Ultrastructural characterization of maturing iPSC-derived nephron structures upon transplantation

Pluripotent stem cell-derived kidney organoids hold great promise as a potential auxiliary transplant tissue for individuals with end-stage renal disease and as a platform for studying kidney diseases and drug discovery. To establish accurate models, it is crucial to thoroughly characterize the morphological features and maturation stages of the cellular components within these organoids. Nephrons, the functional units of the kidney, possess distinct morphological structures that directly correlate with their specific functions. High spatial resolution imaging emerges as a powerful technique for capturing ultrastructural details that may go unnoticed with other methods such as immunofluorescent imaging and scRNA sequencing. In our study, we have applied software capable of seamlessly stitching virtual slides generated from electron microscopy, resulting in high-definition overviews of tissue slides. With this technology, we can comprehensively characterize the development and maturation of kidney organoids when transplanted under the renal capsule of mice. These organoids exhibit advanced ultrastructural developments upon transplantation, including the formation of the filtration barrier in the renal corpuscle, the presence of microvilli in the proximal tubule, and various types of cell sub-segmentation in the connecting tubule similarly to those seen in the adult kidney. Such ultrastructural characterization provides invaluable insights into the structural development and functional morphology of nephron segments within kidney organoids and how to advance them by interventions such as a transplantation. Research Highlights High-resolution imaging is crucial to determine morphological maturation of hiPSC-derived kidney organoids. Upon transplantation, refined ultrastructural development of nephron segments was observed, such as the development of the glomerular filtration barrier.

Characterization of the gene encoding component C3 of the complement system from the spider Loxosceles laeta venom glands: Phylogenetic implications

A transcriptome analysis of the venom glands of the spider Loxosceles laeta, performed by our group, in a previous study (Fernandes-Pedrosa et al., 2008), revealed a transcript with a sequence similar to the human complement component C3. Here we present the analysis of this transcript. cDNA fragments encoding the C3 homologue (Lox-C3) were amplified from total RNA isolated from the venom glands of L. laeta by RACE-PCR. Lox-C3 is a 5178 bps cDNA sequence encoding a 190kDa protein, with a domain configuration similar to human C3. Multiple alignments of C3-like proteins revealed two processing sites, suggesting that Lox-C3 is composed of three chains. Furthermore, the amino acids consensus sequences for the thioester was found, in addition to putative sequences responsible for FB binding. The phylogenetic analysis showed that Lox-C3 belongs to the same group as two C3 isoforms from the spider Hasarius adansoni (Family Salcitidae), showing 53% homology with these. This is the first characterization of a Loxosceles cDNA sequence encoding a human C3 homologue, and this finding, together with our previous finding of the expression of a FB-like molecule, suggests that this spider species also has a complement system. This work will help to improve our understanding of the innate immune system in these spiders and the ancestral structure of C3.

Loxosceles spider venom induces the release of thrombomodulin and endothelial protein C receptor: implications for the pathogenesis of intravascular coagulation as observed in loxoscelism

BACKGROUND

The venom of the spider Loxosceles can cause both local and systemic effects including disseminated intravascular coagulation.

AIM

The aim of this study was to investigate the effects of the venom of Loxosceles intermedia (L. intermedia) and the purified Sphingomyelinase D (SMaseD) toxin upon the Protein C (PC) natural anticoagulant pathway.

RESULTS

Both the venom and e purified SMaseD reduced the cell surface expression of thrombomodulin (TM) and Endothelial PC Receptor on endothelial cells in culture. The reduction of cell surface expression was caused by cleavage from the cell surface mediated by activation of an endogenous metalloproteinase. Reduction of TM and Endothelial PC Receptor on the surface of these cells resulted in an impaired ability of the cells to assist in the thrombin-induced activation of PC.

CONCLUSION

This novel observation gives further insight into the mechanisms of the pathology induced by venom from Loxosceles spiders and may aid the development of a suitable therapy.

Sphingomyelinases D induce direct association of C1q to the erythrocyte membrane causing complement mediated autologous haemolysis

Bites by Loxosceles spiders can induce severe clinical symptoms, including dermonecrosis, thrombosis, vascular leakage, haemolysis and persistent inflammation. The causative toxin is a sphingomyelinase D (SMase D) that cleaves sphingomyelin into choline and ceramide-1-phosphate. A similar enzyme, showing comparable bioactivity, is secreted by certain pathogenic corynebacteria and acts as a potent virulence factor. We have previously found that SMase D toxins led to an increased susceptibility of human erythrocytes (E) to activation of complement (C) via the classical pathway (CP) in the absence of antibodies. In the present study we have investigated the CP initiating components involved in the haemolysis induced by SMases from Corynebacterium pseudotuberculosis (PLD) and from Loxosceles intermedia venom (P1). When P1 or PLD treated E were incubated with C8-depleted human serum, an increase in C1q, serum amyloid protein (SAP) and C-reactive protein (CRP) binding was observed. While purified C1q, SAP and CRP were found to bind to P1 or PLD treated E, depletion of SAP or CRP from human serum did not prevent C-mediated lysis, suggesting that pentraxins are not involved in the initiation of C-activation. However depletion of C1 lead to a greatly reduced haemolysis, demonstrating that the activation of the CP is caused by direct binding of C1q to the SMase treated cells. Binding of fluid phase C-regulators C4b-binding protein and factor H was also observed, however these C-regulators in conjunction with the membrane bound C-regulators were unable to prevent haemolysis, demonstrating the potency of SMase D facilitated binding of C1 and activation of C.

Variations in Loxosceles spider venom composition and toxicity contribute to the severity of envenomation

Envenomation by Loxosceles spiders causes two main clinical manifestations: cutaneous and systemic loxoscelism. The factors contributing to the severity of loxoscelism are not fully understood. We have analysed biochemical and toxicity variations in venom of L. laeta and L. intermedia, with the aim to find a correlation with the seriousness of loxoscelism. Differences in expression of proteins, glycoproteins and sphingomyelinase activity were observed between venom from male and female spiders and between venom from the two species. These differences were reflected in the toxicity of the venoms including the capacity to induce complement-dependent haemolysis, dermonecrosis and lethality. Comparative analysis of gender and species, showed that these biological activities were more prominent in venom from female spiders, especially from L. laeta. Antiserum raised against venom from females L. laeta spiders had the highest efficacy in neutralizing venoms of males and females of both species. These results indicate that the severity of loxoscelism depends, at least partially, on the species and sex of the spider and suggest that for accidents involving L. laeta an specific serum therapy is necessary. Furthermore, it emphasizes the efficacy of the antiserum produced against L. laeta female venom in neutralizing Loxosceles venoms from different species and gender.

Role and regulation of pig CD59 and membrane cofactor protein/CD46 expressed on pig aortic endothelial cells

BACKGROUND

Hyperacute rejection in xenotransplantation is caused by activation of complement (C) on endothelium. We have previously shown that purified C-regulators of the pig (CD59 and membrane cofactor protein [MCP]) are efficient regulators of human C (HuC). The aim of this study was to clarify the role of endogenously expressed C-regulatory molecules on pig endothelium in the protection against hyperacute rejection.

METHODS

Porcine aortic endothelial cells (PAEC) were harvested and cultured for various passages. PAEC were examined for the expression of endogenous pig CD59 and MCP by flow cytometry. PAEC were assessed for their susceptibility to lysis by HuC. The effect of phorbol 12-myristate 13-acetate and various cytokines on the expression of MCP and CD59 and C-susceptibility was assessed.

RESULTS

Primary PAEC showed an initial high level of expression of pig CD59, however, upon culturing, CD59 levels decreased dramatically to about 20% after five passages. In contrast, levels of MCP doubled upon culturing of PAEC to confluency and remained stable during at least five passages. Primary cells and cells in the early passages were more resistant to HuC than cells that were cultured for longer. Blocking the function of CD59 but not of MCP using monoclonal antibody increased the susceptibility to HuC. Purified human CD59 incorporated to a level of expression similar to that of pig CD59 reversed the increased C-susceptibility, suggesting that pig and human CD59 are similarly protective against HuC. Increase of C-resistance and of expression of pig MCP, but not of CD59, was achieved upon incubation with phorbol 12-myristate 13-acetate. Tumor necrosis factor-alpha, interleukin-1beta, interleukin-4, or interferon-gamma had no effect on C-regulator expression or C-susceptibility.

CONCLUSIONS

These data demonstrate the importance of using primary PAEC or cells in the first passages of culturing in in vitro models of xenotransplantation and show that pig MCP and, in particular, pig CD59 play an important role in protection of PAEC from HuC.

Sphingomyelinases in the venom of the spider Loxosceles intermedia are responsible for both dermonecrosis and complement-dependent hemolysis

The bite of spiders of the genus Loxosceles can induce a variety of biological effects, including dermonecrosis and complement (C) dependent haemolysis. The aim of this study was to characterise the toxins in the venom responsible for the different biological effects. We have previously shown that a 35 kDa protein, named F35, purified from Loxosceles intermedia venom, incorporates into the membranes of human erythrocytes and renders them susceptible to the alternative pathway of autologous C. Here we have further purified the F35 protein which was resolved by reversed phase chromatography into three tightly contiguous peaks termed P1, P2, and P3. P1 and P2 were shown to be homogeneous by SDS-PAGE and N-terminal aminoacid analysis, while P3 consisted of two highly homologous proteins. N-terminal sequencing of all four proteins showed a high degree of homology, which was confirmed by cross-reactivity of antisera raised against the individual purified proteins. Functional characterisation of P1 and P2 indicated the presence of sphingomyelinase activity and either protein in isolation was capable of inducing all the in vivo effects seen with whole spider venom, including C-dependent haemolysis and dermonecrosis. In all assays, P2 was more active than P1, while P3 was completely inactive. These data show that different biological effects of L. intermedia venom can be assigned to the sphingomyelinase activity of two highly homologous proteins, P1 and P2. Identification of these proteins as inducers of the principal pathological effects induced by whole venom will aid studies of the mechanism of action of the venom and the development of a effective therapy.

Mechanisms of complement resistance induced by non-lethal complement attack and by growth arrest

Non-lethal complement (C) attack on K562 cells has been shown to induce a transient resistance to lethal amounts of C. We have previously shown that incubation of K562 with phorbol 12-myristate 13-acetate (PMA) caused an increase in both CD59 expression and resistance to C killing and we were interested to examine whether non-lethal C attack caused a similar effect. We here demonstrate that expression of the C inhibitors decay-accelerating factor (DAF), membrane cofactor protein (MCP) and CD59 was unaltered on K562 after non-lethal C attack and that neutralization of these inhibitors with specific blocking antibodies did not reverse the induced resistance. In an effort to understand the mechanisms of resistance we searched for other conditions that might induce C resistance in K562 cells. Growth-arrested cells showed a similar degree of resistance to C killing. The levels of DAF and MCP on these cells were unaltered whereas expression of CD59 was markedly reduced. Non-lethal C attack on these growth-arrested cells induced a further increase in resistance to C killing, suggesting that the mechanisms of resistance were not identical. Indeed, resistance of non-lethally attacked cells was completely lost within 8 hr of attack whereas resistance of growth-arrested cells was detectable for up to 48 hr after returning to cell cycle. These data demonstrate that C resistance induced by two distinct strategies is not mediated by the known membrane C inhibitors. Resistance may be a result of the expression of a novel inhibitor or due to metabolic depletion, a likely common consequence of non-lethal C attack and induction of growth arrest, implying that cells take an active role in C-mediated killing.

The glycosylation of the complement regulatory protein, human erythrocyte CD59

Human erythrocyte CD59 contains N- and O-glycans and a glycosylphosphatidylinositol (GPI) anchor, all of which have been analyzed in this study. The anchor consists principally of the minimum core glycan sequence Manalpha1-2Manalpha1-6Manalpha1-4GlcN-linked to a phosphatidylinositol moiety with the structure sn-1-O-alkyl(C18:0 and C18:1)-2-O-acyl(C20:4)glycerol-3-phospho-1-(2-O-palmitoyl(C16:0))myo- inositol. This structure is essentially identical to that of human erythrocyte cholinesterase (Deeg, M. A., Humphrey, D. R., Yang, S. H. , Ferguson, T. R., Reinhold, V. N., and Rosenberry, T. L. (1992) J. Biol. Chem. 267, 18573-18580). This first comparison of GPI anchors from different proteins expressed in the same tissue suggests that human reticulocytes produce only one type of anchor structure. The N- and O-glycans were sequenced using a novel approach involving digestion of the total glycan pool with multiple enzyme arrays. The N-glycan pool contained families of bi-antennary complex-type structures with and without lactosamine extensions and outer arm fucose residues. The predominant O-glycans were NeuNAcalpha2-3Galbeta1-3GalNAc and Galbeta1-3[NeuNAcalpha2-3]GalNAc. Inspection of a molecular model of CD59, based on the NMR solution structure of the extracellular domain and the structural data from this study, suggested several roles for the glycans, including spacing and orienting CD59 on the cell surface and protecting the molecule from proteases. This work completes the initial structural analysis of CD59, providing the most complete view of any cell surface glycoprotein studied to date.

Molecular cloning, chromosomal localization, expression, and functional characterization of the mouse analogue of human CD59

We have previously described the isolation and cloning of the rat analogue of the human complement inhibitor CD59 (hCD59). Using the rat CD59 (rCD59) coding region as probe, we have isolated positive cDNAs from a mouse kidney cDNA library. Sequence analysis of these clones indicated that they contained an open reading frame encoding a 124 amino acid protein. Comparisons with the known sequences of hCD59 and rCD59 suggested that the clones contained a full-length cDNA encoding the mouse analogue of CD59 (mCD59). The cDNA encoded a 81-bp 5'-flanking region, a 23 amino acid NH2-signal peptide, a 101 amino acid coding region including putative N-glycosylation sites and a glycosyl phosphatidylinositol (GPI) anchoring signal, and approximately 0.8 kb 3'-untranslated flanking region. Reverse transcriptase PCR revealed the presence of mCD59 mRNA in all mouse tissues examined. The gene for mCD59 was mapped by fluorescence in situ hybridization to the E2-E4 region of mouse chromosome 2, a region that includes areas syntenous with the location of the human CD59 gene on chromosome 11p13. Expression of mCD59 in a CD59-negative human cell line conferred protection against lysis by complement from rodent, human, and several other species, confirming that mCD59 was the functional analogue of hCD59 and that function was not species restricted. The expressed protein was glycosyl phosphatidylinositol anchored as demonstrated by its partial removal from U937 cells on treatment with phosphatidylinositol-specific phospholipase C. Abs raised against the expressed protein demonstrated the presence of mCD59 on all mouse blood cell types and on several mouse cell lines and neutralized function of mCD59 on mouse E and expressed on U937. Western blot analysis revealed that both expressed and endogenous mCD59 had a molecular mass of 22 to 24 kDa.

Purification and characterization of the pig analogue of human membrane cofactor protein (CD46/MCP)

A panel of mAbs were raised against pig lymphocytes. Seven mAbs immunoprecipitated a 50- to 60-kDa membrane-bound protein. This protein, termed JM4C8-Ag, was expressed on a wide variety of cells, including all circulating cells and cells of fibroblast, epithelial, and endothelial origin. The JM4C8-Ag was transmembrane-anchored and glycosylated. One of the Abs was used in immunoaffinity chromatography to isolate JM4C8-Ag from erythrocyte membranes. N-terminal amino acid analysis through the first 28 residues showed a 43% homology with the human complement regulatory molecule membrane cofactor protein (MCP; CD46). The purified protein had cofactor activity for factor I-mediated cleavage of human and pig C3b, confirming its identity as the pig analogue of human MCP. The purified protein also strongly inhibited lysis of rabbit erythrocytes by human and pig complement after activation of the classical or alternative pathway. This is the first report of a nonprimate analogue of MCP. The presence of a resident MCP on pig cells capable of acting as a cofactor in the control of human complement activation has consequences for the use of pig organs in xenotransplantation.

Molecular cloning, chromosomal localization, expression, and functional characterization of the mouse analogue of human CD59

We have previously described the isolation and cloning of the rat analogue of the human complement inhibitor CD59 (hCD59). Using the rat CD59 (rCD59) coding region as probe, we have isolated positive cDNAs from a mouse kidney cDNA library. Sequence analysis of these clones indicated that they contained an open reading frame encoding a 124 amino acid protein. Comparisons with the known sequences of hCD59 and rCD59 suggested that the clones contained a full-length cDNA encoding the mouse analogue of CD59 (mCD59). The cDNA encoded a 81-bp 5'-flanking region, a 23 amino acid NH2-signal peptide, a 101 amino acid coding region including putative N-glycosylation sites and a glycosyl phosphatidylinositol (GPI) anchoring signal, and approximately 0.8 kb 3'-untranslated flanking region. Reverse transcriptase PCR revealed the presence of mCD59 mRNA in all mouse tissues examined. The gene for mCD59 was mapped by fluorescence in situ hybridization to the E2-E4 region of mouse chromosome 2, a region that includes areas syntenous with the location of the human CD59 gene on chromosome 11p13. Expression of mCD59 in a CD59-negative human cell line conferred protection against lysis by complement from rodent, human, and several other species, confirming that mCD59 was the functional analogue of hCD59 and that function was not species restricted. The expressed protein was glycosyl phosphatidylinositol anchored as demonstrated by its partial removal from U937 cells on treatment with phosphatidylinositol-specific phospholipase C. Abs raised against the expressed protein demonstrated the presence of mCD59 on all mouse blood cell types and on several mouse cell lines and neutralized function of mCD59 on mouse E and expressed on U937. Western blot analysis revealed that both expressed and endogenous mCD59 had a molecular mass of 22 to 24 kDa.

Purification and characterization of the pig analogue of human membrane cofactor protein (CD46/MCP)

A panel of mAbs were raised against pig lymphocytes. Seven mAbs immunoprecipitated a 50- to 60-kDa membrane-bound protein. This protein, termed JM4C8-Ag, was expressed on a wide variety of cells, including all circulating cells and cells of fibroblast, epithelial, and endothelial origin. The JM4C8-Ag was transmembrane-anchored and glycosylated. One of the Abs was used in immunoaffinity chromatography to isolate JM4C8-Ag from erythrocyte membranes. N-terminal amino acid analysis through the first 28 residues showed a 43% homology with the human complement regulatory molecule membrane cofactor protein (MCP; CD46). The purified protein had cofactor activity for factor I-mediated cleavage of human and pig C3b, confirming its identity as the pig analogue of human MCP. The purified protein also strongly inhibited lysis of rabbit erythrocytes by human and pig complement after activation of the classical or alternative pathway. This is the first report of a nonprimate analogue of MCP. The presence of a resident MCP on pig cells capable of acting as a cofactor in the control of human complement activation has consequences for the use of pig organs in xenotransplantation.

Exogenous glycosyl phosphatidylinositol-anchored CD59 associates with kinases in membrane clusters on U937 cells and becomes Ca(2+)-signaling competent

CD59, an 18-20-kD complement inhibitor anchored to the membrane via glycosyl phosphatidylinositol (GPI), can induce activation of T cells and neutrophils upon cross-linking with antibody. GPI-anchored molecules cocluster in high mol wt detergent-resistant complexes containing tyrosine kinases that are implicated in the signaling pathway. Exogenous, incorporated GPI-anchored molecules are initially unable to induce activation, presumably because they are not associated with kinases. Here we demonstrate that erythrocyte-derived CD59 incorporated in a CD59-negative cell line acquires signaling capacity in a time-dependent manner. Confocal microscopy revealed an initial diffuse distribution of CD59 that became clustered within 2 h to give a pattern similar to endogenous GPI-anchored molecules. Gel filtration of detergent-solubilized cells immediately after incorporation revealed that CD59 was mainly monomeric, but after 3 h incubation all was in high mol wt complexes and had become associated with protein kinases. Newly incorporated CD59 did not deliver a Ca2+ signal upon cross-linking, but at a time when it had become clustered and associated with kinase activity, cross-linking induced a large calcium transient, indicating that CD59 had incorporated in a specialized microenvironment that allowed it to function fully as a signal-transducing molecule.

Regulation of CD59 expression on K562 cells: effects of phorbol myristate acetate, cross-linking antibody and non-lethal complement attack

CD59 is the major membrane attack complex of complement (MAC) inhibiting protein on human cells. Its regulation is therefore an important factor in determining the fate of cells at sites of complement activation. We have chosen the K562 erythroleukaemia cell line as a model for studies of the regulation of CD59 expression, because it has previously been reported that phorbol 12-myristate 13-acetate (PMA) caused a 15-fold up-regulation of CD59 mRNA in these cells, implying a substantial capacity for CD59 synthesis. However, no assessment of CD59 protein expression was made in these studies. We show here that surface expression of CD59, as assessed by flow cytometry, was increased four-fold over a 16-hr incubation with PMA, whereas surface expression of decay-accelerating factor (DAF) (CD55) and membrane cofactor protein (MCP) (CD46) was not altered. The newly expressed CD59 was functionally active and anchored through glycosyl-phosphatidylinositol (GPI). Increased expression was dependent upon de novo protein synthesis. CD59 released into cell supernatant was also increased seven-fold by PMA, this 'secreted' CD59 retained its GPI anchor. Non-lethal complement attack did not alter CD59 expression but antibody cross-linking of CD59 caused a rapid loss of the CD59-antibody complexes. However, CD59 was quickly restored to pre-attack levels. This rapid restoration was not dependent upon protein synthesis, suggesting release from preformed stores.

A rapid method for the isolation of analogues of human CD59 by preparative SDS-PAGE: application to pig CD59

A method for the rapid isolation of functionally active analogues of human CD59 from erythrocytes (E) is described. The method, here applied to pig E, is based on the fractionation of a butanol extract of E ghosts by preparative SDS-PAGE followed by gel filtration on Superose 12. Purification was monitored using a functional complement inhibition assay. SDS-PAGE analysis of the product of this procedure indicated a single protein band with apparent M(r) of 20 kDa under reducing and non-reducing conditions. The preparation could be incorporated into guinea pig E to inhibit both CVF-reactive lysis and lysis through C8 and C9 using preformed C5b-7 sites, demonstrating that it contained a CD59-like activity. PIPLC treatment of the isolated protein abolished the inhibition. In contrast to SDS-PAGE analysis, amino-terminal sequence analysis of the preparation showed that it comprised two components. One was identified from databank searches as a fragment of pig glycophorin. These two components could not be separated by either standard or affinity chromatographic techniques. The second component was novel and had high sequence homology with human CD59, identifying it as the pig analogue. Further functional studies showed that the pig analogue of human CD59 was effective in the protection of guinea pig E against lysis by serum from a variety of species, including human.

Molecular cloning of the rat analogue of human CD59: structural comparison with human CD59 and identification of a putative active site

We have previously described the purification and partial characterization of the rat analogue of the human complement regulatory molecule CD59 [Hughes, Piddlesden, Williams, Harrison and Morgan (1992) Biochem. J. 284, 169-176]. We present here the molecular cloning and full sequence analysis of this molecule. A PCR-based approach utilizing primers designed from the amino-terminal protein sequence was used to isolate a full-length cDNA clone from a rat kidney cDNA library. This clone encoded a 92 bp 5'-flanking sequence, a 66 bp signal peptide and a 315 bp coding region containing putative glycosylation and GPI-anchor signals. The 3' untranslated flanking region was approximately 1.1 kbp long and included the poly-A tail and a CATA repeating sequence. The coding region was 58% identical with the human cDNA at the nucleotide level and 44% identical at the amino acid level. Despite this relatively low overall sequence conservation, several highly conserved stretches were apparent, particularly in the N-terminal portion of the molecule, in the cysteine-rich region immediately preceding the site of glycolipid attachment and in the C-terminal peptide removed during glycolipid attachment. An N-glycosylation site was identified at Asn-16 and a putative glycosylphosphatidylinositol anchor addition site at Asn-79, indicating that the mature processed protein was two residues longer than human CD59. Comparison of the sequences of rat and human CD59, together with consideration of the published three-dimensional structure of human CD59 and functional data, implicates specific regions of the protein in interactions with C-8 and/or C-9.

Complement-inhibiting activities of human CD59 and analogues from rat, sheep, and pig are not homologously restricted

Human erythrocyte CD59 and analogues isolated from erythrocytes of rat, sheep, and pig were examined for their ability to protect erythrocytes from various species against lysis by C from homologous and heterologous sources. In all cases, incorporation of human CD59 or analogues from rat, sheep, and pig efficiently protected guinea pig erythrocytes against lysis by C homologous with the CD59. However, each of the CD59 analogues also conferred on guinea pig erythrocytes protection against C from most heterologous species. These results demonstrate that none of the CD59 analogues tested were species specific in their C-inhibiting activity. Erythrocytes from species other than guinea pig could not be protected by incorporation of any of the available CD59 analogues despite similar incorporation in all erythrocytes tested. We suggest that the presence of endogenous inhibitors on these other erythrocytes masks the activity of incorporated CD59. Evidence that is supportive of this hypothesis was provided by demonstrating that blocking the endogenous CD59 with mAbs rendered erythrocytes susceptible to inhibition by high dosages of incorporated CD59.

Complement-inhibiting activities of human CD59 and analogues from rat, sheep, and pig are not homologously restricted

Human erythrocyte CD59 and analogues isolated from erythrocytes of rat, sheep, and pig were examined for their ability to protect erythrocytes from various species against lysis by C from homologous and heterologous sources. In all cases, incorporation of human CD59 or analogues from rat, sheep, and pig efficiently protected guinea pig erythrocytes against lysis by C homologous with the CD59. However, each of the CD59 analogues also conferred on guinea pig erythrocytes protection against C from most heterologous species. These results demonstrate that none of the CD59 analogues tested were species specific in their C-inhibiting activity. Erythrocytes from species other than guinea pig could not be protected by incorporation of any of the available CD59 analogues despite similar incorporation in all erythrocytes tested. We suggest that the presence of endogenous inhibitors on these other erythrocytes masks the activity of incorporated CD59. Evidence that is supportive of this hypothesis was provided by demonstrating that blocking the endogenous CD59 with mAbs rendered erythrocytes susceptible to inhibition by high dosages of incorporated CD59.

Presence of a dysfunctional form of CD59 on a CD59+ subclone of the U937 cell line

U937 cells are known to be relatively sensitive to C-mediated killing and have been reported to show variable expression of CD59. We have obtained stable CD59+ and CD59- sublines of the U937 cell line. Expression of other C-regulatory proteins, decay-accelerating factor (DAF), MCP and CR1, was similar on both cell lines. Although the sublines were morphologically similar and expressed similar amounts of most surface antigens, qualitative difference in expression of CD13 and CD64 and a quantitative difference in CD15 expression was observed. Sensitivity to C-mediated killing of the cell lines was measured using classical pathway activation. Both cell lines appeared to be equally sensitive to C-mediated killing. Monoclonal antibodies against CD59, which neutralize CD59 and enhance killing of most cell lines (including K562, HL60 and Molt4), did not enhance the killing of the CD59- cells but, surprisingly, also did not enhance killing of the CD59+ U937 subline. CD59 was expressed on the U937 subline at similar levels to that on HL60 and K562 cells, was glycosylphosphatidylinositol (GPI) anchored and could be immunoprecipitated from cell extracts. However, unlike these other cell lines, U937 cell extracts were negative in a Western blot using a variety of anti-CD59 antibodies even when ultrasensitive detection methods were used. These results indicate that the CD59+ U937 cell expresses a form of CD59 which is dysfunctional and structurally abnormal.

Cross-linking of CD59 and of other glycosyl phosphatidylinositol-anchored molecules on neutrophils triggers cell activation via tyrosine kinase

Many membrane proteins are attached via a glycosyl phosphatidylinositol (GPI) anchor. Proteins anchored in this way make no direct contact with the interior of the cell, therefore a role in signaling or activation would seem unlikely. Nevertheless, cross-linking of GPI-anchored proteins on human and murine T lymphocytes has been shown to cause calcium transients and cell activation. Our studies address the non-lethal events caused by the membrane attack complex of complement, which include release of Ca2+ from intracellular stores, and have suggested that the GPI-anchored complement inhibitor CD59 may be involved in signaling these events. We here report that cross-linking of CD59 on human neutrophils using specific monoclonal antibody and second antibody caused a rapid increase in intracellular free Ca2+ concentration (Ca2+ transient) due to release of Ca2+ from stores and also caused neutrophil oxidase activation. All antibodies against CD59 tested were effective and cross-linking of any other GPI-anchored protein expressed on neutrophils also initiated an increase in intracellular free Ca2+ concentration, whereas cross-linking of transmembrane proteins caused little or no response. A tyrosine kinase-dependent activation pathway was indicated by the demonstration of tyrosine phosphorylation on cross-linking and by blocking of the Ca2+ transient with the tyrosine kinase inhibitor herbimycin.

The sheep analogue of human CD59: purification and characterization of its complement inhibitory activity

An inhibitor of the membrane attack complex of complement was isolated from the membranes of sheep erythrocytes. Fast protein liquid chromatography (FPLC) and affinity purification procedures for this sheep complement-inhibiting protein (SCIP) both yielded a pure protein with an apparent M(r) of 19,000 under reducing and non-reducing conditions. Incubation of the denatured protein with neuraminidase and Endo-F reduced the apparent M(r) to 18,000 and 15,000 respectively, while treatment with O-deglycosidase or phosphatidylinositol-specific phospholipase C (PIPLC) did not affect the apparent M(r). SCIP was detectable on erythrocytes and lymphocytes but not on platelets and could partially be removed by PIPLC treatment. Deglycosylation of the pure protein markedly reduced and PIPLC treatment abolished its activity. A monoclonal antibody (mAb) raised against sheep complement-inhibiting protein (SCIP) enhanced the susceptibility of sheep erythrocytes to lysis by homologous complement. SCIP inhibited complement after the stage of C5b-7 formation. Amino-terminal protein sequence was obtained and was shown to be similar to that of human CD59. All these features suggest that SCIP is the sheep equivalent of human CD59. Human CD59 has been reported to be species selective in that it inhibits complement from relatively few species. However, SCIP efficiently inhibited lysis of guinea-pig erythrocytes by complement from a wide range of species tested indicating that it is a potent and non-selective inhibitor of the membrane attack complex of complement (MAC).

Slp is an essential component of an EDTA-resistant activation pathway of mouse complement

Slp (sex-limited protein) is a mouse serum protein encoded by a major histocompatibility complex class III gene. It is considered to be a product of a duplicated complement component C4 gene, but without functional activity. Originally it has been found expressed only in adult males with the S region of the H-2d or H-2s haplotype. In this report we present evidence that Slp is involved in a form of mouse complement activation that occurs after fractionation of serum by polyethylene glycol precipitation. This activation pathway is EDTA-resistant (i.e., independent of classical and alternative pathway activation), is regulated by C1 inhibitor, and leads to the generation of hemolytically active membrane attack complexes. A positive correlation between this EDTA-resistant mouse complement activity and reported Slp levels was found. Direct evidence for a functional role of Slp came from substitution experiments in which purified Slp induced hemolytic activity in polyethylene glycol-fractionated, Slp-deficient mouse serum. Selective depletion of other complement components suggested a role for C1s-, C2, and C5, but not C3, in the Slp-dependent complement activation. A model for this type of mouse complement activation is presented.

C1-inhibitor prevents PEG fractionation-induced, EDTA-resistant activation of mouse complement

Fractionation of mouse serum by precipitation with a critical amount of polyethylene glycol 6000 (PEG; 11% w/v) results in a classical and alternative pathway-independent activation of the terminal complement route. The activation can take place after the separation of an activating principle together with the terminal route components from a natural regulator. The isolation and identification of the regulatory component preventing this activation in serum, is subject of this paper. The regulator was purified by fractionated PEG-precipitation (15-25%), followed by heparin-Sepharose affinity, Mono Q anion-exchange, and Superose 12 gel filtration chromatography. The regulator appeared to be a single-chain protein with a Mr of 96 k. A protein with similar activity purified from human serum had a Mr of 104 k and was functionally and antigenically indistinguishable from C1-INH. The mouse 96 k protein inhibited C1-esterase activity indicating that this protein is indeed C1-INH. Mouse C1-INH regulates the PEG fractionation-induced bypass activation of complement, but does not interfere with the assembly or the lytic activity of membrane attack complexes. alpha 2-Macroglobulin appeared also to be capable of inhibiting the PEG-precipitation-induced activation process, but with lower efficiency.

Rapid isolation of human complement component C9 to verify the specificity of a haemolytic C9 microassay

A sensitive, haemolytic microassay of human complement component C9 was developed. The assay is based on the principle of reactive (C5b6-initiated) haemolysis and uses commercially available C9-depleted serum as reagent for C9. The specificity of the assay was verified by rapid, activity-guided isolation of the haemolytic component from human serum using high-performance liquid chromatography (HPLC) on a system for fast protein liquid chromatography. This isolation yielded a single component with characteristics of C9. The results suggest that rapid, activity-guided isolation as a new application of HPLC can be a useful tool to demonstrate the specificity of a functional assay.

Induction of active immunological hypo/non-responsiveness to C5 in adult C5-deficient DBA/2 mice

Injection of C5-sufficient BALB/c serum rendered DBA/2 mice (C5-deficient) immunologically hypo- or non-responsive to C5. This was indicated by C5-elimination studies in the C5-deficient mice showing similar half-lives for C5 upon single and repeated BALB/c serum injection. Concrete evidence for C5 non-responsiveness came from experiments showing that C5-injected DBA/2 mice were unable to mount an anti-C5 antibody response after active immunization with C5-sufficient serum in Freund's complete adjuvant. C5 hypo/non-responsiveness could be induced in DBA/2 mice via the intravenous as well as the intraperitoneal route, provided the C5-sufficient serum was administered in the very narrow dose range of 10-100 microliters (approximately 0.3-3 micrograms of C5). Upon i.v. C5 injection, C5 non-responsiveness was nearly complete on Day 4 and lasted about 3 weeks. Hyporesponsiveness was still present 6 weeks after serum injection. C3-/C5-depleting cobra venom factor reversed tolerization for C5, at least when applied within 48 hr after i.v. C5 injection. Similarity between the acquired C5 hypo/non-responsiveness of DBA/2 mice and the established C5 tolerance of BALB/c mice was suggested by adoptive cell transfer experiments: spleen cells from naive DBA/2 mice stimulated B cells of C5-sufficient nude mice to produce C5-neutralizing antibodies. In contrast, splenocytes from C5-tolerized DBA/2 mice, like those of BALB/c mice, did not decrease haemolytic C5 levels in C5-sufficient nude mice.

C5 does not play a major role in the immune response of mice to SRBC in vivo

The role of complement component C5 in the immune response of mice to sheep red blood cells (SRBC) was investigated. Congenic C5-sufficient and C5-deficient B10. D2 mice and genetically C5-deficient DBA/2 mice, as such or supplemented with C5-sufficient serum, were used as experimental animals. C5-substitution of the C5-deficient mice resulted in measurable C5 levels for days. The functional half-life of C5 in C5-deficient DBA/2 mice was about 21 h. No significant differences between the IgM-responses of C5-bearing and naive C5-deficient animals were observed. This suggests that C5 does not play a major role in the primary humoral immune response of mice in vivo, although C5 seems to do so in in vitro experiments, even with the same antigen. Antigen-induced C5-production by C5-deficient mice as one of the explanations of the in vitro/in vivo discrepancy could not be confirmed experimentally.

Gramicidin-induced hexagonal HII phase formation in negatively charged phospholipids and the effect of N- and C-terminal modification of gramicidin on its interaction with zwitterionic phospholipids

The effect of gramicidin on macroscopic structure of the negatively charged membrane phospholipids cardiolipin, dioleoylphosphatidylglycerol and dioleoylphosphatidylserine in aqueous dispersions was investigated and compared with the effect of gramicidin on dioleoylphosphatidylcholine. It was shown by small-angle X-ray diffraction, 31P nuclear magnetic resonance and freeze-fracture electron microscopy that in all these lipid systems gramicidin is able to induce the formation of a hexagonal HII phase. 31P-NMR measurements indicated that the extent of HII phase formation in the various lipids ranged from about 40% to 60% upon gramicidin incorporation in a molar ratio of peptide to lipid of 1 : 10. Next, the following charged analogues of gramicidin were prepared: desformylgramicidin, N-succinylgramicidin and O-succinylgramicidin. The synthesis was verified with 13C-NMR and the effect of these analogues on lipid structure was investigated. It was shown that, as with gramicidin itself, the analogues induce HII phase formation in dioleoylphosphatidylcholine, lower and broaden the bilayer-to-HII phase transition in dielaidoylphosphatidylethanolamine and form lamellar structures upon codispersion with palmitoyllysophosphatidylcholine. Differential scanning calorimetry measurements indicated that, again like gramicidin, in phosphatidylethanolamine the energy content of the gel-to-liquid-crystalline phase transition is not affected by incorporation of the analogues, whereas in phosphatidylcholine a reduction of the transition enthalpy is found. These observations were explained in terms of a similar tendency to self-associate for gramicidin and its charged analogues. The results are discussed in the light of the various factors which have been suggested to be of importance for the modulation of lipid structure by gramicidin.