiting physical, biochemical and immunologic driven barrier dysfunctions. One promising test system to study effects of existing or emerging IBD treatments targeting intestinal epithelial cells (IECs) is intestinal organoids (“mini-guts”). However, the fact that healthy intestinal epithelium is in a physiologically hypoxic state has largely been neglected, and studies with intestinal organoids are mainly performed at oxygen concentration of 20%. We hypothesized that lowering the incubator oxygen level from 20% to 2% would recapitulate better the in vivo physiological environment of colonic epithelial cells and enhance the translational value of intestinal organoids as a drug testing platform. In the present study we examine the effects of the key IBD cytokines and drug targets TNF/IL17 on human colonic organoids (colonoids) under atmospheric (20%) or reduced (2%) O2. We show that colonoids derived from both healthy controls and IBD-patients are viable and responsive to IBD-relevant cytokines at 2% oxygen. Because chemokine release is one of the important immunoregulatory traits of the epithelium that may be fine-tuned by IBD-drugs, we also examined chemokine expression and release at different oxygen concentrations. We show that chemokine responses to TNF/IL17 in organoids display similarities to inflamed epithelium in IBD-patients. However, inflammation-associated genes induced by TNF/IL17 were attenuated at low oxygen concentration. We detected substantial oxygen-dependent differences in gene expression in untreated as well as TNF/IL17 treated colonoids in all donors. Further, for some of the IBD-relevant cytokines differences between colonoids from healthy controls and IBD patients were more pronounced in 2% O2 than 20% O2. Our results strongly indicate that an oxygen concentration similar to the in vivo epithelial cell environment is of essence in experimental pharmacology.

High lipoprotein expression and potent activation of host Toll-like receptor-2 (TLR2) are characteristic features of the staphylococcal species. Expression of TLR2 in the host is important for clearance of Staphylococcus aureus infection and host survival. Thus, we hypothesized that bacterial regulation of its intrinsic TLR2-stimulatory capacity could represent a means for immune evasion or host adaptation. We, therefore, compared clinical S. aureus isolates in regards to their TLR2 activation potential and assessed the bacterial factors that modulate TLR2-mediated recognition. S. aureus isolates displayed considerable variability in TLR2-activity with low to absent TLR2-activity in 64% of the isolates tested (68/106). Notably, strain-specific TLR2-activity was independent of the strain origin, e.g. no differences were found between strains isolated from respiratory specimen from cystic fibrosis patients or those isolated from invasive disease specimen. TLR2-activity correlated with protein A expression but not with the agr status. Capsule expression and small colony variant formation had a negative impact on TLR2-activity but any disruption of cell wall integrity enhanced TLR2 activation. Altogether, heterogeneity in host TLR2-activity reflects differences in metabolic activity and cell wall synthesis and/or remodeling.

The intestinal tract is a common site for various types of infections including viruses, bacteria, and helminths, each requiring specific modes of immune defense. The intestinal epithelium has a pivotal role in both immune initiation and effector stages, which are coordinated by lymphocyte cytokines such as IFNγ, IL-13, and IL-22. Here, we studied intestinal epithelial immune responses using organoid image analysis based on a convolutional neural network, transcriptomic analysis, and in vivo infection models. We found that IL-13 and IL-22 both induce genes associated with goblet cells, but the resulting goblet cell phenotypes are dichotomous. Moreover, only IL-13–driven goblet cells are associated with classical NOTCH signaling. We further showed that IL-13 induces the bone morphogenetic protein (BMP) pathway, which acts in a negative feedback loop on immune type 2–driven tuft cell hyperplasia. This is associated with inhibiting Sox4 expression to putatively limit the tuft cell progenitor population. Blocking ALK2, a BMP receptor, with the inhibitor dorsomorphin homolog 1 (DMH1) interrupted the feedback loop, resulting in greater tuft cell numbers both in vitro and in vivo after infection with Nippostrongylus brasiliensis . Together, this investigation of cytokine effector responses revealed an unexpected and critical role for the BMP pathway in regulating type 2 immunity, which can be exploited to tailor epithelial immune responses.