Pulmonary Immune Cell-microbiome Interactions in ARDS

The overall aim is to compare the composition and spatial heterogeneity of the following in critically ill intensive care unit (ICU) patients: i) immune cell populations and their activation patterns, ii) the surrounding cytokine-chemokine milieu, including trans-compartmental fluxes of these mediators between the lung and bloodstream, and iii) the lung microbiome. Main hypotheses: * The immune cell population in bronchoalveolar lavage fluid (BALF) from patients with ARDS is dominated by neutrocytes, while T cells are depleted, and show evidence of hyper-activation and exhaustion * T cell hyper-activation and exhaustion is specifically compartmentalised to the lungs, and much more pronounced in moderate-to-severe than none-to-mild ARDS * Cyto- and chemokines derived from pulmonary immune cells are higher in moderate-to-severe than none-to-mild ARDS with a greater release from lungs to the bloodstream, notably of IL-6 and IL-8. * The differences in T cell profile in BALF, notably the ratio between regulatory T cells and T helper 17 cells, will change with disease severity over time, and can be explained by the presence of tI-IFN antibodies and/or a low microbial diversity of the respiratory tract with low enrichment from the oral cavity.