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    Heterogeneity of post-septic immune responses: a translational approach

    Jean-François Llijtos

    Thursday 24 June, 2021, at 2 pm

    Rosalind Franklin room, 2nd floor

    Institut Cochin, 22 rue méchain Paris 75014

    by videoconference

    Supervisor: Frédéric Pène

    Team: Pulmonary & systemic immune responses during acute and chronic bacterial infections

    Department : Infection, Immunity and Inflammation



    Sepsis following microbial infection is a complex clinical entity, characterized by the simultaneous occurrence of an initial dysregulated inflammatory response and protracted immune suppression associating quantitative and functional abnormalities of immune cells. Most of the immune dysfunction induced by sepsis has been associated with an increased susceptibility to nosocomial infections, first and foremost ventilator-acquired pneumonia (VAP). However, there appears to be variability in the susceptibility to these ICU-acquired infections. First, we identified in patients admitted in the intensive care unit for septic shock an increased risk of VAP after a first pulmonary infection when compared to other sites of infection. Furthermore, in the pandemic context, we have identified that COVID-19 was independently associated with an increased risk of nosocomial pneumonia, thus suggesting that SARS-CoV-2 infection induces particular lung immune. Using two murine models, we then addressed the impact of primary pulmonary and non-pulmonary infectious insults on lung immunity. Mice were first subjected either to polymicrobial peritonitis induced by caecal ligation and puncture (CLP), or to bacterial pneumonia induced by intra-tracheal (i.t.) instillation of Escherichia coli. Respective control mice were subjected to sham surgery or intra-tracheal instillation of phosphate-buffered saline. Seven days later, mice that survived the primary insult were subjected to i.t. instillation of Pseudomonas aeruginosa (PAO1 strain). We assessed survival and pulmonary bacterial clearance of after P. aeruginosa pneumonia, as well as quantitative and functional changes in lung immune cells.  When compared to sham-operated mice, post-CLP animals exhibited increased susceptibility to secondary P. aeruginosa pneumonia as demonstrated by defective lung bacterial clearance and increased mortality rate (50% vs. 0%, p< 0.05). In contrast, all post-pneumonia mice survived and even exhibited improved bacterial clearance as compared to their control counterparts. When addressing whole-lung immune cell distribution prior to second hit (day 7), amounts of alveolar macrophages (AM) were decreased in post-CLP mice while increased in post-pneumonia mice. In contrast to post-CLP, AM from post-pneumonia mice express high MHC-II, a defence-ready transcriptomic signature and promotes antigen-specific CD4 T cells proliferation. Additionally, we observed a TLR2-dependent increase in regulatory T cells (Tregs) proportion among CD4 T cells in the lung of post-CLP mice at day 7 when compared to controls and post-pneumonia mice. CD25-mediated depletion of Tregs prior to P. aeruginosa pneumonia restored survival and bacterial clearance in post-CLP mice. Tregs depletion was associated with restoration of numbers and functions of AMin post-CLP mice. Given the difficulties in accessing tissue macrophages in clinical practice, we evaluated the functions of circulating monocytes using an innovative experimental technique based on the encapsulation of cells in lipid droplets in a microfluidic system. We evaluated in real time the individual secretion of TNFalpha by monocytes from septic patients (n = 7) and healthy volunteers (n = 10) exposed to stimulation by LPS. By comparing the results with those obtained by flow cytometry (membrane expressions of HLA-DR and intracellular expressions of TNFalpha), we were able to demonstrate the existence of a significant temporal variability and a great heterogeneity of the monocytic responses in septic patients.