Timely and reliable distinction of non-infectious systemic inflammatory response syndrome (SIRS), common in critically ill patients, from sepsis to support adequate antimicrobial therapy safes lives but is clinically challenging. Expeditious sepsis biomarkers are thus urgently sought. Blood transcriptional profiling provides insights into sepsis pathophysiology, but variability in leukocyte subtype composition complicates profile interpretation, and reliable reference genes to normalize gene expression in sepsis are lacking. Here, we identified AKIRIN1 as a reference gene, specifically, in peripheral NK cells and granulocytes for differential gene expression analysis between patients with SIRS and septic shock on intensive care unit admission. Discovery by a two-step probabilistic selection from microarray data followed by validation through branched DNA assays in independent patients revealed several candidate reference genes in NK cells, namely, AKIRIN1, PPP6R3, TAX1BP1, and ADRBK1. For in vitro priming of NK cells, GUSB however was confirmed as reference gene of choice. Initially, no candidate genes could be validated in granulocytes, an additional rescreen of known reference genes by RT-PCR included. By serendipity, we could determine equal AKIRIN1 expression levels also in SIRS and septic shock granulocytes and no change by in vitro challenge of granulocytes with LPS. Inspection of four external neutrophil transcriptome datasets further support unchanged AKIRIN1 expression in human systemic inflammation. Invariable AKIRIN1 expression in peripheral NK cells and granulocytes needs further validation in sepsis and other infectious and inflammatory diseases. As a reference gene in these cells and conditions, AKIRIN1 may further our understanding of innate immunity and lead to new biomarkers.
Overall design
We enrolled critically ill patients and collected blood on admission to our surgical intensive care unit if, within the preceding 24 hours, they had a diagnosis of post-traumatic systemic inflammatory response syndrome (SIRS) or septic shock. Hospitalized noncritically ill control patients were recruited on presurgical examination. To transcriptionally characterize the innate immune response to sterile (SIRS) and infectious (septic shock) systemic inflammation, natural killer (NK) cells were isolated from patient blood for RNA extraction and hybridization on Affymetrix microarrays.
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