Mitochondrial Dysfunction of Alveolar and Circulating Immune Cells During Acute Respiratory Distress Syndrome: Impact of Infectious Aggression and Alveolar Stretching as a Result of Mechanical Ventilation.

Sepsis leads to a deregulated host response that can lead to organ failure. During sepsis, experimental and clinical data suggest the occurrence of mitochondrial dysfunctions, particularly in circulating muscle and monocytes, which may contribute to organ failure and death.

Lower respiratory infection is the leading cause of death from infectious causes. Mechanical ventilation (MV) is required in 20% of cases of bacterial pneumopathy with Streptococcus pneumoniae (S.p.) , with mortality reaching 50%. There are then frequently criteria for acute respiratory distress syndrome (ARDS), combining bilateral lung involvement and marked hypoxemia.

Cyclic stretching of lung cells induced by MV causes sterile inflammation and tissue damage (i.e. ventilator-induced lung injury [VILI]), which can cause cellular dysfunction that alter the immune response, particularly during ARDS. This is why the application of a so-called protective MV is then required. However, this does not prevent about one-third of patients from showing signs of alveolar overdistension, as evidenced by an increase in motor pressure (MP) (MP≥ 15 cmH2O), associated with an increase in mortality.

The deleterious effects of MV could be explained by the occurrence of mitochondrial abnormalities. Indeed, the cyclic stretching of lung cells leads to dysfunction in the respiratory chain and the production of free oxygen radicals (FOS), altering membrane permeability. These phenomena could promote VILI, facilitate the translocation of bacteria from the lung to the systemic compartment and lead to alterations in immune response.

In our model of S.p. pneumopathy in rabbits, animals on MV develop more severe lung disorders (lack of pulmonary clearance of bacteria, bacterial translocation in the blood, excess mortality), compared to animals on spontaneous ventilation (SV). Intracellular pulmonary mitochondrial DNA (mtDNA) concentrations, a reflection of the mitochondrial pool, are significantly decreased in ventilated rabbits compared to SV rabbits and in infected rabbits compared to uninfected rabbits. At the same time, the mitochondrial content of circulating cells decreased early (H8) in all infected rabbits, but was only restored in rabbits in SV, those who survived pneumonia (Blot et al, poster ECCMID 2015, submitted article). These data suggest an alteration in the mechanisms that restore mitochondrial homeostasis (mitochondrial biogenesis and mitophagy) during the dual infection/MV agression, which may explain the observed excess mortality. Other work by our team illustrates the importance of these phenomena by showing in a mouse model of polymicrobial infection that inhibition of mitophagia in macrophages promotes survival (Patoli et al, in preparation). Human data on this subject are non-existent.

The phenomena of mitochondrial dysfunction nevertheless deserve to be explored in humans during the combined MV/pneumopathy aggression in order to understand its possible impact on the effectiveness of the host's immune response. In a personalized medicine approach, these data would open up prospects for targeted therapies, capable of activating mitochondrial biogenesis and/or modulating mitophagia, to prevent organ dysfunction and mortality during severe CALs treated with antibiotic therapy.