Flow Cytometry Developments and Perspectives in Clinical Studies: Examples in ICU Patients

Septic syndromes represent a major, although largely under-recognized, healthcare problem worldwide accounting for thousands of deaths every year. Although flow cytometry (FCM) remains a relatively confidential diagnostic tool, it is useful at every step of intensive care unit (ICU) patients’ management. This review will focus on biomarkers measurable by FCM on a routine standardized basis and usable for the diagnosis of sepsis and for prediction of adverse outcome, occurrence of secondary nosocomial infections or guidance of putative immunotherapy relative to innate and adaptive immune dysfunctions in ICU patients. Regarding early diagnosis of infection, neutrophil CD64 has been shown to be a highly sensitive and specific marker for systemic infection and sepsis in adults, neonates, and children. A diminished monocyte HLA-DR expression is a reliable marker for the development of monocyte anergy, secondary nosocomial infection, and death in critically ill patients. Finally, the measurement of an increased CD4⁺ CD25⁺ CD127^low regulatory T cell percentage may represent a reliable marker for the diagnosis of lymphocyte dysfunctions in these patients. These stainings can be performed using lyse-no-wash methods and results are available within 1 h. Ideally, these biomarkers should be part of a panel helping to define ICU patients’ immune status. In the specific clinical context of ICU patients’ monitoring, the increasing potential of FCM is further illustrated by the use of the biomarkers listed above as stratification tools in preliminary clinical studies. The next critical step is to use these standardized FCM protocols in large multicentric clinical trials testing individualized immunotherapy. Importantly, many other markers of immune dysfunction are currently under development that could further enable the administration of targeted individualized therapy in ICU patients.