Immunological Endotyping of Chronic Critical Illness after Severe Trauma or Sepsis

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ABSTRACT Severe traumatic injury and sepsis are acute pro-inflammatory insults that trigger a “genomic and cytokine storm” by the host innate immune response that can result in multiple organ failure (MOF). Whereas many patients previously succumbed to early refractory MOF, progressive advancements in resuscitation and organ support have led to an increasing number of patients surviving to enter a state of chronic critical illness (CCI), defined as a patient with an extended intensive care unit (ICU) stay and non-resolving organ dysfunction. Currently, as many as 25% of trauma and 40% of septic patients in the ICU develop CCI. In addition to a prolonged hospital course, these ICU “survivors” have recurrent infections, are unable to physically rehabilitate, and are frequently discharged to high-resource care facilities with dismal long-term outcomes. Two clinical manifestations dominate the course of the CCI phenotype: 1) recurrent nosocomial and post-discharge infections indicative of a state of chronic immunosuppression, and 2) acute muscle wasting, weakness and physical debilitation indicative of persistent inflammation. There is an expanding body of evidence that an underlying syndrome of persistent inflammation, immunosuppression and catabolism (PICS) is a key mechanistic driver of CCI. We hypothesize that PICS is its own unique immunological endotype independent of the index event, and is the shared mechanistic pathway leading from either trauma or sepsis to the clinical phenotype of CCI. This maladaptive host response is sustained by the ongoing release of endogenous alarmins (DAMPs) associated with end-organ injury, as well as microbial products from primary/secondary infections (PAMPs). This failure to return to immunologic homeostasis also drives the persistent organ dysfunction seen in CCI. Muscle, being both clinically relevant and understudied, serves as a novel area of study from the standpoint of inflammation-mediated end- organ injury both as a driver of acute muscle wasting, as well as a potential source of ongoing alarmin production and release. The goals for our laboratory’s research program over the next five years include the following: 1) characterize the heterogeneity of the host response after severe trauma and sepsis by identifying distinct endotypes based on immune trajectory over time, and whether these endotypes are modified by sex, age and ethnicity/race; 2) determine if the PICS endotype is the common mechanistic pathway after trauma or sepsis to the clinical development of CCI; and 3) determine whether muscle inflammation is both a component of chronic end-organ injury, as well as a mediator of systemic inflammation through the systemic release of endogenous alarmins. The proposed work is novel, innovative and vital. There are currently no therapeutic interventions other than supportive therapies for the increasingly common condition of CCI after trauma or sepsis. We believe that only through a complete understanding of the immunological endotype of CCI can effective therapeutic interventions be designed. Focusing on interactions between host immunity and muscle inflammation is a novel, under-explored area of research for the long-term management of severe trauma and sepsis.