Myeloid-Derived Suppressor Cells in Sepsis.

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells characterized by their immunosuppressive functions. MDSCs expand during chronic and acute inflammatory conditions, the best described being cancer. Recent studies uncovered an important role of MDSCs in the pathogenesis of infectious diseases along with sepsis. Here we discuss the mechanisms underlying the expansion and immunosuppressive functions of MDSCs, and the results of preclinical and clinical studies linking MDSCs to sepsis pathogenesis. Strikingly, all clinical studies to date suggest that high proportions of blood MDSCs are associated with clinical worsening, the incidence of nosocomial infections and/or mortality. Hence, MDSCs are attractive biomarkers and therapeutic targets for sepsis, especially because these cells are barely detectable in healthy subjects. Blocking MDSC-mediated immunosuppression and trafficking or depleting MDSCs might all improve sepsis outcome. While some key aspects of MDSCs biology need in depth investigations, exploring these avenues may participate to pave the way toward the implementation of personalized medicine and precision immunotherapy for patients suffering from sepsis

Trained Immunity Confers Broad-Spectrum Protection Against Bacterial Infections.

The innate immune system recalls a challenge to adapt to a secondary challenge, a phenomenon called trained immunity. Training involves cellular metabolic, epigenetic and functional reprogramming, but how broadly trained immunity protects from infections is unknown. For the first time, we addressed whether trained immunity provides protection in a large panel of preclinical models of infections. Mice were trained and subjected to systemic infections, peritonitis, enteritis, and pneumonia induced by Staphylococcus aureus, Listeria monocytogenes, Escherichia coli, Citrobacter rodentium, and Pseudomonas aeruginosa. Bacteria, cytokines, leukocytes, and hematopoietic precursors were quantified in blood, bone marrow, and organs. The role of monocytes/macrophages, granulocytes, and interleukin 1 signaling was investigated using depletion or blocking approaches. Induction of trained immunity protected mice in all preclinical models, including when training and infection were initiated in distant organs. Trained immunity increased bone marrow hematopoietic progenitors, blood Ly6Chigh inflammatory monocytes and granulocytes, and sustained blood antimicrobial responses. Monocytes/macrophages and interleukin 1 signaling were required to protect trained mice from listeriosis. Trained mice were efficiently protected from peritonitis and listeriosis for up to 5 weeks. Trained immunity confers broad-spectrum protection against lethal bacterial infections. These observations support the development of trained immunity-based strategies to improve host defenses

Trained Immunity Confers Prolonged Protection From Listeriosis.

Trained immunity refers to the ability of the innate immune system exposed to a first challenge to provide an enhanced response to a secondary homologous or heterologous challenge. We reported that training induced with β-glucan one week before infection confers protection against a broad-spectrum of lethal bacterial infections. Whether this protection persists over time is unknown. To tackle this question, we analyzed the immune status and the response to Listeria monocytogenes (L. monocytogenes) of mice trained 9 weeks before analysis. The induction of trained immunity increased bone marrow myelopoiesis and blood counts of Ly6C <sup>high</sup> inflammatory monocytes and polymorphonuclear neutrophils (PMNs). Ex vivo, whole blood, PMNs and monocytes from trained mice produced increased levels of cytokines in response to microbial products and limited the growth of L. monocytogenes. In vivo, following challenge with L. monocytogenes, peripheral blood leukocytes were massively depleted in control mice but largely preserved in trained mice. PMNs were reduced also in the spleen from control mice, and increased in the spleen of trained mice. In transwell experiments, PMNs from trained mice showed increased spontaneous migration and CXCL2/MIP2α-induced chemotaxis, suggesting that training promotes the migration of PMNs in peripheral organs targeted by L. monocytogenes. Trained PMNs and monocytes had higher glycolytic activity and mitochondrial respiration than control cells when exposed to L. monocytogenes. Bacterial burden and dissemination in blood, spleen and liver as well as systemic cytokines and inflammation (multiplex bead assay and bioluminescence imaging) were reduced in trained mice. In full agreement with these results, mice trained 9 weeks before infection were powerfully protected from lethal listeriosis. Altogether, these data suggest that training increases the generation and the antimicrobial activity of PMNs and monocytes, which may confer prolonged protection from lethal bacterial infection

COVID-19 rapidly increases MDSCs and prolongs innate immune dysfunctions.

We used unsupervised immunophenotyping of blood leukocytes and measured cytokine production by innate immune cell exposed to LPS and R848. We show that COVID-19 induces a rapid, transient upregulation of myeloid-derived suppressor cells (MDSCs) accompanied by a rapid, sustained (up to 3 months) hyporesponsiveness of dendritic cells and monocytes. Blood MDSCs may represent biomarkers and targets for intervention strategies in COVID-19 patients

Sirtuin 5 Deficiency Does Not Compromise Innate Immune Responses to Bacterial Infections.

Sirtuin 5 (SIRT5) is a member of the family of NAD <sup>+</sup> -dependent lysine/histone deacetylases. SIRT5 resides mainly in the mitochondria where it catalyzes deacetylation, demalonylation, desuccinylation, and deglutarylation of lysine to regulate metabolic and oxidative stress response pathways. Pharmacologic inhibitors of SIRT5 are under development for oncologic conditions, but nothing is known about the impact of SIRT5 on antimicrobial innate immune defenses. Using SIRT5 knockout mice, we show that SIRT5 deficiency does not affect immune cell development, cytokine production and proliferation by macrophages and splenocytes exposed to microbial and immunological stimuli. Moreover, preclinical models suggest that SIRT5 deficiency does not worsen endotoxemia, Klebsiella pneumoniae and Streptococcus pneumoniae pneumonia, Escherichia coli peritonitis, listeriosis, and staphylococcal infection. Altogether, these data support the safety profile in terms of susceptibility to infections of SIRT5 inhibitors under development

Modes of action and diagnostic value of miRNAs in sepsis.

Sepsis is a clinical syndrome defined as a dysregulated host response to infection resulting in life-threatening organ dysfunction. Sepsis is a major public health concern associated with one in five deaths worldwide. Sepsis is characterized by unbalanced inflammation and profound and sustained immunosuppression, increasing patient susceptibility to secondary infections and mortality. microRNAs (miRNAs) play a central role in the control of many biological processes, and deregulation of their expression has been linked to the development of oncological, cardiovascular, neurodegenerative and metabolic diseases. In this review, we discuss the role of miRNAs in sepsis pathophysiology. Overall, miRNAs are seen as promising biomarkers, and it has been proposed to develop miRNA-based therapies for sepsis. Yet, the picture is not so straightforward because of the versatile and dynamic features of miRNAs. Clearly, more research is needed to clarify the expression and role of miRNAs in sepsis, and to promote the use of miRNAs for sepsis management

Macrophage migration inhibitory factor promotes the migration of dendritic cells through CD74 and the activation of the Src/PI3K/myosin II pathway.

Constitutively expressed by innate immune cells, the cytokine macrophage migration inhibitory factor (MIF) initiates host immune responses and drives pathogenic responses in infectious, inflammatory, and autoimmune diseases. Dendritic cells (DCs) express high levels of MIF, but the role of MIF in DC function remains poorly characterized. As migration is critical for DC immune surveillance, we investigated whether MIF promoted the migration of DCs. In classical transwell experiments, MIF <sup>-/-</sup> bone marrow-derived DCs (BMDCs) or MIF <sup>+/+</sup> BMDCs treated with ISO-1, an inhibitor of MIF, showed markedly reduced spontaneous migration and chemotaxis. CD74 <sup>-/-</sup> BMDCs that are deficient in the ligand-binding component of the cognate MIF receptor exhibited a migration defect similar to that of MIF <sup>-/-</sup> BMDCs. Adoptive transfer experiments of LPS-matured MIF <sup>+/+</sup> and MIF <sup>-/-</sup> and of CD74 <sup>+/+</sup> and CD74 <sup>-/-</sup> BMDCs injected into the hind footpads of homologous or heterologous mice showed that the autocrine and paracrine MIF activity acting via CD74 contributed to the recruitment of DCs to the draining lymph nodes. Mechanistically, MIF activated the Src/PI3K signaling pathway and myosin II complexes, which were required for the migration of BMDCs. Altogether, these data show that the cytokine MIF exerts chemokine-like activity for DC motility and trafficking

High levels of monocytic myeloid-derived suppressor cells are associated with favorable outcome in patients with pneumonia and sepsis with multi-organ failure.

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells with immunosuppressive functions sub-classified into monocytic and polymorphonuclear MDSCs (M-MDSCs and PMN-MDSCs). Clinical studies reported increased levels of MDSCs that were associated with poor outcome in sepsis patients. Since sepsis patients exhibit signs of inflammation and immunosuppression, MDSCs may provide benefit by dampening deleterious inflammation in some patients. To test this hypothesis, we measured MDSCs in critically ill sepsis patients with pneumonia and multi-organ dysfunctions and a high likelihood of death. This was a prospective multicenter observational cohort study performed in eight ICUs in Athens and Thessaloniki, Greece, enrolling critically ill patients with pneumonia and sepsis with multi-organ dysfunctions. A flow cytometry approach using blood collected at study inclusion in tubes containing lyophilized antibodies combined to unsupervised clustering was developed to quantify M-MDSCs and PMN-MDSCs. Forty-eight patients were included, of whom 34 died within 90 days. At study inclusion, M-MDSCs and PMN-MDSCs were increased in sepsis patients when compared to healthy subjects (3.07% vs 0.96% and 22% vs 2.1% of leukocytes, respectively; p < 10 <sup>-4</sup> ). Increased PMN-MDSCs were associated with secondary infections (p = 0.024) and new sepsis episodes (p = 0.036). M-MDSCs were more abundant in survivors than in patients who died within 28 days (p = 0.028). Stratification of patients according to M-MDSC levels revealed that high levels of M-MDSC were associated with reduced 90-day mortality (high vs low M-MDSCs: 47% vs 84% mortality, p = 0.003, hazard ratio [HR] = 3.2, 95% CI 1.4-7.2). Combining high M-MDSC levels with low Acute Physiology and Chronic Health Evaluation (APACHE) II score improved patient stratification (M-MDSCs <sup>high</sup> /APACHE II <sup>low</sup> vs M-MDSCs <sup>low</sup> /APACHE II <sup>low</sup> : 20% vs 80% 90-day mortality, p = 0.0096, HR = 7.2, 95% CI 1.6-32). In multivariate analyses high M-MDSCs remained correlated with improved survival in patients with low APACHE II score (p = 0.05, HR = 5.26, 95% CI 1.0-27.8). This is the first study to associate high levels of M-MDSCs with improved survival in sepsis patients