Critical illness is estimated to cause 58 million adult deaths per year globally, and critically ill patients who require exogenous organ support in an intensive care unit (ICU) are at high risk of secondary infections, commonly pneumonia. Despite its varied aetiology, critical illness is often accompanied by immune dysregulation, including hyperinflammation, immune-mediated organ damage and impaired anti-microbial functions. We have previously shown that pneumonia alters functions of mononuclear phagocytes, which acquire a paralyzed phenotype and are associated with increased risk of respiratory complications, such as acute respiratory distress syndrome (ARDS). Our hypothesis is that ICU patients that experience severe disease with high levels of inflammation will have prolonged susceptibility to secondary infections due to mononuclear phagocyte system dysfunction.
Our Functional Immunophenotyping in Critical Illness (FICI) study is currently recruiting severe trauma and COVID-19 patients, with age and sex matched healthy controls, from the Royal Melbourne Hospital ICU. Peripheral blood is collected at days 1, 3, 5 and, if available, day 28 of ICU stay. We designed and optimised two spectral cytometry panels for use with a 5 laser Cytek Aurora. We use a 25-colour spectral cytometry panel to quantify and phenotype circulating neutrophils, monocytes, and dendritic cells in peripheral blood. A 16-colour panel is used to quantify phagocytosis in peripheral blood cell subsets following 1h incubation with S. aureus coated bioparticles conjugated to the pH-reactive compound pHrodoRed; and the reactive oxygen species reactive molecule dihydrorhodamine 123.
We plan to perform high dimensional analysis to identify immune correlates of initial illness trajectory and organ dysfunction, response to treatment, outcome from initial hospitalisation, subsequent re-infection, and readmission to hospital. Stored PBMCs and plasma will be used to investigate impaired mechanisms of dendritic cells. Outcomes from this study will help guide treatment strategies to reduce post-ICU associated morbidity and mortality.