We tested the hypothesis that pulmonary surfactant-associated lectins -surfactant proteins A and D (SP-A, and -D) contribute to initial protective mechanisms against influenza A viruses (IAVs). SP-D potently inhibited hemagglutination activity of several strains of IAV as well as causing viral aggregation. SP-D enhanced neutrophil binding of 1AV and neutrophil respiratory burst responses to the virus. Neutrophil dysfunction resulting from 1AV exposure was diminished when the virus was pre-incubated with SP-D. Each of these effects was mediated by the calcium-dependent carbohydrate-binding property of SP-D. Native SP-D preparations of both human and rat origin, as well as recombinant rat SP-D, had similar activity. SP-A also inhibited IAV hemagglutination activity. We have previously reported that related mammalian serum lectins (mannose-binding lectin [MBL] and conglutinin) have similar effects. SP-D was at least 10-fold more potent at causing hemagglutination inhibition than were SP-A or MBL. SP-D was shown to contribute to potent anti-IAV activity of human bronchoalveolar lavage fluid. These results suggest that SP-D-alone, and in conjunction with SP-A and phagocytic cells-constitutes an important component of the natural immune response to 1AV infection within the respiratory tract. (J. Clin. Invest.

Gram-positive organisms like Staphylococcus aureus are a major cause of morbidity and mortality worldwide. Humoral response molecules together with phagocytes play a role in host responses to S. aureus. The mannose-binding lectin (MBL, also known as mannose-binding protein) is an oligomeric serum molecule that recognizes carbohydrates decorating a broad range of infectious agents including S. aureus. Circumstantial evidence in vitro and in vivo suggests that MBL plays a key role in first line host defense. We tested this contention directly in vivo by generating mice that were devoid of all MBL activity. We found that 100% of MBL-null mice died 48 h after exposure to an intravenous inoculation of S. aureus compared with 45% mortality in wild-type mice. Furthermore, we demonstrated that neutrophils and MBL are required to limit intraperitoneal infection with S. aureus. Our study provides direct evidence that MBL plays a key role in restricting the complications associated with S. aureus infection in mice and raises the idea that the MBL gene may act as a disease susceptibility gene against staphylococci infections in humans.

SummaryThe macrophage mannose receptor is an integral membrane protein expressed on the surface of tissue macrophages . After ligation ofmannose-rich glycoconjugates or pathogens, the receptor mediates endorytosis and phagocytosis of the bound ligands by macrophages. The cDNAderived primary structure of the mannose receptor predicts a cysteine-rich NH2-terminal domain, followed by a fibronectin type II region. The remainder of the ectodomain is comprised of eight carbohydrate recognition-like domains, followed by a transmembrane region, and a cytoplasmic tail. Transfection of the mannose receptor cDNA into Cos-I cells is necessary for receptor-mediated endorytosis of mannose-rich glycoconjugate as well as phagocytosis of yeasts. Deletion of the cytoplasmic tail results in a mutant receptor that is able to bind but not ingest the ligated pathogens, suggesting that the signal for phagocytosis is contained in the cytoplasmic tail.P lasma clearance and cell uptake experiments (1) indicated the existence ofa receptor that bound glycoproteins bearing high mannose chains. Receptor activity was especially evident in the liver and spleens, although most other tissues were also positive. Subsequent experiments have established that the mannose receptor is expressed on the cell surface of tissue macrophages which reside in a wide anatomical distribution throughout most organs (2) . Although originally defined by its ability to mediate endorytosis ofmannosylated or fucosylated glycoproteins, the mannose receptor's predominant role appears to be in host defense. The receptor engages yeasts (3) and parasites (4) directly resulting in the engulfment of these particles and the release ofbiologically active secretory products like reactive oxygen intermediates (5), arachidonate metabolites (6), neutral proteinases (7), and monokines like IM and tumor necrosis factor (8) . As circulating monocytes do not express the mannose receptor on their cell surface (9), the functional role for the mannose receptor appears to be on tissue macrophages, which in addition to their localization in the reticular endothelial system line the alveolus and form a reticular network beneath epithelial surfaces in the skin, the gastrointestinal tract, kidney, and placenta (10). These sites are the portals of maximum antigen load and underscore the role for the mannose receptor in first line host defense.Recent experiments suggest that a circulating hepatocytederived serum mannose-binding protein is the functional serum equivalent of the tissue macrophage mannose receptor. The human mannose-binding protein (MBP)l is an acute-phase reactant (11) that binds certain viruses (12) and other mannose rich pathogens (13). Mannose-binding proteins after engaging organisms mediate attachment, uptake, and killing of opsonized bacteria by circulating phagocytes that do not express the mannose receptor (13). Underlying structural similarity between the MBP and the mannose receptor, thereby explaining their functional equivalence, was suggested by the discovery that h...

Influenza A viruses (IAVs) cause substantial morbidity and mortality in yearly epidemics, which result from the ability of the virus to alter the antigenicity of its envelope proteins. Despite the rapid replication of this virus and its ability to infect a wide variety of cell types, viremia is rare and the infection is generally limited to the upper respiratory tract. The preimmune host defense response against IAV is generally, therefore, successful. We have previously provided (and summa-

The collectins are a class of collagenous lectin proteins present in serum and pulmonary secretions [pulmonary surfactant protein (SP) A and SP-D] that are believed to participate in innate immune responses to various pathogens. With the use of flow cytometric and fluorescent-microscopic assays, SP-A and SP-D were shown to increase calcium-dependent neutrophil uptake of Escherichia coli, Streptococcus pneumoniae, and Staphylococcus aureus. Evidence is provided that the collectins enhanced bacterial uptake through a mechanism that involved both bacterial aggregation and direct actions on neutrophils. The degree of multimerization of SP-D preparations was a critical determinant of both aggregating activity and potency in enhancing bacterial uptake. The mechanisms of opsonizing activity of SP-D and SP-A differed in important respects from those of opsonizing antibodies. These results provide the first evidence that surfactant collectins may promote neutrophil-mediated clearance of bacteria in the lung independently of opsonizing antibody.

The human mannose-binding protein (MBP)' is an acute phase serum protein of -300 kD comprised of multimers of a 32-kD subunit (1-3). It is a member of an ever-growing family of animal lectins that share at least 18 invariant residues in their carbohydrate recognition domain (CRD). The family can be divided into membrane proteins and soluble proteins, and all bind ligands optimally at neutral pH in the presence of calcium (reviewed in reference 4). For membrane proteins, the CRD is attached to a membrane anchor domain as found in the asialoglycoprotein receptors of rat (5) and human (6, 7), the chicken hepatic lectin (8), the lymphocyte IgE Fc receptor (9), and a rat Kupffer cell-binding protein (10) . This group has recently been expanded to include a lymphocyte homing receptor (11), a granular membrane protein of platelets and endothelium, GMP-140 (12), and ELAM-1, a cytokine-inducible endothelial cell receptor (13) . The function of the attachment domains of the soluble lectins, which include MBP (14, 15), the apoprotein of pulmonary surfactant SP-A (16, 17), cartilage proteoglycan core protein (18), a fly (19) and sea urchin lectin (20), and an acorn barnacle lectin (21) are not known_ Only in the rat and human MBP, and dog, human, and rabbit (22) surfactant A protein, are the noncarbohydrate recognition domains similar. In these proteins there is a collagen-like region that is preceded by a cysteine-rich NH2-terminal region that mediates interchain disulphide bonds. As our knowledge of the function of MBP and surfactant A protein expands, it appears that each domain of these proteins may have a defined purpose.Until recently, the function of MBP was not known. However, a function in host defense is suggested by its ability to bind yeast manvans (1) and interact with the complement cascade (23), and by our finding that MBP prevents infection of H9 lymphoblasts by HIV by binding to the high mannose glycans expressed on the

No abstract

Human mannose binding protein (MBP) is a C-type serum lectin involved in first-line host defense against a variety of bacterial, fungal and viral pathogens. Recently an association was found between low levels of serum MBP and an increased frequency of recurrent infections in infants. A particular genotype, in which glycine is substituted by aspartic acid at codon 54 of MBP in the fifth collagen repeat, shows apparent concordance with the clinical phenotype. We report, however, that this genotype occurs in 5% of the population and encodes a functional protein. Our results indicate that the Gly54Asp allele does not account for a deficiency state, but instead suggest that MBP may have two predominant allelic forms that have overlapping function and differ only in their ability to activate the classical pathway of complement.