Inhibition of the Lectin Pathway of Complement Activation Reduces Acute Respiratory Distress Syndrome Severity in a Mouse Model of SARS-CoV-2 Infection

Most patients with COVID-19 in the intensive care unit develop an acute respiratory distress syndrome characterized by severe hypoxemia, decreased lung compliance, and high vascular permeability. Activation of the complement system is a hallmark of moderate and severe COVID-19, with abundant deposition of complement proteins in inflamed tissue and on the endothelium during COVID-19. Using a transgenic mouse model of SARS-CoV-2 infection, we assessed the therapeutic utility of an inhibitory antibody (HG4) targeting MASP-2, a key enzyme in the lectin pathway. Treatment of infected mice with HG4 reduced the disease severity score and improved survival vs mice that received an isotype control antibody. Administration of HG4 significantly reduced the lung injury score, including alveolar inflammatory cell infiltration, alveolar edema, and alveolar hemorrhage. The ameliorating effect of MASP-2 inhibition on the severity of COVID-19 pathology is reflected by a significant reduction in the proinflammatory activation of brain microglia in HG4-treated mice.

Inhibition of the lectin pathway of complement activation reduces LPS-induced acute respiratory distress syndrome in mice

Acute respiratory distress syndrome (ARDS) is a life-threatening disorder with a high rate of mortality. Complement activation in ARDS initiates a robust inflammatory reaction that can cause progressive endothelial injury in the lung. Here, we tested whether inhibition of the lectin pathway of complement could reduce the pathology and improve the outcomes in a murine model of LPS-induced lung injury that closely mimics ARDS in human. In vitro, LPS binds to murine and human collectin 11, human MBL and murine MBL-A, but not to C1q, the recognition subcomponent of the classical pathway. This binding initiates deposition of the complement activation products C3b, C4b and C5b-9 on LPS via the lectin pathway. HG-4, a monoclonal antibody that targets MASP-2, a key enzyme in the lectin pathway, inhibited lectin pathway functional activity in vitro, with an IC50 of circa 10nM. Administration of HG4 (5mg/kg) in mice led to almost complete inhibition of the lectin pathway activation for 48hrs, and 50% inhibition at 60hrs post administration. Inhibition of the lectin pathway in mice prior to LPS-induced lung injury improved all pathological markers tested. HG4 reduces the protein concentration in bronchoalveolar lavage fluid (p<0.0001) and levels of myeloid peroxide (p<0.0001), LDH (p<0.0001), TNFα and IL6 (both p<0.0001). Lung injury was significantly reduced (p<0.001) and the survival time of the mice increased (p<0.01). From the previous findings we concluded that inhibition of the lectin pathway has the potential to prevent ARDS pathology.

Secondary Complement Deficiency Impairs Anti-Microbial Immunity to Klebsiella pneumoniae and Staphylococcus aureus During Severe Acute COVID-19

A high incidence of secondary Klebsiella pneumoniae and Staphylococcus aureus infection were observed in patients with severe COVID-19. The cause of this predisposition to infection is unclear. Our data demonstrate consumption of complement in acute COVID-19 patients reflected by low levels of C3, C4, and loss of haemolytic activity. Given that the elimination of Gram-negative bacteria depends in part on complement-mediated lysis, we hypothesised that secondary hypocomplementaemia is rendering the antibody-dependent classical pathway activation inactive and compromises serum bactericidal activity (SBA). 217 patients with severe COVID-19 were studied. 142 patients suffered secondary bacterial infections. Klebsiella species were the most common Gram-negative organism, found in 58 patients, while S. aureus was the dominant Gram-positive organism found in 22 patients. Hypocomplementaemia was observed in patients with acute severe COVID-19 but not in convalescent survivors three months after discharge. Sera from patients with acute COVID-19 were unable to opsonise either K. pneumoniae or S. aureus and had impaired complement-mediated killing of Klebsiella. We conclude that hyperactivation of complement during acute COVID-19 leads to secondary hypocomplementaemia and predisposes to opportunistic infections.

Lectin Pathway Mediates Complement Activation by SARS-CoV-2 Proteins

Early and persistent activation of complement is considered to play a key role in the pathogenesis of COVID-19. Complement activation products orchestrate a proinflammatory environment that might be critical for the induction and maintenance of a severe inflammatory response to SARS-CoV-2 by recruiting cells of the cellular immune system to the sites of infection and shifting their state of activation towards an inflammatory phenotype. It precedes pathophysiological milestone events like the cytokine storm, progressive endothelial injury triggering microangiopathy, and further complement activation, and causes an acute respiratory distress syndrome (ARDS). To date, the application of antiviral drugs and corticosteroids have shown efficacy in the early stages of SARS-CoV-2 infection, but failed to ameliorate disease severity in patients who progressed to severe COVID-19 pathology. This report demonstrates that lectin pathway (LP) recognition molecules of the complement system, such as MBL, FCN-2 and CL-11, bind to SARS-CoV-2 S- and N-proteins, with subsequent activation of LP-mediated C3b and C4b deposition. In addition, our results confirm and underline that the N-protein of SARS-CoV-2 binds directly to the LP- effector enzyme MASP-2 and activates complement. Inhibition of the LP using an inhibitory monoclonal antibody against MASP-2 effectively blocks LP-mediated complement activation. FACS analyses using transfected HEK-293 cells expressing SARS-CoV-2 S protein confirm a robust LP-dependent C3b deposition on the cell surface which is inhibited by the MASP-2 inhibitory antibody. In light of our present results, and the encouraging performance of our clinical candidate MASP-2 inhibitor Narsoplimab in recently published clinical trials, we suggest that the targeting of MASP-2 provides an unsurpassed window of therapeutic efficacy for the treatment of severe COVID-19.

Targeted deletions of complement lectin pathway genes improve outcome in traumatic brain injury, with MASP-2 playing a major role

The lectin pathway (LP) of complement activation is believed to contribute to brain inflammation. The study aims to identify the key components of the LP contributing to TBI outcome as possible novel pharmacological targets. We compared the long-term neurological deficits and neuropathology of wild-type mice (WT) to that of mice carrying gene deletions of key LP components after experimental TBI. WT or MASP-2 (Masp2-/-), ficolin-A (Fcna-/-), CL-11 (Colec11-/-), MASP-1/3 (Masp1-/-), MBL-C (Mbl2-/-), MBL-A (Mbl1-/-) or MBL-/- (Mbl1-/-/Mbl2-/-) deficient male C57BL/6J mice were used. Mice underwent sham surgery or TBI by controlled cortical impact. The sensorimotor response was evaluated by neuroscore and beam walk tests weekly for 4 weeks. To obtain a comparative analysis of the functional outcome each transgenic line was rated according to a health score calculated on sensorimotor performance. For selected genotypes, brains were harvested 6 weeks after injury for histopathological analysis. MASP-2-/-, MBL-/- and FCN-A-/- mice had better outcome scores compared to WT. Of these, MASP-2-/- mice had the best recovery after TBI, showing reduced sensorimotor deficits (by 33% at 3 weeks and by 36% at 4 weeks). They also showed higher neuronal density in the lesioned cortex with a 31.5% increase compared to WT. Measurement of LP functional activity in plasma from MASP-2-/- mice revealed the absence of LP functional activity using a C4b deposition assay. The LP critically contributes to the post-traumatic inflammatory pathology following TBI with the highest degree of protection achieved through the absence of the LP key enzyme MASP-2, underlining a therapeutic utility of MASP-2 targeting in TBI.

Deficiency in mannose-binding lectin-associated serine protease-2 does not increase susceptibility to Trypanosoma cruzi infection

Trypanosoma cruzi is the causative agent of Chagas' disease, a chronic illness affecting 10 million people around the world. The complement system plays an important role in fighting microbial infections. The recognition molecules of the lectin pathway of complement activation, mannose-binding lectin (MBL), ficolins, and CL-11, bind to specific carbohydrates on pathogens, triggering complement activation through MBL-associated serine protease-2 (MASP-2). Previous in vitro work showed that human MBL and ficolins contribute to T. cruzi lysis. However, MBL-deficient mice are only moderately compromised in their defense against the parasite, as they may still activate the lectin pathway through ficolins and CL-11. Here, we assessed MASP-2-deficient mice, the only presently available mouse line with total lectin pathway deficiency, for a phenotype in T. cruzi infection. Total absence of lectin pathway functional activity did not confer higher susceptibility to T. cruzi infection, suggesting that it plays a minor role in the immune response against this parasite.

The lectin pathway of complement activation is a critical component of the innate immune response to pneumococcal infection

The complement system plays a key role in host defense against pneumococcal infection. Three different pathways, the classical, alternative and lectin pathways, mediate complement activation. While there is limited information available on the roles of the classical and the alternative activation pathways of complement in fighting streptococcal infection, little is known about the role of the lectin pathway, mainly due to the lack of appropriate experimental models of lectin pathway deficiency. We have recently established a mouse strain deficient of the lectin pathway effector enzyme mannan-binding lectin associated serine protease-2 (MASP-2) and shown that this mouse strain is unable to form the lectin pathway specific C3 and C5 convertases. Here we report that MASP-2 deficient mice (which can still activate complement via the classical pathway and the alternative pathway) are highly susceptible to pneumococcal infection and fail to opsonize Streptococcus pneumoniae in the none-immune host. This defect in complement opsonisation severely compromises pathogen clearance in the lectin pathway deficient host. Using sera from mice and humans with defined complement deficiencies, we demonstrate that mouse ficolin A, human L-ficolin, and collectin 11 in both species, but not mannan-binding lectin (MBL), are the pattern recognition molecules that drive lectin pathway activation on the surface of S. pneumoniae. We further show that pneumococcal opsonisation via the lectin pathway can proceed in the absence of C4. This study corroborates the essential function of MASP-2 in the lectin pathway and highlights the importance of MBL-independent lectin pathway activation in the host defense against pneumococci.

Absence of the lectin activation pathway of complement does not increase susceptibility to Pseudomonas aeruginosa infections

Pseudomonas aeruginosa remains one of the major clinical pathogens that burden immuno-compromised patients and patients with cystic fibrosis. The present study aimed to define the role of the lectin pathway of complement in the immune-defence against P. aeruginosa in a mouse model of invasive pneumonia. Using in vitro assays specific for each of the three complement pathways, we demonstrate that some strains of P. aeruginosa bind lectin pathway recognition sub-components and initiate complement activation in a lectin pathway-specific mode. All of the tested strains activated complement via classical and alternative pathways. We assessed the importance of lectin pathway activation in fighting P. aeruginosa infections by testing a lectin pathway activating strain in a mouse model of intra-nasal infection. MASP-2 (mannan binding lectin associated serine protease-2) deficient mice, which have no lectin pathway activity, had no significant survival disadvantage compared to wild type littermates (72.7% and 81.8% survival, respectively, p=0.48). Likewise, no difference in opsonising activity was seen between MASP-2 sufficient and MASP-2 deficient mouse sera. Moreover, cytokine expression profiles in the lungs of WT mice and MASP-2-/- mice were similar throughout the course of P. aeruginosa infection. We conclude that the lectin pathway does not play an essential role in fighting P. aeruginosa infection in mice.

Small mannose-binding lectin-associated protein plays a regulatory role in the lectin complement pathway

Mannose-binding lectin (MBL) and ficolins are pattern recognition proteins acting in innate immunity, and they trigger the activation of the lectin complement pathway through MBL-associated serine proteases (MASPs). Upon activation of the lectin pathway, MASP-2 cleaves C4 and C2. A truncated form of MASP-2, named small MBL-associated protein (sMAP), is also associated with MBL/ficolin-MASP complexes. To clarify the role of sMAP, we have generated sMAP-deficient (sMAP(-/-)) mice by targeted disruption of the sMAP-specific exon. Because of the gene disruption, the expression level of MASP-2 was also decreased in sMAP(-/-) mice. When recombinant sMAP (rsMAP) and recombinant MASP-2 (rMASP-2) reconstituted the MBL-MASP-sMAP complex in deficient serum, the binding of these recombinant proteins to MBL was competitive, and the C4 cleavage activity of the MBL-MASP-sMAP complex was restored by the addition of rMASP-2, whereas the addition of rsMAP attenuated the activity. Therefore, MASP-2 is essential for the activation of C4 and sMAP plays a regulatory role in the activation of the lectin pathway.

Consumption of mannan-binding lectin during abdominal aortic aneurysm repair

OBJECTIVE

Patients undergoing abdominal aortic aneurysm (AAA) repair are exposed to an ischaemia-reperfusion injury (IRI), which is in part mediated by complement activation. We investigated the role of the novel lectin pathway of complement during IRI in patients undergoing AAA repair.

METHODS

Patients undergoing elective open infrarenal AAA repair had systemic blood samples taken at induction of anaesthesia, prior to aortic clamping, prior to aortic declamping and at reperfusion. Control patients undergoing major abdominal surgery were also included. Plasma was assayed for levels of mannan-binding lectin (MBL) using ELISA techniques. Consumption of plasma MBL was used as a measure of lectin pathway activation.

RESULTS

Twenty-three patients undergoing AAA repair and eight control patients were recruited. No lectin pathway activation could be demonstrated in the control patients. AAA patients experienced a mean decrease in plasma MBL levels of 41% representing significant lectin pathway activation (p = 0.003).

CONCLUSION

Consumption of MBL occurs during AAA repair, suggesting an important role for the lectin pathway in IRI. Specific transient inhibition of lectin pathway activity could be of significant therapeutic value in patients undergoing open surgical AAA repair.

Decoupling of Carbohydrate Binding and MASP-2 Autoactivation in Variant Mannose-Binding Lectins Associated with Immunodeficiency

Mannan-binding lectin (MBL) initiates complement activation by binding to arrays of carbohydrates on the surfaces of pathogenic microorganisms and activating MBL-associated serine proteases (MASPs). Separate point mutations to the collagenous domain of human MBL are associated with immunodeficiency, caused by reduced complement activation by the variant MBLs as well as by lower serum MBL concentrations. In the work reported here, we have used the well characterized rat lectin pathway to analyze the molecular and functional defects associated with two of the variant proteins. Mutations Gly25 --> Asp and Gly28 --> Glu create comparable structural changes in rat MBL but the G28E variant activates complement >10-fold less efficiently than the G25D variant, which in turn has approximately 7-fold lower activity than wild-type MBL. Analysis of mutant MBL . MASP-2 complexes assembled from recombinant components shows that reduced complement activation by both mutant MBLs is caused by failure to activate MASP-2 efficiently on binding to a mannan-coated surface. Disruption of MBL-MASP-2 interactions as well as to changes in oligomeric structure and reduced binding to carbohydrate ligands compared with wild-type MBL probably account for the intermediate phenotype of the G25D variant. However, carbohydrate binding and MASP-2 activation are ostensibly completely decoupled in complexes assembled from the G28E mutant, such that the rate of MASP-2 activation is no greater than the basal rate of zymogen MASP-2 autoactivation. Analogous molecular defects in human MBL probably combine to create the mutant phenotypes of immunodeficient individuals.

Functional MASP2 single nucleotide polymorphism plays no role in psoriasis

BACKGROUND

Psoriasis is a heritable disease and genome-wide scans have implicated several loci of susceptibility. The gene for MASP-2, a protease involved in complement activation, is located within one of these loci on chromosome 1p.

OBJECTIVES

To assess whether partial or total MASP-2 deficiency is a risk factor for developing psoriasis.

METHODS

We screened a cohort of patients affected by plaque psoriasis and their parents by restriction fragment length polymorphism analyses.

RESULTS

We detected a single nucleotide polymorphism that leads to an amino acid exchange, which results in dissociation of MASP-2 from a carbohydrate recognition complex.

CONCLUSIONS

We show that this mutant allele is not associated with psoriasis. There was no favoured transmission from parents to affected offspring. The calculated allele frequency in this psoriasis group (Scottish and English) was 0.0326, and in the unaffected group 0.0379.

Composition of the lectin pathway of complement in Gallus gallus: absence of mannan-binding lectin-associated serine protease-1 in birds

The lectin pathway of complement is activated by multimolecular complexes that recognize and bind to microbial polysaccharides. These complexes comprise a multimeric carbohydrate recognition subunit (either mannan-binding lectin (MBL) or a ficolin), three MBL-associated serine proteases (MASP-1, -2, and -3), and MAp19 (a truncated product of the MASP-2 gene). In this study we report the cloning of chicken MASP-2, MASP-3, and MAp19 and the organization of their genes and those for chicken MBL and a novel ficolin. Mammals usually possess two MBL genes and two or three ficolin genes, but chickens have only one of each, both of which represent the undiversified ancestors of the mammalian genes. The primary structure of chicken MASP-2 is 54% identical with those of the human and mouse MASP-2, and the organization of its gene is the same as in mammals. MASP-3 is even more conserved; chicken MASP-3 shares approximately 75% of its residues with human and Xenopus MASP-3. It is more widely expressed than other lectin pathway components, suggesting a possible function of MASP-3 different from those of the other components. In mammals, MASP-1 and MASP-3 are alternatively spliced products of a single structural gene. We demonstrate the absence of MASP-1 in birds, possibly caused by the loss of MASP-1-specific exons during phylogeny. Despite the lack of MASP-1-like enzymatic activity in sera of chicken and other birds, avian lectin pathway complexes efficiently activate C4.

Organization of the MASP2 locus and its expression profile in mouse and rat

The mouse, rat, and human MASP2 loci are situated on syntenic chromosome regions and are highly conserved. They comprise the genes for MASP-2/ MAp19, TAR DNA binding protein of 43 kDa, FRAP kinase, CDT6, Polymyositis-Scleroderma 100-kDa autoantigen, spermidine synthase, and TERE which were analyzed by annotation of available gene transcript data and cross-species comparison of available genomic sequences. The human and rat genes for spermidine synthase have an additional intron compared to the mouse gene. The mouse and rat genes for Polymyositis-Scleroderma 100-kDa autoantigen have an additional exon compared to the human gene. We find support for the hypothesis that the MAp19-specific exon within the MASP2 gene may have originated in a transposable element. Blocks of highly conserved intronic sequences were found in the MASP2 gene and the TARDBP gene. The expression of all genes within the MASP2 locus was analyzed in mouse and rat. The restricted expression of MASP-2 and MAp19 mRNA in liver contrasts with the ubiquitous expression of all neighboring genes studied.

Antibody-mediated activation of the classical pathway of complement may compensate for mannose-binding lectin deficiency

Deficiency of mannose-binding lectin (MBL), a recognition molecule of the lectin pathway of complement, is associated with increased susceptibility to infections. The high frequency of MBL deficiency suggests that defective MBL-mediated innate immunity can be compensated by alternative defense strategies. To examine this hypothesis, complement activation by MBL-binding ligands was studied. The results show that the prototypic MBL ligand mannan can induce complement activation via both the lectin pathway and the classical pathway. Furthermore, antibody binding to mannan restored complement activation in MBL-deficient serum in a C1q-dependent manner. Cooperation between the classical pathway and the lectin pathway was also observed for complement activation by protein 60 from Listeria monocytogenes. MBL pathway analysis at the levels of C4 and C5b-9 in the presence of classical pathway inhibition revealed a large variation of MBL pathway activity, depending on mbl2 gene polymorphisms. MBL pathway dysfunction in variant allele carriers is associated with reduced MBL ligand binding and a relative increase of low-molecular-mass MBL. These findings indicate that antibody-mediated classical pathway activation can compensate for impaired target opsonization via the MBL pathway in MBL-deficient individuals, and imply that MBL deficiency may become clinically relevant in absence of a concomitant adaptive immune response.

Microglial activation and increased synthesis of complement component C1q precedes blood-brain barrier dysfunction in rats

A reliable way to visualise the state of microglial activation is to monitor the microglial gene expression profile. Microglia are the only CNS resident cells that synthesise C1q, the recognition sub-component of the classical complement pathway, in vivo. C1q biosynthesis in resting ramified microglia is often low, but it increases dramatically in activated microglia. In this study, the expression of C1q was used to monitor microglial activation at all stages of 3-chloropropanediol-induced neurotoxicity, a new model of blood-brain barrier (BBB) breakdown. In rats, 3-chloropropanediol produces very focused lesions in the brain, characterised by early astrocyte swelling and loss, followed by neuronal death and barrier dysfunction. Using in situ hybridisation, immunohistochemistry, and real-time RT-PCR, we found that increased C1q biosynthesis and microglial activation precede BBB dysfunction by at least 18 and peak 48 h after injection of 3-chloropropanediol, which coincides with the onset of active haemorrhage. Microglial activation is biphasic; an early phase of global activation is followed by a later phase in which microglial activation becomes increasingly focused in the lesions. During the early phase, expression of the pro-inflammatory mediators interleukin-1beta (IL1beta), tumour necrosis factor alpha (TNFalpha) and early growth response-1 (Egr-1) increased in parallel with C1q, but was restricted to the lesions. Expression of C1q (but not IL1beta, TNFalpha or Egr-1) remains high after BBB function is restored, and is accompanied by late up-regulation of the C1q-associated serine proteases, C1r and C1s, suggesting that microglial biosynthesis of the activation complex of the classical pathway may support the removal of cell debris by activation of complement.

L-ficolin specifically binds to lipoteichoic acid, a cell wall constituent of Gram-positive bacteria, and activates the lectin pathway of complement

The lectin pathway of complement is activated when a carbohydrate recognition complex and associated serine proteases binds to the surface of a pathogen. Three recognition subcomponents have been shown to form active initiation complexes: mannan-binding lectin (MBL), L-ficolin, and H-ficolin. The importance of MBL in antimicrobial host defense is well recognized, but the role of the ficolins remains largely undefined. This report shows that L-ficolin specifically binds to lipoteichoic acid (LTA), a cell wall component found in all Gram-positive bacteria. Immobilized LTA from Staphylococcus aureus binds L-ficolin complexes from sera, and these complexes initiate lectin pathway-dependent C4 turnover. C4 activation correlates with serum L-ficolin concentration, but not with serum MBL levels. L-ficolin binding and corresponding levels of C4 turnover were observed on LTA purified from other clinically important bacteria, including Streptococcus pyogenes and Streptococcus agalactiae. None of the LTA preparations bound MBL, H-ficolin, or the classical pathway recognition molecule, C1q.

Murine serine proteases MASP-1 and MASP-3, components of the lectin pathway activation complex of complement, are encoded by a single structural gene

Activation of the lectin pathway of complement is initiated by the binding to microbial carbohydrate structures of a multimolecular fluid-phase complex composed of a carbohydrate recognition subcomponent that associates with three specific serine proteases and an enzymatically inert protein of 19 kDa. The first carbohydrate recognition subcomponent of the lectin pathway identified was mannan-binding lectin (MBL), hence the serine proteases were named MBL-associated serine proteases (MASPs) and numbered according to the sequence of their discovery. Here we describe the primary structures of the two distinct serine proteases MASP-1 and MASP-3 in the rat (and of MASP-3 in the mouse), show their association with plasma MBL complexes, and demonstrate that in rat and mouse, as in man, MASP-1 and MASP-3 are encoded by a single structural gene. For both species, we present the genomic region and regulatory elements responsible for the processing of either MASP-1 or MASP-3 mRNA by alternative splicing/alternative polyadenylation. Furthermore, we demonstrate the evolutionary conservation of MASP-3 mRNA in cDNA transcripts from guinea pig, rabbit, pufferfish, and cow.

Differential expression of the murine mannose-binding lectins A and C in lymphoid and nonlymphoid organs and tissues

Mannose-binding lectin (MBL), a member of the collectin family, binds to carbohydrate structures on the surfaces of micro-organisms and may serve as a recognition molecule of the lectin pathway of complement activation. In rodents two forms, MBL-A and MBL-C, were described and shown to be products of two related, but uncoupled, genes. The liver is the main source of MBL biosynthesis. For rat MBL-A, expression has also been described in the kidney. Here we report that the two forms of murine MBL are differentially expressed in a number of nonhepatic tissues. Real-time RT-PCR revealed that the liver is the major site of expression for both MBL genes. Lower copy numbers were found in kidney, brain, spleen, and muscle. In testis, only the MBL-A gene is expressed, whereas MBL-C is exclusively expressed in small intestine. Using in situ hybridization and immunohistochemistry, we demonstrate that both MBLs are synthesized by hepatocytes and show MBL expression in cells of the monocyte/macrophage lineage. In the kidney MBL-A, but not MBL-C, was found to be synthesized. Vice versa, only MBL-C biosynthesis was detected in endothelial cells of the small intestine. The latter finding may support the view that MBL-C, as part of the innate immune system, may be a counterpart of secretory IgA of the acquired immune system in preventing, for example, microbial invasion and colonization. Our findings demonstrate that MBL-A and MBL-C are differentially expressed, implying distinct biological roles for both recognition molecules of the murine lectin pathway of complement.

In vivo biosynthesis of endogenous and of human C1 inhibitor in transgenic mice: tissue distribution and colocalization of their expression

We have produced transgenic mice expressing human C1 inhibitor mRNA and protein under the control of the human promoter and regulatory elements. The transgene was generated using a minigene construct in which most of the human C1 inhibitor gene (C1NH) was replaced by C1 inhibitor cDNA. The construct retained the promoter region extending 1.18 kb upstream of the transcription start site, introns 1 and 2 as well as a stretch of 2.5 kb downstream of the polyadenylation site, and therefore carried all known elements involved in transcriptional regulation of the C1NH gene. Mice with high serum levels of human C1 inhibitor, resulting from multiple tandem integrations of the C1 inhibitor transgene, were selected. Immunohistochemistry in combination with in situ hybridization was applied to localize the sites of C1 inhibitor biosynthesis and to demonstrate its local production in brain, spleen, liver, heart, kidney, and lung. The distribution of human C1 inhibitor-expressing cells was qualitatively indistinguishable from that of its mouse counterpart, but expression levels of the transgene were significantly higher. In the spleen, production of C1 inhibitor was colocalized with that of a specific marker for white pulp follicular dendritic cells. This study demonstrates a stringently regulated expression of both the endogenous and the transgenic human C1 inhibitor gene and reveals local biosynthesis of C1 inhibitor at multiple sites in which the components of the macromolecular C1 complex are also produced.

In vivo pharmacokinetics of calreticulin S-domain, an inhibitor of the classical complement pathway

Inhibition of the complement system is potentially therapeutic in diseases where uncontrolled or overshooting complement activation plays a significant role in the pathogenesis of the disorder. Calreticulin (CRT) is a multifunctional protein whose cell-surface form (ectocalreticulin) is reported to be a C1q receptor. A 124-residue domain within CRT, the S-domain, binds to C1q, prevents the formation of C1 and so inhibits activation of the classical pathway. To assess the usefulness of CRT S-domain as a complement inhibitor, recombinant S-domain was expressed, radiolabeled, and the fate of the radiolabeled peptide followed in vivo. In rats, CRT-S-domain shows a half-life of 1.21 +/- 0.34 and 40.5 +/- 2.7 min in the distribution and elimination phases from plasma, respectively. The peptide remains largely intact, and is cleared from the circulation by the kidneys, where it accumulates in the proximal convoluted tubules, but is not excreted. Much smaller amounts of the peptide accumulate in other tissues, and essentially none crosses the blood-brain barrier.

Mutation detection and physical mapping of the CD11 gene cluster in association with inflammatory bowel disease

A genetic component in the etiology of inflammatory bowel disease (IBD) has clearly been demonstrated by epidemiological and genetic linkage studies. Linkage to IBD on proximal Chromosome (Chr) 16p is well established and replicated. A stratification experiment showed that the recent identification of a disease gene on the q arm does not interfere with the approach on the p arm, and the linkage peak is still significant. Here we present a candidate gene study of the alpha integrins (CD11A-D) on Chr 16. The alpha integrins play a key role in inflammatory processes, including leukocyte adhesion and migration. Their genes are located on the p arm of Chr 16, and therefore represent excellent positional and functional candidates. Since the assignment of the CD11 genes in the genome was not clear, we performed physical, radiation hybrid, and fluorescent in situ hybridization mapping of the gene family. All CD11 genes map on Chr 16p11-12. CD11B-D are arranged in a gene cluster within 300 kb and CD11A is located about 2.5 Mb telomeric. Thirteen new single nucleotide polymorphisms (SNPs) and eight SNPs from databases were identified through full-length sequencing. Case-control statistics demonstrated an association lead in the CD11 gene cluster, which was not confirmed in further family based association/linkage analyses using single markers and haplotypes. It is unlikely that the CD11 genes play an important role in the pathogenesis of IBD. The marginally significant results could indicate a disease gene in the vicinity of the gene cluster.

The human gene for mannan-binding lectin-associated serine protease-2 (MASP-2), the effector component of the lectin route of complement activation, is part of a tightly linked gene cluster on chromosome 1p36.2-3

The proteases of the lectin pathway of complement activation, MASP-1 and MASP-2, are encoded by two separate genes. The MASP1 gene is located on chromosome 3q27, the MASP2 gene on chromosome 1p36.23-31. The genes for the classical complement activation pathway proteases, C1r and C1s, are linked on chromosome 12p13. We have shown that the MASP2 gene encodes two gene products, the 76 kDa MASP-2 serine protease and a plasma protein of 19 kDa, termed MAp19 or sMAP. Both gene products are components of the lectin pathway activation complex. We present the complete primary structure of the human MASP2 gene and the tight cluster that this locus forms with non-complement genes. A comparison of the MASP2 gene with the previously characterised C1s gene revealed identical positions of introns separating orthologous coding sequences, underlining the hypothesis that the C1s and MASP2 genes arose by exon shuffling from one ancestral gene.

Low dosage monophasic oral contraceptive use and intermittent exercise performance and metabolism in humans

Nine untrained women using low dosage monophasic oral contraceptives (OC) performed an intermittent treadmill test on two different occasions within one pill-cycle to determine the effect of OC on performance and some commonly used metabolic markers. The first test was performed after 5-8 days of resuming the OC agents after menstrual bleeding while the other test was performed after 19-21 days. Performance time on the final exhausting run of six intermittent high intensity 20 s runs was no different between trials [mean days 5-8: 22.3 (SEM 1.2) s vs days 19-21: 22.7 (SEM 1.1) s]. There was no difference in heart rate [peak heart rate days 5-8: 183 (SEM 3) beats.min-1 vs days 19-21: 186 (SEM 2) beats.min-1], oxygen consumption during any run [run 5 of days 5-8: 1,392 (SEM 51) ml.min-1 vs run 5 of days 19-21: 1,494 (SEM 3) ml.min-1] or in any of the metabolic variables measured at any time in venous blood [peak blood lactate concentration days 5-8: 8.4 (SEM 0.3) mmol.l-1 vs days 19-21: 8.1 (SEM 0.5) mmol.l-1; peak blood glycerol concentration days 5-8: 0.39 (SEM 0.02) mmol.l-1 vs days 19-21: 0.38 (SEM 0.02) mmol.l-1; resting free fatty acids concentration days 5-8: 0.25 (SEM 0.05) mmol.l-1 vs days 19-21: 0.29 (SEM 0.07) mmol.l-1; peak blood glucose concentration days 5-8: 6.7 (SEM 0.5) mmol.l-1 vs days 19-21: 6.6 (SEM 0.2) mmol.l-1; peak capillary blood ammonia concentration days 5-8: 139 (SEM 18.3) mumol.l-1 vs days 19-21: 170 (SEM 18.0) mumol.l-1]. These results suggest neither intermittent high intensity exercise performance nor energy metabolism change between days 5-8 and days 19-21 of a low dosage monophasic OC pill during one pill-cycle.

Fine mapping of the chromosome 3p susceptibility locus in inflammatory bowel disease

BACKGROUND AND AIMS

Genetic predisposition for inflammatory bowel disease (IBD) has been demonstrated by epidemiological and genetic linkage studies. Genetic linkage of IBD to chromosome 3 has been observed previously. A high density analysis of chromosome 3p was performed to confirm prior linkages and elucidate potential genetic associations.

METHODS

Forty three microsatellite markers on chromosome 3 were genotyped in 353 affected sibling pairs of North European Caucasian extraction (average marker density 2 cM in the linkage interval). Marker order was defined by genetic and radiation hybrid techniques.

RESULTS

The maximum single point logarithm of odds (LOD) score was observed for Crohn's disease at D3S3591. Peak multipoint LOD scores of 1.65 and 1.40 for the IBD phenotype were observed near D3S1304 (distal 3p) and near D3S1283 in the linkage region previously reported. Crohn's disease contributed predominantly to the linkage. The transmission disequilibrium test showed significant evidence of association (p=0.009) between allele 4 of D3S1076 and the IBD phenotype (51 transmitted v 28 non-transmitted). Two known polymorphisms in the CCR2 and CCR5 genes were analysed, neither of which showed significant association with IBD. Additional haplotype associations were observed in the vicinity of D3S1076.

CONCLUSIONS

This study provides confirmatory linkage evidence for an IBD susceptibility locus on chromosome 3p and suggests that CCR2 and CCR5 are unlikely to be major susceptibility loci for IBD. The association findings in this region warrant further investigation.

Effects of moderate dietary manipulation on intermittent exercise performance and metabolism in women

The aim of the current study was to examine the effect of a moderate alteration in pre-exercise diet composition on the performance of, and metabolic response to, intermittent treadmill exercise in a group of normally menstruating females. Eight recreationally active women performed two intermittent, incremental exercise trials, one preceded by 2 days of a high [61 (1)%] carbohydrate (CHO) diet and the other by 2 days of a low [31 (1)%] CHO diet. Oxygen uptake (VO2) was measured during, and blood samples were obtained immediately after, each bout for the determination of blood lactate, glucose, glycerol, plasma free fatty acids and plasma ammonia. Performance, as assessed by time to exhaustion in the final bout, was found to be similar whether preceded by a high- or low-CHO diet [median (range): 28.0 (18-54) s, 29 (18-54) s, respectively]. No significant between trial differences were found in VO2, heart rate, or any of the blood metabolites. The results of the current, study indicate that moderate alterations of pre-exercise diet do not affect intermittent, high-intensity exercise performance in women, despite some evidence of an alteration in the pattern of the metabolic response to exercise.

The Rat and Mouse Homologues of MASP-2 and MAp19, components of the Lectin Activation Pathway of Complement

Recently, we described two novel constituents of the multimolecular initiation complex of the mannan-binding lectin (MBL) pathway of complement activation, a serine protease of 76 kDa, termed MASP-2, and a MASP-2 related plasma protein of 19 kDa, termed MAp19. Upon activation of the MBL/MASPs/MAp19 complex, MASP-2 cleaves the fourth complement component C4, while the role of MAp19 within the MBL/MASP-1/MASP-2/MAp19 complex remains to be clarified. In humans, the mRNA species encoding MASP-2 (2.6 kb) and MAp19 (1.0 kb) arise by an alternative polyadenylation/splicing mechanism from a single structural MASP-2 gene. Here, we report the complete primary structures of the rat homologue of MASP-2 and of rat and mouse MAp19. We show that both MASP-2 and MAp19 are part of the rat MBL pathway activation complex and demonstrate their exclusively hepatic biosynthesis. Southern blot and PCR analyses of rat genomic DNA indicate that as in humans, rat MASP-2 and MAp19 are encoded by a single structural gene.

The rat and mouse homologues of MASP-2 and MAp19, components of the lectin activation pathway of complement

Recently, we described two novel constituents of the multimolecular initiation complex of the mannan-binding lectin (MBL) pathway of complement activation, a serine protease of 76 kDa, termed MASP-2, and a MASP-2 related plasma protein of 19 kDa, termed MAp19. Upon activation of the MBL/MASPs/MAp19 complex, MASP-2 cleaves the fourth complement component C4, while the role of MAp19 within the MBL/MASP-1/MASP-2/MAp19 complex remains to be clarified. In humans, the mRNA species encoding MASP-2 (2.6 kb) and MAp19 (1.0 kb) arise by an alternative polyadenylation/splicing mechanism from a single structural MASP-2 gene. Here, we report the complete primary structures of the rat homologue of MASP-2 and of rat and mouse MAp19. We show that both MASP-2 and MAp19 are part of the rat MBL pathway activation complex and demonstrate their exclusively hepatic biosynthesis. Southern blot and PCR analyses of rat genomic DNA indicate that as in humans, rat MASP-2 and MAp19 are encoded by a single structural gene.

Linkage of inflammatory bowel disease to human chromosome 6p

Inflammatory bowel disease (IBD) is characterized by a chronic relapsing intestinal inflammation. IBD is subdivided into Crohn disease and ulcerative colitis phenotypes. Given the immunologic dysregulation in IBD, the human-leukocyte-antigen region on chromosome 6p is of significant interest. Previous association and linkage analysis has provided conflicting evidence as to the existence of an IBD-susceptibility locus in this region. Here we report on a two-stage linkage and association analysis of both a basic population of 353 affected sibling pairs (ASPs) and an extension of this population to 428 white ASPs of northern European extraction. Twenty-eight microsatellite markers on chromosome 6 were genotyped. A peak multipoint LOD score of 4.2 was observed, at D6S461, for the IBD phenotype. A transmission/disequilibrium test (TDT) result of P=.006 was detected for D6S426 in the basic population and was confirmed in the extended cohort (P=.004; 97 vs. 56 transmissions). The subphenotypes of Crohn disease, ulcerative colitis, and mixed IBD contributed equally to this linkage, suggesting a general role for the chromosome 6 locus in IBD. Analysis of five single-nucleotide polymorphisms in the TNFA and LTA genes did not reveal evidence for association of these important candidate genes with IBD. In summary, we provide firm linkage evidence for an IBD-susceptibility locus on chromosome 6p and demonstrate that TNFA and LTA are unlikely to be susceptibility loci for IBD.

Two constituents of the initiation complex of the mannan-binding lectin activation pathway of complement are encoded by a single structural gene

Mannan-binding lectin (MBL) forms a multimolecular complex with at least two MBL-associated serine proteases, MASP-1 and MASP-2. This complex initiates the MBL pathway of complement activation by binding to carbohydrate structures present on bacteria, yeast, and viruses. MASP-1 and MASP-2 are composed of modular structural motifs similar to those of the C1q-associated serine proteases C1r and C1s. Another protein of 19 kDa with the same N-terminal sequence as the 76-kDa MASP-2 protein is consistently detected as part of the MBL/MASP complex. In this study, we present the primary structure of this novel MBL-associated plasma protein of 19 kDa, MAp19, and demonstrate that MAp19 and MASP-2 are encoded by two different mRNA species generated by alternative splicing/polyadenylation from one structural gene.

Two Constituents of the Initiation Complex of the Mannan-Binding Lectin Activation Pathway of Complement Are Encoded by a Single Structural Gene

Mannan-binding lectin (MBL) forms a multimolecular complex with at least two MBL-associated serine proteases, MASP-1 and MASP-2. This complex initiates the MBL pathway of complement activation by binding to carbohydrate structures present on bacteria, yeast, and viruses. MASP-1 and MASP-2 are composed of modular structural motifs similar to those of the C1q-associated serine proteases C1r and C1s. Another protein of 19 kDa with the same N-terminal sequence as the 76-kDa MASP-2 protein is consistently detected as part of the MBL/MASP complex. In this study, we present the primary structure of this novel MBL-associated plasma protein of 19 kDa, MAp19, and demonstrate that MAp19 and MASP-2 are encoded by two different mRNA species generated by alternative splicing/polyadenylation from one structural gene.

Neuronal expression of fractalkine in the presence and absence of inflammation

Fractalkine is the only as yet known member of a novel class of chemokines. Besides its novel Cys-X-X-X-Cys motif, fractalkine exhibits features which have not been described for any other member of the chemokine family, including its unusual size (397 amino acids human, 395 mouse) and the possession of a transmembrane anchor, from which a soluble form may be released by extracellular cleavage. This report demonstrates the abundant mRNA and fractalkine protein expression in neuronal cells. The neuronal expression of fractalkine mRNA is unaffected by experimentally induced inflammation of central nervous tissue.

Effects of menstrual cycle phase and oral contraceptive use on intermittent exercise

Five women using low-dose, monophasic oral contraceptive (OC) agents (OC group) and ten normally menstruating women (Non-OC group) performed a treadmill protocol to determine the effect of OCs and the menstrual cycle (MC) on intermittent exercise performance and some commonly used metabolic markers. The Non-OC group were tested once in the mid-follicular phase (MFP) and once in the late luteal phase (LLP) of the MC, while the OC group performed their first test within 1 week of taking the OC (T1) and their second test 1 week later (T2). Despite performance time being the same in both groups [mean (SD), Non-OC group: 77.7 (14.9) s versus OC group: 77.7 (21.1)s], plasma ammonia concentration ([NH3]pl) was higher in the Non-OC group when compared to the OC group throughout recovery (P < 0.05). No differences were found in blood lactate (BLa), maximum heart rate or aural temperature (Tau) between groups. Within the Non-OC group Tau increased with exercise in both phases (P < 0.05), however Tau was higher in the LLP at rest [36.1 (0.3) degrees C) and 1 min post-exercise [37.1 (0.6) degrees C), when compared to the MFP [35.8 (0.3) and 36.9 (0.7) degrees C, rest and 1 min post-exercise respectively, P < 0.05]. Within the OC group T1 resulted in a higher peak BLa [11.2 (0.4) mmol/l] and [NH3]pl (143.0 (26.2) Umol/l] when compared to T2 [BLa, 9.6 (0.9); [NH3], 119.4 (48.1), P<0.05]. These results suggest that: (1) exercise performance does not vary between the MFP and the LLP of the MC, nor does it appear to be affected by the number of days using the OC, and (2) an altered metabolism occurs both between groups (Non-OC versus OC) and within the OC group.

Interaction of C1q and the collectins with the potential receptors calreticulin (cC1qR/collectin receptor) and megalin

Several proteins have been identified as candidate cell-surface receptors for the complement protein C1q. Some of these also interact with the structurally-related collectin proteins. Previous descriptions of C1q-binding properties of cells, and information on the cellular distribution of candidate receptors suggest that there is more than one physiologically relevant receptor for C1q. Two such candidate receptors, cell-surface calreticulin (also referred to as cC1qR or collectin receptor) and megalin are discussed in this review.

Characterization of the murine gene of gC1qBP, a novel cell protein that binds the globular heads of C1q, vitronectin, high molecular weight kininogen and factor XII

gC1qBP is a novel cell protein which was found to interact with the globular heads of C1q, high mol. wt kininogen, factor XII and the heparin-binding, multimeric form of vitronectin. The protein sequence shows no homology to any protein family. This paper describes the genomic organization of mouse gC1qBP and the characterization of its 5' flanking region. The mouse gene consists of six exons separated by five introns, and its total length is approximately 6kb. Exon 1 encodes the putative signal peptide, a long stretch of 70 amino acid residues, and the first four amino acid residues found in the mature gC1qBP. Exons 2-5 encode four very hydrophilic domains, whereas exon 6 encodes a neutral domain. The amino acid sequence responsible for binding to the heparin-binding, multimeric form of vitronectin is located in exon 2. A 1kb DNA fragment upstream of the first initiation codon was sequenced, which contained four potential TATA boxes, seven CAAT boxes, six SP1 sites and various putative transcription factor-binding elements, indicating that the promoter region is in close proximity to the first exon. The mouseC1qbp gene was mapped to chromosome 11, closely linked to D11Mit4 using genomic DNAs from a (C57BL/6J x Mus spretus)F1 x Mus spretus backcross.

The C1q and collectin binding site within C1q receptor (cell surface calreticulin)

C1q receptor (C1qR/collectin receptor/cC1qR) has an almost complete amino acid sequence identity with calreticulin (CRT). C1qR/CRT is located on the surface of many cell types. Binding of C1q to C1q receptor elicits a range of immunological responses. C1qR also interacts with the collectins SP-A, MBL, CL43 and conglutinin via a cluster of charged residues on the collagen tails of the ligands. In order to localise C1q and collectin binding activity within C1qR/CRT, recombinant C1qR/CRT domains [N (residues 18-196), P (197-308) and C (309-417)] were produced. Both the N- and P-domains bound to C1q, demonstrating that the binding site spans the intersection of these domains. Amino acid alignment analysis identified a putative CUB module within this region. This S-domain (residues 160-283) was expressed and showed concentration-dependent binding to immobilised C1q, demonstrating that it contains the C1q binding site. Competitive inhibition studies of the S-domain-C1q interaction revealed that the S-domain binds to C1q collagen tails and to the collectin proteins, SP-A, MBL, CL43 and conglutinin. The C1q and collection binding site on C1qR/CRT has therefore been localised to the S-domain.

Characterisation of the rat and mouse homologues of gC1qBP, a 33 kDa glycoprotein that binds to the globular 'heads' of C1q

gC1qBP is a 33 kDa glycoprotein that binds to the globular 'heads' of C1q. We have cloned cDNAs encoding the rat and mouse homologues of gC1qBP. Comparison of the cDNA-derived amino acid sequences of gC1qBP reveals that either of the rodent sequences is 89.9% identical to the reported human sequence. Recombinant rat gC1qBP binds avidly to human C1q. gC1qBP mRNA is abundantly expressed in every rat and mouse tissue analysed. Rat mesangial cells synthesise gC1qBP, but do not express gC1qBP on the cell surface. In rat serum, gC1qBP is present at low levels.

Intracellular localization of the human receptor for the globular domains of C1q

This study was performed to determine the localization of the recently described receptor for the globular domain of C1q, gC1qR. In contrast to previous reports, we were not able to detect significant surface expression of gC1qR on Raji cells, monocytes, neutrophils, human or rat mesangial cells, the endothelial cell line EA.hy 926, or HUVEC using FACS analysis. Only by using digoxigenin-conjugated Abs could some surface staining of gC1qR be observed on rat mesangial cells and neutrophils. However, after permeabilizing these cells with saponin, a strong positive intracellular staining for gC1qR was observed by FACS, fluorescence microscopy on coverslips, and confocal laser scanning microscopic analysis. By reflection contrast microscopy and electron microscopy on ultrathin sections of permeabilized Raji cells, it was shown that gC1qR is present in double membranous cytoplasmic vesicles located in the proximity of the plasma membrane. To determine whether certain conditions could induce surface expression of gC1qR, Raji cells were either stimulated with T cell growth factor, LPS, or driven to apoptosis by incubation with fenretinide or by serum depletion. None of the conditions resulted in significant surface expression of gC1qR. Our hypothesis that gC1qR is not a surface molecule but a soluble molecule that is secreted by cells is supported by the observation that gC1qR is found in significant concentrations in supernatants of several cultured cells and in normal human and rat sera. Our results suggest that the recently described gC1qR is not a cell surface receptor, but a soluble binding protein with affinity for the globular heads of C1q. Excreted gC1qR might act as a potential fluid phase regulator of complement activation.

Intracellular localization of the human receptor for the globular domains of C1q

This study was performed to determine the localization of the recently described receptor for the globular domain of C1q, gC1qR. In contrast to previous reports, we were not able to detect significant surface expression of gC1qR on Raji cells, monocytes, neutrophils, human or rat mesangial cells, the endothelial cell line EA.hy 926, or HUVEC using FACS analysis. Only by using digoxigenin-conjugated Abs could some surface staining of gC1qR be observed on rat mesangial cells and neutrophils. However, after permeabilizing these cells with saponin, a strong positive intracellular staining for gC1qR was observed by FACS, fluorescence microscopy on coverslips, and confocal laser scanning microscopic analysis. By reflection contrast microscopy and electron microscopy on ultrathin sections of permeabilized Raji cells, it was shown that gC1qR is present in double membranous cytoplasmic vesicles located in the proximity of the plasma membrane. To determine whether certain conditions could induce surface expression of gC1qR, Raji cells were either stimulated with T cell growth factor, LPS, or driven to apoptosis by incubation with fenretinide or by serum depletion. None of the conditions resulted in significant surface expression of gC1qR. Our hypothesis that gC1qR is not a surface molecule but a soluble molecule that is secreted by cells is supported by the observation that gC1qR is found in significant concentrations in supernatants of several cultured cells and in normal human and rat sera. Our results suggest that the recently described gC1qR is not a cell surface receptor, but a soluble binding protein with affinity for the globular heads of C1q. Excreted gC1qR might act as a potential fluid phase regulator of complement activation.

Localisation of the C1q binding site within C1q receptor/calreticulin

C1q receptor (C1qR/collectin receptor) is located on many cell types. Binding of C1q to these cells elicits numerous responses. Protein sequencing has shown that C1qR is almost identical to calreticulin (CaR), an abundant multifunctional protein. Radioiodinated C1qR and CaR bind to C1q with identical characteristics. Three recombinant C1qR/CaR domains (N-, C-terminal domains and central P-domain) were expressed using the Thiofusion system, and used to study the interaction with C1q. Both the N- and P-domains were implicated in C1q binding. A region, termed the S-domain, spanning the N and P intersection was expressed, and showed concentration-dependent binding to C1q, demonstrating that the C1q binding site lies within this region.

The effect of increasing end-expiratory pressure on extravascular lung water

The objective of this study was to define the response of extravascular lung water (EVLW) to different levels of positive end-expiratory pressure (PEEP) following a standardized oleic acid injury to the lung. All animals responded to the injection of intravenous oleic acid by the rapid development of hypoxemia. There was a twofold increase in EVLW during the first 3 hours after oleic acid injection which remained stable during the remainder of the experiment, including periods on PEEP. Intrapulmonary shunt (Qs/Qt) increased significantly (P less than 0.001) during the first hour following oleic acid injection. PEEP therapy resulted in an immediate decrease in Qs/Qt and amelioration of the hypoxemia. Return to zero PEEP resulted in a rapid decrease in PaO2 with concomitant increase in Qs/Qt by the end of the experiment. The oxygen transport in the animals did not improve significantly with the addition of PEEP. This was due to the decrease in cardiac output that more than offset the effects of a diminished Qs/Qt with PEEP. This study indicates that the mechanism by which PEEP improves oxygenation does not appear to be mediated by effect on lung water. The study also emphasizes the importance of determining oxygen transport when managing patients on PEEP.