Brucella abortus-infected platelets modulate the activation of neutrophils.

Brucellosis is a contagious disease caused by bacteria of the genus Brucella. Platelets (PLTs) have been widely involved in the modulation of the immune response. We have previously reported the modulation of Brucella abortus-mediated infection of monocytes. As a result, PLTs cooperate with monocytes and increase their inflammatory capacity, promoting the resolution of the infection. Extending these results, in this study we demonstrate that patients with brucellosis present slightly elevated levels of complexes between PLTs and both monocytes and neutrophils. We then assessed whether PLTs were capable of modulating functional aspects of neutrophils. The presence of PLTs throughout neutrophil infection increased the production of interleukin-8, CD11b surface expression and reactive oxygen species formation, whereas it decreased the expression of CD62L, indicating an activated status of these cells. We next analyzed whether this modulation was mediated by released factors. To discriminate between these options, neutrophils were treated with supernatants collected from B. abortus-infected PLTs. Our results show that CD11b expression was induced by soluble factors of PLTs but direct contact between cell populations was needed to enhance the respiratory burst. Additionally, B. abortus-infected PLTs recruit polymorphonuclear (PMN) cells to the site of infection. Finally, the presence of PLTs did not modify the initial invasion of PMN cells by B. abortus but improved the control of the infection at extended times. Altogether, our results demonstrate that PLTs interact with neutrophils and promote a proinflammatory phenotype which could also contribute to the resolution of the infection.

Prokaryotic RNA activates endothelial cells promoting neutrophil transmigration.

Endothelial cell (EC)-neutrophil (PMN) interactions are crucial in the resolution of bacterial infections. Prokaryotic RNA (pRNA) has been reported as a pathogen-associated molecular pattern that is released from bacteria upon death and is able to activate PMN. In this work, we studied the effects of pRNA on EC and investigated whether these effects could modulate EC-PMN interaction. For this purpose, we purified total pRNA from Escherichia coli and used it as a stimulus for Human Umbilical Vein Endothelial Cells (HUVEC). We found that the incubation of pRNA with HUVEC caused the increase of surface intercellular adhesion molecule 1 (ICAM-1 or CD54) expression on HUVEC, and the secretion of IL-8 and von Willebrand factor, characteristics consistent with HUVEC activation, without causing toxic effects. Moreover, pRNA-treated HUVEC also induced PMN adhesion and the conditioned medium obtained from treated-HUVEC was chemotactic for PMN and caused their activation, as determined by CD11b upregulation. As reported previously, the degradation products of pRNA induced similar biological effects. The treatment of HUVEC with endocytosis inhibitors revealed that the entry of pRNA partially relied on a clathrin-dependent mechanism, whereas the effects of degradation products could not be inhibited by any of the inhibitors tested. Using a transwell system, we found that pRNA or degraded pRNA were also able to stimulate HUVEC when recognized from the basolateral side. Our results indicate that pRNA activates EC, resulting in the modulation of EC-PMN interaction by inducing PMN chemotaxis, adhesion and activation. In the context of infection, pRNA sensed by EC and PMN could favor bacterial clearance.

B. abortus RNA is the component involved in the down-modulation of MHC-I expression on human monocytes via TLR8 and the EGFR pathway.

Despite eliciting a potent CD8+ T cell response, Brucella abortus is able to persist and establish a chronic infection inside its host. We have previously reported that the infection of human monocytes/macrophages with B. abortus inhibits the IFN-γ-induced MHC-I cell surface expression down-modulating cytotoxic CD8+ T cell responses. MHC-I down-modulation depends on bacterial viability and results from the capacity of B. abortus to retain the MHC-I molecules within the Golgi apparatus. Furthermore, we recently demonstrated that epidermal growth factor receptor (EGFR) pathway is involved in this phenomenon and that this is an early event during infection. However, the components and mechanisms whereby B. abortus is able to down-modulate MHC-I remained to be elucidated. In this study we demonstrated that the down-modulation of MHC-I expression is not mediated by well-known Brucella virulence factors but instead by B. abortus RNA, a PAMP associated to viability (vita-PAMP). Surprisingly, completely degraded RNA was also able to inhibit MHC-I expression to the same extent as intact RNA. Accordingly, B. abortus RNA and its degradation products were able to mimic the MHC-I intracellular retention within the Golgi apparatus observed upon infection. We further demonstrated that TLR8, a single-stranded RNA and RNA degradation products sensor, was involved in MHC-I inhibition. On the other hand, neutralization of the EGFR reversed the MHC-I inhibition, suggesting a connection between the TLR8 and EGFR pathways. Finally, B. abortus RNA-treated macrophages display diminished capacity of antigen presentation to CD8+ T cells. Overall, our results indicate that the vita-PAMP RNA as well as its degradation products constitute novel virulence factors whereby B. abortus, by a TLR8-dependent mechanism and through the EGFR pathway, inhibits the IFN-γ-induced MHC-I surface expression on human monocytes/macrophages. Thus, bacteria can hide within infected cells and avoid the immunological surveillance of cytotoxic CD8+ T cells.

Massive plasmablast response elicited in the acute phase of hantavirus pulmonary syndrome.

Beside its key diagnostic value, the humoral immune response is thought to play a protective role in hantavirus pulmonary syndrome. However, little is known about the cell source of these antibodies during ongoing human infection. Herein we characterized B-cell subsets circulating in Andes-virus-infected patients. A notable potent plasmablast (PB) response that increased 100-fold over the baseline levels was observed around 1 week after the onset of symptoms. These PB present a CD3neg CD19low CD20neg CD38hi CD27hi CD138+/- IgA+/- surface phenotype together with the presence of cytoplasmic functional immunoglobulins. They are large lymphocytes (lymphoblasts) morphologically coincident with the 'immunoblast-like' cells that have been previously described during blood cytology examinations of hantavirus-infected patients. Immunoreactivity analysis of white blood cell lysates suggests that some circulating PB are virus-specific but we also observed a significant increase of reactivity against virus-unrelated antigens, which suggests a possible bystander effect by polyclonal B-cell activation. The presence of this large and transient PB response raises the question as to whether these cells might have a protective or pathological role during the ongoing hantavirus pulmonary syndrome and suggest their practical application as a diagnostic/prognostic biomarker.

Immature myeloid Gr-1+ CD11b+ cells from lipopolysaccharide-immunosuppressed mice acquire inhibitory activity in the bone marrow and migrate to lymph nodes to exert their suppressive function.

Secondary infections due to post-sepsis immunosuppression are a major cause of death in patients with sepsis. Repetitive inoculation of increasing doses of lipopolysaccharide (LPS) into mice mimics the immunosuppression associated with sepsis. Myeloid-derived suppressor cells (MDSCs, Gr-1(+) CD11b(+)) are considered a major component of the immunosuppressive network, interfering with T-cell responses in many pathological conditions. We used LPS-immunosuppressed (IS) mice to address whether MDSCs acquired their suppressive ability in the bone marrow (BM) and whether they could migrate to lymph nodes (LNs) to exert their suppressive function. Our results showed that Gr-1(+) CD11b(+) cells of IS mice already had the potential to inhibit T-cell proliferation in the BM. Moreover, soluble factors present in the BM from IS mice were responsible for inducing this inhibitory ability in control BM cells. In addition, migration of Gr-1(+) CD11b(+) to LNs in vivo was maximal when cells obtained from the BM of IS mice were inoculated into an IS context. In this regard, we found chemoattractant activity in cell-free LN extracts (LNEs) from IS mice and an increased expression of the LN-homing chemokine receptor C-C chemokine receptor type 7 (CCR7) in IS BM Gr-1(+) CD11b(+) cells. These results indicate that Gr-1(+) CD11b(+) cells found in BM from IS mice acquire their suppressive activity in the same niche where they are generated, and migrate to LNs to exert their inhibitory role. A better understanding of MDSC generation and/or regulation of factors able to induce their inhibitory function may provide new and more effective tools for the treatment of sepsis-associated immunosuppression.

Shiga toxin 1 induces on lipopolysaccharide-treated astrocytes the release of tumor necrosis factor-alpha that alter brain-like endothelium integrity.

The hemolytic uremic syndrome (HUS) is characterized by hemolytic anemia, thrombocytopenia and renal dysfunction. The typical form of HUS is generally associated with infections by Gram-negative Shiga toxin (Stx)-producing Escherichia coli (STEC). Endothelial dysfunction induced by Stx is central, but bacterial lipopolysaccharide (LPS) and neutrophils (PMN) contribute to the pathophysiology. Although renal failure is characteristic of this syndrome, neurological complications occur in severe cases and is usually associated with death. Impaired blood-brain barrier (BBB) is associated with damage to cerebral endothelial cells (ECs) that comprise the BBB. Astrocytes (ASTs) are inflammatory cells in the brain and determine the BBB function. ASTs are in close proximity to ECs, hence the study of the effects of Stx1 and LPS on ASTs, and the influence of their response on ECs is essential. We have previously demonstrated that Stx1 and LPS induced activation of rat ASTs and the release of inflammatory factors such as TNF-α, nitric oxide and chemokines. Here, we demonstrate that rat ASTs-derived factors alter permeability of ECs with brain properties (HUVECd); suggesting that functional properties of BBB could also be affected. Additionally, these factors activate HUVECd and render them into a proagregant state promoting PMN and platelets adhesion. Moreover, these effects were dependent on ASTs secreted-TNF-α. Stx1 and LPS-induced ASTs response could influence brain ECs integrity and BBB function once Stx and factors associated to the STEC infection reach the brain parenchyma and therefore contribute to the development of the neuropathology observed in HUS.

all-trans-retinoic acid improves immunocompetence in a murine model of lipopolysaccharide-induced immunosuppression.

Secondary infections due to post-sepsis immunosuppression are a major cause of death in patients with sepsis. Strategies aimed at restoring immune functions offer a new perspective in the treatment of sepsis. In the present study, we used LPS (lipopolysaccharide)-immunosuppressed mice to analyse the effects of ATRA (all-trans retinoic acid) on different immune parameters. The IS (immunocompromised) group had decreased lymphocyte and increased MDSC (myeloid-derived suppressor cell) counts in lymph nodes. They also had an impaired in vitro T-cell proliferation, mediated by MDSCs. ATRA administration restored T-cell proliferation, which was associated with a decreased number of live MDSCs. The IS group treated with ATRA had an increased number of CD4+ and CD8+ T-cells. ATRA partially improved the primary humoral immune response, even when immunosuppression was established first and ATRA was administered subsequently. Our results demonstrate that ATRA restores immunocompetence by modulating the number of leucocytes and the survival of MDSCs, and thus represents an additional potential strategy in the treatment of the immunosuppressive state of sepsis.

Interleukin-10 controls human peripheral PMN activation triggered by lipopolysaccharide.

Large amounts of anti-inflammatory mediators, such as interleukin (IL)-10, are produced and found early in the course of sepsis. We explore the role of IL-10 on neutrophil (PMN) activation/function using an in vitro model. Isolated human PMN were pre-incubated with lipopolysaccharide (LPS) and/or IL-10 for 18h. Subsequently, a second LPS exposure was performed and CD11b and CD66b up-regulation, and the reactive oxygen species (ROS) generation were measured 2h later. We found that IL-10 prevented PMN activation and the secretion of TNF-α and IL-8 induced by the first LPS contact. In the absence of IL-10, a second LPS exposure induced additive effects that were prevented by IL-10. Only ROS generation was highly affected by the blockade of PMN-secreted TNF-α or IL-8. Additionally, IL-10 prevented other possible mechanisms of LPS priming. Therefore, IL-10 modulates PMN activation preventing autocrine activating loops and priming mechanisms, rendering PMN less responsive to a second LPS exposure.

Cytokine production is altered in monocytes from children with hemolytic uremic syndrome.

PURPOSE: The interaction of Shiga toxin (Stx) and/or lipopolysaccharide (LPS) with monocytes (Mo) may be central to the pathogenesis of hemolytic uremic syndrome (HUS), providing the cytokines necessary to sensitize endothelial cells to Stx action. We have previously demonstrated phenotypical alterations in Mo from HUS patients, including increased number of CD16+ Mo. Our aim was to investigate cytokine production in Mo from HUS patients. METHODS: We evaluated TNF-α and IL-10 intracellular contents and secretion in the different Mo subsets in mild (HUS 1) and moderate/severe (HUS 2 + 3) patients. As controls, we studied healthy (HC) and infected children (IC). We also studied Mo responsive capacity towards LPS, measuring the modulation of Mo surface molecules and cytokine production. RESULTS: In basal conditions, the intracellular measurement of TNF-α and IL-10 revealed that the highest number of cytokine-producing Mo was found in HUS 2 + 3 and IC, whereas LPS caused a similar increase in TNF-α and IL-10-producing Mo for all groups. However, when evaluating the release of TNF-α and IL-10, we found a diminished secretion capacity in the entire HUS group and IC compared to HC in basal and LPS conditions. Similarly, a lower Mo response to LPS in HUS 2 + 3 and IC groups was observed when surface markers were studied. CONCLUSION: These results indicate that Mo from severe cases of HUS, similar to IC but different to mild HUS cases, present functional changes in Mo subpopulations and abnormal responses to LPS.

Neutrophil Extracellular Traps Induced by Shiga Toxin and Lipopolysaccharide-Treated Platelets Exacerbate Endothelial Cell Damage.

Hemolytic uremic syndrome (HUS) is the most common cause of acute renal failure in the pediatric population. The etiology of HUS is linked to Gram-negative, Shiga toxin (Stx)-producing enterohemorrhagic bacterial infections. While the effect of Stx is focused on endothelial damage of renal glomerulus, cytokines induced by Stx or bacterial lipopolysaccharide (LPS) and polymorphonuclear cells (PMNs) are involved in the development of the disease. PMN release neutrophil extracellular traps (NETs) to eliminate pathogens, although NETs favor platelets (Plts) adhesion/thrombus formation and can cause tissue damage within blood vessels. Since thrombus formation and occlusion of vessels are characteristic of HUS, PMN-Plts interaction in the context of Stx may promote netosis and contribute to the endothelial damage observed in HUS. The aim of this study was to determine the relevance of netosis induced by Stx in the context of LPS-sensitized Plts on endothelial damage. We observed that Stx2 induced a marked enhancement of netosis promoted by Plts after LPS stimulation. Several factors seemed to promote this phenomenon. Stx2 itself increased the expression of its receptor on Plts, increasing toxin binding. Stx2 also increased LPS binding to Plts. Moreover, Stx2 amplified LPS induced P-selectin expression on Plts and mixed PMN-Plts aggregates formation, which led to activation of PMN enhancing dramatically NETs formation. Finally, experiments revealed that endothelial cell damage mediated by PMN in the context of Plts treated with LPS and Stx2 was decreased when NETs were disrupted or when mixed aggregate formation was impeded using an anti-P-selectin antibody. Using a murine model of HUS, systemic endothelial damage/dysfunction was decreased when NETs were disrupted, or when Plts were depleted, indicating that the promotion of netosis by Plts in the context of LPS and Stx2 plays a fundamental role in endothelial toxicity. These results provide insights for the first time into the pivotal role of Plts as enhancers of endothelial damage through NETs promotion in the context of Stx and LPS. Consequently, therapies designed to reduce either the formation of PMN-Plts aggregates or NETs formation could lessen the consequences of endothelial damage in HUS.

Model of polymicrobial peritonitis that induces the proinflammatory and immunosuppressive phases of sepsis.

Severe sepsis is associated with early release of inflammatory mediators that contribute to the morbidity and mortality observed during the first stages of this syndrome. Although sepsis is a deadly, acute disease, high mortality rates have been observed in patients displaying evidence of sepsis-induced immune deactivation. Although the contribution of experimental models to the knowledge of pathophysiological and therapeutic aspects of human sepsis is undeniable, most of the current studies using animal models have focused on the acute, proinflammatory phase. We developed a murine model that reproduces the early acute phases but also the long-term consequences of human sepsis. We induced polymicrobial acute peritonitis (AP) by establishing a surgical connection between the cecum and the peritoneum, allowing the exit of intestinal bacteria. Using this model, we observed an acute phase with high mortality, leukopenia, increased interleukin-6 levels, bacteremia, and neutrophil activation. A peak of leukocytosis on day 9 or 10 revealed the persistence of the infection within the lung and liver, with inflammatory hepatic damage being shown by histological examination. Long-term (20 days) derangements in both innate and adaptive immune responses were found, as demonstrated by impaired systemic tumor necrosis factor alpha production in response to an inflammatory stimulus; a decreased primary humoral immune response and T cell proliferation, associated with an increased number of myeloid suppressor cells (Gr-1(+) CD11b(+)) in the spleen; and a low clearance capacity. This model provides a good approach to attempt novel therapeutic interventions directed to augmenting host immunity during late sepsis.

Modulation of neutrophil extracellular traps release by Klebsiella pneumoniae.

One of the main bactericidal mechanisms of polymorphonuclear neutrophils (PMN) is the release of neutrophil extracellular traps (NETs), which capture and destroy pathogens. Klebsiella pneumoniae (Kpn) producer of carbapenemase (KPC) and belonging to the sequence type 258 (ST258), is a hyper epidemic clone that causes a large number of infections worldwide associated with high persistence and mortality. It is necessary to investigate the interaction of Kpn KPC with the immune system to improve prevention and treatment of infections mediated by this bacterium. Based on the hypothesis that Kpn is able to subvert PMN-mediated death, the aim was to assess whether Kpn KPC ST258 could modulate the bactericidal response of PMN, focusing on NETs formation, compared to another opportunistic pathogen, as Escherichia coli (Eco). The results showed that the release of NETs was absent when PMN were challenged with Kpn KPC, while Eco was a strong inducer of NETosis. Moreover, Kpn KPC was able to inhibit NETosis induced by Eco. The inhibition of Kpn KPC-mediated NETs formation still occurred in spite of exogenous addition of hydrogen peroxide (H2 O2 ), did not involve bacterial-released soluble factors or cell wall components, and was dependent on bacterial viability. Moreover, when degranulation was investigated, we found that Kpn KPC affected only the mobilization of primary granules, which harbor the proteins with more potent bactericidal properties and those related to NETosis. In conclusion, Kpn KPC ST258 effectively managed to evade the PMN response by inhibiting the release of NETs, and primary granule mobilization.

Shiga toxin 1-induced inflammatory response in lipopolysaccharide-sensitized astrocytes is mediated by endogenous tumor necrosis factor alpha.

Hemolytic-uremic syndrome (HUS) is generally caused by Shiga toxin (Stx)-producing Escherichia coli. Endothelial dysfunction mediated by Stx is a central aspect in HUS development. However, inflammatory mediators such as bacterial lipopolysaccharide (LPS) and polymorphonuclear neutrophils (PMN) contribute to HUS pathophysiology by potentiating Stx effects. Acute renal failure is the main feature of HUS, but in severe cases, patients can develop neurological complications, which are usually associated with death. Although the mechanisms of neurological damage remain uncertain, alterations of the blood-brain barrier associated with brain endothelial injury is clear. Astrocytes (ASTs) are the most abundant inflammatory cells of the brain that modulate the normal function of brain endothelium and neurons. The aim of this study was to evaluate the effects of Stx type 1 (Stx1) alone or in combination with LPS in ASTs. Although Stx1 induced a weak inflammatory response, pretreatment with LPS sensitized ASTs to Stx1-mediated effects. Moreover, LPS increased the level of expression of the Stx receptor and its internalization. An early inflammatory response, characterized by the release of tumor necrosis factor alpha (TNF-alpha) and nitric oxide and PMN-chemoattractant activity, was induced by Stx1 in LPS-sensitized ASTs, whereas activation, evidenced by higher levels of glial fibrillary acid protein and cell death, was induced later. Furthermore, increased adhesion and PMN-mediated cytotoxicity were observed after Stx1 treatment in LPS-sensitized ASTs. These effects were dependent on NF-kappaB activation or AST-derived TNF-alpha. Our results suggest that TNF-alpha is a pivotal effector molecule that amplifies Stx1 effects on LPS-sensitized ASTs, contributing to brain inflammation and leading to endothelial and neuronal injury.

Interleukin-10 and interferon-gamma modulate surface expression of fractalkine-receptor (CX(3)CR1) via PI3K in monocytes.

The membrane-anchored form of the chemokine fractalkine (CX(3)CL1) has been identified as a novel adhesion molecule that interacts with its specific receptor (CX(3)CR1) expressed in monocytes, T cells and natural killer cells to induce adhesion. In addition, CX(3)CL1 can be cleaved from the cell membrane to induce chemotaxis of CX(3)CR1-expressing leucocytes. Recently, marked variations in CX(3)CR1 monocyte expression have been observed during several pathological conditions. Regulation of CX(3)CR1 in monocytes during basal or inflammatory/anti-inflammatory conditions is poorly understood. The aim of this study was therefore to examine CX(3)CR1 expression during monocyte maturation and the effect of soluble mediators on this process. We found that basal expression of CX(3)CR1 in fresh monocytes was reduced during culture, and that lipopolysacchairde accelerated this effect. In contrast, interleukin-10 and interferon-gamma treatment abrogated CX(3)CR1 down-modulation, through a phosphatidylinositol 3 kinase-dependent pathway. Most importantly, CX(3)CR1 membrane expression correlated with monocyte CX(3)CL1-dependent function. Taken together, our data demonstrate that CX(3)CR1 expression in monocytes can be modulated, and suggest that alterations in their environment are able to influence CX(3)CL1-dependent functions, such as chemotaxis and adhesion, leading to changes in the kinetics, composition and/or functional status of the leucocyte infiltrate.

Klebsiella pneumoniae ST258 Negatively Regulates the Oxidative Burst in Human Neutrophils.

The epidemic clone of Klebsiella pneumoniae (Kpn), sequence type 258 (ST258), carbapenamase producer (KPC), commonly infects hospitalized patients that are left with scarce therapeutic option since carbapenems are last resort antibiotics for life-threatening bacterial infections. To improve prevention and treatment, we should better understand the biology of Kpn KPC ST258 infections. Our hypothesis was that Kpn KPC ST258 evade the first line of defense of innate immunity, the polymorphonuclear neutrophil (PMN), by decreasing its functional response. Therefore, our aim was to evaluate how the ST258 Kpn clone affects PMN responses, focusing on the respiratory burst, compared to another opportunistic pathogen, Escherichia coli (Eco). We found that Kpn KPC ST258 was unable to trigger bactericidal responses as reactive oxygen species (ROS) generation and NETosis, compared to the high induction observed with Eco, but both bacterial strains were similarly phagocytized and cause increases in cell size and CD11b expression. The absence of ROS induction was also observed with other Kpn ST258 strains negative for KPC. These results reflect certain selectivity in terms of the functions that are triggered in PMN by Kpn, which seems to evade specifically those responses critical for bacterial survival. In this sense, bactericidal mechanisms evasion was associated with a higher survival of Kpn KPC ST258 compared to Eco. To investigate the mechanisms and molecules involved in ROS inhibition, we used bacterial extracts (BE) and found that BE were able to inhibit ROS generation triggered by the well-known ROS inducer, fMLP. A sequence of experiments led us to elucidate that the polysaccharide part of LPS was responsible for this inhibition, whereas lipid A mediated the other responses that were not affected by bacteria, such as cell size increase and CD11b up-regulation. In conclusion, we unraveled a mechanism of immune evasion of Kpn KPC ST258, which may contribute to design more effective strategies for the treatment of these multi-resistant bacterial infections.

Role of polymorphonuclear leukocytes in the pathophysiology of typical hemolytic uremic syndrome.

Thrombotic microangiopathy and acute renal failure are cardinal features of post-diarrheal hemolytic uremic syndrome (HUS). These conditions are related to endothelial and epithelial cell damage induced by Shiga toxin (Stx), through an interaction with its globotriaosylceramide (Gb3) receptor. Although, Stx is the main pathogenic factor and necessary for HUS development, clinical and experimental evidence suggest that the inflammatory response is able to potentiate Stx toxicity. Lipopolysaccharides (LPS) and neutrophils (PMN) represent two central components of inflammation during a Gram-negative infection. In this regard, patients with high peripheral PMN counts at presentation have a poor prognosis. In the present review, we discuss the contribution of experimental models and patient's studies in an attempt to elucidate the pathogenic mechanisms of HUS.

Prokaryotic RNA Associated to Bacterial Viability Induces Polymorphonuclear Neutrophil Activation.

Polymorphonuclear neutrophils (PMN) are the first cellular line of antibacterial host defense. They sense pathogens through recognition of pathogen-associated molecular patterns (PAMPs) by innate pattern recognition receptors, such as Toll-like receptors (TLR). The aim of this study was to investigate whether PMN sense bacterial viability and explore which viability factor could be involved in this phenomenon. For this purpose, different functions were evaluated in isolated human PMN using live Escherichia coli (Ec) and heat-killed Ec (HK-Ec). We found that bacterial viability was indispensable to induce PMN activation, as measured by forward-scatter (FSC) increase, CD11b surface expression, chemotaxis, reactive oxygen species (ROS) generation and neutrophil extracellular trap (NET) formation. As uncapped non-polyadenylated prokaryotic mRNA has been recognized as a PAMP associated to bacterial viability by macrophages and dendritic cells, total prokaryotic RNA (pRNA) from live Ec was purified and used as a stimulus for PMN. pRNA triggered similar responses to those observed with live bacteria. No RNA could be isolated from HK-Ec, explaining the lack of effect of dead bacteria. Moreover, the supernatant of dead bacteria was able to induce PMN activation, and this was associated with the presence of pRNA in this supernatant, which is released in the killing process. The induction of bactericidal functions (ROS and NETosis) by pRNA were abolished when the supernatant of dead bacteria or isolated pRNA were treated with RNAse. Moreover, endocytosis was necessary for pRNA-induced ROS generation and NETosis, and priming was required for the induction of pRNA-induced ROS in whole blood. However, responses related to movement and degranulation (FSC increase, CD11b up-regulation, and chemotaxis) were still triggered when pRNA was digested with RNase, and were not dependent on pRNA endocytosis or PMN priming. In conclusion, our results indicate that PMN sense live bacteria through recognition of pRNA, and this sensing triggers potent bactericidal mechanisms.

Novel Use of All-Trans-Retinoic Acid in A Model of Lipopolysaccharide-Immunosuppression to Decrease the Generation of Myeloid-Derived Suppressor Cells by Reducing the Proliferation of CD34+ Precursor Cells.

All-trans-retinoic acid (ATRA) is a derivative of vitamin A with antiproliferative properties. Endotoxin shock and subsequent immunosuppression (IS) by lipopolysaccharide (LPS) stimulates myelopoiesis with expansion of myeloid-derived suppressor cells (MDSC). Since we have previously shown that ATRA reverses the IS state by decreasing functional MDSC, our aim was to investigate if ATRA was able to modulate MDSC generation by regulating myelopoiesis in murine hematopoietic organs. We found that ATRA administration in vivo and in vitro decreased the number of CD34+ precursor cells that were increased in IS mice. When we studied the cellular mechanisms involved, we did not find any differences in apoptosis of CD34+ precursors or in the differentiation of these cells to their mature counterparts. Surprisingly, ATRA decreased precursor proliferation, in vitro and in vivo, as assessed by a reduction in the size and number of colony forming units generated from CD34+ cells and by a decreased incorporation of H-thymidine. Moreover, ATRA administration to IS mice decreased the number of MDSC in the spleen, with a restoration of T lymphocyte proliferation and a restitution of the histological architecture. Our results indicate, for the first time, a new use of ATRA to abolish LPS-induced myelopoiesis, affecting the proliferation of precursor cells, and in consequence, decreasing MDSC generation, having a direct impact on the improvement of immune competence. Administration of ATRA could overcome the immunosuppressive state generated by sepsis that often leads to opportunistic life-threatening infections. Therefore, ATRA could be considered a complementary treatment to enhance immune responses.

Differential expression of function-related antigens on blood monocytes in children with hemolytic uremic syndrome.

Monocytes (Mo) mediate central functions in inflammation and immunity. Different subpopulations of Mo with distinct phenotype and functional properties have been described. Here, we investigate the phenotype and function of peripheral Mo from children with hemolytic uremic syndrome (HUS). For this purpose, blood samples from patients in the acute period of HUS (HUS AP) were obtained on admission before dialysis and/or transfusion. The Mo phenotypic characterization was performed on whole blood by flow cytometry, and markers associated to biological functions were selected: CD14 accounting for lipopolysaccharide (LPS) responsiveness, CD11b for adhesion, Fc receptor for immunoglobulin G type I (FcgammaRI)/CD64 for phagocytosis and cytotoxicity, and human leukocyte antigen (HLA)-DR for antigen presentation. Some of these functions were also determined. Moreover, the percentage of CD14+ CD16+ Mo was evaluated. We found that the entire HUS AP Mo population exhibited reduced CD14, CD64, and CD11b expression and decreased LPS-induced tumor necrosis factor production and Fcgamma-dependent cytotoxicity. HUS AP showed an increased percentage of CD14+ CD16+ Mo with higher CD16 and lower CD14 levels compared with the same subset from healthy children. Moreover, the CD14++ CD16- Mo subpopulation of HUS AP had a decreased HLA-DR expression, which correlated with severity. In conclusion, the Mo population from HUS AP patients presents phenotypic and functional alterations. The contribution to the pathogenesis and the possible scenarios that led to these changes are discussed.

[Activation of innate and specific immune responses in hemolytic uremic syndrome (HUS)-patients]. / Activación de la respuesta inmune innata y específica en los pacientes con síndrome urémico hemolítico. Papel dual a traves de la participacion en los mecanismos patogenicos y protectores.

The central role of the immune system is the preservation of the health against several pathogenic microbes and injury agents. However, on special conditions defensive mechanisms triggered towards the foreign agent can damage the host. Clinical and experimental evidence indicate that inflammatory reaction triggered by the main components of Shiga toxin (Stx)-producing Escherichia coil (STEC), participate in the evolution to the complete form of HUS. When children are diagnosed of HUS, they present evidence that have suffered a very strong and early inflammatory response. These features include: the presence of a marked neutrophilia, the polymorfonuclear leucocytes (PMN) are "deactivated or exhausted" and the monocytes are differentiated towards an inflammatory phenotype (CD14-reduced and CD16-enhanced membrane expression). In addition, HUS-patients show a marked reduction in the absolute and relative number of leucocytes carrying the receptor (CX3CR1) for the chemokine "Fractalkine" (FKN, CX3CL1), which are the classic monocytes and Natural Killer cells (NK). All these cells express a high cytotoxic potencial. The chemokine FKN is expressed in endothelial and epithelial renal cells, and is involved in the pathogenic mechanism of different nephropathies. Noteworthy, we found a significant correlation between the severity of the renal damage (as days of anuria) and the alterations described above. Finally, the protective role of specific immune response, mainly through the antibody production with Stx-neutralizing capacity, is discussed.