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.

Lipopolysaccharides (LPS) originate from the outer membrane of Gram-negative bacteria and trigger an inflammatory response via the innate immune system. LPS consist of a lipid A moiety directly responsible for the stimulation of the proinflammatory cascade and a polysaccharide chain of variable length. LPS form aggregates of variable size and structure in aqueous media, and the aggregation/disaggregation propensity of LPS is known as a key determinant of their biological activity. The aim of the present study was to determine to which extent the length of the polysaccharide chain can affect the nature of LPS structures, their pharmacokinetics, and eventually their proinflammatory properties in vivo . LPS variants of Salmonella Minnesota with identical lipid A but with different polysaccharide moieties were used. The physical properties of LPS aggregates were analyzed by zetametry, dynamic light scattering, and microscopy. The stability of LPS aggregates was tested in the presence of plasma, whole blood, and cultured cell lines. LPS pharmacokinetics was performed in wild-type mice. The accumulation in plasma of rough LPS (R-LPS) with a short polysaccharidic chain was lower, and its hepatic uptake was faster as compared to smooth LPS (S-LPS) with a long polysaccharidic chain. The inflammatory response was weaker with R-LPS than with S-LPS. As compared to S-LPS, R-LPS formed larger aggregates, with a higher hydrophobicity index, a more negative zeta potential, and a higher critical aggregation concentration. The lower stability of R-LPS aggregates could be illustrated in vitro by a higher extent of association of LPS to plasma lipoproteins, faster binding to blood cells, and increased uptake by macrophages and hepatocytes, compared to S-LPS. Our data indicate that a long polysaccharide chain is associated with the formation of more stable aggregates with extended residence time in plasma and higher inflammatory potential. These results show that polysaccharide chain length, and overall aggregability of LPS might be helpful to predict the proinflammatory effect that can be expected in experimental settings using LPS preparations. In addition, better knowledge and control of LPS aggregation and disaggregation might lead to new strategies to enhance LPS detoxification in septic patients.