Interleukin-6 ( Il6), known as a classical pro-inflammatory cytokine in sepsis, is a typical tolerizable gene. Two classes of genes have been categorized in tolerant macrophages in previous studies: genes not inducible described as ‘tolerizable’ genes and genes still inducible named ‘non-tolerizable’ genes ( 5). In the case of sepsis, this tolerization process can be studied in the context of endotoxin tolerance induced by consecutive LPS stimulations ( 4). Ultimately, these deactivated macrophages adopt an immunosuppressive phenotype making them unresponsive to secondary infections, but prone to resolve inflammation and regenerate tissue. The most prominent feature of sepsis-induced immunosuppression is the diminished capacity of innate immune cells, especially monocytes and macrophages, to release pro-inflammatory cytokines in response to pathogen associated molecular patterns (PAMP) ( 3). The failure of resolution process can lead to collateral tissue destruction and multiple organ dysfunction often lethal ( 3). This latter phase often fails to adequately resolve the systemic inflammation and predisposes to immune dysfunction. The initial and uncontrolled systemic inflammatory response (SIRS) is followed by an unchecked compensatory anti-inflammatory response (CARS). Sepsis is caused by a dysregulated host response to infection that consists of two dynamic phases. There is therefore a strong need to better understand the underlying sepsis mechanism to develop new therapeutic strategies ( 2).
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In the last decades, numerous treatments to ameliorate sepsis outcome have failed in clinical trials. Sepsis is a life-threatening condition, which represents the leading cause of death in intensive care units (ICUs) worldwide ( 1). In conclusion, FRA-1 may have a protective role in the tolerance response of sepsis through the regulation of NGAL, leading to resolution of inflammation.
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Collectively, our data indicate that FRA-1 is involved in myeloid cell tolerance responses by mediating the functional reprogramming of Lcn2 transcription in response to prolonged LPS exposure. This suggests that FRA-1 expression supports resolution of inflammation in this model. This was characterized by an increase of neutrophil infiltration in lung and an increase of apoptotic follicular cells in spleen. Moreover, an increased inflammatory state likely dependent of the low level of NGAL was observed in these FRA-1 deficient mice. NGAL secretion was elevated in lung, spleen and serum of wild type tolerant mice, whereas it was significantly lower in tolerant FRA-1 deficient mice. Finally, we used an in vivo septic model of consecutive injection of LPS, in which the second stimulation is performed before the resolution of inflammation, in wild type and FRA-1 deficient mice. Interestingly, ChIP-seq and ChIP-qPCR revealed the binding of FRA-1 on Lcn2 promoter after LPS stimulation in these cells. When using FRA-1 deficient macrophages, we could confirm that FRA-1 regulates NGAL expression during the tolerant state. Identical results were obtained in human PBMC following the endotoxin tolerance model. Moreover, we could correlate FRA-1 expression to the expression of an essential anti-inflammatory molecule involved in sepsis response, Lipocalin 2 aka NGAL. Out of all AP-1 transcription factors tested, Fosl1 gene stood out because of its unique regulation in tolerized cells. Herein, we used the endotoxin tolerance model on murine bone marrow-derived macrophages in which tolerant cells stimulated twice with LPS were compared to naïve cells stimulated once. Surprisingly, there is no report on the role of AP-1 transcription factors in this reprogramming process.
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This immunosuppression can be mediated by the functional reprogramming of gene transcription in monocytes/macrophages in response to prolonged lipopolysaccharide (LPS) exposure. This is followed by an aberrant resolution phase associated to a prolonged period of immune suppression that can ultimately lead to multiple organ dysfunctions. Sepsis is a life-threatening condition characterized by excessive inflammation in its early phase.
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