Collectins and innate immunity in the lung

Impact of MBL-2 coding region polymorphism on modulation of HAND and HIV-1 acquisition

BACKGROUND

Mannose-binding lectin 2 (MBL2) gene has a significant role in the essential protective mechanism of the body. Variations reported in the genetic makeup of this gene influence the circulating MBL levels that could lead to the vulnerability to various viral infections including HIV. Hence, we assessed the MBL2 coding region (52A/D, 54A/B, and 57A/C) variations in HIV-associated neurocognitive disorders (HAND).

METHOD

In this proposed study, 208 HIV seropositive individuals were included, 104 were on ART undergone for IHDS evaluation (44 HAND+60 without HAND), and 104 HIV seropositive individuals naïve to ART, and 130 unrelated HIV uninfected individuals. PCR-RFLP was used to genotype the MBL2 coding region polymorphism (52A/D, 54A/B and 57A/C).

RESULTS

MBL-2 57AC genotype was associated with risk of HAND severity (OR = 4.69, P = 0.0009). MBL-2 57AC and 57C alleles were associated with susceptibility to HAND (OR = 3.14, P = 0.003). Furthermore, the MBL-2 57AC genotype and 57C allele were found to be significantly linked with the susceptibility to HIV disease severity. (OR = 6.34, P = 0.001; 16.82% vs. 3.46%, OR = 5.64, P = 0.001). Haplotype ACA was significantly linked with susceptibility to HAND and its severity (OR = 3.23, P = 0.004, 26.1%-8.1%, OR = 4.70, P = 0.0024), similarly, haplotype ACA was linked with the acquisition of HIV-1 (OR = 4.26, P = 0.005). MBL-2 57AC genotype in presence of tobacco showed a significantly higher risk for HIV disease severity (48.0% vs. 12.5%, OR = 7.00, P = 0.035). Alcohol-taking HIV seropositive individuals on ART showed a greater MBL-2 57AC genotype than with alcohol-taking naïve to ART (32.3% vs. 15.4%, OR = 2.75, P = 0.40).

CONCLUSION

MBL-2 57AC genotype and haplotype ACA were associated with the modulation of HAND. Individuals with haplotype ACA were at higher risk of HIV-1 acquisition.

Knockdown of LncRNA MALAT1 contributes to cell apoptosis via regulating NF-κB/CD80 axis in neonatal respiratory distress syndrome

Neonatal respiratory distress syndrome (NRDS) is a leading cause of morbidity in premature newborns and is a common reason for admission to the neonatal intensive care unit (NICU). Recent studies found that the pathogenesis of NRDS is not simply lung immaturity. Apoptosis is an essential process for the development and maturation of the lungs. In this study, we report a critical role of lncRNA MALAT1 in regulating CD80 transcription in the NRDS-associated apoptosis via binding with NF-κB. We first showed MALAT1 and CD80 were highly expressed in the peripheral blood mononuclear cells of NRDS with infection exposure. Then we found MALAT1 expressions were significantly increased by the treatment of LPS. We confirmed knockdown of MALAT1 promoted cell viability by CCK-8 assays, cell apoptosis by flow cytometric assays and cytoskeleton destruction by immunocytochemistry. We confirmed CD80 expression level was associated with cell apoptosis by affecting PARP and caspase-3. Then we demonstrated knockdown of MALAT1 promoted CD80 transcription in A549 cells. Furthermore, we confirmed that MALAT1 downregulated transcriptional expression of CD80 by interfering with NF-κB activation and disrupting its binding efficiency with the CD80 promoter in the cell nucleus. In conclusion, we first identified lncRNA MALAT1 as an important prognosis maker for NRDS patients. Most significantly, this study then demonstrated a novel regulatory function of knocked-down MALAT1 on the transcriptional level of CD80 by enhancing the binding of NF-κB to CD80 promoter. Since the interaction between MALAT1 and CD80 plays an essential role in the cell apoptosis of NRDS, our findings demonstrate the possibility of using MALAT1 as therapeutic target for treatment of NRDS, and extend existing knowledge about the molecular mechanism that underlies NRDS pathogensis.

How does mother-to-child transmission of HIV differ among African populations? Lessons from MBL2 genetic variation in Zimbabweans

Mannose binding lectin (MBL) is a pathogen pattern recognition protein involved in antimicrobial activities. Variation in MBL2 gene has been extensively implicated in differential outcomes of infectious diseases in studies conducted outside Africa, but virtually very little is known on the role of this candidate gene in the African continent. We investigated human genetic variations in MBL2 in a Zimbabwean pediatric population and their putative associations with HIV infection in perinatally exposed children. One hundred and four children aged 7 to 9 years comprising 68 perinatally exposed to HIV (32 who were born infected and 36 who were uninfected) and 36 unexposed controls were recruited. DNA samples were genotyped for MBL2 polymorphisms using PCR-RFLP and sequencing. HIV infected children had markedly variable and significantly lower mean height (p=0.03) and weight (p=0.005) when compared to the uninfected children. Using all samples, frequencies for MBL2 genetic variants for the Zimbabwean population were calculated. Twelve single nucleotide polymorphisms were observed and minor alleles occurred with the following frequencies: -550C>G (G: 0.02), -435G>A (A: 0.08), -428A>C (C: 0.39), -394A>G (A: 0.39), -328AGAGAA ins/del (AGAGAA ins: 0.44), -245G>A (A: 0.05), -221C>G (C: 0.12), -111A>T (T: 0.10), -70C>T (C: 0.46), +4C>T (C: 0.45), novel -595G>A (A: 0.02), and 170G>A (0.24). We found that the MBL2 +4T variant displayed a trend for association with reduced risk of HIV transmission from mother-to-child but the remaining vast majority of the genetic markers did not show a significant association. We conclude (1) the MBL2 gene is highly polymorphic in the Zimbabwean population, and (2) MBL2 genetic variation does not appear to play a major role in influencing the risk of mother-to-child HIV transmission in our study sample. These observations contest the hitherto significant role of this candidate gene for HIV transmission from mother-to-child in non-African populations and thus, further speak to the limits of extrapolating genomic association studies directly to the African populations from studies conducted elsewhere. It is hoped that more OMICS research in a diverse set of African countries can shed further light on the putative role (or the lack thereof ) of this candidate gene in HIV transmission in the continent, a major global health burden in Africa.

Plasma surfactant D in patients following acute paraquat intoxication

Free radical-induced lung injury is a major problem that occurs because of paraquat intoxication. Serum surfactant protein D (SP-D) reflects the severity of various lung diseases. The purpose of this study is to investigate the changes in plasma SP-D concentrations and to correlate disease severity with SP-D concentrations in patients with acute paraqaut intoxication. Twelve paraquat-intoxicated patients participated in this study. Their paraquat exposure was assessed by their plasma's paraquat level. Serial plasma SP-Ds were measured by ELISA. SP-D was decreased two and three days after the initial measurement within 2 to 72 hours of ingestion. There was no difference in initial SP-D levels between survivors and non-survivors. The SP-D test revealed a significant positive correlation between the SP-D level and PaO(2) (r = 0.384, p = 0.003, N = 57). SP-D did not predict the likelihood of survival, but it was positively correlated with PaO(2). This finding suggests that low concentrations of plasma SP-D could reflect hypoxia due to free radical-induced injury.

Relationship between pulmonary surfactant protein and lipid peroxidation in lung injury due to paraquat intoxication in rats

BACKGROUND

Pulmonary damage resulting from lipid peroxidation is a principal effect of paraquat intoxication. The host-defense functions of surfactant are known to be mediated by the surfactant proteins A and D (SP-A and SP-D, respectively). The primary objective of this study was to evaluate the variations over time in levels of surfactant protein and lipid peroxidation (LPO) in lung tissue following free-radical-induced injury.

METHODS

42 adult, male, Sprague-Dawley rats were administered intraperitoneal injections of paraquat (35 mg/kg body weight). SP-A and SP-D levels were determined via Western blot. LPO in the left lung homogenate was measured via analyses of the levels of thiobarbituric acid-reactive substances.

RESULTS

LPO levels peaked at 6 hours, with no associated histological changes. SP-D levels increased until hour 12 and declined until hour 48; SP-D levels subsequently began to increase again, peaking at hour 72. SP-A levels peaked at hour 6, declining thereafter.

CONCLUSIONS

We suggest that in the early phase of paraquat injury, SP-D levels reflect alveolar damage and that de novo synthesis of SP-D takes 72 hours. Levels of SP-A, on the other hand, reflect abnormalities in the surfactant system in the late stage of paraquat intoxication. Surfactant proteins may play a role in protecting the lungs from reactive oxygen injury. A time-dependent variation has been observed in the levels of surfactant proteins A and D following paraquat injury, and it has been suggested that these proteins play a role in the protection of lung tissue against ROS-induced injuries.

Innate immunity and the lung: defense at the interface between host and environment

The lung serves as a major interface between the host and the external environment. As such, numerous lines of defense protect the host from inhaled potential pathogens. A breach in pulmonary innate immunity can lead to deleterious outcomes, such as pneumonia and disseminated infection. Pulmonary innate immunity, the first line of defense, is mediated by airway and alveolar epithelial cells as well as resident and recruited leukocytes. This article will discuss the key cellular and secreted components of the pulmonary innate immune system.

Mite allergen induces nitric oxide production in alveolar macrophage cell lines via CD14/toll-like receptor 4, and is inhibited by surfactant protein D

BACKGROUND

Previously, we have found that dust mite allergens can directly activate alveolar macrophages (AMs), induce inflammatory cytokines, and enhance T-helper type 2 cytokine production. A molecule of innate immunity in the lung, surfactant protein D (SP-D), is able to bind mite allergens and alleviates allergen-induced airway inflammation.

OBJECTIVES

This study was aimed at investigating the activation pathway of mite allergen (Dermatophagoides pteronyassinus, Der p)-induced nitric oxide (NO) production by AMs, and the role of SP-D in the modulation of activated AMs by mite allergens.

METHODS

Porcine SP-D was purified from bronchoalveolar lavage fluids of Lan-Yu mini-pigs, by affinity chromatography on maltose-sepharose. NO production, inducible expression of lipopolysaccharides (LPS)-related binding and responding surface receptors complex, CD14 and toll-like receptor 4 (TLR4), as well as inducible NO synthase (iNOs) and nuclear factor-kappaB activation were studied in two AMs cell lines, MH-S (BALB/c strain),and AMJ2-C11 (C57BL/6 strain), and one peritoneal macrophage cell line (RAW264.7), after stimulation with LPS, or Der p.

RESULTS

LPS and Der p elicited different responses of NO production in the different cell lines, and the response might depend upon the expression of the cell surface CD14/TLR4 complex in different genetic backgrounds of macrophage cell lines. Pretreatment of macrophages with SP-D could inhibit NO production from Der p or LPS-stimulated alveolar macrophages.

CONCLUSION

Mite allergen-induced alveolar macrophage activation is mediated by CD14/TLR4 receptors and can be inhibited by SP-D; it further supports the concept that SP-D may be an important modulator of allergen-induced pulmonary inflammation.

Serum amyloid P-component in murine tuberculosis: induction kinetics and intramacrophage Mycobacterium tuberculosis growth inhibition in vitro

Serum amyloid P-component (SAP), a pentraxin, is known to play an important role in innate immunity to microbial infections; however, nothing is known about it during tuberculosis (TB). Mice intratracheally infected with Mycobacterium tuberculosis Erdman, showed peak SAP levels (442+/-58.2 microg/ml) on day 21, which declined to background levels by day 60. Their serum interleukin-6 levels paralleled SAP levels, whereas, their serum transforming growth factor-beta levels were paradoxical. During the acute phase of infection, the SAP levels positively correlated with the lung mycobacterial load. Purified mouse SAP (1-50 microg/ml) treatment of M. tuberculosis-infected alveolar macrophages (AMs), in vitro, inhibited their intracellular mycobacterial growth; maximum inhibition (1.1 log10 CFU reduction) occurred at 10 microg/ml, and a 4-day treatment appeared optimal. Treatment of AMs with both rabbit anti-mouse SAP polyclonal antibody and mannose-derived simple sugars, separately, blocked the SAP-induced inhibition of mycobacterial growth. The mycobacterial growth inhibition appeared to be nitric oxide (NO)-dependent as NO synthase inhibitors, both aminoguanidine and N(G)-monomethyl-L-arginine, annulled it. Further, SAP treatment of infected AMs induced significant (P<0.05) elaboration of nitrite (72.1+/-8.3 nM/ml), compared to the controls, and these AMs showed augmented expression of inducible NO synthase. This first study demonstrates that during murine TB the SAP levels were increased, and purified mouse SAP inhibited the intra-AM M. tuberculosis growth, in vitro, apparently via NO-dependent mechanism(s). SAP may thus contribute both to the pathogenesis and pulmonary innate immunity in TB.

Bench-to-bedside review: Paediatric viral lower respiratory tract disease necessitating mechanical ventilation--should we use exogenous surfactant?

Treatment of infants with viral lower respiratory tract disease (LRTD) necessitating mechanical ventilation is mainly symptomatic. The therapeutic use of surfactant seems rational because significantly lower levels of surfactant phospholipids and proteins, and impaired capacity to reduce surface tension were observed among infants and young children with viral LRTD. This article reviews the role of pulmonary surfactant in the pathogenesis of paediatric viral LRTD. Three randomized trials demonstrated improved oxygenation and reduced duration of mechanical ventilation and paediatric intensive care unit stay in young children with viral LRTD after administration of exogenous surfactant. This suggest that exogenous surfactant is the first beneficial treatment for ventilated infants with viral LRTD. Additionally, in vitro and animal studies demonstrated that surfactant associated proteins SP-A and SP-D bind to respiratory viruses, play a role in eliminating these viruses and induce an inflammatory response. Although these immunomodulating effects are promising, the available data are inconclusive and the findings are unconfirmed in humans. In summary, exogenous surfactant in ventilated infants with viral LRTD could be a useful therapeutic approach. Its beneficial role in improving oxygenation has already been established in clinical trials, whereas the immunomodulating effects are promising but remain to be elucidated.

Acute-phase reactants during murine tuberculosis: Unknown dimensions and new frontiers

SETTING

Serum amyloid P-component (SAP) plays important roles in host defense during various infectious diseases; however, nothing is known in tuberculosis (TB).

OBJECTIVE

To study the SAP response of Mycobacterium tuberculosis H37Rv- and H37Ra-infected mice, and to determine the effect(s) of purified mouse SAP both on their intra-alveolar macrophage (AM) uptake and intra-AM growth in vitro.

DESIGN

The SAP levels of mice intratracheally infected with M. tuberculosis H37Rv and H37Ra were determined by ELISA. Mycobacterial AM uptake and intra-AM growth in vitro were determined using fluorescence microscopy and plating, respectively.

RESULTS

M. tuberculosis H37Rv-infected mice showed significantly (p < 0.05) increased SAP levels (352.8+/-36.1 microg/ml) with compared mice infected with M. tuberculosis H37Ra (170+/-18.5 microg/ml). During the acute phase of both these infections, enhanced SAP levels correlated with the lung mycobacterial load. In vitro, purified mouse SAP (1-80 microg/ml) inhibited the AM uptake of both the mycobacteria in a concentration-dependent manners to a similar extent; 20 microg/ml SAP appeared optimal. Mycobacterial uptake inhibition was divalent cation- and pH-dependent, and was unaffected both by heat-inactivated and deglycosylated SAP, separately. Curiously, purified mouse SAP (1-80 microg/ml), in a concentration-dependent manner, inhibited the intra-AM growth of both M. tuberculosis H37Rv and H37Ra in vitro; the effect was 0.8 log10 CFUs greater on the latter. Both the mannose-based simple sugars and rabbit anti-mouse SAP polyclonal antibody, separately, annulled the inhibition of mycobacterial growth in vitro.

CONCLUSION

This initial study demonstrates that both the SAP response of M. tuberculosis-infected mice, and the SAP-induced intra-AM mycobacterial growth inhibition in vitro were apparently dependent on mycobacterial virulence.

The genetics of neonatal respiratory disease

This chapter reviews some of the genetic predispositions that may govern the presence or severity of neonatal respiratory disorders. Respiratory disease is common in the neonatal period, and genetic factors have been implicated in some rare and common respiratory diseases. Among the most common respiratory diseases are respiratory distress syndrome of the newborn and transient tachypnoea of the newborn, whereas less common ones are cystic fibrosis, congenital alveolar proteinosis and primary ciliary dyskinesias. A common complication of neonatal respiratory distress syndrome is bronchopulmonary dysplasia or neonatal chronic lung disease. This review examines the evidence linking known genetic contributions to these diseases. The value and success of neonatal screening for cystic fibrosis is reviewed, and the recently characterised contribution of polymorphisms and mutations in the surfactant protein genes to neonatal respiratory disease is evaluated. The evidence that known variability in the expression of surfactant protein genes may contribute to the risk of development of neonatal chronic lung disease or bronchopulmonary dysplasia is examined.

Collectins

Collectins are a family of collagenous calcium-dependent defense lectins in animals. Their polypeptide chains consist of four regions: a cysteine-rich N-terminal domain, a collagen-like region, an alpha-helical coiled-coil neck domain and a C-terminal lectin or carbohydrate-recognition domain. These polypeptide chains form trimers that may assemble into larger oligomers. The best studied family members are the mannan-binding lectin, which is secreted into the blood by the liver, and the surfactant proteins A and D, which are secreted into the pulmonary alveolar and airway lining fluid. The collectins represent an important group of pattern recognition molecules, which bind to oligosaccharide structures and/or lipid moities on the surface of microorganisms. They bind preferentially to monosaccharide units of the mannose type, which present two vicinal hydroxyl groups in an equatorial position. High-affinity interactions between collectins and microorganisms depend, on the one hand, on the high density of the carbohydrate ligands on the microbial surface, and on the other, on the degree of oligomerization of the collectin. Apart from binding to microorganisms, the collectins can interact with receptors on host cells. Binding of collectins to microorganisms may facilitate microbial clearance through aggregation, complement activation, opsonization and activation of phagocytosis, and inhibition of microbial growth. In addition, the collectins can modulate inflammatory and allergic responses, affect apoptotic cell clearance and modulate the adaptive immune system.

Collectins and their role in lung immunity

The collectins are a small family of secreted glycoproteins that contain C-type lectin domains and collagenous regions. They have an important function in innate immunity, recognizing and binding to microorganisms via sugar arrays on the microbial surface. Their function is to enhance adhesion and phagocytosis of microorganisms by agglutination and opsonization. In the lung, two members of the collectin family, surfactant proteins A and D, are major protein constituents of surfactant. Another collectin, mannan-binding lectin, is also present in the upper airways and buccal cavity and may protect against respiratory infections. Recent work has shown that collectins have roles in resistance to allergy and in the control of apoptosis and clearance of apoptotic macrophage in the lung.

A recombinant fragment of human SP-D reduces allergic responses in mice sensitized to house dust mite allergens

C57Bl6 mice sensitized to Dermatophagoides pteronyssinus and challenged with D. pteronyssinus allergen extract given intranasally followed by treatment with intranasal applications of a 60-kDa truncated, trimeric recombinant form of human SP-D (rfhSP-D) showed a significant reduction in serum IgE, IgG1, peripheral blood eosinophilia and airway hyperresponsiveness compared to saline or bovine serum albumin-treated controls. Intracellular cytokine staining of lung and spleen homogenates showed increases in interleukin (IL)-12 production in lung tissue and normalization of IL-12 and interferon (IFN)-gamma in spleen tissue. In previous studies we demonstrated the effectiveness of native SP-D and rfhSP-D in down-regulating allergic responses to allergens of Aspergillus fumigatus. The results reported here indicate that rfhSP-D can suppress the development of allergic symptoms in sensitized mice challenged with allergens of the common house dust mite.

The potential of recombinant surfactant protein D therapy to reduce inflammation in neonatal chronic lung disease, cystic fibrosis, and emphysema

By lowering surface tension at the air-water interface in the surfactant deficient premature lung, exogenous surfactant replacement therapy for neonatal respiratory distress syndrome has been highly successful in decreasing mortality after preterm birth. It has emerged in recent years that surfactant components not present in current surfactant formulations--particularly surfactant associated proteins A and D (SP-A and SP-D)-have additional roles in host defence distinct from the surface tension lowering effects of surfactant. SP-A and SP-D are calcium dependent carbohydrate binding proteins of the innate immune system important in the first line defence of the lung against microorganisms and in the control of lung inflammation. This review addresses the possibility that recently developed recombinant forms of SP-D could be useful therapeutically in attenuating inflammatory processes in neonatal chronic lung disease, cystic fibrosis, and emphysema.

Bronchoalveolar lavage fluid surfactant protein-A and surfactant protein-D are inversely related to inflammation in early cystic fibrosis

The pulmonary collectins surfactant protein (SP)-A and SP-D play important roles in innate lung defense, enhancing opsonization of microbes and limiting lung inflammatory responses. To quantify relationships among collectins, bacteria, and inflammation in early cystic fibrosis (CF) airway secretions, bronchoalveolar lavage fluids (BALFs) were collected from children undergoing clinically indicated bronchoscopy. Quantitative bacteriology, differential cell counts, and ELISA for SP-A and SP-D were assessed. Significantly increased numbers of neutrophils relative to bacteria were noted in BALF from CF compared with non-CF subjects. Although SP-A levels tended to be lower in CF compared with non-CF, this was only significant in the presence of bacterial infection. Among CF patients, SP-A concentrations in BALF were inversely related to inflammation, bacterial colony-forming units per milliliter, and age. SP-D levels were significantly decreased in CF patients, and SP-D was rarely detectable in the presence of infection. Among CF patients, SP-D correlated inversely with inflammation and bacterial colony-forming units per milliliter, and there was decreased immunostaining of BALF cells for SP-D in CF. Immunohistochemistry of CF autopsy lung sections for SP-A and SP-D confirmed their paucity at sites of infection and inflammation. We conclude that relative collectin deficiency occurs early in CF airways and is inversely related to inflammation in CF airways.

Surfactant protein D reduces alveolar macrophage apoptosis in vivo

Surfactant protein D (SP-D) is a molecule of the innate immune system that recognizes the patterns of surface carbohydrate on pathogens and targets them for phagocytosis and killing. SP-D-deficient mice show an increased number of macrophages in the alveolar space, excess surfactant phospholipid, overproduction of reactive oxygen species, and the development of emphysema. We report here that SP-D-deficient mice have a 5- to 10-fold increase in the number of apoptotic and necrotic alveolar macrophages, as defined by annexin V and propidium iodine staining, respectively. Intrapulmonary administration of a truncated 60-kDa fragment of human recombinant SP-D reduces the number of apoptotic and necrotic alveolar macrophages and partially corrects the lipid accumulation in SP-D-deficient mice. The same SP-D fragment binds preferentially to apoptotic and necrotic alveolar macrophages in vitro, suggesting that SP-D contributes to immune homeostasis in the lung by recognizing and promoting removal of necrotic and apoptotic cells.

Molecular cloning of a mouse scavenger receptor with C-type lectin (SRCL)(1), a novel member of the scavenger receptor family

The cDNA clone encoding a mouse scavenger receptor with C-type lectin (SRCL), a novel member of the scavenger receptor family, has been isolated from a mouse embryonic cDNA library. The predicted cDNA sequence contains a 2226 bp open reading frame encoding a coiled-coil, collagen-like, C-type lectin/carbohydrate recognition domain with an overall sequence identity of 92% to human SRCL. In contrast to human, mouse SRCL mRNA was expressed ubiquitously in various adult tissues including the liver and spleen, in which human SRCL mRNA was under detection limits. Mouse SRCL mRNA was expressed in the macrophage cell line J774A.1 cells at a high level and in the embryo as early as E9.

Domains of surfactant protein A that affect protein oligomerization, lipid structure and surface tension

Surfactant protein A (SP-A) is an abundant protein found in pulmonary surfactant which has been reported to have multiple functions. In this review, we focus on the structural importance of each domain of SP-A in the functions of protein oligomerization, the structural organization of lipids and the surface-active properties of surfactant, with an emphasis on ultrastructural analyses. The N-terminal domain of SP-A is required for disulfide-dependent protein oligomerization, and for binding and aggregation of phospholipids, but there is no evidence that this domain directly interacts with lipid membranes. The collagen-like domain is important for the stability and oligomerization of SP-A. It also contributes shape and dimension to the molecule, and appears to determine membrane spacing in lipid aggregates such as common myelin and tubular myelin. The neck domain of SP-A is primarily involved in protein trimerization, which is critical for many protein functions, but it does not appear to be directly involved in lipid interactions. The globular C-terminal domain of SP-A clearly plays a central role in lipid binding, and in more complex functions such as the formation and/or stabilization of curved membranes. In recent work, we have determined that the maintenance of low surface tension of surfactant in the presence of serum protein inhibitors requires cooperative interactions between the C-terminal and N-terminal domains of the molecule. This effect of SP-A requires a high degree of oligomeric assembly of the protein, and may be mediated by the activity of the protein to alter the form or physical state of surfactant lipid aggregates.

Molecular cloning and functional characterization of a human scavenger receptor with C-type lectin (SRCL), a novel member of a scavenger receptor family

Using a human placenta cDNA library, we cloned a novel member belonging to the scavenger receptor family. Complementary DNA of this clone encodes a type II transmembranous glycoprotein containing a collagen-like domain, which are typical structural characteristics of scavenger receptor class A. This protein also contains a C-type lectin/carbohydrate recognition domain (C-type CRD) located at the C-terminus. We designated this as Scavenger Receptor with C-type Lectin (SRCL) type I. We also isolated human SRCL type II, which lacks the C-type CRD. Northern blot analysis revealed that hSRCL type I and type II mRNAs are abundantly expressed in adult human tissues. When hSRCL type I and type II were expressed in CHO-K1 cells, they were localized in the plasma membrane forming clusters on the surface. Ligand-binding studies of CHO-K1 cells expressing hSRCL type I and type II demonstrated their specific binding capacity in Escherichia coli and Staphylococcus aureus. These results indicate that hSRCL is a novel bacteria-binding receptor containing a C-type CRD and this receptor may play an important role in host defense.