Standardizing Scavenger Receptor Nomenclature

Class A scavenger receptors: SR-A.

Class A scavenger receptors are expressed primarily on tissue macrophages and macrophage subtypes such as Kupffer cells and cortical and medullary thymic macrophages (31). They have also been observed on high endothelial venules (31) and on subpopulations of dendritic cells (32), binding to a variety of foreign and self ligands. Members in this class have a similar structure, comprised of a cytoplasmic tail, transmembrane domain, spacer region, α helical coiled coil domain, collagenous domain, and a C-terminal cysteine-rich domain. SCARA1/MSR1, the prototypical SR-A molecule, was the first scavenger receptor to be cloned and was initially isolated from bovine lung mRNA. It has subsequently been identified in other species, including mice and humans. There are three isoforms, identified as alternative splice variants encoded by the same gene, namely, SR-AI, SR-AII, and SR-AIII (33, 34). Other members of this class of scavenger receptors include MARCO, SCARA5, and scavenger receptor with C-type lectin domain (SRCL)-I/II, also designated collectin from placenta receptor-I (35).

The scavenger receptor currently designated as SCARA1 or MSR1 would now be referred to as SR-A1. SR-A1 is predominantly found in macrophages, monocytes, mast cells, and dendritic cells in both mice and humans (36). A diverse array of ligands is recognized by members of this class. SR-A1 binds to β-amyloid, heat shock proteins, surface molecules of Gram-positive and Gram-negative bacteria, hepatitis C virus (34), and modified LDL such as acetylated LDL and oxidized LDL (oxLDL), but not native LDL.

SR-A1.1, an alternatively spliced form of SR-A1 (currently referred to as SR-AII), is characterized by a shortened C terminus. SR-A1.2, another alternatively spliced form of SR-A1 (currently referred to as SR-AIII), is characterized by a truncated C terminus and remains trapped in the endoplasmic reticulum. The collagenous region of SR-A1 and SR-A1.1 has been identified as a ligand-binding domain (37). There would be no receptor designated as SR-A2 to avoid confusion with the current SR-AII (new designation of SR-A1.1)

Whereas SR-A1 and SR-A6 (currently known as MARCO) are primarily expressed in macrophages, SR-A3 (also referred to as SCARA3, cellular stress response [CSR]), SR-A4 (SCARA4, SRCL, collectin placenta 1 [CL-P1]), and SR-A5 (SCARA5) are expressed in a variety of other tissues and cell types, including the lung, placenta, intestine, heart, and epithelial cells (38). SR-A3 has been associated with protecting cells from the detrimental effects of reactive oxygen species (21). SR-A4 functions as an endocytic receptor for lipoproteins and mediates the recognition, internalization, and degradation of oxidatively modified LDL by vascular endothelial cells (39). SR-A5 (currently known as SCARA5) expression is restricted to epithelial cells within the testis, airway, thymus, and the adrenal gland. SR-A5 has the capacity to bind bacteria and may play an important role in host defense (27). Under normal conditions, in the absence of inflammation, the expression of SR-A6 is restricted to macrophages in the lymph nodes and marginal zone of the spleen. Studies have shown that SR-A6 mediates the clearance of bacteria from the lungs (40) and bloodstream (41).

Class B scavenger receptors: SR-B.

Class B scavenger receptors include receptors currently known as SR-BI, SR-BII, CD36, and LIMP2. Receptors belonging to this class are characterized by two transmembrane domains flanking an extracellular loop, with both the amino and carboxyl termini located within the cytoplasm. The extracellular domain of these receptors is extensively N-linked glycosylated, and this modification provides protection against proteases often found in inflammatory sites. CD36 is the prototype class B type I scavenger receptor and was initially identified as a receptor for thrombospondin (42), and in this capacity it modulates angiogenesis and cell-to-cell interactions. It is one of the most widely studied scavenger receptors and is involved in multiple aspects of macrophage biology, including migration and signaling and inflammatory processes such as foam cell formation. CD36 also plays an important role in the host immune response to fungi and bacteria and binds erythrocytes infected with the malaria parasite Plasmodium falciparum (43), as well as functions as a facilitator of long chain fatty acid uptake (44). It is expressed in many cell types, including insulin-responsive cells; hematopoietic cells such as platelets, monocytes, and macrophages; and endothelial cells and specialized epithelial cells in the breast and eye. Dr. Gerda Endemann and colleagues were the first to characterize CD36 as a receptor for oxLDL, thereby cementing its role as a scavenger receptor (30). CD36 also binds polyanionic ligands of both host (e.g., high-density lipoprotein [HDL]) (45) and pathogen (examples are Staphylococcus aureus and Candida albicans) (20, 46) origin, and it plays an important role in the recognition and endocytic uptake of oxidized phospholipids, apoptotic cells, and amyloid proteins (47). Dr. Cameron Stewart and colleagues have shown that CD36 (SR-B2) cooperates with TLR family members (TLR4 and TLR6) to activate the innate immune response to ligands that accumulate in Alzheimer’s disease and atherosclerosis, inducing proinflammatory mediators such as IL-1β and RANTES.

SR-B1 (also known as SCARB1 or SR-BI) was the first HDL receptor to be identified and mediates the selective transport of lipids, such as cholesteryl esters from HDL, from lipoproteins to cells (48, 49). SR-B1 is most highly expressed by hepatocytes and steroidogenic cells and is also found in cells within the arterial wall and macrophages in human and murine atherosclerotic lesions (50). SR-B1 has been shown to have a protective effect on atherosclerosis development; total body ablation of SR-B1 in apolipoprotein E knockout mice fed a normal chow diet led to severe coronary arterial atherosclerosis, myocardial infarction, and death (51). Ablation of SR-B1 in bone marrow cells of LDL receptor knockout mice also resulted in increased atherosclerosis, but not death, indicative of partial protection against atherosclerosis (52).

SR-BII, an alternatively spliced form of SR-BI, contains a distinctly different cytoplasmic tail than SR-BI. SR-BII is expressed in adrenal glands, testes, and liver, and it has been shown to mediate selective uptake of cholesteryl ester from HDL, with ∼4-fold lower efficiency than SR-BI (53).

One other member of the CD36 family of membrane proteins, LIMP2 (SR-B3), functions primarily in delivering β-glucocerebrosidase from the endoplasmic reticulum to lysosomes. SR-B3 has been identified as a cellular receptor for enterovirus 71 (EV71), coxsackievirus 7 (CVA7), CVA14, and CVA16 entry into host cells. SR-B3 also serves a role in viral uncoating, thereby increasing infection efficiency (21). Expression of SR-B3 on the cell membrane has not been confirmed.

Class C scavenger receptors.

Class C scavenger receptors have been described in Drosophila (5), but there are currently no known mammalian class C scavenger receptors. This scavenger receptor class was not discussed because the workshop participants focused on mammalian scavenger receptors.

Class D scavenger receptors: SR-D.

CD68 is the only known member of the class D scavenger receptors. CD68, also known as macrosialin in mice, is a type I transmembrane glycoprotein that belongs to the lysosome-associated membrane protein family of molecules (54). There is a 300-aa extracellular region that is rich in threonine and serine, which may serve as an attachment site for carbohydrates, and a short cytoplasmic tail. Human CD68 shares 75% amino acid sequence identity to mouse CD68 in the extracellular region. CD68 is found on monocytes and tissue-specific macrophages in the peritoneum, lungs, liver, spleen, Langerhans cells, and microglia, and it serves as a scavenger receptor for oxLDL. CD68 is a differentiation marker of hematopoietic cells of the monocyte/macrophage lineage. It is mostly found in the late endosomal compartment in macrophages and dendritic cells, with limited expression on resting cells. CD68 plays a minor role in the binding and uptake of oxidized lipoproteins and apoptotic cells by macrophages (23). The proposed new nomenclature for this molecule is: SR-D1: currently known as CD68 [prototype].

Class E scavenger receptors: SR-E.

Class E scavenger receptors are type 2 transmembrane proteins with C-type lectin-like domains and demonstrated scavenger receptor activity; sequence homology alone is not sufficient to include a protein in this class of receptors. These proteins structurally belong to a subfamily of the C-type lectin (CLEC)–like NK cell receptor family. The NK receptor gene complex encodes a large number of CLEC-like receptors that are expressed on NK cells and other leukocytes. Analysis of the NK receptor gene complex in five species revealed variations in the sizes of gene clusters in each species that ranged from 22 to 75 clusters and are noted in these species: human (29 clusters), dog (22 clusters), cattle (32 clusters), rat (75 clusters), and mouse (57 clusters) (55). Among these gene clusters, the first two members of the CLEC family, lectin-like oxLDL receptor (LOX-1) and Dectin-1, have been well characterized and shown to have scavenger activity.

LOX-1 is expressed on macrophages and dendritic cells and binds oxidized LDL and the acute phase protein C-reactive protein (56). It was originally cloned from a bovine aortic endothelial cDNA expression library screened for receptors for oxidized LDL (57). LOX-1 is expressed on vascular endothelial cells, platelets, smooth muscle cells, adipocytes, and macrophages. In addition to binding oxidized LDL, LOX-1 has been implicated in recognizing other ligands, including apoptotic cells, Gram-positive bacteria, and Gram-negative bacteria (23, 56). A splicing variant of human LOX-1 lacking a lectin-like domain, called LOXIN, works as a dominant-negative isoform, forming a heterodimeric complex with LOX-1 (58).

Dectin-1 is expressed predominantly on myeloid cells (macrophages, dendritic cells, and neutrophils) and can be regulated by cytokines and microbial stimuli. Human Dectin-1 is alternatively spliced, resulting in two functional forms. This receptor recognizes various bacterial, fungal, and plant carbohydrates (β-1,3- and/or β-1→6-glucans) in addition to intact fungi and parasites (59). Dectin-1 mediates proinflammatory cytokine production in response to β-glucan particles, in conjunction with TLR2. It can also function as a phagocytic receptor through a novel Syk-independent pathway. Dectin-1 has been reported to act as a costimulatory molecule to activate both CD4 and CD8 T cells. Moreover, Dectin-1 is expressed on macrophages and dendritic cells in the medullary regions of the thymus, suggesting a role in thymocyte development (60).

Some CLEC family members are known to recognize pathogen-associated molecules. However, most of the members of the CLEC family have not been shown to have scavenger receptor activity. At this time, only LOX-1 and Dectin-1 would be classified as scavenger receptors using the definition proposed by the workshop participants.

Class F scavenger receptors: SR-F.

There are currently three members belonging to the class F scavenger receptors. The first class F scavenger receptor (SCARF1) was identified as an endothelial receptor for modified LDL and is characterized by the presence of an extracellular ligand-binding domain, with multiple extracellular epidermal growth factor–like repeats, a transmembrane domain, and a long cytoplasmic tail that includes a serine/proline-rich region followed by a glycine-rich region. It was named scavenger receptor expressed by endothelial cells (SREC)-I (61) and later was also named SCARF1. SCARF1/SREC-I is also expressed in macrophages and functions to bind calreticulin and associated peptides, which are transported into the MHC class I pathway in macrophages and dendritic cells (23). SCARF1 also was shown to be a receptor for fungal pathogens and heat shock proteins (20, 34). Interestingly, a Caenorhabditis elegans ortholog of SCARF1, CED-1, plays a role in engulfing apoptotic cells and has been implicated in host defense (23). A recent report indicates that SCARF1, similar to CED-1, is also involved in the clearance of apoptotic cells (62).

A second isoform of SREC-I, SREC-II, was cloned by Dr. Hiroyuki Arai’s group (63) and was shown to be predominantly expressed in heart, placenta, lung, kidney, spleen, small intestine, and ovary. SREC-II is a type I transmembrane receptor containing N-terminal epidermal growth factor–like domains, a transmembrane domain, and a long cytoplasmic tail, and it engages in heterophilic interactions with SREC-I. In contrast to SREC-I that binds and internalizes modified LDL, SREC-II is unable to internalize modified LDL. The cytoplasmic domain of SREC-II is rich in arginine and lysine, whereas SREC-I is rich in serine and proline. The significance of these residues is not known at this time.

Another member of the SCARF family, multiple epidermal growth factor (EGF)–like domains (MEGF)10, recently has been shown to be a receptor for the protein amyloid-β in the brain, and it is thought to be involved in the pathogenesis of Alzheimer’s disease (23). It is not clear yet whether other MEGF family members, such as MEGF8, 9, and 11, also have scavenger receptor function because no functional studies have been published to date on these molecules.

Class G scavenger receptors: SR-G.

Currently, one receptor belongs to scavenger receptor class G, SR-PSOX/CXCL16. Dr. Takeshi Shimaoka and colleagues used expression cloning of a cDNA library from PMA-treated human monocytic THP-1 cells to identify scavenger receptors that bound oxLDL, and they discovered this receptor, which they called SR-PSOX (64). At about the same time, Drs. Jason Cyster and Michael Briskin discovered a new chemokine that was capable of activating the receptor, CXCR6 (also known as BONZO, STRL33 or TYMSTR), resulting in migration of T cells in vitro (65–67). This chemokine was named CXCL16 and was found to be identical to SR-PSOX in its amino acid sequence.

Studies show that the SR-PSOX/CXCL16 gene is located on chromosome 17p13, and its protein exists in both membrane and soluble forms (65). It is a type I transmembrane glycoprotein with a CXC chemokine motif, mucin stalk, and transmembrane and soluble domains. SR-PSOX/ CXCL16 does not share structural homology with other scavenger receptors. SR-PSOX/CXCL16 has the capacity to mediate adhesion and phagocytosis of bacteria by APCs (66). The membrane-bound form can function as an adhesion molecule for CXCR6-expressing T cells without requiring CXCR6-induced signal transduction and integrin activation.

This multifunctional transmembrane protein has a chemokine-like CXC motif and appears to be involved in several phases of an immune response, from Ag recognition to migration of immune cells to formation of inflammatory foci (68). The soluble form of SR-PSOX/CXCL16, produced from cleavage of the membrane-bound form, functions as a chemoattractant for activated T cells and bone marrow plasma cells via its receptor CXCR6 (69). The membrane-bound form acts as a scavenger receptor for oxLDL and facilitates phagocytosis of bacteria by APCs (69).

Class H scavenger receptors: SR-H.

Receptors in this class are transmembrane proteins that consist of Fasciclin, EGF-like and lamin type EGF-like (FEEL) domains. The currently known SR-H members are FEEL-1/Stabilin-1/CLEVER-1 and FEEL-2/stabilin-2/HARE. FEEL-1 is predominantly expressed in macrophages, mononuclear cells, hematopoietic stem cells, and endothelial cells, whereas FEEL-2 is found mainly in sinusoidal endothelial cells. FEEL-1 and FEEL-2 have been shown to be involved in lymphocyte adhesion, transmigration, angiogenesis, apoptotic cell clearance, and intracellular trafficking. FEEL-1 and FEEL- 2 also have been implicated in the clearance of aged RBCs by macrophages (70, 71) in a phosphatidylserine-dependent manner. FEEL-1 has been shown to directly interact with and is sufficient for phagocytosis of phosphatidylserine-coated beads. These results have yet to be confirmed in vivo. However, FEEL-1 and FEEL-2 double knockout mice have been shown to be impaired in the clearance of noxious blood factors (72).

Class I scavenger receptors: SR-I.

Members of the class I scavenger receptors are primarily restricted to the monocytic lineage, of which CD163, the prototypic class I scavenger receptor, is expressed exclusively in monocytes and macrophages and binds to haptoglobin/hemoglobin (Hgb) complexes to promote the clearance of plasma Hgb via endocytosis during intravascular hemolysis. CD163 is a type I transmembrane protein that consists of an extracellular region composed of nine scavenger receptor cysteine-rich (SRCR) domains and a short cytoplasmic tail. CD163 was originally cloned from the spleen of a patient afflicted with hairy cell leukemia, characterized by a large number of tissue macrophages. It is a 130-kDa membrane glycoprotein, and was known as M130 prior to being given a CD designation. CD163 was also given the functional name “hemoglobin scavenger receptor” to describe its primary role in tissue macrophages (73). Its role as an Hgb receptor may contribute to enhancement of an anti-inflammatory response because the proinflammatory heme is converted to anti-inflammatory metabolites (74).

In addition to functioning as an Hgb receptor, CD163 appears to be involved in intracellular signaling, resulting in a protein-kinase–dependent signal and secretion of cytokines such as IL-6 and IL-10 (75). Several CD163 mRNAs arising from an alternatively spliced CD163 gene have been reported (73, 75). Confocal microscopy studies revealed that the shortest tail variant is predominantly present in the cell membrane, whereas the longer variants are located in the endosomal/Golgi cellular compartment (75). One short tail variant and two long tail variants have been shown to bind and internalize haptoglobin/Hgb complexes leading to lysosomal degradation of the complex. The short tail variant is the predominant isoform found in human blood, liver, and spleen, and most of the long tail variants appear in endosomal compartments (75).

CD163-L1 (M160), a long tail variant of CD163, is a type I transmembrane molecule structurally composed of scavenger receptor cysteine-rich domains, expressed by cells of the myeloid lineage, and may be involved in the differentiation of monocytes to macrophages. Two cytoplasmic splice variants of CD163-L1 have been identified thus far; these include the full-length variant (CD163L1α) with a cytoplasmic tail of 71 residues and the short tail variant (CD163-L1β) with 39 residues. CD163-L1α is the predominant membrane-associated form in several human tissues such as spleen, placental, and small intestine, whereas CD163-Lβ is mainly found in intracellular compartments (76). Studies to identify endogenous and microbial ligands of CD163-L1β are currently in progress.

Class J scavenger receptors: SR-J.

RAGE is currently the only member of the class J scavenger receptor group, and it is a member of the Ig superfamily of cell surface molecules capable of interacting with a broad spectrum of ligands, including advanced glycation end products, high mobility group protein box 1, and the S-100 protein. RAGE is composed of a single transmembrane domain that connects the ectodomain with a short cytoplasmic domain and an extracellular domain required for ligand recognition and binding. The extracellular domain comprises an N-terminal sequence and three Ig-like regions. RAGE is a pattern recognition receptor predominantly involved in the recognition of endogenous molecules released in the context of infection, physiological stress, or chronic inflammation (77). Signaling through RAGE mediates processes such as inflammation, oxidative stress, and apoptosis, which can lead to pathologies including atherosclerosis, diabetes, cancer, neurodegenerative disease, and stroke. RAGE signaling also mediates normal neuronal processes such as cell migration and neuronal differentiation. Studies assessing brain pathology in Alzheimer’s disease revealed that diseased brains expressed higher levels of RAGE in the vasculature and on neurons compared with normal brains (23). In addition to a membrane-bound form, a circulating portion of the protein known as “soluble” RAGE also has biological properties.

Other potential scavenger receptors: SR-Others.

The SR-Others group refers to molecules with scavenger activity that do not belong to the current SR classes, but belong to other receptor families. These receptors include LDL receptor family proteins such as LRP1, LRP2/megalin, cubam (the cubilin/amnionless complex), ASGP, MR, and CD11b/CD18α. Classifying these receptors as scavenger receptors will require the input of the wider scientific community.