Mason W. Freeman

Pattern recognition receptors TLR4 and CD14 mediate response to respiratorysyncytial virus

The innate immune system contributes to the earliest phase of the host defense against foreign organisms and has both soluble and cellular pattern recognition receptors for microbial products. Two important members of this receptor group, CD14 and the Toll-like receptor (TLR) pattern recognition receptors, are essential for the innate immune response to components of Gram-negative and Gram-positive bacteria, mycobacteria, spirochetes and yeast. We now find that these receptors function in an antiviral response as well. The innate immune response to the fusion protein of an important respiratory pathogen of humans, respiratory syncytial virus (RSV), was mediated by TLR4 and CD14. RSV persisted longer in the lungs of infected TLR4-deficient mice compared to normal mice. Thus, a common receptor activation pathway can initiate innate immune responses to both bacterial and viral pathogens.

Scavenger receptors class A-I/II and CD36 are the principal receptors responsible for the uptake of modified low density lipoprotein leading to lipid loading in macrophages.

Modification of low density lipoprotein (LDL) can result in the avid uptake of these lipoproteins via a family of macrophage transmembrane proteins referred to as scavenger receptors (SRs). The genetic inactivation of either of two SR family members, SR-A or CD36, has been shown previously to reduce oxidized LDL uptakein vitro and atherosclerotic lesions in mice. Several other SRs are reported to bind modified LDL, but their contribution to macrophage lipid accumulation is uncertain. We generated mice lacking both SR-A and CD36 to determine their combined impact on macrophage lipid uptake and to assess the contribution of other SRs to this process. We show that SR-A and CD36 account for 75–90% of degradation of LDL modified by acetylation or oxidation. Cholesteryl ester derived from modified lipoproteins fails to accumulate in macrophages taken from the double null mice, as assessed by histochemistry and gas chromatography-mass spectrometry. These results demonstrate that SR-A and CD36 are responsible for the preponderance of modified LDL uptake in macrophages and that other scavenger receptors do not compensate for their absence.

Reduced atherosclerosis in MyD88-null mice links elevated serum cholesterol levels to activation of innate immunity signaling pathways

Atherosclerosis, the leading cause of death in developed countries, has been linked to hypercholesterolemia for decades. More recently, atherosclerotic lesion progression has been shown to depend on persistent, chronic inflammation in the artery wall. Although several studies have implicated infectious agents in this process, the role of infection in atherosclerosis remains controversial. Because the involvement of monocytes and macrophages in the pathogenesis of atherosclerosis is well established, we investigated the possibility that macrophage innate immunity signaling pathways normally activated by pathogens might also be activated in response to hyperlipidemia. We examined atherosclerotic lesion development in uninfected, hyperlipidemic mice lacking expression of either lipopolysaccharide (LPS) receptor CD14 or myeloid differentiation protein-88 (MyD88), which transduces cell signaling events downstream of the Toll-like receptors (TLRs), as well as receptors for interleukin-1 (IL-1) and IL-18. Whereas the MyD88-deficient mice evinced a marked reduction in early atherosclerosis, mice deficient in CD14 had no decrease in early lesion development. Inactivation of the MyD88 pathway led to a reduction in atherosclerosis through a decrease in macrophage recruitment to the artery wall that was associated with reduced chemokine levels. These findings link elevated serum lipid levels to a proinflammatory signaling cascade that is also engaged by microbial pathogens.

The role of PPAR-γ in macrophage differentiation and cholesterol uptake

Peroxisome proliferator-activated receptor-γ (PPAR-γ), the transcription factor target of the anti-diabetic thiazolidinedione (TZD) drugs, is reported to mediate macrophage differentiation and inflammatory responses. Using PPAR-γ–deficient stem cells, we demonstrate that PPAR-γ is neither essential for myeloid development, nor for such mature macrophage functions as phagocytosis and inflammatory cytokine production. PPAR-γ is required for basal expression of CD36, but not for expression of the other major scavenger receptor responsible for uptake of modified lipoproteins, SR-A. In wild-type macrophages, TZD treatment divergently regulated CD36 and class A macrophage-scavenger receptor expression and failed to induce significant cellular cholesterol accumulation, indicating that TZDs may not exacerbate macrophage foam-cell formation.

Scavenger Receptors in Atherosclerosis: Beyond Lipid Uptake

Atherosclerotic vascular disease arises as a consequence of the deposition and retention of serum lipoproteins in the artery wall. Macrophages in lesions have been shown to express > or = 6 structurally different scavenger receptors for uptake of modified forms of low-density lipoproteins (LDLs) that promote the cellular accumulation of cholesterol. Because cholesterol-laden macrophage foam cells are the primary component of the fatty streak, the earliest atherosclerotic lesion, lipid uptake by these pathways has long been considered a requisite and initiating event in the pathogenesis of atherosclerosis. Although the removal of proinflammatory modified LDLs from the artery wall via scavenger receptors would seem beneficial, the pathways distal to scavenger receptor uptake that metabolize the modified lipoproteins appear to become overwhelmed, leading to the accumulation of cholesterol-laden macrophages and establishment of a chronic inflammatory setting. These observations have led to the current dogma concerning scavenger receptors, which is that they are proatherogenic molecules. However, recent studies suggest that the effects of scavenger receptors on atherogenesis may be more complex. In addition to modified lipoprotein uptake, these proteins are now known to regulate apoptotic cell clearance, initiate signal transduction, and serve as pattern recognition receptors for pathogens, activities that may contribute both to proinflammatory and anti-inflammatory forces regulating atherogenesis. In this review, we focus on recent advances in our knowledge of scavenger receptor regulation and signal transduction, their roles in sterile inflammation and infection, and the potential impact of these pathways in regulating the balance of lipid accumulation and inflammation in the artery wall.

Loss of receptor-mediated lipid uptake via scavenger receptor A or CD36 pathways does not ameliorate atherosclerosis in hyperlipidemic mice

Macrophage internalization of modified lipoproteins is thought to play a critical role in the initiation of atherogenesis. Two scavenger receptors, scavenger receptor A (SR-A) and CD36, have been centrally implicated in this lipid uptake process. Previous studies showed that these receptors mediated the majority of cholesterol ester accumulation in macrophages exposed to oxidized LDL and that mice with deletions of either receptor exhibited marked reductions in atherosclerosis. This work has contributed to an atherosclerosis paradigm: scavenger receptor-mediated oxidized lipoprotein uptake is required for foam cell formation and atherogenesis. In this study, Apoe-/- mice lacking SR-A or CD36, backcrossed into the C57BL/6 strain for 7 generations, were fed an atherogenic diet for 8 weeks. Hyperlipidemic Cd36-/-Apoe-/- and Msr1-/-Apoe-/- mice showed significant reductions in peritoneal macrophage lipid accumulation in vivo; however, in contrast with previous reports, this was associated with increased aortic sinus lesion areas. Characterization of aortic sinus lesions by electron microscopy and immunohistochemistry showed abundant macrophage foam cells, indicating that lipid uptake by intimal macrophages occurs in the absence of CD36 or SR-A. These data show that alternative lipid uptake mechanisms may contribute to macrophage cholesterol ester accumulation in vivo and suggest that the roles of SR-A and CD36 as proatherosclerotic mediators of modified LDL uptake in vivo need to be reassessed.

CD36 Mediates the Innate Host Response to β-Amyloid

Accumulation of inflammatory microglia in Alzheimers senile plaques is a hallmark of the innate response to β-amyloid fibrils and can initiate and propagate neurodegeneration characteristic of Alzheimers disease (AD). The molecular mechanism whereby fibrillar β-amyloid activates the inflammatory response has not been elucidated. CD36, a class B scavenger receptor, is expressed on microglia in normal and AD brains and binds to β-amyloid fibrils in vitro. We report here that microglia and macrophages, isolated from CD36 null mice, had marked reductions in fibrillar β-amyloid–induced secretion of cytokines, chemokines, and reactive oxygen species. Intraperitoneal and stereotaxic intracerebral injection of fibrillar β-amyloid in CD36 null mice induced significantly less macrophage and microglial recruitment into the peritoneum and brain, respectively, than in wild-type mice. Our data reveal that CD36, a major pattern recognition receptor, mediates microglial and macrophage response to β-amyloid, and imply that CD36 plays a key role in the proinflammatory events associated with AD.

Leukotriene B4 receptor transgenic mice reveal novel protective roles for lipoxins and aspirin-triggered lipoxins in reperfusion

Polymorphonuclear neutrophil (PMN) activation is pivotal in acute inflammation and injury from reperfusion. To elucidate components controlling PMNs in vivo, we prepared novel transgenic mice with the human leukotriene (LT) B4 receptor (BLTR) for functional characterization. Overexpression of BLTR in leukocytes dramatically increased PMN trafficking to skin microabscesses and lungs after ischemia-reperfusion, whereas mice deficient in 5-lipoxygenase (5-LO) showed diminished PMN accumulation in reperfused lungs. Hence, both BLTR expression and LT biosynthesis are critical for PMN infiltration in reperfusion-initiated second-organ injury. Also, in BLTR transgenic mice, 5-LO expression and product formation were selectively increased in exudates, demonstrating that receptor overexpression amplifies proinflammatory circuits. Endogenous lipoxin (LX) A4 was produced in ischemic lungs and elevated by reperfusion. Because LXA4 and aspirin-triggered 15-epimeric LXA4 (ATL) selectively regulate leukocyte responses, they were tested in BLTR transgenic mice. Despite excessive PMN recruitment in BLTR transgenic mice, intravenous injection of ATL sharply diminished reperfusion-initiated PMN trafficking to remote organs, and topical application of LX was protective in acute dermal inflammation. These results demonstrate a direct role for BLTR with positive feedback, involving BLTR and 5-LO signaling in controlling PMNs. Moreover, LXA4 and ATL counter BLTR-amplified networks, revealing a novel protective role for LX and ATL in stress responses that has applications in perioperative medicine.

Divergent Response to LPS and Bacteria in CD14-Deficient Murine Macrophages

Gram-negative bacteria and the LPS constituent of their outer membranes stimulate the release of inflammatory mediators believed to be responsible for the clinical manifestations of septic shock. The GPI-linked membrane protein, CD14, initiates the signaling cascade responsible for the induction of this inflammatory response by LPS. In this paper, we report the generation and characterization of CD14-null mice in which the entire coding region of CD14 was deleted. As expected, LPS failed to elicit TNF-α and IL-6 production in macrophages taken from these animals, and this loss in responsiveness is associated with impaired activation of both the NF-κB and the c-Jun N-terminal mitogen-activated protein kinase pathways. The binding and uptake of heat-killed Escherichia coli, measured by FACS analysis, did not differ between CD14-null and wild-type macrophages. However, in contrast to the findings with LPS, whole E. coli stimulated similar levels of TNF-α release from CD14-null and wild-type macrophages at a dose of 10 bioparticles per cell. This effect was dose dependent, and at lower bacterial concentrations CD14-deficient macrophages produced significantly less TNF-α than wild type. Approximately half of this CD14-independent response appeared to be mediated by CD11b/CD18, as demonstrated by receptor blockade using neutrophil inhibitory factor. An inhibitor of phagocytosis, cytochalasin B, abrogated the induction of TNF-α in CD14-deficient macrophages by E. coli. These data indicate that CD14 is essential for macrophage responses to free LPS, whereas other receptors, including CD11b/CD18, can compensate for the loss of CD14 in response to whole bacteria.

Naturally Occurring Mutations in the Largest Extracellular Loops of ABCA1 Can Disrupt Its Direct Interaction with Apolipoprotein A-I

The ABCA1 transporter contains two large domains into which many of the genetic mutations in individuals with Tangier disease fall. To investigate the structural requirements for the cellular cholesterol efflux mediated by ABCA1, we have determined the topology of these two domains and generated transporters harboring five naturally occurring missense mutations in them. These mutants, unlike wild type ABCA1, produced little or no apoA-I-stimulated cholesterol efflux when transfected into 293 cells, establishing their causality in Tangier disease. Because all five mutant proteins were well expressed and detectable on the plasma membrane, their interaction with the ABCA1 ligand, apolipoprotein (apo) A-I, was measured using bifunctional cross-linking agents. Four of five mutants had a marked decline in cross-linking to apoA-I, whereas one (W590S) retained full cross-linking activity. Cross-linking of apoA-I was temperature-dependent, rapid in onset, and detectable with both lipid- and water-soluble cross-linking agents. These results suggest that apoA-I-stimulated cholesterol efflux cannot occur without a direct interaction between the apoprotein and critical residues in two extracellular loops of ABCA1. The behavior of the W590S mutant indicates that although binding of apoA-I by ABCA1 may be necessary, it is not sufficient for stimulation of cholesterol efflux.