Systemic infl ammatory response syndrome (SIRS) is defi ned by a cluster of clinical signs that include tachycardia, leukocytosis, tachypnoea, and pyrexia ( 1 ). Although SIRS is a major consequence of sepsis (i.e., a deleterious, nonresolving infl ammatory response to infection that can lead to multiple organ dysfunction and septic shock), it can be caused by many pathological events that are not associated with the bacterial infection ( 1 ). It is of major importance to distinguish between noninfectious SIRS, sepsis, and septic shock in critically ill patients because these groups of patients are markedly different in terms of care and clinical outcome. Positive microbiological identifi cation tests [using conventional microbiological tests or bacterial DNA detection ( 2 ) as well as a number of infl ammatory and infectious biomarkers, including cytokines, procalcitonin, immune cell markers or a combination of these ( 3 )] have been proposed, but at this stage, none has proved to be reliable enough to ascertain the occurrence and extent of infection in high-risk patients with SIRS. Interestingly, and as documented further by our group in the prospective multicenter cohort EPISS study ( 4 ), Gramnegative bacilli are the most frequently identifi ed pathogens in patients with septic shock. In this context, the direct quantitation of the culprit component of the Gram-negative bacterium [i.e., lipopolysaccharide (LPS), which triggers SIRS by interacting with the CD14/Toll-like receptor 4 /myeloid differentiation factor 2 receptor complex at the surface of leukocytes ( 1 ) Abbreviations: 3HM, 3-hydroxymyristic acid or 3-hydroxymyristate; LAL, limulus amebocyte lysate; LOD, limit of detection; LOQ, limit of quantifi cation; LPS, lipopolysaccharide; PLTP, phospholipid transfer protein; SIRS, systemic infl ammatory response syndrome; SOFA, sequential organ failure assessment .

Lipopolysaccharides (LPS) or endotoxins are amphipathic, pro-inflammatory components of the outer membrane of Gram-negative bacteria. In the host, LPS can trigger a systemic inflammatory response syndrome. To bring insight into in vivo tissue distribution and cellular uptake of LPS, dual labeling was performed with a bimodal molecular probe designed for fluorescence and nuclear imaging. LPS were labeled with DOTA-Bodipy-NCS, and pro-inflammatory properties were controlled after each labeling step. LPS were then radiolabeled with (111)In and subsequently injected intravenously into wild-type, C57B16 mice, and their in vivo behavior was followed by single photon emission computed tomography coupled with X-ray computed tomography (SPECT-CT) and fluorescence microscopy. Time course of liver uptake of radiolabeled LPS ((111)In-DOTA-Bodipy-LPS) was visualized over a 24-h period in the whole animal by SPECT-CT. In complementary histological analyses with fluorescent microscopy, the bulk of injected (111)In-DOTA-Bodipy-LPS was found to localize early within the liver. Serum kinetics of unlabeled and DOTA-Bodipy-labeled LPS in mouse plasma were similar as ascertained by direct quantitation of β-hydroxymyristate, and DOTA-Bodipy-LPS was found to retain the potent, pro-inflammatory property of the unlabeled molecule as assessed by serum cytokine assays. It is concluded that the dual labeling process, involving the formation of covalent bonds between a DOTA-Bodipy-NCS probe and LPS molecules is relevant for imaging and kinetic analysis of LPS biodistribution, both in vivo and ex vivo. Data of the present study come in direct and visual support of a lipopolysaccharide transport through which pro-inflammatory LPS can be transported from the periphery to the liver for detoxification. The (111)In-DOTA-Bodipy-LPS probe arises here as a relevant tool to identify key components of LPS detoxification in vivo.