Antibody Drug Clearance: An Underexplored Marker of Outcomes with Checkpoint Inhibitors.

Immune-checkpoint inhibitor (ICI) therapy has dramatically changed the clinical landscape for several cancers, and ICI use continues to expand across many cancer types. Low baseline clearance (CL) and/or a large reduction of CL during treatment correlates with better clinical response and longer survival. Similar phenomena have also been reported with other monoclonal antibodies (mAb) in cancer and other diseases, highlighting a characteristic of mAb clinical pharmacology that is potentially shared among various mAbs and diseases. Though tempting to attribute poor outcomes to low drug exposure and arguably low target engagement due to high CL, such speculation is not supported by the relatively flat exposure-response relationship of most ICIs, where a higher dose or exposure is not likely to provide additional benefit. Instead, an elevated and/or increasing CL could be a surrogate marker of the inherent resistant phenotype that cannot be reversed by maximizing drug exposure. The mechanisms connecting ICI clearance, therapeutic efficacy, and resistance are unclear and likely to be multifactorial. Therefore, to explore the potential of ICI CL as an early marker for efficacy, this review highlights the similarities and differences of CL characteristics and CL-response relationships for all FDA-approved ICIs, and we compare and contrast these to selected non-ICI mAbs. We also discuss underlying mechanisms that potentially link mAb CL with efficacy and highlight existing knowledge gaps and future directions where more clinical and preclinical investigations are warranted to clearly understand the value of baseline and/or time-varying CL in predicting response to ICI-based therapeutics.

Decoupling FcRn and tumor contributions to elevated immune checkpoint inhibitor clearance in cancer cachexia.

High baseline clearance of immune checkpoint inhibitors (ICIs), independent of dose or systemic exposure, is associated with cachexia and poor outcomes in cancer patients. Mechanisms linking ICI clearance, cachexia and ICI therapy failure are unknown. Here, we evaluate in four murine models and across multiple antibodies whether altered baseline catabolic clearance of administered antibody requires a tumor and/or cachexia and whether medical reversal of cachexia phenotype can alleviate altered clearance. Key findings include mild cachexia phenotype and lack of elevated pembrolizumab clearance in the MC38 tumor-bearing model. We also observed severe cachexia and decreased, instead of increased, baseline pembrolizumab clearance in the tumor-free cisplatin-induced cachexia model. Liver Fcgrt expression correlated with altered baseline catabolic clearance, though elevated clearance was still observed with antibodies having no (human IgA) or reduced (human H310Q IgG1) FcRn binding. We conclude cachexia phenotype coincides with altered antibody clearance, though tumor presence is neither sufficient nor necessary for altered clearance in immunocompetent mice. Magnitude and direction of clearance alteration correlated with hepatic Fcgrt, suggesting changes in FcRn expression and/or recycling function may be partially responsible, though factors beyond FcRn also contribute to altered clearance in cachexia.

The therapeutic effects of gingival mesenchymal stem cells and their exosomes in a chimeric model of rheumatoid arthritis.

BACKGROUND: Rheumatoid arthritis is a chronic systemic autoimmune disease that involves transformation of the lining of synovial joints into an invasive and destructive tissue. Synovial fibroblasts become transformed, invading and destroying the bone and cartilage of the affected joint(s). Due to the significant role these cells play in the progression of the disease process, developing a therapeutic strategy to target and inhibit their invasive destructive nature could help patients who are afflicted with this debilitating disease. Gingival-derived mesenchymal stem cells are known to possess immunomodulatory properties and have been studied extensively as potential cell-based therapeutics for several autoimmune disorders. METHODS: A chimeric human/mouse model of synovitis was created by surgically implanting SCID mice with a piece of human articular cartilage surrounded by RASF. Mice were injected once with either GMSC or GMSCExo at 5-7 days post-implantation. Histology and IHC were used to assess RASF invasion of the cartilage. Flow cytometry was used to understand the homing ability of GMSC in vivo and the incidence of apoptosis of RASF in vitro. RESULTS: We demonstrate that both GMSC and GMSCExo are potent inhibitors of the deleterious effects of RASF. Both treatments were effective in inhibiting the invasive destructive properties of RASF as well as the potential for these cells to migrate to secondary locations and attack the cartilage. GMSC home to the site of the implant and induce programmed cell death of the RASF. CONCLUSIONS: Our results indicate that both GMSC and GMSCExo can block the pathological effects of RASF in this chimeric model of RA. A single dose of either GMSC or GMSCExo can inhibit the deleterious effects of RASF. These treatments can also block the invasive migration of the RASF, suggesting that they can inhibit the spread of RA to other joints. Because the gingival tissue is harvested with little difficulty, relatively small amounts of tissue are required to expand the cells, the simple in vitro expansion process, and the increasing technological advances in the production of therapeutic exosomes, we believe that GMSCExo are excellent candidates as a potential therapeutic for RA.

Pseudomonas aeruginosa pulmonary infection results in S100A8/A9-dependent cardiac dysfunction.

Pseudomonas aeruginosa (P.a.) infection accounts for nearly 20% of all cases of hospital acquired pneumonia with mortality rates >30%. P.a. infection induces a robust inflammatory response, which ideally enhances bacterial clearance. Unfortunately, excessive inflammation can also have negative effects, and often leads to cardiac dysfunction with associated morbidity and mortality. However, it remains unclear how P.a. lung infection causes cardiac dysfunction. Using a murine pneumonia model, we found that P.a. infection of the lungs led to severe cardiac left ventricular dysfunction and electrical abnormalities. More specifically, we found that neutrophil recruitment and release of S100A8/A9 in the lungs activates the TLR4/RAGE signaling pathways, which in turn enhance systemic inflammation and subsequent cardiac dysfunction. Paradoxically, global deletion of S100A8/A9 did not improve but aggravated cardiac dysfunction and mortality likely due to uncontrolled bacterial burden in the lungs and heart. Our results indicate that P.a. infection induced release of S100A8/9 is double-edged, providing increased risk for cardiac dysfunction yet limiting P.a. growth.

Therapeutic Effects of Gingival Mesenchymal Stem Cells and Their Exosomes in a Chimeric Model of Rheumatoid Arthritis.

Background: Rheumatoid arthritis is a chronic systemic autoimmune disease that involves transformation of the lining of synovial joints into an invasive and destructive tissue. Synovial fibroblasts become transformed, invading and destroying bone and cartilage of the affected joint(s). Due to the significant role these cells play in the progression of the disease process, developing a therapeutic strategy to target and inhibit their invasive destructive nature could help patients who are affiicted with this debilitating disease. Gingival-derived mesenchymal stem cells are known to possess immunomodulatory properties and have been studied extensively as potential cell-based therapeutics for several autoimmune disorders. Methods: A chimeric human/mouse model of synovitis was created by surgically implanting SCID mice with a piece of human articular cartilage surrounded by RASF. Mice were injected once with either GMSC or GMSCExo at 5-7 days post-implantation. Histology and IHC were used to assess RASF invasion of the cartilage. Flow cytometry was used to understand the homing ability of GMSC in vivo and the incidence of apoptosis of RASF in vitro. Results: We demonstrate that both GMSC and GMSCExo are potent inhibitors of the deleterious effects of RASF. Both treatments were effective in inhibiting the invasive destructive properties of RASF as well as the potential of these cells to migrate to secondary locations and attack the cartilage. GMSC home to the site of the implant and induce programmed cell death of the RASF. Conclusions: Our results indicate that both GMSC and GMSCExo can block the pathological effects of RASF in this chimeric model of RA. A single dose of either GMSC or GMSCExo can inhibit the deleterious effects of RASF. These treatments can also block the invasive migration of the RASF, suggesting that they can inhibit the spread of RA to other joints. Because the gingival tissue is harvested with little difficulty, relatively small amounts of tissue are required to expand the cells, the simple in vitro expansion process, and the increasing technological advances in the production of therapeutic exosomes, we believe that GMSCExo are excellent candidates as a potential therapeutic for RA.

Inhibition of BET Proteins Regulates Fcγ Receptor Function and Reduces Inflammation in Rheumatoid Arthritis.

Overactivation of immune responses is a hallmark of autoimmune disease pathogenesis. This includes the heightened production of inflammatory cytokines such as Tumor Necrosis Factor α (TNFα), and the secretion of autoantibodies such as isotypes of rheumatoid factor (RF) and anticitrullinated protein antibody (ACPA). Fcγ receptors (FcγR) expressed on the surface of myeloid cells bind Immunoglobulin G (IgG) immune complexes. Recognition of autoantigen-antibody complexes by FcγR induces an inflammatory phenotype that results in tissue damage and further escalation of the inflammatory response. Bromodomain and extra-terminal protein (BET) inhibition is associated with reduced immune responses, making the BET family a potential therapeutic target for autoimmune diseases such as rheumatoid arthritis (RA). In this paper, we examined the BET inhibitor PLX51107 and its effect on regulating FcγR expression and function in RA. PLX51107 significantly downregulated expression of FcγRIIa, FcγRIIb, FcγRIIIa, and the common γ-chain, FcϵR1-γ, in both healthy donor and RA patient monocytes. Consistent with this, PLX51107 treatment attenuated signaling events downstream of FcγR activation. This was accompanied by a significant decrease in phagocytosis and TNFα production. Finally, in a collagen-induced arthritis model, PLX51107-treatment reduced FcγR expression in vivo accompanied by a significant reduction in footpad swelling. These results suggest that BET inhibition is a novel therapeutic approach that requires further exploration as a treatment for patients with RA.

Influenza A virus modulates ACE2 expression and SARS-CoV-2 infectivity in human cardiomyocytes.

Influenza A virus (IAV) and SARS-CoV-2 virus are both acute respiratory viruses currently circulating in the human population. This study aims to determine the impact of IAV infection on SARS-CoV-2 pathogenesis and cardiomyocyte function. Infection of human bronchial epithelial cells (HBEC), A549 cells, lung fibroblasts (HLF), monocyte derived macrophages (MDMs), cardiac fibroblasts (HCF) and hiPSC-derived cardiomyocytes with IAV enhanced the expression of ACE2, the SARS-CoV-2 receptor. Similarly, IAV infection increased levels of ACE2 in the lungs of mice and humans. Of interest, we detected heavily glycosylated form of ACE2 in hiPSC-CMs and poorly glycosylated ACE2 in other cell types. Also, prior IAV infection enhances SARS-CoV-2 spike protein binding and viral entry in all cell types. However, efficient SARS-CoV-2 replication was uniquely inhibited in cardiomyocytes. Glycosylation of ACE2 correlated with enzymatic conversion of its substrate Ang II, induction of eNOS and nitric oxide production, may provide a potential mechanism for the restricted SARS-CoV-2 replication in cardiomyocytes.

Molecular profiling of kidney compartments from serial biopsies differentiate treatment responders from non-responders in lupus nephritis.

The immune pathways that define treatment response and non-response in lupus nephritis (LN) are unknown. To characterize these intra-kidney pathways, transcriptomic analysis was done on protocol kidney biopsies obtained at flare (initial biopsy (Bx1)) and after treatment (second biopsy (Bx2)) in 58 patients with LN. Glomeruli and tubulointerstitial compartments were isolated using laser microdissection. RNA was extracted and analyzed by nanostring technology with transcript expression from clinically complete responders, partial responders and non-responders compared at Bx1 and Bx2 and to the healthy controls. Top transcripts that differentiate clinically complete responders from non-responders were validated at the protein level by confocal microscopy and urine ELISA. At Bx1, cluster analysis determined that glomerular integrin, neutrophil, chemokines/cytokines and tubulointerstitial chemokines, T cell and leukocyte adhesion genes were able to differentiate non-responders from clinically complete responders. At Bx2, glomerular monocyte, extracellular matrix, and interferon, and tubulointerstitial interferon, complement, and T cell transcripts differentiated non-responders from clinically complete responders. Protein analysis identified several protein products of overexpressed glomerular and tubulointerstitial transcripts at LN flare, recapitulating top transcript findings. Urine complement component 5a and fibronectin-1 protein levels reflected complement and fibronectin expression at flare and after treatment. Thus, transcript analysis of serial LN kidney biopsies demonstrated how gene expression in the kidney changes with clinically successful and unsuccessful therapy. Hence, these insights into the molecular landscape of response and non-response may help align LN management with the pathogenesis of kidney injury.

The Neonatal Fc Receptor Is Elevated in Monocyte-Derived Immune Cells in Pancreatic Cancer.

The neonatal Fc receptor (FcRn) is responsible for recycling of IgG antibodies and albumin throughout the body. This mechanism has been exploited for pharmaceutic delivery across an array of diseases to either enhance or diminish this function. Monoclonal antibodies and albumin-bound nanoparticles are examples of FcRn-dependent anti-cancer therapeutics. Despite its importance in drug delivery, little is known about FcRn expression in circulating immune cells. Through time-of-flight mass cytometry (CyTOF) we were able to characterize FcRn expression in peripheral blood mononuclear cell (PBMC) populations of pancreatic ductal adenocarcinoma (PDAC) patients and non-cancer donors. Furthermore, we were able to replicate these findings in an orthotopic murine model of PDAC. Altogether, we found that in both patients and mice with PDAC, FcRn was elevated in migratory and resident classical dendritic cell type 2 (cDC2) as well as monocytic and granulocytic myeloid-derived suppressor cell (MDSC) populations compared to tumor-free controls. Furthermore, PBMCs from PDAC patients had elevated monocyte, dendritic cells and MDSCs relative to non-cancer donor PBMCs. Future investigations into FcRn activity may further elucidate possible mechanisms of poor efficacy of antibody immunotherapies in patients with PDAC.

Serum Albumin: Early Prognostic Marker of Benefit for Immune Checkpoint Inhibitor Monotherapy But Not Chemoimmunotherapy.

BACKGROUND: Cancer cachexia exhibits decreased albumin and associates with short overall survival (OS) in patients with non-small cell lung cancer (NSCLC), but whether on-treatment albumin changes associate with OS in NSCLC patients treated with immune checkpoint inhibitors (ICIs) and combination chemoimmunotherapy has not been thoroughly evaluated. PATIENTS AND METHODS: We conducted a single-center retrospective study of patients with advanced NSCLC who received first-line ICI with or without chemotherapy between 2013 and 2020. The association of pretreatment albumin and early albumin changes with OS was evaluated using Kaplan-Meier method and Cox regression models. RESULTS: A total of 210 patients were included: 109 in ICI cohort and 101 in ICI + Chemo cohort. Within a median of 21 days from treatment initiation, patients with ≥ 10% of albumin decrease had significantly shorter OS compared to patients without albumin decrease in ICI cohort. Pretreatment albumin and albumin decrease within the first or second cycle of treatment were significantly and independently associated with OS in ICI cohort, but not in ICI + Chemo cohort. The lack of association between albumin and OS with the addition of chemotherapy was more pronounced among patients with ≥ 1% PD-L1 expression in subgroup analysis. CONCLUSION: Pretreatment serum albumin and early albumin decrease in ICI monotherapy was significantly associated with OS in advanced NSCLC. Early albumin change, as a routine lab value tested in clinic, may be combined with established biomarkers to improve outcome predictions of ICI monotherapy. The underlying mechanism of the observed association between decreased albumin and ICI resistance warrants further investigation.

Stabilin receptors clear LPS and control systemic inflammation.

Lipopolysaccharides (LPSs) cause lethal endotoxemia if not rapidly cleared from blood circulation. Liver sinusoidal endothelial cells (LSEC) systemically clear LPS by unknown mechanisms. We discovered that LPS clearance through LSEC involves endocytosis and lysosomal inactivation via Stabilin-1 and 2 (Stab1 and Stab2) but does not involve TLR4. Cytokine production was inversely related to clearance/endocytosis of LPS by LSEC. When exposed to LPS, Stabilin double knockout mice (Stab DK) and Stab1 KO, but not Stab2 KO, showed significantly enhanced systemic inflammatory cytokine production and early death compared with WT mice. Stab1 KO is not significantly different from Stab DK in circulatory LPS clearance, LPS uptake and endocytosis by LSEC, and cytokine production. These data indicate that (1) Stab1 receptor primarily facilitates the proactive clearance of LPS and limits TLR4-mediated inflammation and (2) TLR4 and Stab1 are functionally opposing LPS receptors. These findings suggest that endotoxemia can be controlled by optimizing LPS clearance by Stab1.

A Novel Inflammatory Dendritic Cell That Is Abundant and Contiguous to T Cells in the Kidneys of Patients With Lupus Nephritis.

The mechanisms that promote local inflammatory injury during lupus nephritis (LN) flare are largely unknown. Understanding the key immune cells that drive intrarenal inflammation will advance our knowledge of disease pathogenesis and inform the development of new therapeutics for LN management. In this study, we analyzed kidney biopsies from patients with proliferative LN and identified a novel inflammatory dendritic cell (infDC) population that is highly expressed in the LN kidney, but minimally present in healthy human kidneys. During an agnostic evaluation of immune transcript expression in the kidneys of patients with proliferative LN, the most abundantly overexpressed transcript from isolated glomeruli was FCER1G, which encodes the Fc receptor gamma chain (FcRγ). To identify the cell types expressing FcRγ that infiltrate the kidney in LN, studies were done on kidney biopsies from patients with active LN using confocal immunofluorescence (IF) microscopy. This showed that FcRγ is abundantly present in the periglomerular (PG) region of the kidney and to a lesser extent in the tubulointerstitium (TI). Further investigation of the surface markers of these cells showed that they were FcRγ+, MHC II+, CD11c+, CD163+, CD5-, DC-SIGN+, CD64+, CD14+, CD16+, SIRPα+, CD206-, CD68-, CD123-, CD3-, and CD11b-, suggesting the cells were infDCs. Quantification of the infDCs showed an average 10-fold higher level of infDCs in the LN kidney compared to the healthy kidneys. Importantly, IF identified CD3+ T cells to be adjacent to these infDCs in the PG space of the LN kidney, whereas both cell types are minimally present in the healthy kidney. Thus, we have identified a previously undescribed DC in lupus kidneys that may interact with intrarenal T cells and play a role in the pathogenesis of kidney injury during LN flare.

Accelerated Clearance and Degradation of Cell-Free HIV by Neutralizing Antibodies Occurs via FcγRIIb on Liver Sinusoidal Endothelial Cells by Endocytosis.

Neutralizing Abs suppress HIV infection by accelerating viral clearance from blood circulation in addition to neutralization. The elimination mechanism is largely unknown. We determined that human liver sinusoidal endothelial cells (LSEC) express FcγRIIb as the lone Fcγ receptor, and using humanized FcγRIIb mouse, we found that Ab-opsonized HIV pseudoviruses were cleared considerably faster from circulation than HIV by LSEC FcγRIIb. Compared with humanized FcγRIIb-expressing mice, HIV clearance was significantly slower in FcγRIIb knockout mice. Interestingly, a pentamix of neutralizing Abs cleared HIV faster compared with hyperimmune anti-HIV Ig (HIVIG), although the HIV Ab/Ag ratio was higher in immune complexes made of HIVIG and HIV than pentamix and HIV. The effector mechanism of LSEC FcγRIIb was identified to be endocytosis. Once endocytosed, both Ab-opsonized HIV pseudoviruses and HIV localized to lysosomes. This suggests that clearance of HIV, endocytosis, and lysosomal trafficking within LSEC occur sequentially and that the clearance rate may influence downstream events. Most importantly, we have identified LSEC FcγRIIb-mediated endocytosis to be the Fc effector mechanism to eliminate cell-free HIV by Abs, which could inform development of HIV vaccine and Ab therapy.

Dietary patterns and nutrient intake of individuals with rheumatoid arthritis and osteoarthritis in the United States.

OBJECTIVE: The aim of this study was to investigate the differences in adherence to the Mediterranean diet, assessed by the alternative Mediterranean Diet (aMED) score, and diet quality, assessed by Healthy Eating Index 2015 to 2020 (HEI-2015), between presence and type of arthritis (rheumatoid arthritis and osteoarthritis). Additionally, the study investigated the association between aMED scores and HEI-2015 scores and the presence of arthritis. METHODS: Cross-sectional data from four cycles (2007-2014) of the National Health and Nutrition Examination Survey were used and weighted to produce a nationally representative sample. Arthritis information was extracted from the Medical Conditions file and recoded into relevant variables. Food group and nutrient data from the 24-h recall was transformed to provide aMED and HEI-2015 scores. RESULTS: Individuals with arthritis had significantly worse adherence to the Mediterranean diet and diet quality. aMED scores were 3.43 ± 0.04 for individuals with arthritis and 3.54 ± 0.03 for individuals without arthritis (P = 0.014). HEI-2015 scores were also lower in individuals with arthritis (51.41 ± 0.37) than in those without (53.50 ± 0.28; P < 0.001). There were no significant differences in aMED or HEI-2015 scores between individuals with rheumatoid arthritis and those with osteoarthritis. There were also no associations between aMED scores or HEI-2015 scores and the presence of arthritis. CONCLUSIONS: Individuals diagnosed with arthritis can take steps to improve their diet quality as a possible route to reduce arthritis symptoms and maintain a healthy body weight. Further research on dietary patterns and their potential to treat and manage arthritis is warranted.

Mouse Liver Sinusoidal Endothelium Eliminates HIV-Like Particles from Blood at a Rate of 100 Million per Minute by a Second-Order Kinetic Process.

We crafted human immunodeficiency virus (HIV)-like particles of diameter about 140 nm, which expressed two major HIV-1 proteins, namely, env and gag gene products, and used this reagent to simulate the rate of decay of HIV from the blood stream of BALB/c male mice. We found that most (~90%) of the particles were eliminated (cleared) from the blood by the liver sinusoidal endothelial cells (LSECs), the remainder from Kupffer cells; suggesting that LSECs are the major liver scavengers for HIV clearance from blood. Decay was rapid with kinetics suggesting second order with respect to particles, which infers dimerization of a putative receptor on LSEC. The number of HIV-like particles required for saturating the clearance mechanism was approximated. The capacity for elimination of blood-borne HIV-like particles by the sinusoid was 112 million particles per minute. Assuming that the sinusoid endothelial cells were about the size of glass-adherent macrophages, then elimination capacity was more than 540 particles per hour per endothelial cell.

Cardiac Electrical and Structural Changes During Bacterial Infection: An Instructive Model to Study Cardiac Dysfunction in Sepsis.

BACKGROUND: Sepsis patients with cardiac dysfunction have significantly higher mortality. Although several pathways are associated with myocardial damage in sepsis, the precise cause(s) remains unclear and treatment options are limited. This study was designed to develop a new model to investigate the early events of cardiac damage during sepsis progression. METHODS AND RESULTS: Francisella tularensis subspecies novicida (Ft.n) is a Gram-negative intracellular pathogen causing severe sepsis syndrome in mice. BALB/c mice (N=12) were sham treated or infected with Ft.n through the intranasal route. Serial electrocardiograms were recorded at multiple time points until 96 hours. Hearts were then harvested for histology and gene expression studies. Similar to septic patients, we illustrate both cardiac electrical and structural phenotypes in our murine Ft.n infection model, including prominent R' wave formation, prolonged QRS intervals, and significant left ventricular dysfunction. Notably, in infected animals, we detected numerous microlesions in the myocardium, previously observed following nosocomial Streptococcus infection and in sepsis patients. We show that Ft.n-mediated microlesions are attributed to cardiomyocyte apoptosis, increased immune cell infiltration, and expression of inflammatory mediators (tumor necrosis factor, interleukin [IL]-1ß, IL-8, and superoxide dismutase 2). Finally, we identify increased expression of microRNA-155 and rapid degradation of heat shock factor 1 following cardiac Ft.n infection as a primary cause of myocardial inflammation and apoptosis. CONCLUSIONS: We have developed and characterized an Ft.n infection model to understand the pathogenesis of cardiac dysregulation in sepsis. Our findings illustrate novel in vivo phenotypes underlying cardiac dysfunction during Ft.n infection with significant translational impact on our understanding of sepsis pathophysiology.

Blood-Borne Lipopolysaccharide Is Rapidly Eliminated by Liver Sinusoidal Endothelial Cells via High-Density Lipoprotein.

During Gram-negative bacterial infections, excessive LPS induces inflammation and sepsis via action on immune cells. However, the bulk of LPS can be cleared from circulation by the liver. Liver clearance is thought to be a slow process mediated exclusively by phagocytic resident macrophages, Kupffer cells (KC). However, we discovered that LPS disappears rapidly from the circulation, with a half-life of 2-4 min in mice, and liver eliminates about three quarters of LPS from blood circulation. Using microscopic techniques, we found that ∼75% of fluor-tagged LPS in liver became associated with liver sinusoidal endothelial cells (LSEC) and only ∼25% with KC. Notably, the ratio of LSEC-KC-associated LPS remained unchanged 45 min after infusion, indicating that LSEC independently processes the LPS. Most interestingly, results of kinetic analysis of LPS bioactivity, using modified limulus amebocyte lysate assay, suggest that recombinant factor C, an LPS binding protein, competitively inhibits high-density lipoprotein (HDL)-mediated LPS association with LSEC early in the process. Supporting the previous notion, 3 min postinfusion, 75% of infused fluorescently tagged LPS-HDL complex associates with LSEC, suggesting that HDL facilitates LPS clearance. These results lead us to propose a new paradigm of LSEC and HDL in clearing LPS with a potential to avoid inflammation during sepsis.

Scavenger receptor B1, the HDL receptor, is expressed abundantly in liver sinusoidal endothelial cells.

Cholesterol from peripheral tissue, carried by HDL, is metabolized in the liver after uptake by the HDL receptor, SR-B1. Hepatocytes have long been considered the only liver cells expressing SR-B1; however, in this study we describe two disparate immunofluorescence (IF) experiments that suggest otherwise. Using high-resolution confocal microscopy employing ultrathin (120 nm) sections of mouse liver, improving z-axis resolution, we identified the liver sinusoidal endothelial cells (LSEC), marked by FcγRIIb, as the cell within the liver expressing abundant SR-B1. In contrast, the hepatocyte, identified with ß-catenin, expressed considerably weaker levels, although optical resolution of SR-B1 was inadequate. Thus, we moved to a different IF strategy, first separating dissociated liver cells by gradient centrifugation into two portions, hepatocytes (parenchymal cells) and LSEC (non-parenchymal cells). Characterizing both portions for the cellular expression of SR-B1 by flow cytometry, we found that LSEC expressed considerable amounts of SR-B1 while in hepatocytes SR-B1 expression was barely perceptible. Assessing mRNA of SR-B1 by real time PCR we found messenger expression in LSEC to be about 5 times higher than in hepatocytes.