Significance: The role of metabolic processes during T cell activation has been increasingly acknowledged, and recent data suggest an impact of T cell immunometabolism on T cell function and T cell-mediated autoimmunity. The factors regulating metabolic function in T cells are not clear, however. We identify the nuclear receptor Nur77 as central regulator of T cell immunometabolism, controlling oxidative phosphorylation and aerobic glycolysis during T cell activation. Functionally, Nur77 restricts murine and human T cell activation and proliferation and limits inflammation in autoimmune conditions in animal models of CNS autoimmunity, contact dermatitis, and arthritis. These findings identify Nur77 as a central regulator of T cell immunometabolism that restricts T cell-mediated autoimmunity, which might open up new avenues for a more tailored therapeutic approach.

The molecular mechanisms regulating noncanonical protein transport across cellular membranes are poorly understood. Cross-presentation of exogenous antigens on MHC I molecules by dendritic cells (DCs) generally requires antigen translocation from the endosomal compartment into the cytosol for proteasomal degradation. In this study, we demonstrate that such translocation is controlled by the endocytic receptor and regulated by ubiquitination. Antigens internalized by the mannose receptor (MR), an endocytic receptor that targets its ligands specifically toward cross-presentation, were translocated into the cytosol only after attachment of a lysin48-linked polyubiquitin chain to the cytosolic region of the MR. Furthermore, we identify TSG101 as a central regulator of MR ubiquitination and antigen translocation. Importantly, we demonstrate that MR polyubiquitination mediates the recruitment of p97, a member of the ER-associated degradation machinery that provides the driving force for antigen translocation, toward the endosomal membrane, proving the central role of the endocytic receptor and its ubiquitination in antigen translocation.

The inflammatory responsiveness of phagocytes to exogenous and endogenous stimuli is tightly regulated. This regulation plays an important role in systemic inflammatory response syndromes (SIRS). In SIRS phagocytes initially develop a hyper-inflammatory response followed by a secondary state of hypo-responsiveness, a so called tolerance. This hypo-responsiveness can be induced by endotoxin stimulation of Toll-like receptor 4 (TLR4) resulting in an ameliorated response after subsequent re-stimulation. This modification of inflammatory response patterns has been described as innate immune memory. Interestingly, tolerance can also be triggered by endogenous TLR4 ligands, such as alarmins MRP8 (myeloid-related protein) and MRP14, under sterile conditions. However, signaling pathways that trigger hypo-responsiveness of phagocytes in clinically relevant diseases are only barely understood. Through our work, we have now identified two main signaling cascades that are activated during MRP-induced tolerance of phagocytes. We demonstrate that the PI3K/AKT/GSK-3β pathway interferes with NF-kB-driven gene expression and that inhibition of GSK-3β mimics tolerance in vivo. Moreover, we identified interleukin-10 triggered activation of transcription factors STAT3 and BCL-3 as master regulators of MRP-induced tolerance. Accordingly, patients with dominant negative STAT3 mutations show no tolerance development. In a clinically relevant condition of systemic sterile stress, cardiopulmonary bypass surgery, we confirmed the initial induction of MRP-expression and the tolerance induction of monocytes associated with nuclear translocation of STAT3 and BCL-3 as relevant mechanisms. Our data indicate that the use of pharmacological JAK-STAT inhibitors may be promising targets for future therapeutic approaches to prevent complications associated with secondary hypo-responsiveness during SIRS. Key Points MRP alarmins induce an IL-10-, STAT-3- and BCL-3-dependent hypo-responsiveness of monocytes after cardiopulmonary bypass surgery.

The proinflammatory alarmins S100A8 and S100A9 are among the most abundant proteins in neutrophils and monocytes but are completely silenced after differentiation to macrophages. The molecular mechanisms of the extraordinarily dynamic transcriptional regulation of S100a8 and S100a9 genes, however, are only barely understood. Using an unbiased genome-wide CRISPR/Cas9 knockout (KO)-based screening approach in immortalized murine monocytes, we identified the transcription factor C/EBPδ as a central regulator of S100a8 and S100a9 expression. We showed that S100A8/A9 expression and thereby neutrophil recruitment and cytokine release were decreased in C/EBPδ KO mice in a mouse model of acute lung inflammation. S100a8 and S100a9 expression was further controlled by the C/EBPδ antagonists ATF3 and FBXW7. We confirmed the clinical relevance of this regulatory network in subpopulations of human monocytes in a clinical cohort of cardiovascular patients. Moreover, we identified specific C/EBPδ-binding sites within S100a8 and S100a9 promoter regions, and demonstrated that C/EBPδ-dependent JMJD3-mediated demethylation of H3K27me₃ is indispensable for their expression. Overall, our work uncovered C/EBPδ as a novel regulator of S100a8 and S100a9 expression. Therefore, C/EBPδ represents a promising target for modulation of inflammatory conditions that are characterized by S100a8 and S100a9 overexpression.

Traditionally, Staphylococcus aureus has been considered an extracellular pathogen. However, among other factors, the frequent failure of antimicrobial therapy and the ability of the pathogen to cause recurrent disease have established the concept of eukaryotic invasion of the pathogen, thereby evading the host’s immune system.

It remains largely unclear how thymocytes translate relative differences in T cell receptor (TCR) signal strength into distinct developmental programs that drive the cell fate decisions towards conventional (Tconv) or regulatory T cells (Treg). Following TCR activation, intracellular calcium (Ca²⁺) is the most important second messenger, for which the potassium channel K_2P18.1 is a relevant regulator. Here, we identify K_2P18.1 as a central translator of the TCR signal into the thymus-derived Treg (tTreg) selection process. TCR signal was coupled to NF-κB-mediated K_2P18.1 upregulation in tTreg progenitors. K_2P18.1 provided the driving force for sustained Ca²⁺ influx that facilitated NF-κB- and NFAT-dependent expression of FoxP3, the master transcription factor for Treg development and function. Loss of K_2P18.1 ion-current function induced a mild lymphoproliferative phenotype in mice, with reduced Treg numbers that led to aggravated experimental autoimmune encephalomyelitis, while a gain-of-function mutation in K_2P18.1 resulted in increased Treg numbers in mice. Our findings in human thymus, recent thymic emigrants and multiple sclerosis patients with a dominant-negative missense K_2P18.1 variant that is associated with poor clinical outcomes indicate that K_2P18.1 also plays a role in human Treg development. Pharmacological modulation of K_2P18.1 specifically modulated Treg numbers in vitro and in vivo. Finally, we identified nitroxoline as a K_2P18.1 activator that led to rapid and reversible Treg increase in patients with urinary tract infections. Conclusively, our findings reveal how K_2P18.1 translates TCR signals into thymic T cell fate decisions and Treg development, and provide a basis for the therapeutic utilization of Treg in several human disorders.